研究者データベース

谷本 陽一(タニモト ヨウイチ)
地球環境科学研究院 地球圏科学部門 気候力学分野
教授

基本情報

所属

  • 地球環境科学研究院 地球圏科学部門 気候力学分野

職名

  • 教授

学位

  • 博士(理学)(東北大学)

ホームページURL

科研費研究者番号

  • 00291568

J-Global ID

研究キーワード

  • 大気海洋相互作用   気候変動   海面水温   10年スケール   海面フラックス   低気圧活動   数値実験   海洋表層   観測データ   水温前線   GPSラジオゾンデ観測   黒潮・黒潮続流域   数値モデル   海上気象観測   潜熱蒸発   季節遷移期   年々変動   海上風   亜寒帯前線   雲レジーム   梅雨   黒潮続流域   黒潮続流   熱帯及び中緯度海域   大気境界層   海洋前線帯   力学モード   異常気象   テレコネクション   ブロッキング   

研究分野

  • 自然科学一般 / 大気水圏科学

職歴

  • 2000年 - 2012年 北海道大学 大学院地球環境科学研究科(研究院) 准教授
  • 1998年10月 - 2000年03月 地球フロンティア研究システム サブリーダー
  • 1996年10月 - 1998年09月 東京都立大学理学部 助手
  • 1995年04月 - 1996年09月 ワシントン大学 日本学術振興会海外特別研究員
  • 1994年04月 - 1995年03月 日本学術振興会 日本学術振興会特別研究員(PD)

学歴

  • 1989年04月 - 1994年03月   東北大学   理学研究科   地球物理学専攻

所属学協会

  • アメリカ地球物理学連合   日本地理学会   アメリカ気象学会   日本気象学会   日本海洋学会   

研究活動情報

論文

  • Vivek Shilimkar, Hiroto Abe, Mathew Koll Roxy, Youichi Tanimoto
    Journal of Oceanography 2022年05月14日 
    Abstract The Indonesian throughflow (ITF) transports a significant amount of warm freshwater from the Pacific to the Indian Ocean, making it critical to the global climate system. This study examines decadal ITF variations using ocean reanalysis data as well as climate model simulations from the Coupled Model Inter-comparison Project Phase 5 (CMIP5). While the observed annual cycle of ITF transport is known to be correlated with the annual cycle of sea surface height (SSH) difference between the Pacific and Indian Oceans, ocean reanalysis data (1959–2015) show that the Pacific Ocean SSH variability controls more than 85% of ITF variation on decadal timescales. In contrast, the Indian Ocean SSH variability contributes less than 15%. While those observed contributions are mostly reproduced in the CMIP5 historical simulations, an analysis of future climate projections shows a 25–30% increase in the Indian Ocean SSH variability to decadal ITF variations and a corresponding decrease in the Pacific contribution. These projected changes in the Indian Ocean SSH variability are associated with a 23% increase in the amplitudes of negative zonal wind stress anomalies over the equatorial Indian Ocean, along with a 12º eastward shift in the center of action in these anomalies. This combined effect of the increased amplitude and eastward shift in the zonal wind stress increases the SSHA variance over the Indian Ocean, increasing its contribution to the ITF variation. The decadal ITF changes discussed in this study will be crucial in understanding the future global climate variability, strongly coupled to Indo-Pacific interactions.
  • Shoichiro Kido, Masami Nonaka, Youichi Tanimoto
    Geophysical Research Letters 2021年12月09日 [査読有り][通常論文]
  • Shoichiro Kido, Masami Nonaka, Youichi Tanimoto
    Journal of Geophysical Research: Oceans 126 8 2021年08月 [査読有り][通常論文]
  • Shimpo Akihiko, Takemura Kazuto, Wakamatsu Shunya, Togawa Hiroki, Mochizuki Yasushi, Takekawa Motoaki, Tanaka Shotaro, Yamashita Kazuya, Maeda Shuhei, Kurora Ryuta, Murai Hirokazu, Kitabatake Naoko, Tsuguti Hiroshige, Mukougawa Hitoshi, Iwasaki Toshiki, Kawamura Ryuichi, Kimoto Masahide, Takayabu Izuru, Takayabu Yukari N, Tanimoto Youichi, Hirooka Toshihiko, Masiunoto Yukio, Watanabe Masahiro, Tsuboki Kazuhisa, Nakamura Hisashi
    SOLA 15A A 13 - 18 2019年06月15日 [査読有り][通常論文]
     
    An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named "the Heavy Rain Event of July 2018", which caused widespread havoc. It was followed by heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of tropical origins confluent persistently into western Japan and largescale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan.
  • Simultaneous formation of the oceanic Rossby waves in the eastern North and South Pacific during the ENSO events.
    Abe, H, Y. Tanimoto, T. Hasegawa, N. Ebuchi
    Journal of Physical Oceanography 46 11 3397 - 3414 2016年11月 [査読有り][通常論文]
  • Hiroto Abe, Youichi Tanimoto, Takuya Hasegawa, Naoto Ebuchi
    JOURNAL OF PHYSICAL OCEANOGRAPHY 46 11 3397 - 3414 2016年11月 [査読有り][通常論文]
     
    The present study examined ENSO-related wind forcing contribution to off-equatorial Rossby wave formations in the eastern tropical regions of the North and South Pacific using satellite altimeter data and atmospheric reanalysis data during the period of 1993-2013. After mature phases of ENSO events, the sea surface height anomaly fields showed that off-equatorial Rossby waves propagated westward along 118N and 88S from the eastern Pacific. Starting longitudes of the westward propagation were distant from the eastern coast, especially for weak El Nino events in the 2000s, in contrast to the strong 1997/98 El Nino event in which the propagations started from the coast. Based on observational data, it was hypothesized that the Rossby waves could be formed by off-equatorial zonal belts of wind stress curl anomalies (WSCAs) in 1358-908W rather than by wave emissions from the eastern coast. A numerical model forced only by WSCAs, that is, without wave emissions from the coast, successfully reproduced observed features of the Rossby waves in 1808-1208W, supporting the study's hypothesis. During mature phases of El Nino events, equatorially symmetric negative sea level pressure anomalies (SLPAs) resulting from hydrostatic adjustment to the underlying warm sea surface temperature anomalies dominated over the eastern tropical Pacific. Anomalous surface easterlies blowing around the negative SLPA area as geostrophic winds were a major contributor in forming the anticyclonic WSCAs. The polarity of the anomalies is reversed during La Nina events. Therefore, spatial patterns of the SLPAs associated with the ENSO events are necessary to understand the Rossby wave formations.
  • Naoki Sato, Masami Nonaka, Yoshikazu Sasai, Hideharu Sasaki, Youichi Tanimoto, Ryuichi Shirooka
    JOURNAL OF OCEANOGRAPHY 72 5 697 - 705 2016年10月 [査読有り][通常論文]
     
    The seasonal cycle of the meridional sea-surface temperature (SST) gradient in the upstream Kuroshio Extension (KE) region was examined using satellite observation data and model simulations. In general, the meridional SST gradient is small in summer. However, in early summer (June and July), the SST front is sustained or intensified on the northern side of the KE near the coast of eastern Japan. This observed seasonal cycle was successfully simulated in the North Pacific Ocean model for the Earth Simulator (NP-OFES). Analysis of the forecast data revealed that the vertical profiles of temperature and salinity are shifted upward along the KE in early summer. As a result, the permanent thermocline depth is shallowest during summer, causing a relatively small SST tendency. In addition, significant cyclonic vorticity in the lower atmosphere related to the southwesterly sea-surface wind was found to the south of the KE, associated with the Baiu frontal zone (BFZ). It was inferred that the positive vorticity causes Ekman upwelling over the KE region, resulting in suppressed SST warming on the northern side of the KE. These results suggest that the BFZ contributes to maintaining or strengthening the SST front.
  • Shuhei Maeda, Yusuke Urabe, Kazuto Takemura, Tamaki Yasuda, Youichi Tanimoto
    SOLA 12 282 - 286 2016年 [査読有り][通常論文]
     
    We describe the atmospheric features observed over the North Pacific during the summer-fall of 2014 and investigate their association with the convective activity in the intertropical convergence zone (ITCZ) and underlying oceanic conditions. During this duration, while the NINO. 3 index, the area-averaged sea surface temperature anomalies (SSTAs) in the eastern equatorial Pacific, fluctuates around the threshold value for the onset of El Nino events, off-equatorial SSTAs display an equatorial antisymmetric pattern with positive (negative) anomalies north (south) of the equator. As expected from the wind-evaporation-SST feedback, the equatorial antisymmetric SSTAs accompany with anomalous southerlies, converging into a zonal belt of 5 degrees N-10 degrees N that induces enhanced convection in the ITCZ. Thus, the oceanic and atmospheric features in the eastern tropical Pacific are different from those in the typical El Nico events. In contrast, the observed weaker subtropical high and the shallower upper-tropospheric trough over the North Pacific are similar to the features typically found during the El NiCo events. The amplitude of those anomalies, however, is much larger than that of regressed anomalies onto the NINO. 3 index. A linear baroclinic model experiment indicates that the enhanced convective heating in the ITCZ contributes to sustain the anomalous atmospheric circulation.
  • Shuhei Maeda, Yusuke Urabe, Kazuto Takemura, Tamaki Yasuda, Youichi Tanimoto
    SOLA 12 17 - 21 2016年 [査読有り][通常論文]
     
    We investigated features of the atmosphere and ocean to seek a possible candidate that suppressed the growth of the El Nino event in 2014. In the boreal summer-fall season, equatorially antisymmetric sea surface temperature (SST) anomalies with a positive (negative) sign to the north (south) of the equator prevailed in the central and eastern tropical Pacific. In association with the SST anomalies, cumulus convective activity was enhanced in the region of the climatological Intertropical Convergence Zone (ITCZ). Anomalous southerly surface winds flowing across the equator toward the ITCZ induced upward latent heat flux anomalies and lowered SST in the near-equatorial region. These coherent spatial patterns between SST, wind, and latent heat flux anomalies suggested that the wind-evaporation-SST (WES) feedback sustained the suppression of the El Nino growth. A linear baroclinic model experiment indicated that the enhanced convective heating in the ITCZ also contributed to sustain the anomalous surface southerlies across the equator by the intense meridional atmospheric circulation over the equator. These results indicate that the anomalous southerlies across the equator sustained by the WES feedback and intense convective heating in the ITCZ contributed to the suppression of the El Nino growth.
  • Ryusuke Masunaga, Hisashi Nakamura, Takafumi Miyasaka, Kazuaki Nishii, Youichi Tanimoto
    JOURNAL OF CLIMATE 28 5 1764 - 1787 2015年03月 [査読有り][通常論文]
     
    Mesoscale structures of the wintertime marine atmospheric boundary layer (MABL) as climatological imprints of oceanic fronts within the Kuroshio Oyashio Extension (KOE) region east of Japan are investigated by taking advantage of high horizontal resolution of the ERA-Interim global atmospheric reanalysis data, for which the resolution of sea surface temperature (SST) data has been improved. These imprints, including locally enhanced sensible and latent heat fluxes and local maxima in cloudiness and precipitation in association with locally strengthened surface-wind convergence in the vicinities of SST fronts along the warm Kuroshio Extension and cool Oyashio to its north, are also identified in high-resolution satellite data. In addition to these mesoscale MABL features, meridionally confined near-surface baroclinic zones and zonally oriented sea level pressure (SLP) minima associated with the dual SST fronts are represented in ERA-Interim only in the period of high-resolution SST, but those imprints of the Oyashio front are missing in the low-resolution SST period. In the presence of the prevailing monsoonal northerlies, latitudinal displacements of the SLP trough, baroclinic zone, and the peak meridional gradient of the turbulent heat fluxes from each of the corresponding SST fronts are also found to be sensitive to the frontal width that depends on the SST resolution. The analysis herein suggests that the converging surface northerlies into the SLP minima can contribute positively to the formation of a surface baroclinic zone along the Kuroshio Extension, while a stronger baroclinic zone along the Oyashio front is maintained primarily through the pronounced cross-frontal contrast in sensible heat release from the ocean.
  • Youichi Tanimoto, Kou Shimoyama, Shoichi Mori
    JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 32 1 3 - 21 2015年01月 [査読有り][通常論文]
     
    This paper describes a new initiative in which in situ observations of the marine atmospheric boundary layer (MABL) are made by a helicopter shuttle connecting six islands south of Tokyo. This observation method aims to make frequent measurements of temperature and moisture in the MABL across an ocean front, where direct measurements of the MABL have been limited. An onboard observation system to meet flight regulations was developed. Observed temperature and moisture as a function of pressure at 1-s intervals provided vertical profiles up to the 900-hPa level above each of the islands, from 24 December 2010 to 6 April 2011, with the exception of an accidental power down inmid-February 2011. The observed values are validated by intercomparison with surface measurements from weather stations, atmospheric soundings, and mesoscale weather analysis provided by the Japan Meteorological Agency. Temperature and moisture values obtained using the system described here at the surface are significantly correlated with those from the weather station. The meridional changes revealed by the observed vertical profiles depict rich MABL structures, such as a cold-air intrusion and a strong near-surface inversion, that are not captured by the mesoscale weather analysis. However, this discrepancy is probably due to insufficient treatment in the mesoscale numerical model rather than observational errors. Additional intercomparisons indicate no influence from either artificial mixing by the helicopter rotor or by dynamic pressure caused by the fast-moving helicopter when obtaining the vertical profiles. Following these validations, the continuation of the initiative will allow for examining the influence of the ocean front on the overlying MABL on a synoptic time scale.
  • Hiroto Abe, Youichi Tanimoto, Takuya Hasegawa, Naoto Ebuchi, Kimio Hanawa
    JOURNAL OF OCEANOGRAPHY 70 2 165 - 174 2014年04月 [査読有り][通常論文]
     
    This study investigated the eastern Pacific Intertropical Convergence Zone (ITCZ) as an atmospheric forcing to the ocean by using various observed and reanalysis data sets over 29 years. Climatologically, a zonal band of positive wind stress curl (WSC) with a 10 degrees meridional width was exhibited along the ITCZ. A southward shift of the positive WSC band during the El Nino phase induced a negative (positive) WSC anomaly along the northern (southern) portion of the ITCZ, and vice versa during the La Nina phase. This meridional dipole accounted for more than 25 % of interannual variances of the WSC anomalies (WSCAs), based on analysis of the period 1993-2008. The negative (positive) WSCA in the northern portion of the ITCZ during the El Nino (La Nina) phase was collocated with a positive (negative) sea surface height anomaly (SSHA) that propagated westward as a Rossby wave all the way to the western North Pacific. This finding indicates that this off-equatorial Rossby wave is induced by the WSCA around the ITCZ. Our analysis of a 1.5-layer reduced gravity model revealed that the Rossby waves are mostly explained by wind stress forcing, rather than by reflection of an equatorial Kelvin wave on the eastern coastal boundary. The off-equatorial Rossby wave had the same SSHA polarity as the equatorial Kelvin wave, and generation of a phase-preserving Rossby wave without the Kelvin wave reflection was explained by meridional movement of the ITCZ. Thus, the ITCZ acts as an atmospheric bridge that connects the equatorial and off-equatorial oceanic waves.
  • Kunihiro Aoki, Shoshiro Minobe, Youichi Tanimoto, Yoshikazu Sasai
    JOURNAL OF PHYSICAL OCEANOGRAPHY 43 9 1899 - 1910 2013年09月 [査読有り][通常論文]
     
    The present study investigates meridional heat transport induced by oceanic mesoscale variability in the World Ocean using a degrees global ocean general circulation model (OGCM) running on the Earth Simulator. The results indicate prominent poleward eddy heat transport around the western boundary currents and the Antarctic Circumpolar Current, and equatorward eddy heat transport in the equatorial region, consistent with the previous studies using coarse-resolution OGCMs. Such poleward eddy heat transport in midlatitude oceans suggests that the eddies act to reduce meridional background temperature gradients across the currents, as would be expected based on baroclinic instability. Interestingly, however, along the southern flanks of the eastward jets of the Kuroshio Extension and the Gulf Stream, southward eddy heat transport occurs in subsurface layers. This is likely due to the southward migration of warm water cores originating from southern areas adjacent to these currents. Southward movement of these cores is caused by interactions with unsteady meanders and cold eddies detaching from the meanders. The potential impact on biological production in the subtropical surface layers of these southward-traveling warm water cores is also discussed.
  • Mathew Roxy, Youichi Tanimoto, B. Preethi, Pascal Terray, R. Krishnan
    CLIMATE DYNAMICS 41 1 45 - 61 2013年07月 [査読有り][通常論文]
     
    The SST-precipitation relationship in the intraseasonal variability (ISV) over the Asian monsoon region is examined using recent high quality satellite data and simulations from a state of the art coupled model, the climate forecast system version 2 (CFSv2). CFSv2 demonstrates high skill in reproducing the spatial distribution of the observed climatological mean summer monsoon precipitation along with its interannual variability, a task which has been a conundrum for many recent climate coupled models. The model also exhibits reasonable skill in simulating coherent northward propagating monsoon intraseasonal anomalies including SST and precipitation, which are generally consistent with observed ISV characteristics. Results from the observations and the model establish the existence of spatial variability in the atmospheric convective response to SST anomalies, over the Asian monsoon domain on intraseasonal timescales. The response is fast over the Arabian Sea, where precipitation lags SST by similar to 5 days; whereas it is slow over the Bay of Bengal and South China Sea, with a lag of similar to 12 days. The intraseasonal SST anomalies result in a similar atmospheric response across the basins, which consists of a destabilization of the bottom of the atmospheric column, as observed from the equivalent potential temperature anomalies near the surface. However, the presence of a relatively strong surface convergence over the Arabian Sea, due to the presence of a strong zonal gradient in SST, which accelerates the upward motion of the moist air, results in a relatively faster response in terms of the local precipitation anomalies over the Arabian Sea than over the Bay of Bengal and South China Sea. With respect to the observations, the ocean-atmosphere coupling is well simulated in the model, though with an overestimation of the intraseasonal SST anomalies, leading to an exaggerated SST-precipitation relationship. A detailed examination points to a systematic bias in the thickness of the mixed layer of the ocean model, which needs to be rectified. A too shallow (deep) mixed layer enhances (suppress) the amplitude of the intraseasonal SST anomalies, thereby amplifying (lessening) the ISV and the active-break phases of the monsoon in the model.
  • Mathew Roxy, Youichi Tanimoto
    CLIMATE DYNAMICS 39 5 1209 - 1218 2012年09月 [査読有り][通常論文]
     
    The objective of this study is to examine, based on recently available high resolution satellite and observational data, the evolution and role of sea surface temperature (SST) in influencing the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM). The study focuses on the 30-60 day timescale when the northward propagating anomalies are dominant over the SCS. Composite analysis of the SST maximum events during SCS SM shows that increased SST anomalies over the SCS are significantly influenced by the downward shortwave radiation flux anomalies, with the suppressed surface latent heat flux anomalies supplementing to it. A thermal damping of the positive SST anomalies induces positive upward heat fluxes, which then destabilize the lower atmosphere between 1,000 and 700 hPa. The positive SST anomalies lead the positive precipitation anomalies over the SCS by 10 days, with a significant correlation (r = 0.44) between the SST-precipitation anomalies. The new findings here indicate an ocean-to-atmosphere effect over the SCS, where underlying SST anomalies tend to form a favorable condition for convective activity and sustain enhanced precipitation during the SCS SM. It is also argued, based on our observations, that the negative sea level pressure anomalies induced by the positive SST anomalies play a role in enhancing the northward propagation of the intraseasonal anomalies over the SCS.
  • Kazuhiro Oshima, Youichi Tanimoto, Shang-Ping Xie
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 90A 385 - 396 2012年02月 [査読有り][通常論文]
     
    Regional patterns of wintertime sea level pressure (SLP) trends over the North Pacific and their uncertainty were investigated based on the phase 3 of the Coupled Model Intercomparison Project (CMIP3) multi-model projections under the Special Report on Emissions Scenarios (SRES) A1B emission scenario for the 21st century (2000-2099). While the 24-model ensemble mean of the 100-yr SLP trend over the North Pacific shows a northward shift of the Aleutian low (AL), regional patterns of the SLP change vary among the models. Projected changes deepen the AL in several models but it shifts northward in some others. The different response of the AL results in a large inter-model spread over the North Pacific, which is largest of the Northern Hemisphere and comparable in magnitude to the ensemble mean in the same region. This large spread means a high degree of uncertainty in the 100-yr SLP trend over the North Pacific. For the total uncertainty in the SLP trends over the North Pacific, we examined the relative importance of the internal climate variability and model uncertainty due to different treatments of physical processes and computational scheme. To evaluate each of contributions, a single-realization ensemble using a subset of 10 CMIP3 models is compared to a multi-realization ensemble for the same models in the A1B projections. Additionally the control simulations under preindustrial conditions are examined to evaluate the background internal variability in each of the CMIP3 models. Our analysis shows that both the model uncertainty and internal climate variability contribute to the total uncertainty in the 100-yr SLP trends during the 21st century, while the internal climate variability largely explains the total uncertainty in the 50-yr SLP trends during the first half of the 21st century. The changes in surface heat flux and North Pacific subtropical gyre in association with the different response of the AL affect regional patterns of the sea surface temperature trends among models.
  • Youichi Tanimoto, Tomohisa Kanenari, Hiroki Tokinaga, Shang-Ping Xie
    JOURNAL OF CLIMATE 24 16 4419 - 4434 2011年08月 [査読有り][通常論文]
     
    Atmospheric effects of sea surface temperature (SST) fronts along the Kuroshio and Kuroshio Extension (K-KE) are investigated by examining spatial characteristics of the climatological sea level pressure (SLP), surface winds and surface heat flux (Q) fields based on an in situ observation dataset. A hydrostatic effect of the SST front is observed during the northwesterly monsoon characterized by a westward-extending low-SLP wedge (trough) slightly south of the peak in Q along the K-KE. Ageostrophic surface westerlies crossing SLP isobars toward a trough center are found north of the low-SLP wedge apparently because of the eastward acceleration by the vertical mixing effect. This feature of the ageostrophic winds is less pronounced south of it, a north-south asymmetry arising as vertical mixing accelerates (decelerates) eastward ageostrophic winds north (south) of it. While the SLP trough near the SST front is found nearly year-round except for summer, its meridional location seasonally migrates probably due to the change in the mean surface flow. Regional atmospheric model experiments are conducted to examine the effect of the SST front on the overlying atmosphere. The observed features in winter are adequately simulated when high-resolution SST is prescribed as the boundary condition. The strong Q along the K-KE and resultant SLP trough and ageostrophic surface winds are absent when the SST boundary condition is smoothed in space to weaken the SST front. These results illustrate that the cross-frontal change in Q is of great importance to leave the SST imprint on the overlying atmosphere via hydrostatic and vertical mixing adjustments.
  • Yoshimitsu Chikamoto, Youichi Tanimoto, Hitoshi Mukougawa, Masahide Kimoto
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 88 2 183 - 202 2010年04月 [査読有り][通常論文]
     
    Formation processes of negative (positive) sea surface temperature anomalies (SSTAs) in the subtropical North and South Pacific associated with the ENSO warm (cold) events are examined using reanalysis and in-situ observational datasets. During the premature stage of the ENSO warm events, negative SSTAs appear over the subtropical North Pacific in the February March period and over the subtropical South Pacific after April, and vice versa in the ENSO cold events. One month prior to the formation of these subtropical negative SSTAs, the negative air humidity anomaly and anomalous downward motion appear at the same location in either the Northern or Southern hemisphere. Associated with these atmospheric anomalies, the strengthened descending branch of local Hadley circulation is observed during the January-February period in the Northern hemisphere and after March in the Southern hemisphere, which coincides with the seasonal transition of the climatological local Hadley circulation from the Northern to Southern hemisphere. Our linear decomposition analysis of surface heat flux anomalies indicates that the negative air humidity anomaly, as well as anomalies in wind speed, contributes to the formation of the subtropical negative SSTAs through the enhanced latent heat flux induced by the anomalous air sea humidity difference. These results suggest that the anomalous downward motion associated with the changes in local Hadley circulation can induce the subtropical negative SSTAs through the surface humidity variability. A possible mechanism for the subtropical air sea interaction associated with the local Hadley circulation is discussed.
  • Youichi Tanimoto, Takushi Kajitani, Hideki Okajima, Shang-Ping Xie
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 88 1 79 - 90 2010年02月 [査読有り][通常論文]
     
    Seasonal migration of tropical rainfall is examined in the South American sector and compared with that over Africa, using satellite and reanalysis data. While the African rain band moves continuously back and forth across the equator following the seasonal march of the sun, the South American one displays a peculiar asymmetry between its northward and southward migration. The rain band moves gradually northward from October to July from the Amazon toward the Caribbean Sea, while its return to the Amazon is an abrupt event, with convection developing rapidly in October around 10 degrees S without going through the equator. Over the equatorial Amazon during July October, equivalent potential temperature (theta(e)) is kept low by the easterly advection of low temperature and humidity air from the equatorial Atlantic, coinciding with the seasonal development of ocean upwelling and a cold tongue in sea surface temperature. The low theta(e) values prevent convection from developing in the equatorial Amazon while warm SST supports convection on the north coast of South America during August October. Meanwhile solar radiation continues to heat up the land surface to the south, eventually triggering the onset of deep convection there in October. Atmospheric general circulation model experiments were conducted to examine the effect of the Atlantic cold tongue on tropical rain bands. Without the seasonal development of the Atlantic cold tongue, surface theta(e) remains high in September, and rainfall in the equatorial Amazon switches to a pronounced semi-annual cycle with a peak in each equinox. These results illustrate the role of the Atlantic cold tongue in the peculiar meridional migration of the observed South American rain band.
  • Hiroki Tokinaga, Youichi Tanimoto, Shang-Ping Xie, Takeaki Sampe, Hiroyuki Tomita, Hiroshi Ichikawa
    JOURNAL OF CLIMATE 22 16 4241 - 4260 2009年08月 [査読有り][通常論文]
     
    A suite of shipboard and satellite observations are analyzed and synthesized to investigate the three-dimensional structure of clouds and influences from sea surface temperature fronts over the western North Pacific. Sharp transitions are observed across the Kuroshio Extension (KE) front in the marine atmospheric boundary layer (MABL) and its clouds. The ocean's influence appears to extend beyond the MABL, with higher cloud tops in altitude along the KE front than the surroundings. In winter, intense turbulent heat release from the ocean takes place on the southern flank of the KE front, where the cloud top penetrates above the MABL and reaches the midtroposphere. In this band of high cloud tops, frequent lightning activity is observed. The results of this study suggest a sea level pressure mechanism for which the temperature gradient in the MABL induces strong surface wind convergence on the southern flank of the KE front, deepening the clouds there. In early summer, sea fog frequently occurs on the northern flank of the subtropical KE and subarctic fronts under southerly warm advection that suppresses surface heat flux and stabilizes the surface atmosphere. Sea fog is infrequently observed over the KE front even under southerly conditions, as the warm ocean current weakens atmospheric stratification and promotes vertical mixing. The KE front produces a narrow band of surface wind convergence, helping support a broad band of upward motion at 700 hPa that is associated with the eastward extension of the baiu rainband from Japan in June-July.
  • Kazuhiro Oshima, Youichi Tanimoto
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 87 4 755 - 770 2009年08月 [査読有り][通常論文]
     
    Reproducibility of the Pacific Decadal Oscillation (PDO) is evaluated in the sea surface temperature (SST) anomaly field in "the 20th century climate in coupled models" (20C3M) simulations of the 24 CMIP3 models. In this evaluation, we examine how well patterns of the PDO match between the observations and simulations by calculating a metric of the patterns that is a function of their spatial correlation and their standard deviation. Among the CMIP3 models, the models with the high PDO metric reproduce the decadal SST variability with opposing polarities between the central North Pacific and the tropical Pacific. As observed, temporal correlation between the PDO and decadal-ENSO indices in those simulations are negatively correlated at the statistically-significant level. The sea level pressure and outgoing longwave radiation anomalies onto the decadal-ENSO index in those simulations are realistic both in the tropical Pacific and North Pacific, indicating that this tropics-extratropics linkage in the SST anomaly field is induced by atmospheric teleconnection. This notion is consistent with the previous studies for the natural climate variability. In contrast, the models with the low PDO metric fail to reproduce those characteristics. In the simulations under a middle-range IPCC greenhouse gas emissions scenario (A I B), the PDO indices during the 21st century still represent SST variations on the decadal timescales with superimposition on a linear warming trend. Several models which reproduce the observed PDO pattern in the 20th century record tend to simulate a similar pattern over the 21st century. This indicates that the models with the high PDO metric have their own properties that tend to simulate the natural climate variations with the observed pattern under the global warming condition.
  • Youichi Tanimoto, Shang-Ping Xie, Kohei Kai, Hideki Okajima, Hiroki Tokinaga, Toshiyuki Murayama, Masami Nonaka, Hisashi Nakamura
    JOURNAL OF CLIMATE 22 6 1360 - 1374 2009年03月 [査読有り][通常論文]
     
    The baiu and Kuroshio Extension (KE) fronts, both zonally oriented and nearly collocated east of Japan, are the dominant summertime features of the atmosphere and ocean, respectively, over the midlatitude northwest Pacific. An atmospheric sounding campaign was conducted on board the R/V Roger Revelle during the 2005 summer. Transects of soundings across the KE front are analyzed to study its effects on the atmosphere, along with continuous surface meteorological and ceilometer cloud-base observations. While the KE front remained nearly stationary during the cruise, the baiu front displayed large meridional displacements that changed wind direction across the KE front. The presence of sharp sea surface temperature (SST) gradients anchored by the KE enhanced the thermal and moisture advection, causing substantial changes in the marine atmospheric boundary layer (MABL) structure. When the baiu front was displaced north of the KE front, southwesterly winds advected warm, humid air from the subtropics over the cold water, producing a surface inversion favorable to fog formation. When the baiu front was to the south, on the other hand, northerly winds across the KE front destabilized the MABL, leading to the formation of a solid low-cloud deck beneath a strong capping inversion. The wind changes with the meridional displacement of the baiu front thus caused large variations in near-surface atmospheric stability and surface turbulent heat fluxes, with potential feedback on deep convection and fog/low-cloud formation around the front.
  • Local and remote influences of the Kuroshio Extension on the atmosphere
    Hiroki Tokinaga, Shang-Ping Xie, Fumiaki Kobashi, Youichi Tanimoto
    U.S. CLIVAR Variations 7 1 1 - 4 2009年 [査読無し][通常論文]
  • Masami Nonaka, Hisashi Nakamura, Youichi Tanimoto, Takashi Kagimoto, Hideharu Sasaki
    JOURNAL OF CLIMATE 21 23 6283 - 6303 2008年12月 [査読有り][通常論文]
     
    Output of an eddy-resolving OGCM simulation is used to investigate mechanisms for interannual-to-decadal variability in the Oyashio and its influence on the subarctic frontal zone in the western North Pacific. Lag correlation analysis reveals that positive anomalies both in basin-scale wind stress curl and in local Ekman pumping can intensify the southward Oyashio almost simultaneously via barotropic and baroclinic Rossby wave propagations, respectively. The Oyashio strength can also be influenced by anomalous Ekman pumping that is exerted in the western portion of the basin through the baroclinic wave propagation with the lag of 3 yr, which appears to arise from a periodicity in the wind field. The intensification of the Oyashio is accompanied by negative anomalies both in the sea surface temperature and height off of Hokkaido Island of Japan and is followed by their eastward development along the southern branch of the Oyashio Extension and associated subarctic frontal zone in association with a southward displacement of their axes. These changes are associated with cool sea surface temperature anomalies and low potential vorticity anomalies at the thermocline level in the frontal zone. The surface cooling, thus induced in the frontal zone by those oceanic processes, accompanies anomalous downward surface heat fluxes, indicative of ocean-to-atmosphere feedback forcing associated with the Oyashio variations.
  • Kathleen Donohue, D. Randolph Watts, Karen Tracey, Mark Wimbush, Jae-Hun Park, Nicholas Bond, Meghan Cronin, Shuiming Chen, Bo Qiu, Peter Hacker, Nelson Hogg, Steven Jayne, Julie MCClean, Luc Rainville, Humio Mitsudera, Youichi Tanimoto, Shang-Ping Xie
    Eos 89 17 161 - 162 2008年04月22日 [査読有り][通常論文]
  • Mathew Roxy, Youichi Tanimoto
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 85 3 349 - 358 2007年06月 [査読有り][通常論文]
     
    Intraseasonal variability (10-60 days) of sea surface temperature (SST) over the north Indian Ocean and its influence on regional precipitation variability over the Indian subcontinent are examined. SST, cloud liquid water and precipitation over the Indian Ocean of the Tropical Rainfall Measuring Mission (TRMM), precipitation of Climate Prediction Center Merged Analysis of Precipitation (CMAP), and low-level atmospheric parameters of National Center for Environmental Prediction (NCEP) II reanalysis are utilized for this study. Western Ghats (WG) in the southwest and the Ganges-Mahanadi Basin (GB) in the northeast of the Indian subcontinent are observed to be the regions of maximum precipitation with large standard deviations of the intraseasonal variability. Active (break) phases of precipitation occur in these regions by the northward propagation of positive (negative) precipitation anomalies over the Arabian Sea and the Bay of Bengal. Latitude-time plots during the active phase of the WG region shows that the positive SST anomalies over the Arabian Sea formed by suppressed surface latent heat flux and increased downward shortwave radiation flux lead the positive precipitation anomalies. Surface air temperature anomalies follow the SST anomalies and then destabilize the lower atmosphere between 1000 hPa and 700 hPa. These results indicate that, in the northward propagating dynamical surface convergence, underlying SST anomalies tend to form a favorable condition for convective activity and may sustain enhanced precipitation over the convergence region. This results in enhanced precipitation anomalies over the WG region that move further northeastward and merge with the northward propagating precipitation anomalies from the Bay of Bengal, enhancing the active phase of the GB region.
  • Yoshimitsu Chikamoto, Youichi Tanimoto
    GEOPHYSICAL RESEARCH LETTERS 33 19 L19702-L19702-6  2006年10月 [査読有り][通常論文]
     
    After the mature stage of the ENSO events in the boreal winter, SST and latent heat flux anomalies over the tropical Atlantic during the following spring show large amplitude north of the equator but small one south of the equator. The linear decomposition analyses of the latent heat flux anomalies indicate that the contribution from wind speed anomaly shows an equatorial antisymmetric structure with same magnitude but opposite polarity between north and south of the equator, while the contribution from anomalous air-sea humidity difference counters to that from wind speed anomaly south of the equator. These results suggest an important role of anomalous air-sea humidity difference on forming latent heat flux anomaly and significantly modifies the conventional view of wind speed as the dominant effect in ENSO-induced tropical Atlantic SST changes.
  • Masami Nonaka, Hisashi Nakamura, Youichi Tanimoto, Takashi Kagimoto, Hideharu Sasaki
    JOURNAL OF CLIMATE 19 10 1970 - 1989 2006年05月 [査読有り][通常論文]
     
    Through analysis of a hindcast integration of an eddy-resolving quasi-global ocean general circulation model, decadal variability in the Kuroshio-Oyashio Extension region is investigated, with particular emphasis on that of the subarctic (Oyashio) and the Kuroshio Extension (KE) fronts. The KE front is deep and is accompanied by a sharp sea surface height (SSH) gradient with modest sea surface temperature (SST) gradient. In contrast, the subarctic front is shallow and is recognized as a zone of tight gradient in SST but not SSH. As a decadal-scale change from a warm period around 1970 to a cool period in the mid-1980s, those fronts in the model migrate southward as observed, and the associated pronounced cooling is confined mainly to those frontal zones. Reflecting the distinctive vertical structure of the fronts, the mixed layer cooling is the strongest along the subarctic front, whereas the subsurface cooling and the associated salinity changes are most pronounced along the KE front. Concomitantly with their southward migration, the two fronts have undergone decadal-scale intensification. Associated with reduced heat release into the atmosphere, the cooling in the frontal zones can be attributed neither to the direct atmospheric thermal forcing nor to the advective effect of the intensified KE, while the advective effect by the intense Oyashio can contribute to the cooling in the subarctic frontal zone. In fact, their time evolution is not found to be completely coherent, suggesting that their variability may be governed by different mechanisms. Decadal SSH variability in the KE frontal zone seems to be largely explained by propagation of baroclinic Rossby waves forced by anomalous Ekman pumping in the central North Pacific. This process alone cannot fully explain the corresponding variability in the subarctic frontal zone, where eastward propagating SSH anomalies off the Japanese coast seem to be superimposed on the Rossby wave signals.
  • H Tokinaga, Y Tanimoto, M Nonaka, B Taguchi, T Fukamachi, SP Xie, H Nakamura, T Watanabe, Yasuda, I
    GEOPHYSICAL RESEARCH LETTERS 33 4 L04703 - L04703 2006年02月 [査読有り][通常論文]
     
    Shipboard radiosonde surveys were conducted during the 2003-04 winter east of Japan to study atmospheric boundary layer (ABL) structure over the Kuroshio Extension. ABL displayed large variations in vertical structure, most of which are attributable to changes in atmospheric surface stability. Where the surface atmosphere was unstable (neutral) as measured by the sea-air temperature difference, surface turbulent heat flux increased (decreased) and a mixed-layer developed (undeveloped) with weakened (intensified) vertical wind shear. A linear regression analysis indicates that ABL height tends to increase by 1km as the sea-air temperature difference increases by 7 degrees C or surface turbulent heat flux by 500 Wm(-2). While meridional thermal advection by weather disturbances seems to cause much of atmospheric stability variability during the 43-day surveys, the strong sensitivity of vertical mixing and wind shear to stability is consistent with the observed in-phase covariability of SST and surface wind from satellite on monthly and longer timescales.
  • H Tokinaga, Y Tanimoto, SP Xie
    JOURNAL OF CLIMATE 18 17 3470 - 3482 2005年09月 [査読有り][通常論文]
     
    The confluence of the Brazil-Malvinas Currents maintains strong sea surface temperature (SST) fronts in the midlatitude southwestern Atlantic year-round. SST effects on near-surface stability and surface wind variations are examined in this region using satellite and in situ datasets. Satellite observations show strong (weak) surface wind speeds over the warm Brazil (cold Malvinas) Current. A novel feature of this study is the construction of a high-resolution surface meteorological dataset that is based on historical ship observations. Analysis of this new in situ dataset reveals an increased (reduced) sea-air temperature difference over the Brazil (Malvinas) Current, indicating destabilization (stabilization) in the atmospheric boundary layer. These results are consistent with the SST-induced vertical mixing mechanism for wind adjustment. The SST effect on the near-surface atmosphere is observed both in the climatology and on interannual time scales in the Brazil-Malvinas confluence. Along a zonal SST front at 49 degrees S northeast of the Malvinas/Falkland Islands, there is a collocated line of surface wind divergence, with moderate convergence to the north. Vertical mixing does not explain this divergence pattern because the prevailing surface winds are westerly, blowing in parallel with the front. An additional mechanism is proposed for boundary layer wind adjustment.
  • Chikamoto Y, Youichi Tanimoto
    J. Meteor. Soc. Japan 83 6 959 - 975 Meteorological Society of Japan 2005年 [査読有り][通常論文]
     
    Remote influences of El Nino-Southern Oscillation (ENSO) on tropical Atlantic sea surface temperature (SST) are examined using reanalysis and in-situ observational datasets. During both the warm and cold events of ENSO, latent heat flux anomalies are the major mechanism for SST anomalies over the Caribbean Sea. Results from a linear decomposition of the latent heat flux anomalies indicate that the anomalous air-sea difference in specific humidity (Δq´) is the dominant term in January, one month after the ENSO's mature phase. Since anomalies of SST and saturation specific humidity at the sea surface are still small in January, Δq´ is due mostly to changes in specific humidity in the lower atmosphere. Changes in surface air humidity and temperature, and their relationship to temperature variability in the upper troposphere during ENSO warm events are discussed.
  • SP Xie, M Nonaka, Y Tanimoto, H Tokinaga, HM Xu, WS Kessler, RJ Small, WT Liu, J Hafner
    BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY 85 8 1060 - 1062 2004年08月 [査読有り][通常論文]
  • H Tokinaga, Y Tanimoto
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 82 4 1007 - 1018 2004年08月 [査読有り][通常論文]
     
    We investigated seasonal transition of dominant modes of sea surface temperature anomalies (SSTAs) in the tropical Indian Ocean, analyzing the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis products (NCEP/NCAR reanalyses), the Global sea-Ice and SST dataset (GISST2.3b), and the Simple Ocean Data Assimilation (SODA). During the coincidence years when the Indian Ocean Dipole (IOD) is followed by the major El Ni (n) over tildeo during boreal autumn-winter season, surface dipole structure in the tropical Indian Ocean tends to turn into the basinwide warm pattern in the November-December period. In contrast, the subsurface dipole keeps its structure from boreal autumn to winter. Such a surface-confined transition of SSTA is induced by latent heat flux anomalies in the eastern Indian Ocean. These latent heat flux anomalies are associated with changes in scalar wind speed anomalies. The zonal direction of climatological surface winds changes from easterly into westerly over the eastern Indian Ocean in November-December, while the anomalous Walker circulation during the El Ni (n) over tildeo induces easterly surface wind anomalies to persist there. As a result, deceleration of scalar wind speed takes place during boreal winter, and leads to warming of SST through suppressed evaporation. In addition to these latent heat flux anomalies, incoming solar radiation anomalies contribute to the net surface warming during this period. Furthermore, we discuss the role of the ocean dynamics for keeping the warm SSTAs in the western Indian Ocean.
  • H Nakamura, T Sampe, Y Tanimoto, A Shimpo
    EARTH'S CLIMATE: THE OCEAN-ATMOSPHERE INTERACTION 147 329 - 345 2004年 [査読有り][通常論文]
     
    An association is discussed among a midlatitude storm track, a westerly polar-front jet stream and an underlying oceanic frontal zone. Their close association is observed when a subtropical jet stream is weak, as in the Southern Hemisphere summer or in the North Atlantic. Along a near-surface baroclinic zone that tends to be anchored around a frontal zone, storm track activity is enhanced within a well-defined polar-front jet with modest core. velocity. This eddy-driven jet exhibits a deep structure with the strong surface westerlies maintained mainly through a poleward eddy heat flux. The westerly wind stress exerted along the frontal zone acts to maintain it by driving the oceanic current system, suggestive of a feedback loop via midlatitude atmosphere-ocean interaction. It is argued that the context of this feedback must be included in interpreting the tropospheric general circulation and its variability. In fact, decadal-scale sea-surface temperature anomalies observed in the North Pacific subarctic frontal zone controlled the anomalous heat release to the atmosphere. Seemingly, the local storm track responded consistently to the decadal-scale shift of the frontal axis, acting to reinforce basin-scale flow anomalies. Over the North and South Pacific, the association is disturbed in winter by an intensified subtropical jet that traps eddy activity into its sharp core. The trapping impairs baroclinic interaction of upper-level eddies with the surface baroclinicity along a midlatitude oceanic front, leading to the suppression of eddy activity as observed in midwinter over the North Pacific.
  • Y Tanimoto, H Nakamura, T Kagimoto, S Yamane
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 108 C10 2003年10月 [査読有り][通常論文]
     
    [1] Temporal and spatial structures of turbulent latent and sensible heat flux anomalies are examined in relation to dominant patterns of sea surface temperature anomalies (SSTA) observed over the North Pacific. Relative importance among observed anomalies in SST, surface air temperature, and wind speed in determining the anomalous turbulent heat fluxes is assessed through linearizing the observed flux anomalies. Over the central basin of the North Pacific, changes in the atmospheric variables, including air temperature and wind speed, are primarily responsible for the generation of local SST variations by changing turbulent heat flux, which supports a conventional view of extratropical air-sea interaction. In the region where ocean dynamics is very important in forming SSTAs, in contrast, SSTAs that have been formed in early winter play the primary role in determining mid- and late-winter turbulent heat flux anomalies, indicative of the SST forcing upon the overlying atmosphere. Specifically, both decadal scale SSTAs in the western Pacific subarctic frontal zone and El Nino related SSTAs south of Japan are found to be engaged actively in such forcing on the atmosphere. The atmospheric response to this forcing appears to include the anomalous storm track activity. The observed atmospheric anomalies, which may be, in part, forced by the preexisting SSTAs in those two regions, act to force SSTAs in other portions of the basin, leading to the time evolution of SSTAs as observed in the course of the winter season.
  • 季節内から数十年スケール気候変動の数値的研究
    山崎孝治, 向川 均, 谷本陽一, 石渡正樹, 稲津將, 謝尚平, 岡島秀樹, 奥村夕子
    東京大学気候システム研究センター 平成14年度共同研究報告書 pp.101-130 2003年 [査読無し][通常論文]
  • H Hashizume, SP Xie, M Fujiwara, M Shiotani, T Watanabe, Y Tanimoto, WT Liu, K Takeuchi
    JOURNAL OF CLIMATE 15 23 3379 - 3393 2002年12月 [査読有り][通常論文]
     
    Tropical instability waves (TIWs), with a typical wavelength of 1000 km and period of 30 days, cause the equatorial front to meander and result in SST variations on the order of 1degrees-2degreesC. Vertical soundings of temperature, humidity, and wind velocity were obtained on board a Japanese research vessel, which sailed through three fully developed SST waves from 140degrees to 110degreesW along 2degreesN during 21-28 September 1999. A strong temperature inversion is observed throughout the cruise along 2degreesN, capping the planetary boundary layer (PBL) that is 1-1.5 km deep. Temperature response to TIW-induced SST changes penetrates the whole depth of the PBL. In response to an SST increase, air temperature rises in the lowest kilometer and shows a strong cooling at the mean inversion height. As a result, this temperature dipole is associated with little TIW signal in the observed sea level pressure (SLP). The cruise mean vertical profiles show a speed maximum at 400-500 m for both zonal and meridional velocities. SST-based composite profiles of zonal wind velocity show weakened ( intensified) vertical shear within the PBL that is consistent with enhanced ( reduced) vertical mixing, causing surface wind to accelerate ( decelerate) over warm (cold) SSTs. Taken together, the temperature and wind soundings indicate the dominance of the vertical mixing over the SLP-driving mechanism. Based on the authors' measurements, a physical interpretation of the widely used PBL model proposed by Lindzen and Nigam is presented.
  • SP Xie, J Hafner, Y Tanimoto, WT Liu, H Tokinaga, HM Xu
    GEOPHYSICAL RESEARCH LETTERS 29 24 2002年12月 [査読有り][通常論文]
     
    [1] Whether and how the atmosphere reacts to changes in extratropical sea surface temperature (SST) is under intense debate and this lack of understanding has been a major obstacle in the study of non-El Nino climate variability. Using new satellite measurements, we detect clear ocean-to-atmospheric feedback in the Yellow and East China (YEC) Seas that is triggered by the submerged ocean bottom topography. Under intense surface cooling in winter, water properties are well mixed up to 100 m deep. Ocean depth thus has a strong influence on SST of the continental shelf, leading to a remarkable collocation of warm tongues and deep channels. High winds and increased cloudiness are found over these warm tongues; one such band of ocean-atmospheric co-variation meanders through the basin, following a deep channel for an amazing distance of 1000 km. In addition to these climatic effects, the Kuroshio Front-where the warm current meets the much colder shelf water-strengthens the growth of storms.
  • Y Tanimoto, SP Xie
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 80 5 1199 - 1219 2002年10月 [査読有り][通常論文]
     
    Atlantic decadal climate variations are studied using marine meteorological observations. To remove artificial interhemispheric correlation, we perform empirical orthogonal function (EOF) analysis of sea surface temperature (SST) variability separately for the North and South Atlantic. The first EOF for the North (South) Atlantic in the decadal (8-16 years) band features a meridional tripole (dipole). In the tropics, the northern and southern leading EOFs form a meridional dipole with a center of action at 15degrees on either side of the equator. The leading sea level pressure (SLP) EOFs for the North and South Atlantic each feature a center of action that is displaced poleward of the tropical SST extreme, at 30degrees latitude. The SLP center of action in the North Atlantic has a barotropic structure and contributes significantly to surface wind variability in the tropics. Despite being derived from statistically independent data samples, the principle components for the leading SST and SLP EOFs (four in total) are significantly correlated with one another, indicative of the existence of an interhemispheric mode spanning the entire Atlantic Ocean. The same analysis for a longer SST record suggests that this pan-Atlantic decadal variability exists throughout the 20th century. In the North Atlantic, composite analysis of wind velocity and heat fluxes based on the PCs of the leading SST modes indicates that wind-induced latent heat flux is the major forcing for decadal SST variability. In the South Atlantic, by contrast, wind anomalies are neither organized in space nor in geostrophic balance with SLP, a problem likely due to poor sampling there as indicated by a comparison with well-sampled satellite measurements. Spatially coherent anomalies of low-level cloud cover are found to be associated with the tropical Atlantic dipole, with increased (decreased) cloudiness over the cold (warm) lobe. These low-level cloud anomalies do not appear to be associated with significant surface wind convergence, unlike the deep convective clouds near the equator. By shielding solar radiation, these low-level cloud anomalies act to reinforce the underlying SST anomalies, reducing the Newtonian cooling rate for SST by as much as, 30%.
  • Y Okumura, SP Xie, A Numaguti, Y Tanimoto
    GEOPHYSICAL RESEARCH LETTERS 28 8 1507 - 1510 2001年04月 [査読有り][通常論文]
     
    An atmospheric general circulation model (AGCM) is forced with a prescribed SST dipole anomaly in the tropical Atlantic to investigate the cause of crossequatorial SST gradient (CESG) variability and its teleconnection tb the extratropics. The model response bears a striking resemblance to observations in both the tropics and extratropics. The tropical response is robust and can act to reinforce the prescribed SST anomalies through wind-induced evaporation. A new feedback mechanism involving low-level stratiform clouds in the subtropics is also identified in the model and observations. The tropical SST dipole forces a barotropic teleconnection into the extratropics that projects onto the North Atlantic Oscillation (NAO). It further induces the extratropical portion of the North Atlantic SST tripole when the AGCM is coupled with an ocean mixed layer model. CESG variability thus appears to be the centerpiece of a pan-Atlantic climate pattern observed to extend from the South Atlantic to Greenland.
  • A Suzuki, H Kawahata, Y Tanimoto, H Tsukamoto, LP Gupta, Yukino, I
    GEOCHEMICAL JOURNAL 34 4 321 - 329 2000年 [査読有り][通常論文]
     
    Based on skeletal oxygen and carbon isotope analyses, we found a remarkable decrease in skeletal linear extension rate of a Porites colony during the bleaching event of 1998 around the Ryukyu Islands. Water temperature in the summer of 1998 was recorded as the highest for at least last 50 years in the region. This colony, growing in a channel of fringing reef in Ishigaki Island, has experienced complete bleaching, as evidenced from direct observations of discoloration of the colony. Growth increment between the summer of 1997 and winter of 1997-98 is about 7 mm long while the corresponding increment between the summer of 1998 and winter of 1998-99 is only 2 mm long. Since the surface colonization by algae and excavating sponges cannot be recognized by visual inspection of X-radiographs, the slow growth might have been caused by the temporal perturbations in coral-algal symbiotic system, which inhibits calcification. The shift of skeletal carbon isotope ratios toward negative during bleaching also suggests the decrease of photosynthetic activity of symbiotic algae. Presumably, high temperature during the summer of 1998 could have led to massive decrease in carbonate production by coral reefs around the Ryukyu Islands through a large-scale coral bleaching event.
  • 八ヶ岳南麓における高標高気温逆転現象
    田中博春, 谷本陽一, 三上岳彦
    地学雑誌 109 703 - 718 2000年 [査読有り][通常論文]
  • SP Xie, Y Tanimoto, H Noguchi, T Matsuno
    GEOPHYSICAL RESEARCH LETTERS 26 11 1609 - 1612 1999年06月 [査読有り][通常論文]
     
    The tropical Pacific and Atlantic Oceans share many common climatological features such as easterly trade winds, eastward shoaling thermocline, an eastern cold tongue and a northerly intertropical convergence zone (ITCZ). However, a comparison of climate variability between the two oceans reveals more differences than similarities. The Pacific is dominated by the equatorially symmetric El Nine/Southern Oscillation (ENSO) while the Atlantic ITCZ is controlled by changes in interhemispheric sea surface temperature (SST) gradient. To understand the causes of these differences in variability, a dynamic ocean-atmosphere coupling model is developed that includes both the Bjerknes and wind-evaporation-SST feedbacks. Equatorially symmetric and antisymmetric modes emerge from the model, displaying distinct growth rate dependence on zonal wavenumber. Consistent with observations, an equatorially trapped ENSO mode dominates an ocean with a longitudinal size of the Pacific while in a smaller Atlantic-size one, a monopole mode with a broad meridional scale coexists with an equatorially antisymmetric dipole mode.
  • 鈴木 淳, 谷本 陽一, 川幡 穂高
    地球化学 33 1 23 - 44 日本地球化学会 1999年03月 [査読有り][通常論文]
     
    We describe recent progress in palaeo-environmental studies using coral skeletal records. Coral skeletons are excellent archives for considerable and diverse environmental information from tropical oceans. A high precision micro-sampling technique has allowed us to reconstruct high-resolution proxy climate records using the annual density bands of massive corals such as Porites spp. The combination of high-precision oxygen stable isotope analysis and Sr/Ca thermometry is one of the most promising methods for the reconstruction of palaeo-seawater temperature and water budgets including rainf...
  • Y Tanimoto, SP Xie
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 77 1 31 - 46 1999年02月 [査読有り][通常論文]
     
    Ocean and atmosphere variability is investigated over the whole Atlantic basin in a 51-year record of sea surface temperature (SST). In the tropics, SSTs variations are separated into two time scales: decadal (8-16 years) and interannual (< 5 years). A strong cross-equatorial gradient mode dominates decadal SST anomalies with centers of action having opposite polarities at 15 degrees N and 15 degrees S, while interannual variations are characterized by SST anomalies of the same polarity throughout the tropics. Sea level pressure (SLP) regressions onto the cross-equatorial gradient index reveal extratropical teleconnections associated with the North Atlantic Oscillation (NAO), and with the South Atlantic. At the same time, a Pan-Atlantic spatial pattern is also found in SST regressions. Lagged regressions of SLP on the cross-equatorial SST gradient index shows the extratropical decadal oscillation that is out of phase between hemispheres. These results suggest that an extratropical forcing could excite the tropical Atlantic variability.
  • SP Xie, Y Tanimoto
    GEOPHYSICAL RESEARCH LETTERS 25 12 2185 - 2188 1998年06月 [査読有り][通常論文]
     
    Sea surface temperature (SST) variabilities on time scales of 10-14 years have been documented in various parts of the Atlantic Ocean. Here we present observational evidence that these regional variabilities are part of a coherent pan-Atlantic decadal oscillation (PADO) characterized by zonal bands of SST and wind anomalies stacked in the meridional direction with alternate polarities from the South Atlantic all the way to Greenland, We propose that the interaction of wind, evaporation and SST is key to establishing this interhemispherical PADO, based on results from a new ocean-atmosphere coupling model. Forced by extratropical wind forcing, the model successfully reproduces the observed decadal oscillation in both SST and wind velocity over the Tropics. This decadal extratropical forcing of the Tropics is in sharp contrast to the well-known Tropics-to-extratropics teleconnection that operates on interannual time scales in association with El Nino/Southern Oscillation (ENSO).
  • Y Tanimoto, N Iwasaka, K Hanawa
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 75 4 831 - 849 1997年08月 [査読有り][通常論文]
     
    The relationships of sea surface temperature (SST) anomalies with atmospheric general circulation and with momentum and heat fluxes are investigated in order to show the dominant structures in the North Pacific in the ENSO cycle (ES: 2-5 years) and decadal (DC: longer than 5 years) time scales, using newly computed monthly 5 degrees x 5 degrees (longitude x latitude) gridded datasets for SST, wind stress, and net surface heating fields and National Meteorological Center (NMC) geopotential height data for the 40-year period of 1951-1990 The Pacific/North American (PNA) like pattern that has three centers in the 150 degrees W-170 degrees W longitudinal band in the wintertime 500 hPa height field is prominent in the decadal changes of wintertime SST anomalies. Stronger (weaker) westerlies are located south (north) of the normal jet stream position during periods of below-normal (above-normal) SST anomalies in most of the entire extratropical North Pacific. Those SST anomalies are caused by the enhanced (suppressed) heat release along the Kuroshio and its extension, and by stronger (weaker) southward Ekman transport. However the dominant pattern of the wind field on the ES time scale is quite different from that on the DC time scale. The westerly jet in the central North Pacific is stronger than normal, but shifts northward of its mean position in years of ENSO warm episodes. On the other hand, the northwesterly wind is weaker, hence heat release is suppressed in the western North Pacific. A Western Pacific (WP) like pattern then appears in the atmospheric geopotential height field over the ocean with a below-normal center in the Bering Sea and an above-normal center south of Japan. The situation is reversed in the years of ENSO cold episodes. The differences in the spatial patterns of SST anomalies on the DC and ES time scales are associated with differences in the atmospheric circulation, such as the predominance of PNA-like versus WP-like patterns. The corresponding change in the wind system then brings about a change of heat release and Ekman transport that form the SST anomalies. It appears that the distinctions between these two atmospheric patterns are responsible for the differences in the spatial structure of tropical SST anomalies on the DC and ES time scales.
  • K Hanawa, S Ishizaki, Y Tanimoto
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 74 5 715 - 721 1996年10月 [査読有り][通常論文]
     
    Strengthening of the wintertime mid-latitude westerlies over the North Pacific since the mid-1970s is examined in wind velocity and sea level pressure (SLP) fields computed from the ship-report data archived in the Comprehensive Ocean-Atmosphere Data Set. Changes in the observed wind fields between 1966-75 and 1976-85 are consistent with geostrophic winds based on the difference of the SLP fields between the two decades. This fact confirms that the strengthening of wintertime mid-latitude westerlies in the latter decade over the North Pacific occurred in the real world. Wind fields of the annual mean and the other three seasons are also examined by the same method, and the ship-report wind velocity fields for these averages also have features consistent with the wind fields estimated from SLP.
  • K HANAWA, R SANNOMIYA, Y TANIMOTO
    JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN 73 3 757 - 763 1995年06月 [査読有り][通常論文]
     
    The static relationship between anomalies of sea surface temperatures (SSTs) and air-sea heat fluxes in the North Pacific is investigated using the datasets of SSTs and heat fluxes through the sea surface computed from GOADS for 40 years (1951 to 1990). In the low-latitude ocean, through a whole year, the downward heat fluxes (oceanic heat gain) negatively correlate with SST anomalies: when SST anomalies are negative (positive), then the ocean gains more (less) heat. On the contrary, in winter and spring in the mid-latitude ocean, the downward heat fluxes positively correlate with SST anomalies: when SST anomalies are positive (negative), then the ocean gains more (less) heat. The static relationship between SST anomalies and the upward heat fluxes (oceanic heat loss) is also examined. It is found that the situation is almost completely opposite with those between SST anomalies and the downward heat fluxes.
  • Y TANIMOTO, N IWASAKA, K HANAWA, Y TOBA
    JOURNAL OF CLIMATE 6 6 1153 - 1160 1993年06月 [査読有り][通常論文]
     
    Temporal evolution and spectral structure of sea surface temperature (SST) anomalies in the North Pacific over the last 37 years are investigated on the three characteristic time scales: shorter than 24 months (HF), 24-60 months (ES), and longer than 60 months (DC). The leading empirical-orthogonal function (EOF) for the DC time scale is characterized by a zonally elongated monopole centered at around 40-degrees-N, 180-degrees. The leading EOF for the HF time scale is somewhat similar to that for the DC time scale, although there are two centers of action with the same polarity at the mid and western Pacific. The leading EOF for the ES time scale, however, exhibits a different pattern whose center of action at the mid Pacific is located farther southeastward. In the time evolution of the SST anomalies associated with the leading EOF of the DC time scale, several anomaly periods can be identified that last five years or longer. The transition from a persistent period to another with the opposite polarity is generally very brief, except for the one that lasts throughout the late 1960s. The EOF analysis was repeated separately on these persistent anomaly periods and the long transition period. The spatial structure of the leading EOF of the SST variability with the ES time scale is found to be sensitive to the polarity of the decadal anomaly. These results are suggestive of the possible influence of the decadal SST variability upon the spatial structure of the variability with shorter time scales.
  • Variations with multiple time scales of the global sea surface temperature fields and their relationship with the atmospheric general circulation
    谷本陽一
    1993年 [査読無し][通常論文]

書籍

講演・口頭発表等

  • 海面水温フロント上における気温と運動量の鉛直構造  [通常講演]
    谷本陽一
    Sixth China-Korea-Japan Joint Conference on Meteorology 2013年10月 口頭発表(一般) 南京 
    セッションコンビナー 座長
  • 黒潮続流上における大気下層の調節過程  [招待講演]
    谷本陽一
    日本気象学会2012年春季大会専門分科会 「東アシアモンスーンと黒潮(II) ―中緯度大気海洋相互作用に着目した新たな研究のハラタイム」 2012年05月 口頭発表(招待・特別)

その他活動・業績

受賞

  • 2011年05月 日本気象学会 2010年度JMSJ award受賞
     
    受賞者: 谷本陽一
  • 2002年10月 日本気象学会 2002年度正野・山本論文賞受賞
     
    受賞者: 谷本陽一

共同研究・競争的資金等の研究課題

  • 気候系のhot spot:熱帯と寒帯が近接するモンスーンアジアの大気海洋結合変動
    JSPS:科研費
    研究期間 : 2010年04月 -2015年03月 
    代表者 : 中村尚
  • 中緯度大気海洋系10年スケール変動の再現性とその将来変化に関する研究
    環境省:地球環境研究総合推進費
    研究期間 : 2007年04月 -2012年03月 
    代表者 : 谷本陽一
  • 文部科学省:科学研究費補助金(基盤研究(B))
    研究期間 : 2010年 -2012年 
    代表者 : 谷本 陽一, 中村 尚
     
    黒潮本流は,非大蛇行流路を取る場合,伊豆諸島付近で概ね三宅島・八丈島付近の間を横切り,この付近に明瞭な水温前線を形成する.この水温前線を跨ぐ大気の鉛直構造を伊豆諸島付近におけるマルチサウンディングと定期ヘリコプター便を活用したモニタリング観測を実施した.本研究では伊豆諸島を結ぶ定期ヘリコプター便の機体に気温,湿度,気圧計を搭載し,ヘリコプターの離陸・着陸時の上昇・下降を利用した大気下層の鉛直モニタリングシステムの開発を進めた.平成22年度は,航空機観測に適するような,時定数の小さいセンサーから構成される小型・省電力の観測システムを開発し,観測テストを行った.定期ヘリコプター便に搭載可能となるように観測システムの安定性と安全性の実績を積み,平成22年1月からヘリコプターによるモニタリングを開始した.平成22年3月には,八丈島と黒潮続流上で船舶の2点におけるデュアルサウンディングを実施する予定であったが,東日本大震災の影響により白鳳丸航海が中止なり,八丈島における観測を限定的に行った.これらの現場観測を互いに比較し,観測記録の精度を検証し,本研究で開発したモニタリングシステムが大気境界層における気温・湿度の鉛直構造を充分に捉えていることが示す.今後は,本モニタリングシステムの維持・改良につとめ,大気下層における気温・湿度構造が水温前線に跨いでどのように変質するかを連続的に捉え,温...
  • 黒潮と親潮上における大気海洋相互作用
    住友財団:住友財団研究助成
    研究期間 : 2004年04月 -2007年03月 
    代表者 : 谷本陽一
  • 文部科学省:科学研究費補助金(基盤研究(B))
    研究期間 : 2005年 -2007年 
    代表者 : 谷本 陽一, 中村 尚, 小橋 史明
     
    日本東方における海面水温分布の特徴のひとつに黒潮続流に沿う水温前線を挙げることができる.この前線付近では季節を通じて水温傾度が5℃/200km程度であり,水温が空間的に大きく変化する.このような黒潮続流水温前線を横切るGPSラジオゾンデ観測を2005年7月にスクリップス海洋研究所のR/V Roger RevelleのKESS2005航海と2006年の1月の東京大学海洋研究所研究船白鳳丸KH-06-01次航海にて行った.初夏に黒潮続流海域を覆う大気特性は総観規模で大きく変化するものの,黒潮続流フロント近傍における海洋の中規模渦を伴った水温場はそれほど敏感に応答せず,数週間ほぼ同じ水温分布を維持していた.黒潮続流フロントを横断する鉛直断面におけるラジオゾンデ観測とシーロメータ観測から,海面近傍の温度差の変化が海洋性大気境界層の鉛直構造を変化させ,高温湿潤の大気が覆う場合には積雲から霧を含む層雲へのレジームシフト,低温湿潤の大気が覆う場合は明瞭な雲底を持つ層雲から隙間の多い薄い層雲へのレジームシフトが生じることを明らかにした.一方,冬季における海面付近の静的安定度は常に不安定である.このため,仮温位の鉛直勾配の小さい混合層が大気下層に形成され,その上端に逆転層が見られた,観測期間中の混合層高度と海面熱フラックスの変動は5-12時間のラグでよく一致していて,混合層の上端と雲底高度は概...
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 2003年 -2004年 
    代表者 : 谷本 陽一, 渡部 雅浩, 山崎 孝治
     
    西部北太平洋等に見られる黒潮続流水温前線や亜寒帯水温前線では大気の変形半径スケールより小さい数百km程度の空間スケールで水温が大きく変化する.このような中緯度における水温変化に対し大気がどのような変調を受けるかについて,熱帯域における現象と対比させながら,大気下層の安定度,大気下層における温度・水蒸気・運動量について総合的な解析を行った.黒潮続流,マルビーナス海流,南極前線等の相対的に暖かい水温を持つ海域では海洋からの熱放出により海面付近では大気の鉛直方向の安定度が低下し,1000-1500mまで惑星境界層が発達する.この発達に伴い境界層内では熱・運動量の混合が活発になるため,海上風が増加していることが示された.中緯度の大気海洋間交換過程において,高風速が潜熱放出を促し水温を低下させるという大気から海洋の作用がこれまでの研究において主に捉えられてきていたが,本研究の解析はこれとは逆の海洋から大気への影響があることを指摘した.このような境界層の変質は鉛直積分として海面気圧の分布にはあまり寄与せず,地衡風調節は少なくとも中緯度では示されていない.むしろ,水温分布は,大気下層の南北気温傾度の変化すなわち傾圧性の変化には影響し,総観規模の高低気圧の変調にまで海洋の効果が及ぶことが示唆された.総観規模における変調はアリューシャン低気圧の変動にも関わるため,黒潮続流海域における長期的な水...
  • 文部科学省:科学研究費補助金(奨励研究(A))
    研究期間 : 1997年 -1998年 
    代表者 : 谷本 陽一
     
    気候変動に対する海洋の役割を解明するためには,大気海洋相互作用の実態を明らかにする必要がある.数値モデルによる大気海洋間の熱・運動量の交換には未だ問題があり,人為的に調整をして数値実験を行っているに過ぎない.これらの問題を解決するには,全球海洋における海面熱フラックスの研究を観測面から充実させる必要があった.本研究では,全球海洋における海上気象要素の観測値を整備し,旬平均緯度経度2度格子の海上気象要素ー海面熱フラックス・運動量フラックスのデータセットを1950年から1995年にわたって整備した.実測値に基づいたデータセットを用い,環大西洋域における10年スケール変動について詳しく調べた.熱帯における海面水温場は10年スケール(8-16年)と年々変動(5年以下)のスケールに分離された.10年スケール変動では,海面水温の強い南北傾度が北緯15度と南緯15度付近の間に現れた.一方,年々変動では熱帯全域で同符号の変動を示し南北傾度はほとんど示されなかった.熱帯の強い南北傾度に対する海面気圧場・海面水温場の回帰係数は中高緯度へのテレコネクションを示した.特に北半球の気圧場ではNorth Atlantic Oscillation(NAO)と関連していた.熱帯海面水温の強い南北傾度に伴うラグ回帰は,中高緯度の海面気圧アノマリが南北半球間でほぼ逆位相の関係を持ち10年スケール規模で変動してい...

教育活動情報

主要な担当授業

  • 気候変動特論
    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 環境科学院
    キーワード : 気候,気候変動,相互作用,放射過程 climate, climate variability, interaction, radiation process
  • 地球惑星科学Ⅱ
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : 宇宙,惑星,大気と海洋の運動と構造,気候変動,地球環境
  • 環境と人間
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 全学教育
    キーワード : 地球環境変動 地球温暖化 オゾンホール 天気予報 大気大循環 海洋循環 エルニーニョ 海氷 古気候 異常気象

大学運営

学内役職歴

  • 2021年10月1日 - 2023年9月30日 教育研究評議会評議員
  • 2021年10月1日 - 2023年9月30日 大学院環境科学院長
  • 2021年10月1日 - 2023年9月30日 大学院地球環境科学研究院長

委員歴

  • 2013年06月 - 現在   気象庁   異常気象分析検討会委員
  • 2007年 - 2013年   気象庁異常気象分析作業部会部会   部会員


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