研究者データベース

MENSAH Vigan Joel Sebastien(メンサ ビガン ジヨエル セバスチヤン)
低温科学研究所 共同研究推進部
特任助教

基本情報

所属

  • 低温科学研究所 共同研究推進部

職名

  • 特任助教

J-Global ID

研究キーワード

  • Climate change   Thermohaline circulation   Large scale Oceanic circulation   Sea ice-Ocean interaction   Physical Oceanography   

研究分野

  • 自然科学一般 / 大気水圏科学 / 物理海洋学

職歴

  • 2020年12月 - 現在 北海道大学 低温科学研究所 特任助教
  • 2017年08月 - 2020年11月 北海道大学 低温科学研究所
  • 2015年11月 - 2017年07月 台湾大学

学歴

  • 2011年09月 - 2015年06月   National Taiwan University
  • 2005年01月 - 2007年09月   ENSTA Bretagne (Formerly known as ENSIETA).
  • 2003年01月 - 2004年07月   French Navy Hydrographic Service (SHOM)

研究活動情報

論文

  • Vigan Mensah, Yoshihiro Nakayama, Masakazu Fujii, Yoshifumi Nogi, Kay I. Ohshima
    Ocean Modelling 165 101843 - 101843 2021年09月 [査読有り]
  • Yoshihiro Nakayama, Chad A. Greene, Fernando S. Paolo, Vigan Mensah, Hong Zhang, Haruhiko Kashiwase, Daisuke Simizu, Jamin S. Greenbaum, Donald D. Blankenship, Ayako Abe‐Ouchi, Shigeru Aoki
    Geophysical Research Letters 2021年08月16日 [査読有り]
  • Vigan Mensah, Kay. I. Ohshima
    Scientific Reports 11 1 2021年07月 [査読有り]
     
    AbstractThe western subarctic Pacific exhibits major biological productivity fed by the Oyashio Current and its two source waters: Western Subarctic Water, which supplies nutrients from the subarctic Pacific, and cold Okhotsk Sea Intermediate Water (OSIW), which supplies iron from the Sea of Okhotsk. We created seasonal climatologies of water properties to understand how the long-term trend (~ 50 years) and 18.6-year tidal cycle affect the Oyashio Intermediate Water (OYW). We found that over the trend, decreased OSIW outflow due to weakening of North Pacific overturning modifies OYW in winter. Meanwhile, OSIW outflow increases (decreases) in strong (weak) tide years. We predict that the opposite effects of the trend and strong tide will lead to stagnation of OYW properties until the mid-2020s, followed by accelerated warming until the mid-2030s (weak tide). A predicted 1 °C increase in OYW temperature and 50% decrease in OSIW content between 1960 and 2040 potentially have significant impact on biological productivity and carbon drawdown in the North Pacific.
  • Vigan Mensah, Sen Jan, Magdalena Andres, Ming-Huei Chang
    JOURNAL OF OCEANOGRAPHY 76 4 271 - 288 2020年08月 
    Two-years of data from an array of pressure-sensor equipped inverted echo sounders (PIESs) deployed across the Kuroshio east of Taiwan are analyzed in this study. Cross sections of temperature, absolute, barotropic and baroclinic velocities, as well as time series of volume transports and maximum velocity axis location are analyzed to better understand the response of the Kuroshio to local arrivals of mesoscale eddies and to upstream variations. A measure of Kuroshio position, X-vmax, defined by the velocity in the upper 500 m, does not always coincide with the position of the Kuroshio core defined by the velocity in the upper 150 m. These respond differently to single eddies, dipole eddies and upstream variations. Single mesoscale eddies affect the Kuroshio baroclinicity via a see-saw like vertical displacement of isopycnals from the surface to at least 1000 m depth. Single anticyclonic (cyclonic) eddies deepen (shoal) the isopycnals on the offshore side of the X-vmax, and shoal (deepen) the isopycnals on the inshore side. As a result of single anticyclonic (cyclonic) eddy-Kuroshio interactions, only the X-vmax shifts towards the east (west) while the Kuroshio core location is unchanged. Upstream variations of the Kuroshio in the Luzon Strait can weaken the Kuroshio and favor the eastward migration of both X-vmax and the Kuroshio core east of Taiwan. Coincident with this, a southward current is observed below 200 m depth off the coast of Taiwan. Dipole eddies were also observed to generate migrations of both the Kuroshio core and X-vmax.
  • Vigan Mensah, Kay Ohshima
    JOURNAL OF PHYSICAL OCEANOGRAPHY 50 8 2289 - 2313 2020年08月 
    A time series analysis of the sea surface height anomaly (SSHa) was conducted in the Kuril Basin of the Sea of Okhotsk. The mapping of the satellite-derived SSHa data was optimized to mitigate the effects of sea ice on the SSHa field during winter and early spring. Complex empirical orthogonal functions (CEOFs) were then used to analyze the SSHa field, revealing that the first three modes account for 55% of the signal variance. Mode 1 mainly represents the coherent variability trapped over the shelves all along the coastal regions and the Kuril Islands. Both seasonal and interannual variations are strongly correlated with the alongshore wind stress and can be well explained by an arrested topographic wave. Mode 3 is a propagating mode that exhibits trains of southwestward-propagating, high-amplitude anomalies. One possible mechanism for this is first-mode baroclinic Rossby waves, whose energy propagates from the Kuril Straits toward the Kuril Basin. However, mode 3 can be better interpreted as barotropic Rossby normal modes generated in the deep Kuril Basin. Mode 2 is a standing mode that may encompass the baroclinic variability in the basin. The monthly mean of the SSHa in the Kuril Basin is primarily governed by variability in mode 1, with mode 2 contributing to a lesser extent, and mode 3 being insignificant.
  • Ming-Huei Chang, Sen Jan, Chih-Lun Liu, Yu-Hsin Cheng, Vigan Mensah
    JOURNAL OF PHYSICAL OCEANOGRAPHY 49 11 2997 - 3016 2019年11月 
    Oceanic vortex evolution on the lee side of Taiwan's Green Island (similar to 7 km in diameter), where the Kuroshio flows at a speed of 1-1.5 m s(-1), is observationally examined and compared to theories and the preceding results of laboratory experiments. In the near wake, recirculation occurs with a relative vorticity of zeta similar to 20f (where f is the planetary vorticity) and subsequently sheds at a combination of periods resulting from the tidal oscillations and the intrinsic time scale of eddy evolution. The tidal oscillations are the predominant processes. Our analysis suggests that an island positioned in the Kuroshio with periodic and cross-stream tidal excursions is analogous to a cross-stream oscillating cylinder. Consequently, the shedding period of the vortex is synchronized to a tidal period occurring close to the intrinsic period. The free shear layer, which is characterized by an similar to 30f relative vorticity band (2 km wide) and a wavy thermal front, develops between the Kuroshio and recirculation. The frontal wave occurring over a time period of 0.5-2 h resembles Kelvin-Helmholtz instability corresponding to high Re values. For the far wake, repeated cross-wake surveys suggest that cyclonic and anticyclonic vortices are alternatively present at a period close to the period of M-2 tides in agreement with near-wake measurements. Repeated along-wake surveys reveal a cyclonic eddy shedding downstream at a speed of 0.35 m s(-1), 1/3 of the upstream current speed, from the near wake. In comparing our observations with the results of previous water tank experiments, an Re value of O(10(3)) for the submesoscale wake regime is expected.
  • Lia Siegelman, Fabien Roquet, Vigan Mensah, Pascal Riviere, Etienne Pauthenet, Baptiste Picard, Christophe Guinet
    JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 36 5 745 - 760 2019年05月 
    Most available CTD Satellite Relay Data Logger (CTD-SRDL) profiles are heavily compressed before satellite transmission. High-resolution profiles recorded at the sampling frequency of 0.5 Hz are, however, available upon physical retrieval of the logger. Between 2014 and 2018, several loggers deployed on elephant seals in the Southern Ocean have been set in continuous recording mode, capturing both the ascent and descent for over 60 profiles per day during several months, opening new horizons for the physical oceanography community. Taking advantage of a new dataset made of seven such loggers, a postprocessing procedure is proposed and validated to improve the quality of all CTD-SRDL data: that is, both high-resolution profiles and compressed low-resolution ones. First, temperature and conductivity are corrected for a thermal mass effect. Then salinity spiking and density inversion are removed by adjusting salinity while leaving temperature unchanged. This method, applied here to more than 50 000 profiles, yields significant and systematic improvements in both temperature and salinity, particularly in regions of rapid temperature variation. The continuous high-resolution dataset is then used to provide updated accuracy estimates of CTD-SRDL data. For high-resolution data, accuracies are estimated to be of +/- 0.02 degrees C for temperature and +/- 0.03 g kg(-1) for salinity. For low-resolution data, transmitted data points have similar accuracies; however, reconstructed temperature profiles have a reduced accuracy associated with the vertical interpolation of +/- 0.04 degrees C and a nearly unchanged salinity accuracy of +/- 0.03 g kg(-1).
  • Vigan Mensah, Kay I. Ohshima, Takuya Nakanowatari, Stephen Riser
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 144 115 - 131 2019年02月 
    The seasonal variability of the hydrography and anticyclonic circulation in the Kuril Basin of the Sea of Okhotsk, from which the North Pacific ventilation originates, is studied from bi-monthly climatologies of temperature, salinity, dynamic height and current velocity. The subsurface hydrography of the Kuril Basin is characterized by the dominant role of fresh and cold waters transported by the East Sakhalin Current during winter and spring, and relatively warm and saline waters flowing with the Soya Warm Current. At the intermediate layer, the influence of dense shelf water is maximum from May to August, and this water mixes with surrounding waters in around half a year to form a well homogenized Okhotsk Sea Intermediate Water, the source of North Pacific ventilation. The anticyclonic circulation typical of the Kuril Basin exhibits a strong seasonality, being absent in winter and showing a maximum amplitude in summer. Its formation is mainly related to the dynamic response of isopycnals to wind forcing. A dynamic height maximum appears along the coastal regions from Sakhalin Island to the southern Kuril Straits in winter, likely migrates towards the Kuril Basin in March April and forms the anticyclonic system between May and October. The dynamic height and thus the flow field is mainly determined by isopycnal displacement in and around the Kuril Basin, whereas advection of water masses has a negligible effect. Surface intensification and decay of the anticyclonic circulation seems to be related to other processes such as heat exchange with the atmosphere.
  • Vigan Mensah, Fabien Roquet, Lia Siegelman-Charbit, Baptiste Picard, Etienne Pauthenet, Christophe Guinet
    JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 35 6 1237 - 1252 2018年06月 
    The effect of thermal mass on the salinity estimate from conductivity-temperature-depth (CTD) tags sensor mounted on marine mammals is documented, and a correction scheme is proposed to mitigate its impact. The algorithm developed here allows for a direct correction of the salinity data, rather than a correction of the sample's conductivity and temperature. The amplitude of the thermal mass-induced error on salinity and its correction are evaluated via comparison between data from CTD tags and from Sea-Bird Scientific CTD used as a reference. Thermal mass error on salinity appears to be generally O(10(-2)) g kg(-1), it may reach O(10(-1)) g kg(-1), and it tends to increase together with the magnitude of the cumulated temperature gradient (T-HP) within the water column. The correction we propose yields an error decrease of up to similar to 60% if correction coefficients specific to a certain tag or environment are calculated, and up to 50% if a default value for the coefficients is provided. The correction with the default coefficients was also evaluated using over 22 000 in situ dive data from five tags deployed in the Southern Ocean and is found to yield significant and systematic improvements on the salinity data, including for profiles whose T-HP was weak and the error small. The correction proposed here yields substantial improvements in the density estimates, although a thermal mass-induced error in temperature measurements exists for very large T-HP and has yet to be corrected.
  • Ming-Huei Chang, Sen Jan, Vigan Mensah, Magdalena Andres, Luc Rainville, Yiing Jang Yang, Yu-Hsin Cheng
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 131 1 - 15 2018年01月 
    Variability of the Kuroshio east of Taiwan was observed at a cross-stream transect similar to 50 km south of the PCM-1 line with an array of three moored ADCPs measuring for similar to 23 months, supplemented with eleven repeated shipboard surveys. Observations of the Kuroshio's velocity structure reveal the absence of an obvious regular seasonal signal, but significant variability at 70-200 day period for both maximum velocity axis migration and transport due to interactions with mesoscale eddies. Empirical orthogonal function (EOF) analysis shows the migration and transport modes explain 46% and 29% of the total variance, respectively, which is in contrast to the findings at the PCM-1 line where the transport mode explained more variance than did the migration mode. The Kuroshio transport in the upper 500 m across a 150 km section is 17.2 Sv with a standard deviation of 5 Sv. The estimated Kuroshio transport is 4.3 Sv lower than that reported for the PCM-1 line, likely due to the interannual variations related to abundance of mesoscale eddies in the Subtropical Counter Current (STCC) region. Transport variability east of Taiwan is mostly caused by Kuroshio-eddy interactions. When single anticyclonic (cyclonic) eddies encounter the Kuroshio, they enhance (reduce) poleward transport, presumably by increasing (decreasing) the sea level anomaly (SLA) along the eastern flank of the Kuroshio (correlation = 0.82). When a pair of eddies impinges on the Kuroshio, the upstream confluence and diffluence caused by the dipole eddies increases and decreases the Kuroshio transport, respectively. Furthermore, the eastward (westward) currents that result from either the single eddy or the dipole eddy produce flow divergence (convergence) adjacent to the Kuroshio's eastern edge, favoring the offshore (onshore) migration of the Kuroshio axis.
  • Sen Jan, Vigan Mensah, Magdalena Andres, Ming-Huei Chang, Yiing Jang Yang
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 122 12 9744 - 9764 2017年12月 
    Quasi-geostrophic mesoscale eddies regularly impinge on the Kuroshio in the western North Pacific, but the processes underlying the evolution of these eddy-Kuroshio interactions have not yet been thoroughly investigated in the literature. Here this interaction is examined with results from a semi-idealized three-dimensional numerical model and observations from four pressure-sensor equipped inverted echo sounders (PIESs) in a zonal section east of Taiwan and satellite altimeters. Both the observations and numerical simulations suggest that, during the interaction of a cyclonic eddy with the Kuroshio, the circular eddy is deformed into an elliptic shape with the major axis in the northwest-southeast direction, before being dissipated; the poleward velocity and associated Kuroshio transport decrease and the sea level and pycnocline slopes across the Kuroshio weaken. In contrast, for an anticyclonic eddy during the eddy-Kuroshio interaction, variations in the velocity, sea level, and isopycnal depth are reversed; the circular eddy is also deformed to an ellipse but with the major axis parallel to the Kuroshio. The model results also demonstrate that the velocity field is modified first and consequently the SSH and isopycnal depth evolve during the interaction. Furthermore, due to the combined effect of impingement latitude and realistic topography, some eddy-Kuroshio interactions east of Taiwan are found to have remote effects, both in the Luzon Strait and on the East China Sea shelf northeast of Taiwan.Plain Language Summary Mesoscale eddies are everywhere in the ocean. These ocean swirls of either clockwise or counterclockwise spinning with diameter of about 100-300 km and rounding current speed of about 0.5 m/s, carrying energy and certain type of water mass, move westward and eventually reach the western boundary of each ocean. The evolution of these eddies and the interaction which occurs when they encounter the western boundary current, e.g. the Kuroshio in the western North Pacific, is important in redistributing ocean energy and, in turn, shaping the large scale ocean circulation. This study focuses on the processes underlying the interaction of nonlinear mesoscale eddies with the Kuroshio, which have not yet been thoroughly investigated in the literature. Using pressure-sensor equipped echo sounder and satellite observations interpreted in the context of semi-idealized numerical simulations, this study find (1) locally, eddy arrivals modify velocity structure in the Kuroshio first, followed by changes in sea level and isopycnal depths leading to seesaw-like variations of the sea level and density slopes across the Kuroshio, and (2) modeled remote effects, i.e., Kuroshio intrusions, manifest in the Luzon Strait and on the East China Sea shelf and depend on the eddies' impingement latitude, strength, and polarity.
  • Takuya Nakanowatari, Kay I. Ohshima, Vigan Mensah, Yoko Mitani, Kaoru Hattori, Mari Kobayashi, Fabien Roquet, Yasunori Sakurai, Humio Mitsudera, Masaaki Wakatsuchi
    POLAR SCIENCE 13 56 - 65 2017年09月 
    The Sea of Okhotsk is a challenging environment for obtaining in situ data and satellite observation in winter due to sea ice cover. In this study, we evaluated the validity of hydrographic observations by marine mammals (e.g., seals and sea lions) equipped with oceanographic conductivity-temperaturedepth (CTD) sensors. During 4-yr operations from 2011 to 2014, we obtained total of 997 temperature-salinity profiles in and around the Soya Strait, Iony Island, and Urup Strait. The hydrographic data were mainly obtained from May to August and the maximum profile depth in shelf regions almost reaches to the seafloor, while valuable hydrographic data under sea ice cover were also obtained. In strong thermoclines, the seal-derived data sometimes showed positive biases in salinity with spikelike signal. For these salinity biases, we applied a new thermal mass inertia correction scheme, effectively reducing spurious salinity biases in the seasonal thermocline. In the Soya Strait and the adjacent region, the detailed structure of the Soya Warm Current including the cold-water belt was well identified. Dense water up to 27.0 sigma(theta), which can be a potential source of Okhotsk Sea Intermediate Water, has flowed from the Soya Strait into the Sea of Okhotsk in mid-winter (February). In summer, around the Iony Island and Urup Strait, remarkable cold and saline waters are localized in the surface layers. These regions are also characterized by weak stratification, suggesting the occurrence of tidally induced vertical mixing. Thus, CTD-tag observations have a great potential in monitoring data-sparse regions in the Sea of Okhotsk. (C) 2017 The Authors. Published by Elsevier B.V.
  • Vigan Mensah, Magdalena Andres, Ren-Chieh Lien, Barry Ma, Craig M. Lee, Sen Jan
    JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 33 10 2185 - 2203 2016年10月 
    This study presents amended procedures to process and map data collected by pressure-sensor-equipped inverted echo sounders (PIESs) in western boundary current regions. The modifications to the existing methodology, applied to observations of the Kuroshio from a PIES array deployed northeast of Luzon, Philippines, consist of substituting a hydrography-based mean travel time field for the PIES-based mean field and using two distinct gravest empirical mode (GEM) lookup tables across the front that separate water masses of South China Sea and North Pacific origin. In addition, this study presents a method to use time-mean velocities from acoustic Doppler current profilers (ADCPs) to reference (or "level'') the PIES-recorded pressures in order to obtain time series of absolute geostrophic velocity. Results derived from the PIES observations processed with the hydrography-based mean field and two GEMs are compared with hydrographic profiles sampled by Seagliders during the PIES observation period and with current velocity measured concurrently by a collocated ADCP array. The updated processing scheme leads to a 41% error decrease in the determination of the thermocline depth across the current, a 22% error decrease in baroclinic current velocity shear, and a 61% error decrease in baroclinic volume transports. The absolute volume transport time series derived from the leveled PIES array compares well with that obtained directly from the ADCPs with a root-mean-square difference of 3.0 Sv (1 Sv = 10 6 m(3) s(-1)), which is mainly attributed to the influence of ageostrophic processes on the ADCP-measured velocities that cannot be calculated from the PIES observations.
  • Magdalena Andres, Sen Jan, Thomas B. Sanford, Vigan Mensah, Luca R. Centurioni, Jeffrey W. Book
    OCEANOGRAPHY 28 4 84 - 95 2015年12月 
    In the subtropical western North Pacific Ocean, the Kuroshio delivers heat, salt, and momentum poleward, much like its North Atlantic analog, the Gulf Stream. Though the Kuroshio generally flows along the western boundary from Taiwan to southeastern Japan as an "attached" current, the Kuroshio's strength, vertical structure, and horizontal position undergo significant temporal and spatial variability along this entire route. Ubiquitous mesoscale eddies and complicated topography associated with a string of marginal seas combine to make the western North Pacific a region with complex circulation. Here, we synthesize results from the recent US Origins of the Kuroshio and Mindanao Currents and Taiwan Observations of Kuroshio Transport Variability observational programs with previous findings to build a comprehensive picture of the Kuroshio on its route from northeastern Taiwan to southeastern Japan, where the current finally transitions from a western boundary current into the Kuroshio Extension, a vigorously meandering free jet.
  • Ren-Chieh Lien, Barry Ma, Craig M. Lee, Thomas B. Sanford, Vigan Mensah, Luca R. Centurioni, Bruce D. Cornuelle, Ganesh Gopalakrishnan, Arnold L. Gordon, Ming-Huei Chang, Steve R. Jayne, Yiing Jang Yang
    OCEANOGRAPHY 28 4 54 - 63 2015年12月 
    Current structure, transport, and water mass properties of the northward-flowing Kuroshio and the southward-flowing Luzon Undercurrent (LU) were observed for nearly one year, June 8, 2012-June 4, 2013, across the Kuroshio path at 18.75 degrees N. Observations were made from four platforms: an array of six subsurface ADCP moorings, two Seagliders, fivepressure inverted echo sounders (PIES), and five horizontal electric field (HEF) sensors, providing the most detailed time series of the Kuroshio and Luzon Undercurrent water properties to date. Ocean state estimates of the western boundary current system were performed using the MIT general circulation model-four-dimensional variational assimilation (MITgcm-4D-Var) system. Prominent Kuroshio features from observations are simulated well by the numerical model. Annual mean Kuroshio transport, averaged over all platforms, is similar to 16 Sv with a standard deviation similar to 4 Sv. Kuroshio and LU transports and water mass pathways east of Luzon are revealed by Seaglider measurements. In a layer above the salinity maximum associated with North Pacific Tropical Water (NPTW), Kuroshio transport is similar to 7 Sv and contains North Equatorial Current (NEC) and Western Philippine Sea (WPS) waters, with an insignificant amount of South China Sea water on the shallow western flank. In an intermediate layer containing the core of the NPTW, Kuroshio transport is similar to 10 Sv, consisting mostly of NEC water. In the lower layer of the Kuroshio, transport is similar to 1.5 Sv of mostly North Pacific Intermediate Water (NPIW) as a part of WPS waters. Annual mean Luzon Undercurrent southward transport integrated to 1,000 m depth is similar to 2.7 Sv with a standard deviation similar to 2 Sv, carrying solely WPS waters below the salinity minimum of the NPIW. The transport of the western boundary current integrated over the full ocean depth east of Luzon Island is similar to 14 +/- 4.5 Sv. Sources of the water masses in the Kuroshio and Luzon Undercurrent are confirmed qualitatively by the numerical model.
  • Yiing Jang Yang, Sen Jan, Ming-Huei Chang, Joe Wang, Vigan Mensah, Tien-Hsia Kuo, Cheng-Ju Tsai, Chung-Yaung Lee, Magdalena Andres, Luca R. Centurioni, Yu-Heng Tseng, Wen-Der Liang, Jian-Wu Lai
    OCEANOGRAPHY 28 4 74 - 83 2015年12月 
    The Kuroshio is important to climate, weather prediction, and fishery management along the northeast coast of Asia because it transports tremendous heat, salt, and energy from east of the Philippines to waters southeast of Japan. In the middle of its journey northward, the Kuroshio's velocity mean and its variability east of Taiwan crucially affect its downstream variability. To improve understanding of the Kuroshio there, multiple platforms were used to collect intensive observations off Taiwan during the three-year Observations of the Kuroshio Transports and their Variability (OKTV) program (2012-2015). Mean Kuroshio velocity transects show two velocity maxima southeast of Taiwan, with the primary velocity core on the onshore side of the Kuroshio exhibiting a mean maximum velocity of similar to 1.2 m s(-1). The two cores then merge and move at a single velocity maximum of similar to 1 m s(-1) east of Taiwan. Standard deviations of both the directly measured poleward (v) and zonal (u) velocities are similar to 0.4 m s(-1) in the Kuroshio main stream. Water mass exchange in the Kuroshio east of Taiwan was found to be complicated, as it includes water of Kuroshio origin, South China Sea Water, and West Philippine Sea Water, and it vitally affects heat, salt, and nutrient inputs to the East China Sea. Impinging eddies and typhoons are two of the principal causes of variability in the Kuroshio. This study's models are more consistent with the observed Kuroshio than with high-frequency radar measurements.
  • Cheng-Ju Tsai, Magdalena Andres, Sen Jan, Vigan Mensah, Thomas B. Sanford, Ren-Chieh Lien, Craig M. Lee
    GEOPHYSICAL RESEARCH LETTERS 42 19 8098 - 8105 2015年10月 
    The influence and fate of westward propagating eddies that impinge on the Kuroshio were observed with pressure sensor-equipped inverted echo sounders (PIESs) deployed east of Taiwan and northeast of Luzon. Zero lag correlations between PIES-measured acoustic travel times and satellite-measured sea surface height anomalies (SSHa), which are normally negative, have lower magnitude toward the west, suggesting the eddy-influence is weakened across the Kuroshio. The observational data reveal that impinging eddies lead to seesaw-like SSHa and pycnocline depth changes across the Kuroshio east of Taiwan, whereas analogous responses are not found in the Kuroshio northeast of Luzon. Anticyclones intensify sea surface and pycnocline slopes across the Kuroshio, while cyclones weaken these slopes, particularly east of Taiwan. During the 6month period of overlap between the two PIES arrays, only one anticyclone affected the pycnocline depth first at the array northeast of Luzon and 21days later in the downstream Kuroshio east of Taiwan.
  • Vigan Mensah, Sen Jan, Ming-Huei Chang, Yiing-Jang Yang
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 120 8 5473 - 5489 2015年08月 
    The variability of the intermediate water (IW) east of Luzon and Taiwan is investigated using data acquired from moored instrumented lines and shipboard hydrographic and current velocity surveys. The IW is defined as the water mass with a local salinity minimum along the Kuroshio path. An empirical formula is developed to estimate the IW salinity minimum east of Taiwan using temperature measurements around 580 m depth. Properties of the IW east of Taiwan vary greatly as a result of variable contributions from three water masses including the high-salinity South China Sea Intermediate Water (SCSIW), the low-salinity North Pacific Intermediate Water (NPIW), and the intermediate-salinity Kuroshio Intermediate Water (KIW). Our analysis concludes that NPIW is predominantly found east of Taiwan, and the northward transport of KIW from northeast of Luzon to east of Taiwan is not a steady process. Concurrent mooring measurements at these two locations enable us to correlate the variations of the layer thickness of the Kuroshio near its origin (KLTo) northeast of Luzon to the nature of the IW east of Taiwan. When the Kuroshio is deep, i.e., large KLTo, KIW is transported northward across the Luzon Strait, where its salinity increases presumably due to turbulence mixing with SCSIW. This modified KIW is then transported to the east of Taiwan. When the Kuroshio is shallow, i.e., small KLTo, the KIW transport east of Luzon is nil or southward. East of Taiwan, NPIW feeds in below the Kuroshio and is transported northward beyond the I-Lan Ridge.
  • Sen Jan, Yiing Jang Yang, Joe Wang, Vigan Mensah, Tien-Hsia Kuo, Ming-Da Chiou, Ching-Sheng Chern, Ming-Huei Chang, Hwa Chien
    JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS 120 3 1825 - 1840 2015年03月 
    Synoptic features of the Kuroshio at 23.75 degrees N were quantified using nine ship-based surveys between September 2012 and September 2014. The new ship-based data set provides an unprecedented view of the Kuroshio east of Taiwan and suggest tremendous variability in its velocity, hydrography, volume, heat, and salt transports, and water masses. The Kuroshio maximum velocity varied in 0.7-1.4 m s(-1); the core current width, delineated with the limit v0.2 m s(-1), ranged from 85 to 135 km, and the thickness varied from 400 to 600 m. A dual velocity maximum in the Kuroshio core current, though unexpected, was observed in three of nine cruises. The Kuroshio core transport, integrated from the directly measured velocity, varied between 10.46 and 22.92 Sv (1 Sv=10(6) m(3) s(-1)). The corresponding heat transport referenced to 0 degrees C was 0.838-1.793 x 10(15) W, and the salt transport was 345.0-775.9 x 10(6) kg s(-1). The geostrophic transport estimated using the thermal wind relation with the observed hydrographic data and reference velocity at 900 m is comparable to the directly measured Kuroshio transport during most of the surveys, suggesting the directly measured velocity is mostly in geostrophic balance.
  • Vigan Mensah, Sen Jan, Ming-Da Chiou, Tien Hsia Kuo, Ren-Chieh Lien
    DEEP-SEA RESEARCH PART I-OCEANOGRAPHIC RESEARCH PAPERS 86 68 - 81 2014年04月 
    This study examines the evolution of the Kuroshio Tropical Water (KTW) from the Luzon Strait to the I-Lan Ridge northeast of Taiwan. Historical conductivity temperature depth (CTD) profiles are analyzed using a method based on the calculation of the root mean square (rms) difference of the salinity along isopycnals. In combination with analysis of the distribution of the salinity maximum, this method enables water masses in the Kuroshio and the vicinity, to be tracked and distinguished as well as the detection of the areas where water masses are modified. Vertical and horizontal eddy diffusivities are then calculated from hydrographic and current velocity data to elucidate the dynamics underlying the KTW interactions with the surrounding water masses. Changes in KTW properties mainly occur in the southern half of the Luzon Strait, while moderate variations are observed east of Taiwan on the right flank of the Kuroshio. In spite of a front dividing the KTW from the South China Sea Tropical Water (SCSTW) on Kuroshio's western side, mixing between these two water masses seemingly occurs in the Luzon Strait. These water masses' interaction is not evident east of Taiwan. The estimation of eddy diffusivities yields high horizontal diffusivities (K-h similar to 10(2) m(2) s(-1)) all along the Kuroshio path, due to the high current shear along the Kuroshio's flanks. The vertical diffusivity approaches 10(-3) m(2) s(-1), with the highest values in the southern Luzon Strait. Instabilities generated when the Kuroshio encounters the rough topography of this region may enhance both vertical and horizontal diffusivities there. (C) 2014 Elsevier Ltd. All rights reserved.
  • Vigan Mensah, Marc Le Menn, Yves Morel
    JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 26 3 665 - 672 2009年03月 
    This paper revisits the thermal mass inertia correction of Sea-Bird Electronics, Inc., (SBE4) conductivity probes for the calculation of salinity. In particular, it is shown that the standard parameters recommended for the correction method are not satisfactory for the data collected during recent campaigns at sea. A method, based on Morison et al., is proposed to determine optimal values for the correction parameters from selected datasets. Values valid for general cases are then proposed that yield significant improvements in the reduction of salinity errors that occur during the upcasts and downcasts of CTD profilers in areas with sharp thermoclines. The sources of the differences found between the recommended coefficient values and the ones proposed here are also discussed.

受賞

  • 2016年05月 Taiwan Geological Society (2016) Vei Chow Juan Best Ph.D. Thesis
  • 2013年05月 Taiwan Oceanographic Society Best graduate student's poster


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