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Oral Presentations

OS13 - Western Boundary Currents, Eddies And Continental Shelf Processes
Tuesday, August 02, 2016 | 401 (L4S) | 14:00-15:30
OS13-D2-PM1-401(L4S)-001 (OS13-A021)
Energy Spectra of Submesoscale Coastal Ocean Currents
Sung Yong KIM#+
Korea Advanced Institute of Science and Technology, South Korea
#Corresponding author: +Presenter

This paper describes submesoscale kinetic energy spectra of coastal ocean currents observed from multiple platforms of satellite altimeters, shore-based high-frequency radars, and shipboard acoustic Doppler current profilers. One-dimensional wavenumber energy spectra of coastal currents show a continuous transition of oceanic energy between mesoscale and submesoscale, decaying with a slope of approximately $k^{-2}$ at a wavenumber ($k$) of 0.5 km$^{-1}$. The spatial covariance of surface currents, equivalent to their two-dimensional wavenumber spectra, has an anisotropic exponential shape with decorrelation length scales of {\it O}(10) km close to the shore and {\it O}(100) km offshore and principal axes nearly parallel with the shoreline, exhibiting coastal boundary effects on surface currents. The estimated exponentially decaying spatial covariance function is consistent with submesoscale wavenumber spectra of a $k^{-2}$ decay slope.

OS13-D2-PM1-401(L4S)-002 (OS13-A020)
Influences of Meso- and Submeso-Scale Oceanic Variability on the Primary Production in the Kuroshio Region Off Japan
Yota SUZUE1#+, Yusuke UCHIYAMA1, Hidekatsu YAMAZAKI2
1 Kobe University, Japan, 2 Tokyo University of Marine Science and Technology, Japan
#Corresponding author: +Presenter

The Kuroshio is one of the most energetic western boundary currents accompanied by vigorous eddy activity both at meso- and submesoscales. In general, these eddies are induced by baroclinic, frontal and shear instabilities, significantly affecting biogeochemical productivity in the Pacific marginal sea off Japan. We examine such processes with a synoptic downscaling ocean modeling based on the UCLA version of ROMS (Shchepetkin and McWilliams, 2005; 2008) coupled with an NPZD (nutrient, phyto/zooplanktons and detritus) nitrogen-based biogeochemical model (e.g., Fasham et al., 1990). The hydrodynamic model is initialized and forced by the JCOPE2 assimilative oceanic reanalysis (Miyazawa et al., 2009) with a horizontal grid resolution of 1/12o (dx ≈ 10 km) to successively convey basin-scale information including the Kuroshio to the parent ROMS-L1 model (dx = 3 km) and further to the child ROMS-L2 model (dx = 1 km) with a one-way offline nesting technique (Mason et al., 2011). The biogeochemical model is configured according to the modeling work conducted by Gruber et al. (2006). The result demonstrates that the ROMS-NPZD model is capable of reproducing the surface Chl.a obtained by MODIS/Aqua satellite reasonably well. In the Kuroshio region off Japan, cyclonic mesoscale eddies develop in the coastal area near Cape Omaezaki at Kii Peninsula, leading to upward nutrient supply to euphotic zone with promoting active primary production. In contrast, Chl.a is much less prominent around the Kuroshio path and the Kuroshio extension where the higher eddy kinetic energy (EKE) is distributed. The downward eddy-induced vertical nutrient flux occurs at the depth of 500 m around the Kuroshio path and the reduction of nutrient consequently takes place. Therefore, a substantial amount of nutrient is removed from the near-surface layer by eddy-driven downward nutrient transport, resulting in suppressing the primary production there. 

OS13-D2-PM1-401(L4S)-003 (OS13-A019)
Submesoscale Meanders and Frontal Eddies Along the East Australian Current Observed by HF Radars
University of New South Wales, Australia
#Corresponding author: +Presenter

The East Australian Current (EAC) is the major feature of the ocean circulation along south-eastern Australia, influencing the water temperature, phytoplankton to fish distribution and regional climate. While the shedding of mesoscale warm core eddies at the EAC separation of the coast is relatively well understood, little is known about its sub-mesoscale instabilities upstream.

More than 1 year of surface currents from HF Radars, in conjunction with mooring measurements, satellite sea surface temperature and ocean color imagery, highlight for the first time the occurrence and characteristics of frontal cyclonic structures along the EAC. Investigation into the flow field kinematics shows high Rossby numbers and a strong impact on horizontal divergence and particle dispersion, associated with cold water uplift at the eddy centers. While large cold core eddies appear to be locally generated though unstable velocity shear between the EAC and a wind-driven shelf current, most of frontal eddies propagate with the EAC meanders. Their downstream advection speed is of ~0.3 m/s, in agreement with meander phase speed evidenced in other western boundary currents. These coherent structures can migrate poleward as far as 100s km south, being a major mechanism for the transport of coastal waters and physical and biological connectivity.

OS13-D2-PM1-401(L4S)-004 (OS13-A022)
Surface Mixing and Dispersion of Radioactive Tracer Due to Submesoscale Eddies Off the Northeastern Pacific Coast of Japan
Yuki KAMIDAIRA1,2#+, Hideyuki KAWAMURA1, Takuya KOBAYASHI1, Yusuke UCHIYAMA2
1 Japan Atomic Energy Agency, Japan, 2 Kobe University, Japan
#Corresponding author: +Presenter

A multiple nesting technique enables us to examine submesoscale eddies and their effects on surface mixing and associated material transport. In this study, downscaling simulations for the coastal margin off Fukushima are conducted by using Regional Ocean Modeling System (ROMS) with a horizontal resolution of 1 km. The oceanic initial and boundary conditions are given by an oceanic data assimilation system MOVE developed at Meteorological Research Institute (MRI), Japan Meteorological Agency (JMA). The surface wind stress is provided by the JMA’s GPV-MSM atmospheric reanalysis while surface heat and freshwater fluxes are from the monthly climatological data. The modeled result is compared with a satellite altimetry data to demonstrate a quite close agreement. Energy conversion analysis exhibits that surface eddy potential energy to eddy kinetic energy (EKE) conversion through baroclinic instability is substantial in vorticity generation in this area.

A further simulation is performed on oceanic dispersion of a radioactive tracer released from the Fukushima Dai-ichi Nuclear Power Plant to investigate eddy-induced mixing using an offline, oceanic tracer dispersion model SEA-GEARN developed at Japan Atomic Energy Agency (JAEA). A spatial moment analysis with respect to the center of gravity of the concentration reveals that tracer dispersion occurs with two distinct regimes leading to anisotropic transport in the zonal and meridional directions. The first regime lasting for about 5–8 days after the tracer release are mainly caused by coastally-trapped alongshore jet and submesoscale eddy-mixing. The subsequent regime occurs after the tracer is sufficiently dispersed offshore where mesoscale eddies play more dominant roles. Moreover, time series of zonal and meridional tracer variances fluctuate with enstrophy, area averaged density anomaly and EKE. These results clearly illustrate that fluctuating submesoscale and mesoscale eddy field induced by baroclinic insatiability evidently affects the tracer dispersion off Fukushima.

OS13-D2-PM1-401(L4S)-005 (OS13-A014)
On the Reynolds Stress Caused by Oceanic Mesoscale Eddies
Kunihiro AOKI#+, Yukio MASUMOTO
The University of Tokyo, Japan
#Corresponding author: +Presenter

A vast amount of energetic oceanic mesoscale eddies are generated in the region of the eastward jet such as the Kuroshio Extension. These eddies yields horizontal Reyonlds stress, which potentially accelerate or decelerate the jet. In this study, we analytically explore the spatial property of the Reynolds stress and its impact on background mean flow in the circumstance in which axisymmetric eddies occur with the probability following a two-dimensional Gaussian function.  

The Reynolds stress is generally decomposed into isotropic part expressed by eddy kinetic energy and anisotropic part, both of which depend on the horizontal scale and occurrence probability of the eddies. The isotropic part is dominant in the area of high occurrence probability of eddies and this area extends with decreasing the horizontal scale of eddies. The anisotropic part, however, becomes larger in the outside of this area, which leads to the spatial distribution of the meridional momentum flux divergence which is positive (negative) to the west (east) of the maximum occurrence of eddies, similar to the divergence field shown in previous studies on the eastward jet on the basis of umerical model experiment and some observations. Those divergence and convergence have acceleration and deceleration tendencies, respectively, but do not affect the eastward jet in the quasi-geostrophic framework because the Reynolds stress produced by the axisymmetric eddies does not yield voriticity in this case. 

This Reyonlds stress instead causes a large-scale ageostrophic circulation, which results from the curvature effect associated with velocities circulate around the individual eddies. The eddy kinetic energy gives the streamfunction of the ageostrophic circulation in the eddy-rich region. The relation between the eddy kinetic energy and the ageostrophic flow is reminiscent of that between pressure and velocity under the geostrophic balance.

OS13-D2-PM1-401(L4S)-006 (OS13-A007)
Recent Changes in the Properties of Kuroshio Eddies Observed from Argo
Peter OKE1#+, Tatiana RYKOVA1, Bo QIU2
1 Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia, 2 University of Hawaii, United States
#Corresponding author: +Presenter

The Kuroshio Current is the western boundary current of the North Pacific basin. The Kuroshio extension is characterised by a meandering stream and a complex and energetic mesoscale eddy field. In this study, we construct a canonical (mean) cyclonic and anticyclonic eddy by combining in situ profiles from Argo floats and gridded sea-level anomaly fields from satellite altimetry. We analyse the seasonal and interannual variability of the properties of Kuroshio eddies and find that they have freshened and warmed over the past 10 years (2005-2015). We relate these changes to local and remote surface fluxes – and to changes in the Kuroshio Current transport. Noting similar observations in East Australian Current eddies, we speculate that the changes in eddy properties may be more wide-spread than expected.   

OS13 - Western Boundary Currents, Eddies And Continental Shelf Processes
Tuesday, August 02, 2016 | 401 (L4S) | 16:00-18:00
OS13-D2-PM2-401(L4S)-007 (OS13-A008)
Observations of the Merging of Two Anticyclonic Eddies in the East Australian Current Region
Tatiana RYKOVA#+, Peter OKE
Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia
#Corresponding author: +Presenter

Analysis of temperature and salinity from a single Argo float in the East Australian Current region shows evidence of the merging of two mesoscale eddies. The Argo float appears to be trapped in an intense anticyclonic eddy that is characterized by a deep surface mixed layer. The float then moves out of the eddy for a few weeks – apparently during an eddy-merging event – before moving back into the resultant eddy that now displays two distinct mixed layers. The surface mixed layer spans the top 400 m of the water column with characteristics that match the original eddy; and the second mixed layer spans depths of 500-900 m. The Argo float remains in the double-core eddy for several months. The double-core eddy has large temperature and salinity anomalies between the surface and almost 1200 m depth. Glider observations in the same region for a different period also seem to show evidence of a double-core eddy, leading us to speculate that such eddies may be common-place.

OS13-D2-PM2-401(L4S)-008 (OS13-A004)
An Observational Study of the Biophysical Characteristics of Two Contrasting Cyclonic Eddies in the Tasman Sea
University of New South Wales, Australia
#Corresponding author: +Presenter

Mesoscale cold core eddies are known to be highly productive regions of the ocean due to their cyclonic rotation which drives upwelling at the core.  Lesser known however are the dynamics and productivity of smaller frontal eddies that form on the inside edge of western boundary currents. In this study we investigate the physical and biogeochemical properties of two contrasting cyclonic eddies in the Tasman Sea.

The first being a frontal eddy that formed from a shelf water billow at ~ 32S on the continental shelf of south east Australia and was advected offshore along the East Australian Current (EAC) front. The second is a larger meso scale cyclonic eddy that formed at ~ 28S and was trapped off Brisbane, blocking the southward flow of the EAC.

We present results from a dedicated research voyage on the RV Investigator to study the biophysical interactions and productivity in cyclonic eddies of the Tasman Sea. Our results show that not all cyclonic eddies are created equal, i.e. the smaller frontal eddy is significantly more a-geostrophic, more energetic and more productive than the mesoscale cyclone, despite its small size and short life. We show that frontal eddies contribute significantly to the net productivity of the Tasman Sea region.

OS13-D2-PM2-401(L4S)-009 (OS13-A012)
Advances in Coastal Circulation and Ecosystem Studies in China Seas: Observation, Modeling and Physics
Jianping GAN#+
The Hong Kong University of Science and Technology, Hong Kong SAR
#Corresponding author: +Presenter

The China Seas have a vast continental shelf from the South China Sea to the East China Sea. The coastal circulation and ecosystem in the China shelf seas are characterized by highly variable shelf circulation in both along- and cross-shelf directions as well as by the associated unique ecosystem responses. Besides the monsoon wind-forced shelf circulation, along-isobath geostrophic dynamics arising from flow-shelf topography interaction are the key physics that shape the variability of shelf circulation and ecosystem. The shelf circulation in the China shelf seas regulates nutrient loadings from Pearl River and Changjiang discharges to form distinctly the biogeochemical hot spots along the shelf seas. We provide an overview of our coastal circulation and ecosystem studies in the China Seas, based on both observational and modeling studies, to illustrate the processes and physics invoked in the intensified shelf transports induced by the steep shelf east of Hainan Island, by the broad shelves in Gulf of Zhanjian and off east of Guangdong in the northern South China Sea, and by the coastal promontory and submerged valley over the shelf in the East China Sea. Corresponding ecosystem responses will be assessed by both observational evidences and results from a new coupled circulation-NPPZD (nitrogen, phosphorus, phytoplankton, zooplankton, and detritus (NPPZD) ecosystem model.

OS13-D2-PM2-401(L4S)-010 (OS13-A006)
Asymmetric Oceanic Responses Around Okinawa Island in the East China Sea
Sachika ODANI1#+, Yusuke UCHIYAMA1, Takafumi YAMANISHI1, Yuki KAMIDAIRA1,2, Satoshi MITARAI3
1 Kobe University, Japan, 2 Japan Atomic Energy Agency, Japan, 3 Okinawa Institute of Science and Technology Graduate University, Japan
#Corresponding author: +Presenter

Okinawa Island is located in the subtropical region of Japan, hosting ecologically abundant coral reefs even thought they lie beyond the northernmost extreme of the habitable region. On the east side of the island, the coral ecosystem is maintained by persistent intrusions of the Kuroshio warm water through eddy-induced lateral mixing (Kamidaira et al., 2016), while the Ryukyu Under Current is a major source of warm water from the lower latitude on the east coast. The island is situated on a ridge of the Ryukyu Arc that separates the shallow East China Sea (ECS) and the deep Ryukyu Trench (RT) to the Pacific Ocean, preconditioning oceanic asymmetry between the both sides of the island. We thus conduct a triple nested JCOPE2-ROMS oceanic downscaling reanalysis with the horizontal grid spacing of 3 km, 1 km and 250 m to investigate asymmetric structure around the island. Ten principal tidal constituents are introduced from the TPXO 7.0 reanalysis to include tides in the innermost model for more realistic analysis.

The harmonic analysis exhibits that amplitude and phase of semi-diurnal and diurnal tides are altered spatially on the both side of the island with appreciable phase lags and increased amplitude only on the ECS side, contributing to the asymmetry. The southernmost area of the island, Tokashiki Channel (TC), is found to affect the bidirectional tidal propagation and to prevent topographically trapped internal Kelvin waves, and thus plays substantial roles in controlling the asymmetric responses. In spring, the transports from ECS to RT through TC are strengthened due to the Kuroshio Counter Current (KCC), which consists of the clockwise-rotating mesoscale eddies shed from the Kuroshio path located at 150 – 200 km west of the island. On the contrary, in winter, KCC is reduced by the development of the submesoscale eddies.    

OS13-D2-PM2-401(L4S)-011 (OS13-A005)
Seasonal Exchanges of the Kuroshio and Shelf Waters and Their Impacts on the Shelf Currents of the East China Sea
Jia WANG1,2+, Leo OEY3#
1 Xiamen University, China, 2 National Central University of Taiwan, Taiwan, 3 National Central University, Taiwan
#Corresponding author: +Presenter

Previous in situ observations and modeling studies have indicated that, through mass and momentum exchanges across the shelf edge, the Kuroshio can significantly  influence the shelf currents of the East China Sea (ECS). Here, instead of localized observations, we use 25 years of drifter data,  supported by satellite and other data to identify seasonal cross-shelf exchanges along the entire shelf edge. We show that Kuroshio meanders onshore from fall through winter and offshore from spring through summer, with largest amplitude northeast of Taiwan. The influence is limited to the shelf-edge when Kuroshio meanders offshore in spring and summer. By  contrast, strong on-shelf intrusions and cross-shelf exchanges occur when the Kuroshio meanders onshore in fall and winter. Drifters intrude on-shelf northeast of Taiwan and spread as far north as the 30N against the strong northeasterly wind. The forcing on the shelf is identified as a northward down-sloping of the sea level which is steepest north of Taiwan from 25~28 N, but which is 3-times weaker farther north. Vorticity budget computed from a numerical model indicates that intrusion during fall and winter is primarily a result of balance between on-shelf advection of ambient potential vorticity and vorticity production by the along-isobath pressure gradient acting on the changing mass of water column across the continental slope.

OS13-D2-PM2-401(L4S)-012 (OS13-A003)
Coupled Circulation and Dynamics Between a Bay and Adjacent Shelf Around Hong Kong: Observational and Modeling Studies
Zhiqiang LIU1+, Jianping GAN2#, Xiaoyan WU2
1 Guangzhou HKUST Fok Ying Tung Research Institute, China, 2 The Hong Kong University of Science and Technology, Hong Kong SAR
#Corresponding author: +Presenter

We conducted observational and numerical modeling studies to investigate the interactive circulation and underlying dynamics in Mirs Bay (Hong Kong) and adjacent shelf during an extensively upwelling season. We synthesized long-term and short-term observations to characterize the circulations in the bay-shelf region and identified their linkage. We utilized a three-dimensional circulation model with realistic topography that is forced with time-dependent wind stress, tides and lateral buoyancy flux from the nearby Pearl River to identify the processes and physics in the circulation of the coupled bay-shelf regime. It is found that a strong northeastward jet existed persistently over the shelf with highly variable topography off the bay. Strong upslope current occurred at a sharp convex topography, and intruded into the bay as a cold water stream via the western coast of the bay at water depth below 10 m.  The intruded waters moved upward and formed an anti-cyclonic circulation inside the bay with a seaward outflow in the upper layer (< 10 m) along the eastern coast of the bay. We conducted momentum and vorticity balances analyses based on the validated model and showed that the intrusive bay-ward transport in the shelf circulation is intensified by a southwestward along-isobath pressure gradient force (PGF) over the convex isobaths in the lee of Hong Kong. This PGF is contributed by a negative relative vorticity advection from the jet in which relative vorticity was decreasing downstream. The anticyclonic circulation inside the bay formed a high elevation at its center and weakened the bay-shelf exchanges by offsetting a considerable portion of the southwestward PGF over the shelf immediately next to the bay entrance.

OS13-D2-PM2-401(L4S)-013 (OS13-A009)
Age and Residence Time of Terrestrial Source Water in the Northwest Atlantic Shelf Seas
Ruoying HE#+, Austin TODD
North Carolina State University, United States
#Corresponding author: +Presenter

Coastal river mouths and bays are the junctions where terrestrial-source water meets and mixes with water from the open ocean. Once the riverine water reaches the coastal ocean, its eventual fate is largely unknown and difficult to trace. Rivers that flow into the ocean may contain high levels of nutrients and organic matter, so understanding the fate of terrestrial source water is important for a variety of biogeochemical processes that occur in the shelf seas. The fate of this terrestrial source water may be described in terms of its mean age (the time since it reached the ocean) and its residence time (the time it remains on the continental shelf). Using a high-resolution ocean model, we apply the constituent-oriented age and residence time (CART) theory to a large region encompassing the northwest Atlantic shelf seas to calculate the mean age of terrestrial source water and its residence time. For this application, 196 river mouths are used as sources of terrestrial water from South America to Nova Scotia. We investigate the spatial and seasonal variability of the water’s mean age and compute the residence time within four different shelf regions: the Caribbean Sea, the Gulf of Mexico, the South Atlantic Bight, and the Mid-Atlantic Bight/Gulf of Maine. From the estimates of mean age and residence time, we describe the impact of the coastal circulation on the eventual fate of terrestrial waters, and provide conjecture on how varying transport time scales may affect the general biogeochemical processes in the coastal ocean.

Poster Presentations

  OS13-D4-PM2-P-014 (OS13-A001)
Spatial Geostrophic Adjustment and the on-Shelf Intrusion of the Kuroshio Northeast of Taiwan
Xiaohui LIU1#+, Jilan SU2, Dake CHEN3, Changming DONG4,5
1 Second Institute of Oceanography, China, 2 State Key Laboratory of Satellite Ocean Environment Dynamics (SOED), China, 3 State Oceanic Administration, China, 4 Nanjing University of Information Science & Technology, China, 5 University of California, Los Angeles, United States
#Corresponding author: +Presenter

Intrusion of the Kuroshio onto the East China Sea northeast of Taiwan exerts strong influence on the circulation system and marine ecosystems of the China Seas.

The seasonal features of the Kuroshio intrusion were obtained by analyzing the along-track satellite altimeter data, the sea-surface drifter data and the numerical model results. (1) In summer, the on-shelf intrusion of the Kuroshio mainly manifests as the upwelling of the subsurface cold water. The “Cold Dome” formed by the upwelled water often can be seen in surface layer. The Kuroshio surface water also intrudes directly onshore in summer at the location of the sharp eastward turning continental slope northeast of Taiwan. (2) In winter, on-shelf intrusion of the surface Kuroshio water follows two routes. Near the northern shore of Taiwan on-shelf intrusion of the Kuroshio from the geostrophic adjustment manifests as a large anti-cyclonic loop current, accompanied by a strong thermal front between the Kuroshio water and the shelf water.

The dynamics of on-shelf intrusion of the Kuroshio northeast of Taiwan was studied with ROMS (Regional Ocean modeling System) model. Analysis of the momentum balance from the results of the numerical model suggests that spatial geostrophic adjustment of the Kuroshio, due to loss of the Taiwan Island in support of Kuroshio’s cross-shelf pressure gradient of the Kuroshio, is the principal mechanism for its on-shelf intrusion northeast of Taiwan. The advection term in the momentum equation plays a major role in the process of the adjustment. The bottom friction term influences mainly the adjustment of the subsurface layer of the Kuroshio. The horizontal viscosity plays a secondary role in the momentum balance. The eastward turning of the shelf break orientation further north affects the spatial extent, as well as the pattern, of the geostrophic adjustment.

  OS13-D4-PM2-P-015 (OS13-A002)
Features and Variability of the South China Sea Western Boundary Current from 1992 to 2011
Qi QUAN1+, Huijie XUE2#, Huiling QIN1, Xuezhi ZENG1, Shiqiu PENG1
1 South China Sea Institute of Oceanology, China, 2 University of Maine, United States
#Corresponding author: +Presenter

Different from the traditional definition of the South China Sea western boundary current (SCSWBC), in this paper only the southwestward and southward currents along the northern and western slopes in the SCS, which are closely associated with the basin-wide wind stress curl, are defined as the SCSWBC, while the flows on the southwestern shelf driven directly by the local wind stress are regarded as part of the shelf circulation. Using a new reanalysis dataset of the SCS in conjunction with the in situ and remote sensing data, the main features and variability of the SCSWBC from 1992 to 2011 were studied. Dictated by the prevailing monsoonal winds and in- and outflows, the SCSWBC in winter extended the full length of the western slope and reached its maximum intensity on the southern segment of the western slope off the southeast coast of Vietnam, while in summer the southward flowing SCSWBC was cut off by the Vietnam Offshore Current (VOC) and its main body was limited to the northern segment of the western slope. Moreover, the respective seasonal patterns of the SCSWBCshowed pronounced interannual variations in its structure, including the axis, the width and the maximum depth. The strength of the SCSWBC, with a mean transport of -11.8 Sv in winter and -3.0 Sv in summer off the central coast of Vietnam, also varied significantly from year to year. It was demonstrated that the monsoonal forcing over the SCS, the interannual variability of which was closely associated with El Niño events, played an important role in modulating the interannual variability of the SCSWBC. The impact of the upper-layer Luzon Strait transport was secondary, which was largely limited in the northern SCS with a weak teleconnection to the southwestern corner of the SCS deep basin.

  OS13-D4-PM2-P-016 (OS13-A011)
Kuroshio’s Long-Term Variability East of Taiwan: Altimeter Observations and Mechanisms
Hsuan-Chang SHIH1#+, Chau-Ron WU2, Cheinway HWANG3, Huang-Hsiung HSU4, Ya-Chi LIU5
1 National Taipei university, Taiwan, 2 National Taiwan Normal University, Taiwan, 3 Wuhan University, Taiwan, 4 Academia Sinica, Taiwan, 5 National Chiao Tung University, Taiwan
#Corresponding author: +Presenter

It is now widely recognized that global warming leads to warming sea surface temperature and rising sea level. The change in the intensity and frequency of the El Niño Southern Oscillation is potentially caused by the change in the climate-ocean interaction. The slowing of the ocean currents has been predicted from model experiments and case studies, despite uncertainties. The lack of field measurements makes it difficult to assess climate-induced changes in ocean currents. Sea surface height observations from satellite altimetry have provided accurate estimates of the sea surface height for over 21 years, allowing to detect the time-varying trends of ocean currents. Using altimeter-derived absolute dynamic topography, we computed two decades of Kuroshio volume transport time series at three sections located offshore southeastern and northeastern Taiwan to study Kuroshio’s secular changes and variabilities at different time scales. Large intra-seasonal transport fluctuations were found in the years of higher-than-normal eddy activity in the Subtropical Counter Current. The Kuroshio seasonal migrations are strengthened in summer and weakened in winter, and the migrations take place offshore southeastern to northeastern Taiwan. We detected a decreasing trend and an increasing trendin the Kuroshio volume transports northeast and southeast of Taiwan. Using the Hilbert-Huang transform and empirical orthogonal function analysis, we identify two forcings that cause the interannual change in Kuroshio volume transport: wind and eddy. These two forcings also drive Kuroshio’s long-term change east of Taiwan. We elaborate how the Kuroshio transport responds to the two forcings in this paper.