Model-based estimate of the heat budget in the East China Sea

Using a global ocean model with regionally focused high resolution (1/10°) in the East China Sea (ECS), we studied the oceanic heat budget in the ECS. The modeled sea surface height variability and eddy kinetic energy are consistent with those derived from satellite altimetry. Significant levels of eddy kinetic energy are found east of the Ryukyu Islands and east of Taiwan, where the short-term variability is spawned by active mesoscale eddies coalescing with the circulation. Furthermore, the simulated vertical cross-stream structure of the Kuroshio (along the Pollution Nagasaki line) and the volume transport through each channel in the ECS are in good agreement with the observational estimates. The time-averaged temperature fluxes across the Taiwan Strait (TWS), Tsushima Strait (TSS), and the 200 m isobath between Taiwan and Japan are 0.20 PW, 0.21 PW, and 0.05 PW, respectively. The residual heat flux of 0.04 PW into the ECS is balanced by the surface heat loss. The eddy temperature flux across the 200 m isobath is 0.005 PW, which accounts for 11.2% of the total temperature flux. The Kuroshio onshore temperature flux has two major sources: the Kuroshio intrusion northeast of Taiwan and southwest of Kyushu. The Ekman temperature flux induced by the wind stress in the ECS shows the same seasonal cycle and amplitude as the onshore temperature flux, with a maximum in autumn and a minimum in summer. We conclude that the Ekman temperature flux dominates the seasonal cycle of Kuroshio onshore flux

Eddy induced Kuroshio intrusions onto the continental shelf of the East China Sea

The Kuroshio is known to intrude onto the continental shelf in the southern East China Sea (ECS) northeast of Taiwan. Two types of intrusions are observed: large and small, depending on how far the Kuroshio penetrates onto the ECS continental shelf, and on the location where it crosses the shelf break. This study demonstrates that cyclonic eddies from the western Pacific induce some of these large Kuroshio intrusions. The large intrusions are identified from more than 20 years of drifter tracks archived in the Global Drifter Program historical database and from weekly and biweekly drifter deployments carried out between April 2008 and September 2009 west of the Green Island (Taiwan). Kuroshio intrusions are observed in all seasons. Cyclonic mesoscale eddies, generated in the Subtropical Countercurrent and North Equatorial Current regions of the northwest Pacific Ocean, propagate westward into the Kuroshio and are well correlated with the observed intrusions. During the intrusions, the mean sea level anomaly computed from AVISO gridded maps shows a well-defined cyclonic circulation southeast of the I-Lan ridge. The mean sea level anomaly also shows the meandering pattern of the Kuroshio when it intrudes onto the continental shelf of the southern East China Sea. The high correlation between the Kuroshio volume transport in the East Taiwan Channel (observed with moorings) and the satellite sea level anomaly permits us to use sea level anomaly as a proxy for the Kuroshio volume transport. When direct transport measurements are not available, this proxy is used to verify that intrusions due to the westward propagating eddies occur when the Kuroshio transport is low. An analytical reduced gravity model of an incident baroclinic current upon a step shelf is used to explain the difference between the large and small intrusions

Structure and evolution of the cold dome off northeastern Taiwan : a numerical study

Author Posting. © The Oceanography Society, 2013. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 26, no. 1 (2013): 66–79, doi:10.5670/oceanog.2013.06.Numerous observational and modeling studies of ocean circulation surrounding Taiwan have reported occurrences of cold water and doming of isotherms (called the cold dome) that result in the formation of coastal upwelling on the northeastern Taiwan shelf. We use a high-resolution (1/24°) ocean model based on the Massachusetts Institute of Technology general circulation model to study the evolution of this distinct shelf-slope circulation phenomenon. We performed a number of model simulations spanning a five-year period (2004–2008) using realistic atmospheric forcing and initial and open boundary conditions. The model solutions were compared with satellite measurements of sea surface height (SSH), sea surface temperature (SST), and historical temperature and salinity observations. The model showed a realistically shaped cold dome with a diameter of ~ 100 km and temperature of ~ 3°C below the ambient shelf waters at 50 m depth. The occurrences of simulated cold dome events appeared to be connected with the seasonal variability of the Kuroshio Current. The model simulations showed more upwelling events during spring and summer when the core of the Kuroshio tends to migrate away from the east coast of Taiwan, compared to fall and winter when the core of the Kuroshio is generally found closer to the east coast of Taiwan. The model also reproduced weak cyclonic circulation associated with the upwelling off northeastern Taiwan. We analyzed the spatio-temporal variability of the cold dome using the model solution as a proxy and designed a "cold dome index" based on the temperature at 50 m depth averaged over a 0.5° × 0.5° box centered at 25.5°N, 122°E. The cold dome index correlates with temperature at 50 m depth in a larger region, suggesting the spatial extent of the cold dome phenomenon. The index had correlation maxima of 0.78 and 0.40 for simulated SSH and SST, respectively, in and around the cold dome box region, and we hypothesize that it is a useful indicator of upwelling off northeastern Taiwan. In addition, both correlation and composite analysis between the temperature at 50 m depth and the East Taiwan Channel transport showed no cold dome events during low-transport events (often in winter) and more frequent cold dome events during high-transport events (often in summer). The simulated cold dome events had time scales of about two weeks, and their centers aligned roughly along a northeastward line starting from the northeastern tip of Taiwan.This work was supported by Office of Naval Research grant N00014-08- 1-0587

Topographic beta spiral and onshore intrusion of the Kuroshio Current

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 287–296, doi:10.1002/2017GL076614.The Kuroshio intrusion plays a vitally important role in carrying nutrients to marginal seas. However, the key mechanism leading to the Kuroshio intrusion remains unclear. In this study we postulate a mechanism: when the Kuroshio runs onto steep topography northeast of Taiwan, the strong inertia gives rise to upwelling over topography, leading to a left-hand spiral in the stratified ocean. This is called the topographic beta spiral, which is a major player regulating the Kuroshio intrusion; this spiral can be inferred from hydrographic surveys. In the world oceans, the topographic beta spirals can be induced by upwelling generated by strong currents running onto steep topography. This is a vital mechanism regulating onshore intruding flow and the cross-shelf transport of energy and nutrients from the Kuroshio Current to the East China Sea. This topographic beta spiral reveals a long-term missing link between the oceanic general circulation theory and shelf dynamic theory.Strategic Priority Research Program of the Chinese Academy of Sciences Grant Numbers: XDA11020104, XDA110203052; National Natural Science Foundation of China (NSFC) Grant Numbers: 41576023, 41376030, 41476019; Foundation for Innovative Research Groups of NSFC Grant Number: 41421005; NSFC-Shandong Joint Fund for Marine Science Research Centers Grant Number: U1406401; Aoshan Sci-Tec Innovative Project of Qingdao National Laboratory for Marine Science and Technology Grant Number: 2016ASKJ02; National Key Research and Development Program of China Grant Numbers: 2017YFC1404000, 2016YFC1401601; National Key research and development Plan Sino-Australian Center for Healthy Coasts Grant Number: 2016YFE01015002018-07-1

Mean structure and variability of the cold dome northeast of Taiwan

Author Posting. © The Oceanography Society, 2011. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 24 no. 4 (2011): 100–109, doi:10.5670/oceanog.2011.98.The "cold dome" off northeastern Taiwan is one of the distinctive oceanic features in the seas surrounding Taiwan. The cold dome is important because persistent upwelling makes the region highly biologically productive. This article uses historical data, recent observations, and satellite-observed sea surface temperatures (SST) to describe the mean structure and variability of the cold dome. The long-term mean position of the cold dome, using the temperature at 50 m depth as a reference, is centered at 25.625°N, 122.125°E. The cold dome has a diameter of approximately 100 km, and is maintained by cold ( 34.5) waters upwelled along the continental slope. The ocean currents around the cold dome, although weak, flow counterclockwise. The monsoon-driven winter intrusion of the Kuroshio current onto the East China Sea shelf intensifies the upwelling and carries more subsurface water up to the cold dome than during the summer monsoon season. On a shorter timescale, the cold dome's properties can be significantly modified by the passage of typhoons, which creates favorable physical conditions for short-term Kuroshio intrusions in summer. The surface expression of the cold dome viewed from satellite SST images is often not domelike but instead is an irregular shape with numerous filaments, and thus may contribute substantially to shelf/slope exchange. As a result of persistent upwelling, typhoon passage, and monsoon forcing, higher chlorophyll a concentrations, and thus higher primary productivity, are frequently observed in the vicinity of the cold dome.The National Science Council (NSC) of Taiwan sponsored this study under grant NSC98-2611-M-002-019-MY3. NSC supported C.-C. Chen under grant NSC98-2611-M-003-001-MY3

Circulation and intrusions northeast of Taiwan : chasing and predicting uncertainty in the cold dome

Author Posting. © The Oceanography Society, 2011. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 24 no. 4 (2011): 110–121, doi:10.5670/oceanog.2011.99.An important element of present oceanographic research is the assessment and quantification of uncertainty. These studies are challenging in the coastal ocean due to the wide variety of physical processes occurring on a broad range of spatial and temporal scales. In order to assess new methods for quantifying and predicting uncertainty, a joint Taiwan-US field program was undertaken in August/September 2009 to compare model forecasts of uncertainties in ocean circulation and acoustic propagation, with high-resolution in situ observations. The geographical setting was the continental shelf and slope northeast of Taiwan, where a feature called the "cold dome" frequently forms. Even though it is hypothesized that Kuroshio subsurface intrusions are the water sources for the cold dome, the dome's dynamics are highly uncertain, involving multiple scales and many interacting ocean features. During the experiment, a combination of near-surface and profiling drifters, broad-scale and high-resolution hydrography, mooring arrays, remote sensing, and regional ocean model forecasts of fields and uncertainties were used to assess mean fields and uncertainties in the region. River runoff from Typhoon Morakot, which hit Taiwan August 7–8, 2009, strongly affected shelf stratification. In addition to the river runoff, a cold cyclonic eddy advected into the region north of the Kuroshio, resulting in a cold dome formation event. Uncertainty forecasts were successfully employed to guide the hydrographic sampling plans. Measurements and forecasts also shed light on the evolution of cold dome waters, including the frequency of eddy shedding to the north-northeast, and interactions with the Kuroshio and tides. For the first time in such a complex region, comparisons between uncertainty forecasts and the model skill at measurement locations validated uncertainty forecasts. To complement the real-time model simulations, historical simulations with another model show that large Kuroshio intrusions were associated with low sea surface height anomalies east of Taiwan, suggesting that there may be some degree of predictability for Kuroshio intrusions.We thank the National Science Council of Taiwan as well as the Office of Naval Research for generous support of this effort

Mean structure and variability of the Kuroshio from northeastern Taiwan to southwestern Japan

Author Posting. © The Oceanography Society, 2015. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 28, no. 4 (2015): 84-95, doi:10.5670/oceanog.2015.84.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.ONR sponsored many of the field programs that are reported on in this study, including grant N00014-12- 1-0445 to MA and grant N00014-10-1-0468 to TBS. Additionally, MA received support from The Andrew W. Mellon Foundation Endowed Fund for Innovative Research. LC and the drifter work were supported by ONR grant N0001-10-1-0273 and NOAA grant NA10OAR4320156, “The Global Drifter Program.” SJ was sponsored by the Ministry of Science and Technology, ROC (Taiwan) grant NSC-101-2611-M- 002-018-MY3

Numerical Study of Cold Dome Variability induced by Typhoon Morakot (2009) off Northeastern Taiwan

The ocean response to the passage of Typhoon Morakot (2009) near the continental shelf of the East China Sea off northeastern Taiwan was evaluated using a numerical ocean model to clarify how the permanent upwelling feature in this region was changed during this storm event. Several studies have identified the presence of the Kuroshio subsurface water in this Cold Dome region, which results from the interactions among the monsoon, the Kuroshio and the shelf topography. This study shows how tropical cyclone Morakot\u27s passage quickly disturbed the circulation around Taiwan and induced a short-period intrusion of the Kuroshio water onto the continental shelf. The intrusion began during the second half of the forced period and lasted for approximately two days. The upwelling and northward flow were greatly enhanced during this period, allowing the subsurface water from the upstream Kuroshio to be transported onto the shelf and to reach the Cold Dome. The intrusion-induced cold anomaly along the north coast of Taiwan was much more significant than what can be achieved by local vertical mixing. The cold anomaly later formed an eddy, which gradually propagated with the Kuroshio to the northeast

Tidal Effects on Circulation in and near the East China Sea

We incorporate tidal currents into a previously validated, three-dimensional, subtidal circulation model to assess tidal effects on the circulation in and around the East China Sea. Of particular interest is the tide-enhanced Changjiang plume dispersal and circulation in the southern East China Sea. The modeling results show that without tides, the Changjiang plume in summer presents itself as a stagnant, expansive pool in regions bordering the northern East China Sea and Yellow Sea, too far north and too accumulating relative to observations. The winter plume dispersal pushed by the north-northeast monsoon follows the China coastline southeastward as a coastal current that matches more closely with observations with or without tides. Incorporating the effect of tides brings the model closer to observation, especially in summer. During summer the Taiwan Warm Current shifts to lower latitudes, enhances upwelling off southeast China and induces a southward tidal residual coastal flow off southeast China. Tides also induce the observed seaward detachment of the summer plume. In winter, the prevailing north-northeast monsoon suppresses the Taiwan Warm Current to the minimum. However, if the winter monsoon is weakened for a few weeks, the Taiwan Warm Current reappears and these three mechanisms begin to operate as in summer. CTD surveys and satellite observations south of the Changjiang River estuary contribute to a better understanding of the tidal effects on regional ocean currents

PICES Press, Vol. 6, No. 2, July 1998

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