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Ocean currents in the southern East China Sea (ECS) are complex and have long lacked systematic observational data. Observations from coastal radars in northern Taiwan, along with several drifters and satellite data, reveal a detailed ECS surface flow structure and thermal front. Drifter trajectories followed the seasonal flow field and were oscillatory and trapped due to strong tidal currents. Three drifters crossed the oceanic front off the northern coast of Taiwan following the tidal motion with a rapid change in the sea surface temperature by 2.5°C, which was recorded within a small distance of 500 m. A significant seasonal water mass exchange cycle occurred in the southern ECS. Starting in October, the Kuroshio Current intrudes onto the ECS shelf, causing the formation of a southwestward Northern Taiwan Coastal Current that mixes with the China Coastal Current to form a southward flow in the northern Taiwan Strait. Beginning in March, the ocean current off northwestern Taiwan shifts northward and then gradually flows eastward into the Kuroshio region. With satellite‐derived and model‐simulated surface currents contradicting each other over long periods of time, hourly coastal radar data have not only successfully explained the spatiotemporal variations in the sea surface temperature, salinity, and chlorophyll concentration observed by satellites but have also resolved the long‐standing disputes over the surface currents in the southern ECS. Plain Language Summary: In the southern East China Sea (ECS), the ocean surface currents observed by satellite altimeters and estimated based on numerical models have considerably differed for a long time, especially in winter. The data observed by shore‐based high‐frequency radar have yielded new discoveries. Beginning in October, the Kuroshio Current intrudes onto the ECS shelf and pushes Kuroshio waters with high temperatures, high salinities, and low chlorophyll concentrations to the west, causing the current from northern Taiwan to flow southwestward into the Taiwan Strait (TS). Then, starting in March, ocean currents off northwestern Taiwan turn northward, and the water off northern Taiwan is pushed eastward back to the Kuroshio region. These flow field variations cause a significant oceanic front in the 121°E−122°E area in the southern ECS. We deployed four surface velocity program (SVP) drifters to observe the ocean flow and used virtual drifters to simulate the distribution probability of the track in each month. The drifters basically followed the movement direction of the seasonal flow field but could have been affected by the significant tidal currents and become trapped. With this study, we aim to better understand the southern ECS environment from a geophysical perspective. Key Points: Coastal radar data and long‐term multisatellite data are used to observe the formation of oceanic fronts driven by ocean currentsThe Kuroshio Current's intrusion of the shelf causes the Northern Taiwan Coastal Current to flow southward along western Taiwan in winterVirtual drifter experiments present the possible trajectories of and seasonal changes in near‐surface ocean currents [ABSTRACT FROM AUTHOR]

Field observations were conducted along three west‐east transects in the East China Sea Kuroshio Front in July 2017. The microstructure observations results show that the surface turbulence kinetic energy (TKE) dissipation rates in the northern and southern transects are about 2 orders larger than those in the middle transect. The sea surface temperature, mixed layer depth, and the submesoscale motions (with horizontal scales smaller than 30 km) also suggest the surface turbulences are more active in the northern and southern transects. The current velocity data and sea level anomaly show that the middle transect was located at the northern edge of a cyclonic eddy. The eddy‐generated cross‐front geostrophic current was observed to flow toward the opposite direction of the cross‐front Ekman transport and believed to counteract the Ekman buoyancy flux (EBF)‐induced turbulence enhancement. The TKE dissipation rates can be well scaled by a scaling considering the EBF and the wind in the northern and southern transects but wind scaling only in the middle transect, suggesting the EBF does not contribute to the surface turbulence in the middle transect. An improved empirical scaling considering the cross‐front geostrophic current is defined and can scale the TKE dissipation rates better, indicating an eddy plays a key role in surface turbulent mixing in the East China Sea Kuroshio front. Plain Language Summary: Ocean surface boundary layer is an important mediator for air‐sea interaction. Recent field observations have shown that the turbulence mixing within the ocean surface boundary layer can be significantly elevated at the front when the cross‐front Ekman transport carries dense cold water over less dense warm water. Our observations show that the mesoscale eddy also plays an important role in the surface turbulence at the front by generating cross‐front geostrophic current. This result may have important implications on the ocean model and climate forecast. Key Points: Expected Ekman buoyancy flux‐induced surface turbulence enhancement was not observed in the Kuroshio FrontEddy‐generated cross‐front geostrophic current counteracted the Ekman buoyancy flux‐induced surface turbulence enhancementNewly defined empirical scaling considering geostrophic current can better scale observed turbulent dissipation rates [ABSTRACT FROM AUTHOR]

Abstract The interaction of coastal water in I-Lan Bay, a bay near northeast Taiwan, and the Kuroshio Current is studied using the data from hydrologic survey and satellite remote sensing. An index for water mass distinguished is used for clarifying the origin of water mass in I-Lan Bay. The velocity profile from acoustic Doppler current profile data indicates a countercurrent that flows southward along the northeast coast of Taiwan toward I-Lan Bay with a speed around 0.1–0.2 m/s. The index of water mass indicates that the water of I-Lan Bay is mainly affected by the northern shelf waters of Taiwan and mixed with Kuroshio nearshore water, thus forming a clear ocean front between these two areas. The near surface water temperature in the I-Lan Bay is about 2 °C lower than that in the Kuroshio region. The seasonal average near surface salinity of I-Lan Bay can exist 0.4 psu fresher than the Kuroshio area. From the analysis of dynamic process, the coastal countercurrent in I-Lan Bay is primarily affected by 1) the occurrence of cold dome in northeast of Taiwan mainly occurs in summer, and 2) the wind-driven current from the Taiwan Strait along the north coast and the northeast coast of Taiwan caused by the southwesterly monsoon makes the countercurrents strongly in summer. The dynamics of the countercurrent occurrence can be explained by the Ekman transport theory. Highlights • Ship-board acoustic Doppler current profiles are used to observe flow structure from I-Lan Bay to Kuroshio region. • Simplified index of water mass based on CTD data isused to analyze compositions and source of water in I-Lan Bay. • The water of I-Lan Bay is mainly affected by the northern shelf waters of Taiwan and mixed with Kuroshio nearshore water. • Dynamic processes of the occurrence of countercurrent in I-Lan Bay are explained. [ABSTRACT FROM AUTHOR]

Abstract: This study examines the horizontal scale and spatial characteristics of island-induced ocean vortex trains (IOVTs) in the Kuroshio region southeast of Taiwan Island using European remote sensing satellite ERS-1 SAR imagery. US Aqua satellite Moderate Resolution Imaging Spectroradiometer (MODIS) data are used to analyze the sea surface temperature (SST) features of the study area. Seasonal composites of SST images show that the IOVTs are current-induced vortexes rather than wind-induced ones. Furthermore, using the HYbrid Coordinate Ocean Model/Navy Coupled Ocean Data Assimilation (HYCOM/NCODA) system that generated current and sea surface height anomaly data, the temporal and spatial variability of the Green Island IOVTs is analyzed. The variability of IOVTs within this region shows a distinct seasonality. This seasonal variability of IOVTs is closely associated with the shoreward shift of Kuroshio mainstream driven by the winter northeasterly monsoon. This scenario is verified by vector empirical orthogonal function analysis focused on the weak IOVT period in 2012. In addition to meandering of the Kuroshio, westward-propagating mesoscale eddies and the arrival of typhoons play an important role in modifying the variability of IOVTs at intraseasonal timescale. [Copyright &y& Elsevier]

Sea surface currents observed by high-frequency (HF) radars have been widely used in ocean circulation research. In this study, hourly sea surface currents observed by the Taiwan Coastal Ocean Dynamics Applications Radar (CODAR) system from 2015 to 2019 were analyzed by the empirical orthogonal function (EOF) analysis to reveal the characteristics of the sea surface currents around Taiwan Island. The study area is divided into two regions, the Kuroshio region east of Taiwan Island and the Taiwan Strait west of Taiwan Island. In the Kuroshio region, the first EOF mode shows that the Kuroshio is characterized by higher current speeds with greater variability in summer. The second and third EOF modes present a dipole eddy pair and single eddy impingement on the Kuroshio during different periods. The seasonal variation of the dipole eddy pair indicates that the cyclonic/anticyclonic eddy on the north/south side appears more frequently in summer. Single eddy impingement occurs at multiple periods, including daily, intraseasonal, interseasonal, and annual periods. For the Taiwan Strait, the first EOF mode displays the tide signals. The tides enter the Taiwan Strait from the north and south, forming strong sea surface currents around the northern tip of Taiwan Island and the Penghu Archipelago. The second EOF mode exhibits the seasonal changes of the sea surface currents driven by the monsoon winds. The sea surface currents in the northern Taiwan Strait are relatively strong, possibly due to the narrow and shallow terrain there. The high spatiotemporal resolution of sea surface currents derived from CODAR observations provide more detailed characteristics of sea surface circulation around Taiwan Island. [ABSTRACT FROM AUTHOR]

The Kuroshio Current has its origin in the northwestern Pacific, flowing northward to the east of Taiwan and the northern part of Luzon Island. As the Kuroshio Current flows northward, it quasi-periodically intrudes (hereafter referred to as Kuroshio intrusion (KI)) into the northern South China Sea (SCS) basin through the Luzon Strait. Despite the complex generation mechanisms of KI, the purpose of this study is to improve our understanding of the effects of KI through the Luzon Strait on the regional atmospheric and weather variations. Long-term multiple satellite observations, including absolute dynamic topography, absolute geostrophic currents, sea surface winds by ASCAT, multi-scale ultra-high resolution sea surface temperature (MURSST) level-four analysis, and research-quality three-hourly TRMM multi-satellite precipitation analysis (TMPA), was used to systematically examine the aforementioned scientific problem. Analysis indicates that the KI is interlinked with the consequential anomalous precipitation off southwestern Taiwan. This anomalous precipitation would lead to ~560 million tons of freshwater influx during each KI event. Subsequently, independent moisture budget analysis suggests that moisture, mainly from vertical advection, is the possible source of the precipitation anomaly. Additionally, a bulk formula analysis was applied to understand how KI can trigger the precipitation anomaly through vertical advection of moisture without causing an evident change in the low-level flows. These new research findings might reconcile the divisiveness on why winds are not showing a synchronous response during the KI and consequential anomalous precipitation events. [ABSTRACT FROM AUTHOR]

The spatial and temporal variability of the Kuroshio surface axis northeast of Taiwan Island is investigated using 24 years of surface geostrophic currents derived from satellite altimeter data from 1993 to 2016. The Kuroshio surface axis is derived by an extraction method with three selected parameters, including the length of the subsidiary line, the intervals between two adjacent points, and the distance between the two adjacent subsidiary lines. The empirical mode decomposition analysis on the 24-year Kuroshio axes reveals that the mean periods of intra-seasonal and inter-annual variability, which are the two dominant components, are about 3.2 months and 1.3 years, respectively. The self-organizing map analysis reveals that the variation of Kuroshio axis northeast of Taiwan Island has four best matching unit (BMU) patterns: straight-path (BMUS), meandering-path (BMUM) and two transition stages (BMUT1 and BMUT2). The straight-path pattern shows strong seasonality: more likely occurring in summer. The meandering-path pattern is less frequent than straight-path pattern. During a typical period from November 26, 2012 to January 27, 2013, which is chosen as an independent example, the analysis on the satellite altimeter and sea surface temperature data shows that the patterns of the Kuroshio axis change successively in order of BMUT1→BMUM→BMUT2→BMUS, i.e., the Kuroshio axis migrates from the meandering-path to the straight-path pattern. During the typical period the warm water intrusion and a mesoscale eddy occur at the second stage corresponding to BMUM and migrate northwestward gradually at the last two stages corresponding to BMUT2 and BMUS. The transient order appears only during this typical period but it is not common for the whole study period. The monthly mean relatively vorticity is calculated and analyzed to evaluate the impact of the eddies on the Kuroshio surface axis variability, the results show that the anticyclonic (cyclonic) eddies can promote the Kuroshio surface axis to present the meandering-path (straight-path) pattern because of the potential vorticity conservation. The impacts of the anticyclonic eddies and the cyclonic eddies on the variability of the Kuroshio surface axis are opposite. The long-term day-to-day detection contributes to improving understanding the variability of Kuroshio surface axis northeast of Taiwan Island. [ABSTRACT FROM AUTHOR]

Highlights • Nanmadol interacted with the Kuroshio in a western boundary gap within two days. • Experiments were performed to demonstrate the importance of the ocean feedback. • Changes in the thermal structure and momentum budget in the LS is revealed. Abstract In this study, a coupled ocean–atmosphere model was used to simulate the evolution of Typhoon Nanmadol (2011), an intense typhoon that drifted over the Kuroshio Current over a 2-day period, encountering land twice, once in the Philippines and once in Taiwan. Sensitivity experiments were performed to elucidate the influence of the three-dimensional ocean structure on predictions of typhoon intensity, with a focus on the interaction between the typhoon and the Kuroshio Current in the Luzon Strait (LS) and the unique path of Nanmadol. The cooling of the sea surface, which was associated with frontal movement in the LS, significantly reduced the intensity of the typhoon. The improved predictions of Nanmodal's intensity resulting from this complex interaction between the ocean and atmosphere occur only in model runs which include full two-way coupling between oceanic and atmospheric processes. [ABSTRACT FROM AUTHOR]

In this study, we used satellite observations, in-situ measurements, and numerical modelling to investigate an extreme temperature change triggered by a typhoon in the ocean near the Kuroshio region off eastern Taiwan. With the westward passage of Typhoon Morakot in 2009 through Taiwan, a distinct cool wake was generated at the southeastern corner of Taiwan (CWSET) and moved towards the downstream Kuroshio region; it involved a precipitous cooling of at least 4 °C within 10–20 km of the coast. Rapid and drastic temperature drops triggered by the CWSET and advected by the strong conveyor belt effect of the Kuroshio Current are highly probable sources of cold shocks in summer. We clarified the mechanism that generated the CWSET through a series of sensitivity experiments using the Regional Oceanic Modeling System. The cold shock was mainly triggered by local wind stress associated with the typhoon. In addition, the Kuroshio Current was demonstrated to have played a crucial role in both the generation of upwelling off the southeastern coast of Taiwan during the passage of the typhoon and the transporting of this impact downstream. This process was verified through a systematic analysis of all typhoons moving westward through Taiwan from 2005 to 2013. Cold-shock stress is thought to be linked with naturally occurring ‘fish kills’, and obtaining a more thorough understanding of the CWSET will be helpful for protecting aquaculture off the eastern coast of Taiwan from the impacts of cold shocks triggered by typhoons moving westward through Taiwan in summer. [ABSTRACT FROM AUTHOR]

Typhoon Fungwong, a category-2 but deadly typhoon in the 2008 Pacific typhoon season, made landfall first on Taiwan and then on Southeast China. During its approach toward Taiwan, it triggered an extreme sea surface temperature (SST) drop of 12.5 °C, which was the strongest SST drop recorded by Longdong buoy northeast of Taiwan coast from 1998 to 2017. In this study, including moored buoy temperature measurements, Argo float temperature profiles, satellite-observed SSTs, and a suite of numerical experiments performed using the Regional Ocean Modeling System were used to unveil the detailed processes of how Fungwong triggered such an extreme cooling. Subsequently, the source of the cold waters feeding the extreme cooling, possible mechanisms triggering the cooling, and consequential effects of cooling on the ambient ocean environment were systematically investigated. Results show that the extreme cooling was triggered mainly by a process of uplift of subsurface cold water tied to shore-ward Kuroshio intrusion driven by easterly/northeasterly winds and consequential entrainment mixing, while coastal upwelling driven by persistent longshore (southerly) winds plays a minor role. Nevertheless, the southerly winds still help the enhancement of entrainment mixing and thus the sea surface cooling. Finally, modeled float trajectories with temperature tracers identified where the cold water goes and indicate that the temperature drop might extend all the way toward the south end of Japan (Kyushu) along the flowing path of Kuroshio. • Typhoon Fungwong (2008) triggers an extreme sea surface cooling of 12.5 °C. • It is one of the strongest cooling responses ever recorded by in-situ instruments. • How Fungwong triggers such an extreme cooling was systematically resolved. • Combined effect of Kuroshio intrusion and enhanced entrainment triggers it. • Newly formed cold water impacts oceanic environment along Kuroshio downstream. [ABSTRACT FROM AUTHOR]