East China Sea Kuroshio Research Articles

The Intensifying East China Sea Kuroshio and Disappearing Ryukyu Current in a Warming Climate

AbstractThe East China Sea Kuroshio (ECS‐Kuroshio) and the Ryukyu Current are the major poleward heat carriers in the North Pacific. Anomalous changes of ECS‐Kuroshio and Ryukyu Current could exert substantial influence on the climate in mid‐latitude regions. However, owing to limited observations and coarse resolution of climate models, how they might change under anthropogenic warming remains unknown. Here, we find an accelerating ECS‐Kuroshio (1.5 Sv) and a decelerating (−2.2 Sv) Ryukyu Current using in‐situ observation during 1958–2022, equivalent to 7% strengthening and 20% weakening in the 65 years. The trend is also simulated by four high‐resolution climate models, with multi‐model ensemble‐mean acceleration (deceleration) of the ECS‐Kuroshio (Ryukyu Current) of 1.2 ± 0.6 Sv (−6.2 ± 2.5 Sv) over 1950–2050. The weakening subtropical wind field reduces their summed transport o. Enhanced stratification, which induces uplift of current system and weaker topography‐flow interaction, leads to the intensifying ECS‐Kuroshio and disappearing Ryukyu Current.

Latent Heat Flux Trend and Its Seasonal Dependence over the East China Sea Kuroshio Region

Investigating latent heat flux (LHF) variations in the western boundary current region is crucial for understanding air–sea interactions. In this study, we examine the LHF trend in the East China Sea Kuroshio Region (ECSKR) from 1959 to 2021 using atmospheric and oceanic reanalysis datasets and find that the LHF has a significant strengthening trend. This strengthening can be attributed to sea surface warming resulting from the advection of sea surface temperatures. More importantly, the LHF trend has an apparent seasonal dependence: the most substantial increasing trend in LHF is observed in spring, while the trends are weak in other seasons. Further analysis illustrates that the anomaly of air–sea humidity difference plays a pivotal role in controlling the seasonal variations in LHF trends. Specifically, as a result of the different responses of the East Asian Trough to global warming across different seasons, during spring, the East Asian Trough significantly deepens, resulting in northerly winds that facilitate the intrusion of dry and cold air into the ECSKR region. This intensifies the humidity difference between the sea and air, promoting the release of oceanic latent heat. These findings can contribute to a better understanding of the surface heat budget balance within western boundary currents.

Seasonal Reversal of ENSO Impacts on SST in the East China Sea–Kuroshio Region

Abstract Sea surface temperature (SST) variability in the East China Sea–Kuroshio (EK) region has important implications for the surrounding weather, climate, and marine ecology. The year-to-year variations of the EK SST are expectedly linked to El Niño–Southern Oscillation (ENSO), the predominant predictability source of seasonal-to-interannual climate variability. Surprisingly, no significant SST signal is observed in the EK region when focusing on the ENSO autumn–winter season with the persistent and pronounced SST anomalies in the tropical Pacific. We find that a remarkable seasonal reversal appears in the ENSO–EK SST connection, shifting from a negative relationship in autumn [Aug(0)–Oct(0)] to a positive relationship in winter [Dec(0)–Feb(1)]. This reversal is mainly attributed to the seasonally varying ENSO-associated western North Pacific (WNP) atmospheric circulation patterns. During ENSO autumns, the anomalous WNP anticyclone is confined south of 20°N, which is accompanied with cyclonic circulation anomalies in the EK region. The associated anomalous northerly wind tends to enhance the background northerly wind, thereby facilitating the local SST cooling mainly via the wind–evaporation–SST effect. In the subsequent winter, the ENSO-related WNP anticyclonic anomalies intensify and extend toward the EK region. Consequently, the weakened background northerly wind induced by southerly wind anomalies leads to the increase of downward latent and sensible heat flux in the EK region, fostering the local SST warming. The observed seasonal reversal of ENSO impacts can be evidenced by the tropical Pacific pacemaker experiments, emphasizing the importance of seasonally modulated ENSO teleconnection and holding implications for the local SST climate prediction.

The Impacts of East China Sea Kuroshio Front on Winter Heavy Precipitation Events in Southern China

AbstractThe wintertime Kuroshio sea surface temperature (SST) front has the significant climate effects on Southern China. The study demonstrates a close relationship between heavy precipitation over Southern China and Kuroshio SST front in winter. More than half winter heavy rainfall events in Southern China are proved to be resulted from strong High‐frequency Variability events of the sea surface Wind Coupled with Precipitation (HV‐WCP events) over Kuroshio SST front. One day before strong HV‐WCP events, the initial precipitation appears over Middle‐lower Yangtze River due to the significantly enhanced frontal intensity. Then, precipitation generates low‐level cyclone and southeasterly wind anomalies, after it moving into Kuroshio front area because of the winter monsoon. The significant marine atmospheric boundary layer (MABL) height gradient over Kuroshio leads to plentiful moisture transporting from MABL into the free atmosphere and enhances the local precipitation again. This process further causes the large‐scale stratus rainband extending to Southern China and enhancing the heavy rainfall locally. Especially in 2008 winter, several processes of a strong HV‐WCP event followed by continuous weak ones are conducive to the low‐temperature‐precipitation disaster in Southern China.

Environmental factors controlling zooplankton accumulation in the near-surface layer in the western North Pacific marginal seas

Vertical distribution of zooplankton abundance is essential for understanding the linking primary production to higher-trophic marine organisms. In this study, we investigated the zooplankton abundance at the near-surface layer (0–20 m depth; ZA 20 ) and those at 0–200 m depth (ZA 200 ) from 240 stations in the Sea of Japan (SOJ) and the East China Sea–Kuroshio (ECSK) area during summer to evaluate the environmental parameters controlling zooplankton accumulation in the near-surface layer. The total numerical ZA 20 was 22 ± 15% (mean ± standard deviation) and 15 ± 11% of ZA 200 in the SOJ and ECSK, respectively. Temperature and chlorophyll a concentration at 20 m depth, as well as the sampling time, had a significant impact on the degree of surface accumulation (ZA 20 : ZA 200 ratio) in a generalized additive model (GAM); the ZA 20 : ZA 200 ratio increased in cold and eutrophic conditions and during the nighttime. The difference between SOJ and ECSK was insignificant, while the zooplankton communities were significantly different between the areas. Therefore, the vertical distribution of zooplankton is seamlessly explained by environmental parameters when setting the zooplankton as one ecological functional group. Members belonging to six orders, Calanoida, Harpacticoida, Cyclopoida, Ctenopoda ( Penilia avirostris ), Onychopoda (family Podonidae ), and Copelata (appendicularian), were dominant based on microscopic observations. When the ZA 20 : ZA 200 ratio was estimated at the order level, the responses to temperature differed between copepods and non-copepods groups: the copepods stay in the subsurface water when the surface water is warm. • We compared the zooplankton abundance in the ≤20 m and ≤ 200 m depth. • Zooplankton accumulated the ≤20 m depth in cold and eutrophic water and at night. • Almost a quarter of zooplankton was in the ≤20 m depth. • Copepods stayed in the subsurface water when the surface water is warm.

Synergistic effect of El Ni\xf1o and the North Pacific Oscillation on wintertime precipitation over Southeastern China and the East China Sea Kuroshio area

Wintertime precipitation in China is most pronounced over the southeastern area, and the Kuroshio in the East China Sea anchors a prominent precipitation band over the warm side of the sea surface temperature front. Previous studies have suggested that many factors contribute to the interannual variation of the precipitation over southeastern China (SC), whereas less attention has been paid to precipitation variability over the East China Sea Kuroshio (ECSK) area. This study focuses on the interannual variation of wintertime precipitation over the SC and ECSK areas. Empirical orthogonal function analysis reveals a spatially uniform pattern from SC to the ECSK area. Composite analysis shows that an El Niño event intensifies wintertime precipitation over our target region, and this effect is tripled when an El Niño follows a positive North Pacific Oscillation (NPO) event in the previous winter. The positive NPO event in the previous winter intensifies the El Niño event via the Victoria mode ocean bridge and the subsequent Bjerknes feedback. In comparison with single-factor El Niño events, a much weaker Walker cell induced by the joint event induces a much weaker regional Hadley cell through anomalous descending motion over the western tropical Pacific. The weakened regional Hadley circulation over the western Pacific directly enhances the precipitation over the SC and ECSK area. In this study, the synergistic effect of an El Niño event and a positive NPO event indicates that the influence of the El Niño event can be amplified by the positive NPO event in the previous winter.

Abundance and habitats of marine cladocerans in the Sea of Japan over two decades

Marine cladocerans are secondary producers in marine neritic ecosystems. To investigate the population dynamics and species-specific habitats of marine cladocerans in the Sea of Japan, marine cladoceran abundance was evaluated in the southern Sea of Japan and the East China Sea-Kuroshio using three mesh-size (0.06, 0.10, and 0.33 mm) plankton nets at several depths (n = 4674) over two decades from 1997 to 2018. We observed seven out of eight species, including Penilia avirostris, Evadne nordmanni, E. spinifera, Pseudevadne tergestina, and Pleopis schmackeri, from > 10% of samples, and Pleopis polyphemoides and Podon leuckartii from < 2% of samples. Abundance and diversity were higher in the Sea of Japan than in the East China Sea-Kuroshio during summer. In the East China Sea-Kuroshio, E. spinifera and Ps. tergestina abundances were high and they were distributed widely; however, other species were rare and were sparsely distributed. These suggested that the environmental conditions in the Sea of Japan are optimal for the reproduction of marine cladocerans because horizontal advective transport of marine cladocerans is not expected from the East China Sea-Kuroshio to the Sea of Japan. The habitats of the five common species in the Sea of Japan were estimated using generalized additive models (GAMs), revealing different optimal temperature ranges. Evadne nordmanni performed optimally under cold water, and the other four performed optimally under warm water. Penilia avirostris, E. spinifera, Ps. tergestina, and Pl. schmackeri coexisted in time and space in the Sea of Japan, although the vertical habitats were potentially different. Based on the estimated habitats, steep and shallow thermocline was responsible for the high marine cladoceran abundance and rich diversity in the southern Sea of Japan during summer.

Effect of a Mesoscale Eddy on Surface Turbulence at the Kuroshio Front in the East China Sea

AbstractField 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.

Nitrogen Fixation by Trichodesmium and unicellular diazotrophs in the northern South China Sea and the Kuroshio in summer

Distribution of diazotrophs and their nitrogen fixation activity were investigated in the northern South China Sea (nSCS) and the Kuroshio from July 16th to September 1st, 2009. N2 fixation activities in whole seawater and <10 μm fraction at the surface were measured by acetylene reduction assay. Higher activities were observed at the East China Sea (ECS) Kuroshio and the nSCS shelf. The nSCS basin showed a low N2 fixation activity. The <10 μm fractions (unicellular diazotrophs) contributed major portion to the whole-water activity in the survey time, indicating that nanoplanktonic cyanobacterias were the major diazotrophs in the survey area. Daily N2 fixation rates of Trichodesmium ranged from 0.11 to 9.83 pmolNtrichome−1 d−1 with an average of 4.03 pmolNtrichome−1 d−1. The Luzon Strait and the ECS Kuroshio had higher N2 fixation rates of Trichodesmium than the nSCS shelf and basin. Calculated activities of Trichodesmium at most stations were moderately low compared with that of the whole-water. The contribution of N2 fixation by the whole-water to primary production ranged from 1.7% to 18.5%. The estimated amount of new nitrogen introduced by Trichodesmium contributed up to 0.14% of the total primary production and 0.41% of the new production in the Luzon Strait.

Low-Cloud Transitions across the Kuroshio Front in the East China Sea

Abstract The East China Sea Kuroshio (ECSK) flows in the East Asian monsoon region where the background atmospheric circulation varies significantly with season. A sea surface temperature (SST) front associated with the ECSK becomes narrower and sharper from winter to spring. The present study investigates how low clouds respond to the ECSK front in different seasons by synthesizing spaceborne lidar and surface visual observations. The results reveal prominent cross-frontal transitions in low clouds, which exhibit distinct behavior between winter and spring. In winter, cloud responses are generally confined below 4 km by the strong background descending motion and feature a gradual cloud-top elevation from the cold to the warm flank of the front. The ice clouds on the cold flank of the ECSK front transform into liquid water clouds and rain on the warm flank. The springtime clouds, by contrast, are characterized by a sharp cross-frontal transition with deep clouds reaching up to 7 km over the ECSK. In both winter and spring, the low-cloud morphology exhibits a large transformation from the cold to the warm flank of the ECSK front, including increases in cloud-top height, a decline in smoothness of cloud top, and the transition from stratiform to convective clouds. All this along with the atmospheric soundings indicates that the decoupling of the marine atmospheric boundary layer (MABL) is more prevalent on the warm flank of the front. Thus, long-term observations reveal prominent cross-frontal low-cloud transitions in morphology associated with MABL decoupling that resemble a large-scale cloud-regime transition over the eastern subtropical Pacific.

Inter‐annual and decadal fluctuations of the Kuroshio in East China Sea and connection with surface fluxes of momentum and heat

AbstractDespite attempts in the literature to link large‐scale wind to long‐term variations of the Kuroshio in East China Sea (ECS), the driving mechanism(s) are unknown. Here we use satellite altimetry data, wind, surface heat fluxes and sea‐surface temperatures (SST) to explain the low‐frequency fluctuations of Kuroshio path (KP) in ECS. The dominant fluctuations occur northeast of Taiwan. The KP correlates best with the PTO index of Chang and Oey (2012), less with the PDO index and a Kuroshio transport index, and poorly with other climate indices. The forcing are wind stress curl and surface heat flux northeast of Taiwan, which produce a thermocline tilt along the Kuroshio. Shelf's SST warms and cools in response to onshore and offshore KP, but prominent change occurs at a localized coastal zone shoreward of the above dominant KP‐fluctuations. Over the past 2 decades, the KP has shifted onshore, coincident with a coastal warming trend.

The formation of wind curl in the marine atmosphere boundary layer over the East China Sea Kuroshio in spring

Various data are used to investigate the characteristics of the surface wind field and rainfall on the East China Sea Kuroshio (ESK) in March and April, 2011. In March, the wind speed maximum shows over the ESK front (ESKF) in the 10 meter wind field, which agrees with the thermal wind effect. A wind curl center is generated on the warm flank of the ESKF. The winds are much weaker in April, so is the wind curl. A rainband exists over the ESKF in both the months. The Weather Research and Forecasting (WRF) model is used for further researches. The winds on the top of the marine atmosphere boundary layer (MABL) indicate that in March, a positive wind curl is generated in the whole MABL over the warm flank of the ESKF. The thermal wind effect forced by the strong SST gradient overlying the background wind leads to strong surface northeasterly winds on the ESKF, and a positive shearing vorticity is created over the warm flank of the ESKF to generate wind curl. In the smoothed sea surface temperature experiment, the presence of the ESKF is responsible for the strong northeast winds in the ESKF, and essential for the distribution of the rainfall centers in March, which confirms the mechanism above. The same simulation is made for April, 2011, and the responses from the MABL become weak. The low background wind speed weakens the effect of the thermal wind, thus no strong Ekman pumping is helpful for precipitation. There is no big difference in rainfall between the control run and the smooth SST run. Decomposition of the wind vector shows that local wind acceleration induced by the thermal wind effect along with the variations in wind direction is responsible for the pronounced wind curl/divergence over the ESKF.

Two Types of Surface Wind Response to the East China Sea Kuroshio Front*

Abstract Effects of the sea surface temperature (SST) front along the East China Sea Kuroshio on sea surface winds at different time scales are investigated. In winter and spring, the climatological vector wind is strongest on the SST front while the scalar wind speed reaches a maximum on the warm flank of the front and is collocated with the maximum difference between sea surface temperature and surface air temperature (SST − SAT). The distinction is due to the change in relative importance of two physical processes of SST–wind interaction at different time scales. The SST front–induced sea surface level pressure (SLP) adjustment (SF–SLP) contributes to a strong vector wind above the front on long time scales, consistent with the collocation of baroclinicity in the marine boundary layer and corroborated by the similarity between the thermal wind and observed wind shear between 1000 and 850 hPa. In contrast, the SST modulation of synoptic winds is more evident on the warm flank of the SST front. Large thermal instability of the near-surface layer strengthens temporal synoptic wind perturbations by intensifying vertical mixing, resulting in a scalar wind maximum. The vertical mixing and SF–SLP mechanisms are both at work but manifest more clearly at the synoptic time scale and in the long-term mean, respectively. The cross-frontal variations are 1.5 m s−1 in both the scalar and vector wind speeds, representing the vertical mixing and SF–SLP effects, respectively. The results illustrate the utility of high-frequency sampling by satellite scatterometers.

Spatial Change of Salinity at Kuroshio in East China Sea with Seasons

Kuroshio influences hydrological-climate environment of East China Sea by exchanging heat and salinity. It has great influence on climate-environment in East China. Understanding of seasonal variability of salinity at Kuroshio in the East China Sea (ECS Kuroshio) can provide a scientific basis to reveal the climate-environment change in East China. In this paper, Using salinity database of World Ocean Atlas 2009(WOA09) issued by NOAA in 2010, the change of salinity distribution and the high-salt areas at ECS Kuroshio with seasons have been analyzed, the results show that: upper low-salinity water between 0~125m is obviously changing with seasons. From spring to winter, the horizontal stratification at upper low-salinity water first increases and then decreases, but the phenomenon of vertical mixing is opposite with the horizontal stratification. The horizontal stratification is the most obvious in summer, it forms 0.33/m of halocline strength above 10m; the vertical mixing is the strongest in winter, upper low-salinity water mixes with higher-salinity water in Okinawa trough, and salinity is homogenous in vertical above 250m. There are seasonal variations of depth and range of the high-salt areas in different segments along ECS Kuroshio, and there are three high salinity cores in the high-salt areas: two of the high salinity cores appear in the sea area east of Taiwan in autumn and winter, the third core appears in the sea area northwest of Miyako-jima in spring.

Interaction between the East China Sea Kuroshio and the Ryukyu Current as revealed by the self‐organizing map

The self‐organizing map (SOM) is used to study the linkage between the two western boundary currents, the Kuroshio Current and the Ryukyu Current, through the Kerama Gap. Four coherent ocean current patterns, extracted from a numerical model output for the Kerama Gap area, are used to describe the variability of the Kuroshio current axis and the mesoscale eddies in the Ryukyu Current system. The temporal evolution of these four patterns shows a robust cycle with an average period of 120 days. The shift of the Kuroshio current axis is found to be a dominant factor in determining the water exchange in the Kerama Gap, and the eddies associated with the Ryukyu Current are also important in affecting the strength of eddies in the Kerama Gap. The interaction between the Kuroshio Current and the Ryukyu Current through the Kerama Gap as revealed by the SOM provides new insights in understanding the water exchange between the East China Sea and the northwest Pacific.

The effect of the East China Sea Kuroshio Front on the marine atmospheric boundary layer

Various satellite data, JRA-25 (Japan reanalysis of 25 years) reanalyzed data and WRF (Weather Research Forecast) model are used to investigate the in situ effect of the ESKF (East China Sea Kuroshio Front) on the MABL (marine atmospheric boundary layer). The intensity of the ESKF is most robust from January to April in its annual cycle. The local strong surface northerly/northeasterly winds are observed right over the ESKF in January and in April and the wind speeds decrease upward in the MABL. The thermal wind effect that is derived from the baroclinic MABL forced by the strong SST gradient contributes to the strong surface winds to a large degree. The convergence zone existing along the warm flank of the ESKF is stronger in April than in January corresponding to the steeper SST (sea surface temperature) gradient. The collocations of the cloud cover maximum and precipitation maximum are basically consistent with the convergence zone of the wind field. The clouds develop higher (lower) in the warm (cold) flank of the ESKF due to the less (more) stable stratification in the MABL. The lowest clouds are observed in April on the cold flank of the ESKF and over the Yellow Sea due to the existence of the pronounced temperature inversion. The numerical experiments with smoothed SST are consistent with the results from the ovservations.

Southward recirculation of the East China Sea Kuroshio west of Okinawa Island

The surface current of the Kuroshio recirculation region in the East China Sea is investigated. The surface currents in the west of Okinawa Island observed by HF radars and the assimilation results of the Japan Coastal Ocean Predictability Experiment (JCOPE) are analyzed. It appeared that currents in the easternmost edge of the Kuroshio bifurcate southward and flow into the west of Okinawa Island from JCOPE data. The currents are called Kuroshio southward recirculation currents. The currents are controlled by the bottom topography. The wind stress curl in the area where the bifurcated current can be seen in JCOPE data is negative, while the curl is positive in the eastern portion of the Kuroshio. The Kuroshio southward recirculation currents are sometimes enhanced, and their enhancement coincides with that of the positive and negative wind stress curls.

Multi-core structure of the Kuroshio in the East China Sea from long-term transect observations

Long-term hydrographic temperature and salinity transect data in the East China Sea from June 1955 to November 2001 are analyzed in order to examine the geostrophic velocity and the structure of the Kuroshio current. The structure of the Kuroshio Current is divided into three basic forms, a single-core structure, a double-core structure and a multi-core structure; the appearance percentage of the three forms are 53.1%, 31.4%, and 15.4%, respectively. The analysis suggests that multi-core structures have significant seasonal and interannual variabilities that are not fully understood but may relate to variations in transport and associated flow instabilities. The Kuroshio's spatial character is also analyzed in detail by applying a simplified model of motion instability into this multi-core structure of the East China Sea Kuroshio. The theoretical results are found to be consistent with the observations, suggesting that the instability of the Kuroshio in the East China Sea may bring forth the formation of the observed multi-core structure.

Multi-core structure of the main part of the Kuroshio at G-PN section in the East China Sea

Sectional velocity distribution of the East China Sea Kuroshio is one of the basic points in the study of the Kuroshio. Hydrographic temperature and salinity data at G-PN section in the East China Sea from June 1955 to November 2001 are collected and properly processed to calculate the geostrophic current using dynamic height method at the transect of the Kuroshio. After analysis of calculation results, the basic current structure of the Kuroshio in its main part is examined together with scalar estimate and characters of multi- core structure, and spacial-temporal variations of current cores' position. Main result shows that (1) single-core structure, double-core structure and multi-core structure are basic forms in axial part of the Kuroshio; (2) abvious temporal variations exist in current structure of the Kuroshio; (3) the current of structure of the Kuroshio has distinctly seasonal association. The number of current cores is on the high side of core numbers in average and multi-core stucture appears in fall mostly.

A motion instability formation mechanism of the multi-core structure of the East China Sea Kuroshio

On the basis of the inductive analysis of observation and calculation results, this paper presents a motion instability formation mechanism of the multi-core structure of the East China Sea Kuroshio. This theoretical work includes presentation of the simplified model, derivation of the existence condition of the instable solution and some scaling estimation of the multi-core structure. Theoretical results are in good consistence with the observations.