Spatio-temporal variability of surface chlorophyll a in the Yellow Sea and the East China Sea based on reconstructions of satellite data of 2001-2020

Chlorophyll-a(Chl-a)concentration is a primary indicator for marine environmental monitoring.The spatio-temporal variations of sea surface Chl-a concentration in the Yellow Sea(YS)and the East China Sea(ECS)in 2001-2020 were investigated by reconstructing the MODIS Level 3 products with the data interpolation empirical orthogonal function(DINEOF)method.The reconstructed results by interpolating the combined MODIS daily+8-day datasets were found better than those merely by interpolating daily or 8-day data.Chl-a concentration in the YS and the ECS reached its maximum in spring,with blooms occurring,decreased in summer and autumn,and increased in late autumn and early winter.By performing empirical orthogonal function(EOF)decomposition of the reconstructed data fields and correlation analysis with several potential environmental factors,we found that the sea surface temperature(SST)plays a significant role in the seasonal variation of Chl a,especially during spring and summer.The increase of SST in spring and the upper-layer nutrients mixed up during the last winter might favor the occurrence of spring blooms.The high sea surface temperature(SST)throughout the summer would strengthen the vertical stratification and prevent nutrients supply from deep water,resulting in low surface Chl-a concentrations.The sea surface Chl-a concentration in the YS was found decreased significantly from 2012 to 2020,which was possibly related to the Pacific Decadal Oscillation(PDO).

Evaluation on monthly sea surface wind speed of four reanalysis data sets over the China seas after 1988

This study investigates the long-term changes of monthly sea surface wind speeds over the China seas from 1988 to 2015. The 10-meter wind speeds products from four major global reanalysis datasets with high resolution are used: Cross-Calibrated Multi-Platform data set(CCMP), NCEP climate forecast system reanalysis data set(CFSR),ERA-interim reanalysis data set(ERA-int) and Japanese 55-year reanalysis data set(JRA55). The monthly sea surface wind speeds of four major reanalysis data sets have been investigated through comparisons with the longterm and homogeneous observation wind speeds data recorded at ten stations. The results reveal that(1) the wind speeds bias of CCMP, CFSR, ERA-int and JRA55 are 0.91 m/s, 1.22 m/s, 0.62 m/s and 0.22 m/s, respectively.The wind speeds RMSE of CCMP, CFSR, ERA-int and JRA55 are 1.38 m/s, 1.59 m/s, 1.01 m/s and 0.96 m/s,respectively;(2) JRA55 and ERA-int provides a realistic representation of monthly wind speeds, while CCMP and CFSR tend to overestimate observed wind speeds. And all the four data sets tend to underestimate observed wind speeds in Bohai Sea and Yellow Sea;(3) Comparing the annual wind speeds trends between observation and the four data sets at ten stations for 1988-1997, 1988–2007 and 1988–2015, the result show that ERA-int is superior to represent homogeneity monthly wind speeds over the China seaes.

Comparison among four kinds of data of sea surface wind stress in the South China Sea

By using remote sensing (ERS) data, FSU data, COADS data and Hellerman & Rosen-stein objective analysis data to analyze the sea surface wind stress in the South China Sea, it is found that the remote sensing data have higher resolution and more reasonable values. Therefore we suggest that remote sensing data be chosen in the study of climatological features of sea surface wind stress and its seasonal variability in the South China Sea, especially in the study of small and middle scale eddies.

Typhoon-induced wind waves in the northern East China Sea during two typhoon events:the impact of wind field and wave-current interaction

We examined the influences of the wind fi eld and wave-current interaction(WCI)on the numerical simulation results of typhoon-induced wind waves in the northern East China Sea(NECS)using the coupled Simulating Waves Nearshore+Advanced Circulation(SWAN+ADCIRC)model.The simulations were performed during two typhoon events(Lekima and Muifa),and two widely used reanalysis wind fields,the Climate Forecast System Version 2(CFSv2)from the National Centers for Environmental Prediction(NCEP)and the fifth-generation European Centre for Medium-Range Weather Forecasts(ECMWF)Reanalysis(ERA5),were compared.The results indicate that the ERA5 and CFSv2 wind fields both reliably reproduced the wind variations measured by in-situ buoys,and the accuracy of the winds from ERA5 were generally better than those from CFSv2 because CFSv2 tended to overestimate the wind speed and the simulated significant wave height(SWH),particularly the peak SWH.The WCI effects between the two wind field simulations were similar;these effects enhanced the SWH throughout the nearshore NECS during both typhoons but suppressed the SWH on the right side of the Typhoon Muifa track in the deep and off shore sea areas.In summary,variations in the water depth and current propagation direction dominate the modulation of wave height.

Spatio-temporal Variations of Sea Surface Wind in Coral Reef Regions over the South China Sea from 1988 to 2017

The seasonal and interannual variabilities of sea surface wind(SSW)in the South China Sea(SCS),especially in coral reef regions such as Nansha Islands,Xisha Islands,Zhongsha Islands and Dongsha Islands were investigated in detail using the Blended Sea Winds dataset(1988-2017).Annual and monthly variations of SSW and sea surface temperature(SST)in the four zones were investigated.Empirical Orthogonal Function(EOF)analysis of wind field was performed to aid in better understanding the different spatial patterns.The results indicate that,as observed in the spatial distribution of the first mode of monthly mean wind speed anomaly,the mag-nitudes in the four island zones are all negative and are similar to each other,showing that the variations of SSW in the four island zones are consistent.In the second mode,the magnitudes in Nansha Islands are opposite to those in the other three zones.The spatial distribution of the third mode reflects regional differences.The maximum annual SSW appears in Dongsha Islands,and the minimum appears in Nansha Islands.The interannual variations of SSW in all island zones are basically concurrent.The island zones with high SSW mostly have low SST,and vice versa.There may be an inverse relationship between SSW and SST in coral reef regions in the SCS.The multiyear monthly variations of SSW in the island zones present a'W'-shaped structural variation.Each island undergoes two months of minimum SSW every year,one during March-May(MAM)and the other during September-November(SON).Both months are in mon-soon transition periods.During the months with low SSW,high SST appears.The SST peaks almost correspond to the SSW troughs.This further indicates that SSW and SST may have opposite changes in coral reef regions.Coral bleaching events often correspond to years of high SST and low SSW.

Surface distribution of alkalinity and specific alkalinity and their application to water mass tracing in Kuroshio area of the East China Sea

Surface distribution and seasonal variation of alkalinity and specific alkalinity in Kuroshio area of the East ChinaSea and their application to the water mass tracing are discussed in this paper. Results show a distinct seasonal variation of the alkalinity, which is concerned with the process of vertical mixing. Different specific alkalinity in various water masses has been found. On the basis of the difference of the specific alkalinity and the distribution of alkalinity, two water fronts in summer season, located at 27°-30°N and 124°-1 27°E, (Ⅰ), and at the northern waters about one latitude from the Taiwan Island, (Ⅱ); one in winter season at about one longitude from coast of mainland of China and 26°-30°N were found. In summer season, about 1-2 longitudes eastward shift of front (Ⅰ) is found by comparison of data in May and August. And the high alkalinity of the northern East China Sea in summer season may be caused by the Huanghe River runoff flowing southward along with the Huanghai Sea Coastal Current.

Impact of Sea Surface Temperature Front on Stratus-Sea Fog over the Yellow and East China Seas-A Case Study with Implications for Climatology

A stratus-sea fog event that occurred over the Yellow and East China Seas on 3 June 2011 is investigated using observa-tions and a numerical model, with a focus on the effects of background circulation and Sea Surface Temperature Front (SSTF) on the transition of stratus into sea fog. Southerly winds of a synoptic high-pressure circulation transport water vapor to the Yellow Sea, creating conditions favorable for sea fog/stratus formation. The subsidence from the high-pressure contributes to the temperature inversion at the top of the stratus. The SSTF forces a secondary circulation within the ABL (Atmospheric Boundary Layer), the sink-ing branch of which on the cold flank of SSTF helps lower the stratus layer further to reach the sea surface. The cooling effect over the cold sea surface counteracts the adiabatic warming induced by subsidence. The secondary circulation becomes weak and the fog patches are shrunk heavily with the smoothed SSTF. A conceptual model is proposed for the transition of stratus into sea fog over the Yellow and East China Seas. Finally, the analyses suggest that sea fog frequency will probably decrease due to the weakened SSTF and the reduced subsidence of secondary circulation under global warming.

Temporal-Spatial Variation of Surface Suspended Matter and Controlling Factors in the Inner Shelf of the East China Sea in Winter

Based on the observed surface suspended matter in the East China Sea in February 2007 and June 2015, an empirical model was established using L1 b's band 4 data to retrieve surface suspended matter from the Moderate Resolution Imagine Spectroradiometer Terra imagery. The squared correlation coefficient is 0.8358, and the root mean square error is 0.4285 mg L-1. The model reflects the distribution characteristics of surface suspended matter in the inner shelf of the East China Sea. In this paper, the satellite images of the study area were retrieved in January from 2001 to 2015, and the monthly distribution of surface suspended matter were obtained. The inter-annual distribution of the study area is similar, and the concentration of surface suspended matter is higher near the shore than offshore. A large amount of surface suspended matter is transported southeast under the influence of Zhejiang and Fujian coastal current and Taiwan warm current. Only a small amount of surface suspension can reach the Kuroshio area. The surface suspended matter concentration changes obviously near the estuary because of the effect of differences in the flux of the Yangtze River. Meanwhile, winter monsoon, temperature front, El Ni?o events, and other factors affect the distribution of surface suspended matter in 100 m isobath to coastal water but minimally influence the distribution in 100 m isobath to deep sea.

Major Mid-Late Holocene Cooling in the East China Sea Revealed by an Alkenone Sea Surface Temperature Record

Although the mid-late Holocene cold and dry event about 4000 years ago(the 4 ka event) has been observed almost globally, it was most prominent in terrestrial climate proxies from the lower latitudes. Here we evaluate the oceanic response to this event in terms of a Holocene sea surface temperature(SST) record reconstructed using the K'37U index for Core B3 on the continental shelf of the East China Sea. The record reveals a large temperature drop of about 5℃ from the mid-Holocene(24.7℃ at 5.6 ka) to the 4 ka event(19.2℃ at 3.8 ka). This mid-late Holocene cooling period in Core B3 correlated with(i) decreases in the East Asia summer monsoon intensity and(ii) the transition period with increased El Nino/Southern Oscillation activities in the Equatorial Pacific. Our SST record provides oceanic evidence for a more global nature of the mid-late Holocene climate change, which was most likely caused by a southward migration of the Intertropical Converge Zone in response to the decreasing summer solar insolation in the Northern Hemisphere. However, the large SST drop around Core B3 indicates that the mid-late Holocene cooling was regionally amplified by the initiation/strengthening of eddy circulation/cold front which caused upwelling and resulted in additional SST decrease. Upwelling during the mid-late Holocene also enhanced with surface productivity in the East China Sea as reflected by higher alkenone content around Core B3.

Study on the growth of wind wave frequency spectra generated by cold waves in the northern East China Sea

The growth of frequency spectra and spectral parameters of wind waves generated by cold waves, a kind of severe weather system, in the northern East China Sea is studied in this paper. Based on a third-generation wave action model(the Simulating WAves Nearshore model), simulations were developed to analyze the spatiotemporal characteristics of wind waves and to output spectral data. It is shown that the cold wave-induced spectra can be well described by the modified Joint North Sea Wave Project spectral form. The growth of wave spectra is comprehensively reflected by the evolution of the three characteristic parameters: peak frequency, spectral peak and wave energy. Besides, the approximations of dependences between spectral parameters and the three types of universal induced factors are obtained with the least squares method and compared systematically. Fetch and peak frequency turn out to be suitable parameters to describe the spectral parameters, while the dependences on the inverse wave age vary in different sea areas. In general, the derived relationships improve on results from previous studies for better practical application of the wind wave frequency spectrum in the northern East China Sea.

Surface diurnal warming in the East China Sea derived from satellite remote sensing

Process of sea surface diurnal warming has drawn a lot of attention in recent years, but that occurs in shelf seas was rarely addressed. In the present work, surface diurnal warming strength in the East China Sea was calculated by the sea surface temperature(SST) data derived from the MODIS sensors carried by the satellites Aqua and Terra. Due to transit time difference, both the number of valid data and the surface diurnal warming strength computed by the MODIS-Aqua data are relatively larger than Terra. Therefore, the 10-year MODIS-Aqua data from 2005 to 2014 were used to analyze the monthly variability of the surface diurnal warming. Generally, the surface diurnal warming in the East China sea is stronger in summer and autumn but weaker in winter and spring, while it shows different peaks in different regions. Large events with ΔT≥5 K have also been discussed. They were found mainly in coastal area, especially near the Changjiang(Yangtze) River estuary. And there exists a high-incidence period from April to July. Furthermore, the relationship between surface diurnal warming and wind speed was discussed. Larger diurnal warming mainly lies in areas with low wind speed. And its possibility decreases with the increase of wind speed. Events with ΔT ≥2.5 K rarely occur when wind speed is over 12 m/s. Study on surface diurnal warming in the East China Sea may help to understand the daily scale air-sea interaction in the shelf seas. A potential application might be in the marine weather forecasts by numerical models. Its impact on the coastal eco-system and the activities of marine organisms can also be pursued.

Derivation of sea surface current field from sequential satellite images of the East China Sea

A series of NOAA AVHRR data over the East China Sea were collected from the ground station of the Second Institute of Oceanography, Hangzhou, China. Three methods, including a functional analytic method (FAM), a maximum cross correlation (MCC)'method and a correlation relaxation (C - R) method, are applied to derive the sea surface current field from sequential satellite images in the area of the East China Sea. Several preprocessing steps, such as geometric correction, SST determination, image projection, image navigation and grey value normalization as well as land and cloud mask are performed. The results from the three methods reflect the general current system in this area reasonably.

Generation and propagation of 21-day bottom pressure variability driven by wind stress curl in the East China Sea

Between June 2015 and June 2017,two pressure-recording inverted echo sounders(PIESs)and five current and pressure-recording inverted echo sounders(CPIESs)deployed along a section across the Kerama Gap acquired a dataset of ocean bottom pressure records in which there was significant 21-day variability(Pbot21).The Pbot21,which was particularly strong from July-December 2016,was coherent with wind stress curl(WSC)on the continental shelf of the East China Sea(ECS)with a squared coherence of 0.65 for a 3-day time lag.A barotropic ocean model demonstrated the generation,propagation,and dissipation of Pbot21.The modeled results show that the Pbot21 driven by coastal ocean WSC in the ECS propagated toward the Ryukyu Island Chain(RIC),while deep ocean WSC could not induce such variability.On the continental shelf,the Pbot21 was generated nearly synchronously with the WSC from the coastline to the southeast but dissipated within a few days due to the effect of bottom friction.The detection of Pbot21 by the moored array was dependent on the 21-day WSC patterns on the continental shelf.The Pbot21 driven southeast of the Changjiang Estuary by the WSC was detected while the Pbot21generated northeast of the Changjiang Estuary was not.

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.

An Objective Analysis Method for Surface Residual Current Field in the East China Seas

Since the volume transport across the pycnocline is much smaller than that in the mixed layer, the current in the mixed layer can be regarded as non-divergent. An objective analysis method is deduced based on this hypothesis. The linear combination method is used to solve the non-divergent component of the current field of an ocean basin containing islands,which is equivalent to a mathematical problem of solving a Poisson equation in a multi-connected domain. The method is applied to the Bohai Sea, the Yellow Sea and the East China Sea (ECS). The modeled result is consistent with the current maps constructed by other oceanographers.

Two drifting paths of Sargassum bloom in the Yellow Sea and East China Sea during 2019-2020

The macroalgal blooms of floating brown algae Sargassum horneri are increasing in the Yellow Sea and East China Sea during the past few years.However,the annual pattern of Sargassum bloom is not well characterized.To study the developing pattern and explore the impacts from hydro-meteorologic environment,high resolution satellite imageries were used to monitor the distribution,coverage and drifting of the pelagic Sargassum rafts in the Yellow Sea and East China Sea from September 2019 to August 2020.Sargassum blooms were detected from October 2019 to June 2020 and presented two successive drifting paths that both initiated from around 37°N.The first path spanned smaller spatial scale and shorter period,starting with a bloom of 3 km^(2) distribution area near the eastern tip of Shandong Peninsula in late October 2019 and drifted southwards,hit the Pyropia aquaculture area in early January 2020,then vanished in the northwest of East China Sea(ca.32°N)around end of January.The second path began with a large distribution area of 23000 km^(2) east of 123°E in late January 2020,firstly moved southwards in the central Yellow Sea and northern East China Sea(north of 29°N)till late April,then turned northwards with monsoon wind and vanished from late June to August.The mean sea surface temperature of 8℃ to 20℃ in the Sargassum bloom areas corresponded to in situ observed temperature range for vegetative growth and floating of S.horneri.There was no observed floating Sargassum blooms during July through September in the Yellow Sea and East China Sea.The results indicate that floating S.horneri is unable to complete life cycle in the Yellow Sea and East China Sea,and provide insights to the future management of Sargassum blooms.Further studies are needed to validate the pattern and source of annual Sargassum bloom in the Yellow Sea and East China Sea.

Warming trend in northern East China Sea in recent four decades

Global warming has become a notable trend especially since an abrupt climate change in 1976. Response of the East China Sea (ECS) to the global warming trend, however, is not well understood because of sparse long-term observation. In this paper, hydrographic observation data of 1957–1996 are collected and reviewed to study climatological variability in northern ECS. Significant warming trends are found in both summer and winter. In summer, the average SST is about 0.46°C higher during the period of 1977-1996 than that of 1957-1976, and the Taiwan Warm Current Water (TWCW) was strengthened. In winter, despite of the cooling effect in the coastal areas adjacent to the Changjiang (Yangtze) River Estuary (CRE), the average SST increase was about 0.53°C during the same period. The causes of this SST warming up in summer are different from in winter. The warming trend and intensification of the TWCW in summer were primarily influenced by the strengthening of the Kuroshio transport, while the warming in winter was mainly induced by the variability of the climate system.

Numerical simulation and preliminary analysis of typhoon waves during three typhoons in the Yellow Sea and East China Sea

In this study,typhoon waves generated during three typhoons(Damrey(1210),Fung-wong(1416),and Chan-hom(1509))in the Yellow Sea and East China Sea were simulated in a simulating waves nearshore(SWAN)model,and the wind forcing was constructed by combining reanalyzed wind data with a Holland typhoon wind model.Various parameters,such as the Holland fitting parameter(B)and the maximum wind radius?,were investigated in sensitivity experiments in the Holland model that affect the wind field construction.Six different formulations were considered and the parameters determined by comparing the simulated wind results with in-situ wind measurements.The key factors affecting wave growth and dissipation processes from deep to shallow waters were studied,including wind input,whitecapping,and bottom friction.Comparison with in-situ wave measurements suggested that the KOMEN scheme(wind input exponential growth and whitecapping energy dissipation)and the JONSWAP scheme(dissipation of bottom friction)resulted in good reproduction of the significant wave height of typhoon waves.A preliminary analysis of the wave characteristics in terms of wind-sea and swell wave revealed that swell waves dominated with the distance of R to the eye of the typhoon,while wind-sea prevailed in the outer region up to six to eight times the R values despite a clear misalignment between wind and waves.The results support the hypothesis that nonlinear wave-wave interactions may play a key role in the formation of wave characteristics.

Holocene Temperature Records from the East China Sea Mud Area Southwest of the Cheju Island Reconstructed by theU_(37)~K~' and TEX_(86) Paleothermometers

As an important marginal sea under the influences of both the Changjiang River and the Kuroshio,the East China Sea (ECS) environment is sensitive to both continental and oceanic forcing.Paleoenvironmental records are essential for understanding the long-term environmental evolution of the ECS and adjacent areas.However,paleo-temperature records from the ECS shelf are currently very limited.In this study,the'K37U and TEX86paleothermometers were used to reconstruct surface and subsurface temperature changes of the mud area southwest of the Cheju Island (Site F10B) in the ECS during the Holocene.The results indicate that temperature changes of F10B during the early Holocene (11.6 6.2 kyr) are associated with global climate change.During the period of 6.2 2.5 kyr,the similar variability trends of smoothing average of T (the difference between surface and subsurface temperature) of Site F10B and the strength of the Kuroshio suggest that the Kuroshio influence on the site started around 6.2 kyr when the Kuroshio entered the Yellow Sea and continued to 2.5 kyr.During the late Holocene (2.5 1.45 kyr) ,apparent decreases of'K37U sea surface temperature (SST) and T imply that the direct influence of the Kuroshio was reduced while cold eddy induced by the Kuroshio gradually controlled hydrological conditions of this region around 2.5 kyr.

3D Velocity Structure and Its Tectonic Implications in the East China Sea and Yellow Sea

3D structure of the crust and upper mantle in the studied area has been analyzed from surface wave tomography. The velocity distribution in the uppermost crust is symmetrical on two sides of the central line of the sea, and coincides with the structure of crystalline basement. The essential difference in tectonics between the East China Sea and the Yellow Sea mainly lies in that the velocity structures of their lower crust and upper mantle are identical to those of South China and North China respectively. In the upper mantle there exists a high-velocity zone with a nearly EW strike from the Hangzhou Bay, China, to the Tokara Channel, Japan, along about the latitude of 30°N. It is found that between the East China Sea and the Yellow Sea there are systematical differences in geomorphology, geology, seismicity, heat flow, quality factor and gravity and aeromagnetic anomalies, which is related to both left-lateral shear dislocation and right-lateral tear of the Benioff zone from the Hangzhou Bay to the Tokara Channel.It is inferred that the East China Sea was formed by Cenozoic back-arc extension. The boundary between the North China and South China crustal blocks stretches along the southern piedmont of Mts. Daba-Dabie-Hangzhou Bay-Tokara Channel, and the subduction zone at the Okinawa trench is the eastern boundary of the South China crustal block. The movements of the Pacific plate, Indian plate and upper mantle rather than the Philippine plate subduction have played a dominant role for the modern tectonic movements in East Asia.