Water Exchange Through Kerama Gap And It's Influence On The ECS-kuroshio

The Kerama Gap,located between the Okinawa Islands and Miyakojima Island(~50Km),is the deepest channel in the Miyako strait and the Ryukyu Island chain with a sill depth of 1050 m,which plays an important role in the water exchange between the East China Sea(ECS)and Northwestern Pacific(NP).Although the mean Kerama Gap transport(KGT)is small,the variation of KGT is large,resulting a significant influence on the variation between the Kuroshio upstream and the downstream.Due to the deficiency of observational data,we investigate the water exchange through the Kerama Gap by using the combination of public observation data and model.Then we analysis the effects of water exchange through the Kerama Gap on the transport variation and water properties between the Kuroshio upstream and downstream.Finally,we discuss the dynamic mechanism of the effects of mesoscale eddies,Kuroshio and Ryukyu Currents on the water exchange through the Kerama Gap.The main results and conclusions are as follows:(1)The mean KGT is 1.93 Sv from the NP into the ECS,but the monthly KGT has a large variability with a maximum of 13.9 Sv and a minimum of-9.9Sv with a standard deviation of 4.3Sv.Pacific water flow into the ECS through the Kerama Gap mainly in the subsurface layer(300-500m),which may be induced by the east Ryukyu Currents.The seasonal variability of KGT is significant,which is strongest in spring,second stronger in spring,weaker in winter and weakest in summer.The inter-annual variability of KGT has an increasing tendency in spring and winter and decreasing tendency in summer and autumn.The most significant increasing tendency is in spring and the most decreasing tendency is in autumn.(2)Volume transport between the Kuroshio upstream transects were spatially consistent,but there was a significant discrepancy between the Kuroshio upstream transport and downstream transport;This discrepancy was contributed by the KGT and the continental shelf water,but the variation of KGT plays the most important role.The variation of KGT shows a positive correlation with the variation of Kuroshio transport across the PN line(KNT).(3)The intrusion of NP into the ECS via the Kerama Gap has a significant influence on the water properties of the downstream Kuroshio,which increases the salinity of the Kuroshio subsurface water and decreases the salinity of Kurshio intermediate water.Besides,the subsurface water depth become much shallower and the intermediate water depth becomes much deeper.The seasonal variation of the subsurface water salinity and the intermediate water salinity between the upstream and downstream Kurshio is not consistent.The seasonal variation of subsurface water salinity may be influenced by the combination of the variation of KGT,KMT and the surface freshwater flux,which is maximum in winter,second in autumn,third in spring,and minimum in summer.However,the seasonal variation of intermediate water salinity is mainly influenced by the variation of KGT and KMT,which is maximum in autumn,second in spring,third in winter,and maximum in summer.(4)The variation of KGT is mainly determined by the variation of sea level difference(SLD)across the Kerama Gap,and these differences are dominated by the westward propagating mesoscale eddies.The effects of mesoscale eddies on the variation of KGT depend on the meridional location of these eddies.When the anticyclonic(cyclonic)eddy is located in the northeast of the Kerama Gap or the cyclonic(anticyclonic)eddy is located in the southwest of the Kerama Gap,the KGT shows a strong positive(negative)anomaly.However,when the anticyclonic or cyclonic eddy is located in the central region of the Kerama Gap,the KGT shows a relatively smaller anomaly.The dynamic mechanism underlying the effects of mesoscale eddies on the variation of KGT may be explained by the mass balance analysis and barotropic island rule.The former indicates that the relative strength of the mass convergence to divergence makes a great contribution to the variation of KGT.The latter suggests that energy dissipation in the boundary between the eddy and the Okinawa Islands or Miyakojima Island induced the streamer flowing around the islands,which caused the variation of KGT.(5)The ECS-Kuroshio and Ryukyu Currents also have an influence on the variation of KGT.The influence of Kuroshio is mainly determined by the KMT and the shift of Kuroshio central position(KCP).When the KMT is high(low)or the KCP shifts toward(away from)the Kerama Gap,there will be more(less)water flowing from the ECS to the NP through the Kerama Gap,resulting the decreasing(increasing)of KGT and finally the KNT.The influence of Ryukyu Currents is mainly determined by the transport variation of Ryukyu currents between the northeastern and southwestern of the Gap,which shows a positive correlation with the KGT.Besides,the increasing(decreasing)volume transport in southwest of the Gap results the decreasing(increasing)KGT.However,the increasing(decreasing)volume transport in northeast of the Gap results the increasing(decreasing)KGT.