Seasonal Variation Of Hydrologic Characteristics In The Miyako Strait And Its Adjacent Seas

The Miyako strait locates between the Okinawa and Miyako islands. As a major deep channel in linking the East China Sea to the Pacific Ocean, it plays an important role in the water exchange between them. The hydrologic characteristics around the Miyako strait also has an important effect on fishery and shipping. Based on the reanalysis dataset of JCOPE2(Japan Coastal Ocean Predictability Experiments 2), the temporal and spatial variations of temperature, salinity and velocity are analyzed. Special attentions were also paid to the water exchange across the Miyako strait.The water mass around the strait was characterized by four layers, with the maximum and minimum salinity water located at the subsurface and intermediate layers respectively. The potential density of subsurface water and intermediate water of North Pacific were 23.8-25.0 and 26.5-27.0. The North Pacific has a higher salinity and lower salinity than the adjacent East China Sea in the subsurface and intermediate layer, respectively. In the Miyako strait, the temperature and salinity are higher in the southwest which is partly influenced by the water exchange in the strait. The characteristics of water in the Miyako strait are similar with the adjacent seas. It has a clear seasonal variation of temperature and salinity in the upper layer with high salinity and low temperature in winter and high temperature and low temperature in summer. The mixed layer depth(MLD) is thichest in winter and thinnest in summer. The seasonal and total variations of MLD are mainly affected by change of heat flux, while the interannual variation of MLD are controlled by the wind stress.At the typical section (section KR), there are significant seasonal variations of the hydrologic characteristics and sound velocity in the upper 200 m, but the seasonal variation disappears in the water deeper than 200m. The density is mainly controlled by temperature. The longitudinal-depth variations of isopycnals and isothermals both show a depression around the Kuroshio. In the east of 126°E in section KR, the thermocline undergoes a seasonal cycle including well mixing, slight stratification, strong stratification and decaying periods. Halocline only appears from July to September in the slope and it’s extremely thin. The total variability (interannual variance plus seasonal variance) of squared buoyancy frequency is larger at the depth less than 100 m comparing with the depth larger than 100 m. The variability of the pycnocline shows the predominant interannual variability at the depth larger than 200 m and seasonal variability at the depth less than 200 m.In Section S across the Miyako strait, The mean volume transport is 3.97Sv from 1993 to 2012 with the standard deviation reaches 6.61Sv. The volume transport is highest in October and lowest in December. Section S is divided into west shallow area and east deep area by126.2°E. Seasonal variability of volume transport is large in the east where standard deviation is 0.17Sv, and is small in the west of where standard deviation is 1.36Sv. The seasonal variation of the volume transport is determined by the east area. The velocity structure shows that water flows from the Pacific to East China Sea mostly with a cycle of about 2 years. Volume transport in section S shows a negative correlation (r=-0.61) with the transport of the strait east of Tai Wan and a positive correlation (r=0.41) with section PN.