The Study Of Intermediate Water, Circulation In South China Sea And Its Intermediate Water Exchange With Northwest Pacific

Based on the analysis of a total of 13748 temperature-salinity profiles from historical hydrological data set WOD05 and ARGO and the three nested simulations using HYCOM, the characteristic and seasonal variation of intermediate water, circulation in South China Sea (SCS) and its intermediate water exchange with Northwest Pacific are studied in this paper.South China Sea Intermediate Water (SCSIW) in the most significant feature of the salinity minimum, is located at potential density 26.5-27.0σθor neutral density 26.5-27.2σN layer, which is sourced from North Pacific Intermediate Water (NPIW) formed in Middle and high latitudes.NPIW finally reaches the Luzon Strait after an excursion, that is, the source waters sink by cabbeling process and are transformed to NPIW subtropical gyre along the subarctic-tropical frontal zone (SATFZ), then spread from northeast North Pacific southwestward toward the Luzon Strait.SCSIW communicates with NPIW mainly through the Luzon Strait which is the only deep channel around the SCS.There are significant differences in water mass property distribution between SCSIW and NPIW,and the intermediate water near the Luzon Strait has a notable mixing property distribution of these two water masses.The depth range of SCSIW is about 340-740m, shallower than NPIW, while the ranges of temperature and salinity is about 6.4-10.6°and 34.4-34.5psu respectively, higher than NPIW. The pattern of thickness distribution of SCSIW is thick in northern part and thin in the southern part, both parts are thicker than NPIW. There are high-pressure, low-temperature and low-salinity tongues entering from the Northwest Pacific to the South China Sea by the Luzon Strait. In the northern SCS,both of the intermediate water property distribution and the intermediate circulation pattern show a obvious cyclonic feature, which is one of the evidences showing that the intermediate water, circulation in the SCS are mainly controlled by monsoon.The characteristic and seasonal variation of intermediate water exchange between the Northwest Pacific and the South China Sea are also controlled by monsoon. The SCS is dominated by winter northeast monsoon on annual mean, therefore the NPIW shows an invasion into the SCS and the annual mean volume transport of NPIW calculated from the Luzon Strait section is about 0.75Sv. The strong winter monsoon plays a dominant role on the intrusion of NPIW into SCS,while the weaker summer monsoon can only weaken the intensity of intrusion, rather than reverse the inflow to outflow. The inflow to SCS of NPIW is strongest in spring and summer, followed by winter and weakest in fall.A time lag exists in the response of the invasion of NPIW to monsoon.The pathway of the NPIW entering into SCS is mostly from the northern part of the Luzon Strait, the analyses of property distribution of intermediate water, pattern of intermediate circulation and transport of the section in the strait all demonstrate the existence of a cyclonic extending from the North Pacific to SCS through the northern part of the strait. In the middle and southern parts of the Luzon Strait, the transports of both inflow and outflow are weak compared to the northern part, although the transport of outflow is slightly larger than that of inflow.