Interannual Variability Of The Intermediate Water In The South China Sea

The characteristic and interannual variation of the intermediate water in the South China Sea (SCS) and its change mechanism are studied based on the analysis of 14261 temperature-salinity profiles from the historical hydrographic data set WOD09, Grid Point Value of the Monthly Objective Analysis using the Argo data provided by the Japan Agency for Marine-Earth Science and Technology, and a nested HYCOM model simulation results.The most significant feature of the intermediate water is the water mass located at the salinity minimum layer, and the T-S scatter diagram shows that the salinity minimum lies in between 26.5σ_θand 27.2σ_θ, which corresponding to the South China Sea Intermediate Water (SCSIW) and the North Pacific Intermediate Water (NPIW) respectively. The minimum salinity of SCSIW is around 34.5psu, and the NPIW is 34.2psu, while the Luzon strait intermediate water minimum salinity is about 34.2-34.5psu. .The distributions of the climatological intermediate water mass indicate that in theσ_N =26.9 neutral density layer, the average depth of SCSIW core is about 510m and the NPIW core depth exceeds 600m. The salinity and potential temperature of SCSIW core are about 34.44psu and 8.2℃respectively, and the TS is slightly higher in the southern area than in the northern area. The NPIW core temperature and salinity values are less than 7.6℃and 34.3psu. The thickness of SCSIW is larger than the NPIW. In the northern SCS and the Vietnam coastal areas the thickness of SCSIW is more than 500m while the NPIW is less than 400m. The distribution pattern of the intermediate water thickness demonstrates that there is a strong vertical mixing after NPIW entering into the South China Sea and in the horizontal direction the intermediate water basically spreads southward along the northern continental shelf of the SCS and the coast of Vietnam.The analysis of a 10-year time series of SCSIW and NPIW core layer indicates that the 10-year average temperature of the northeast SCSIW is 8.13℃, salinity is 34.415psu, depth is 530m, while the NPIW's is 7.22℃, 34.228psu, 644m respectively. The seasonal change signals of the temperature and salinity of the intermediate water core in the northeast SCS are stronger compared with NPIW, reflecting the northeast SCSIW response to the SCS monsoon, and the interannual variation signal of the NPIW in the west Pacific is much stronger.Comparing the results of HYCOM with the data provided by a variety of data sets shows that HYCOM can well simulate the temperature, salinity and flow field of the SCS. The 26 years averaging Luzon Strait Volume transport (LST) results shows a significant seasonal variation. The westward transportation occurs all the year round except in June. The maximum value in December can be to 7.24Sv, while in June the transport is eastward and has a value of 0.27Sv. When El Nino occurs, the annual LST tends to be relatively larger, and during La Ni?a years the value of LST is much smaller. The intermediate water volume transportation shows that the maximum outflow transport occurs in December and the largest inflow occurs in August. The northern part of Luzon Strait is the main path of the NPIW into the SCS. In the central and southern strait, the transport is mainly out of the SCS, showing the interaction of SCSIW and NPIW.The distributions of 26 years averaged intermediate water mass core layer eigenvalue show that the depth of SCSIW is about 550m, salinity is 34.4psu and potential temperature is roughly 8℃. The potential temperature and salinity in the southern SCS are slightly higher than those in the northern SCS. These characteristics are consistent with the results of the historical observations. HYCOM simulates the intermediate water masses of the SCS well. The annual change of the SCSIW is weak, while in the northeast of the SCS near the Luzon Strait the annual variation is slightly higher. From 1979 to 2004 the temperature and salinity of SCSIW core layer have a clear upward trend. In theσ_N =26.9 neutral density surface, the temperature and salinity on average increase 0.05℃and 0.009psu per 10 years, after 1996 the increasing rate accelerates.