Spatial-temporal variation and salt transport in the intermediate water of the northern South China Sea

To explore the spatial-temporal distribution of salinity in the intermediate water and water exchange with the external waters of the northern South China Sea(SCS), the monthly mean oceanic assimilation data of past 140-year from 1871 to 2010 (SODA2.2.4) were selected, on which the spatial-temporal variation, periodicity, and salt transport of intermediate water salinity in the SCS and adjacent northwest Pacific Ocean were analyzed using wavelet analysis and EOF decomposition. The result show that the monthly salinity variation of intermediate water in the northern SCS is higher in January-March than that of the central part due to northeastward extending of high salinity water from the eastern coast of Vietnam (＞34.45 psu). The salinity distribution of the intermediate water in the northern SCS is uniform in April, and the salinity ranged from 34.40 to 34.45 psu. In May-August, there is a cyclonic circulation in the intermediate water of the northern SCS in summer, which can carry the North Pacific Intermediate Water (NPIW) to the central part of SCS, and then forms a low-salinity water lens (＜34.42 psu) in the north-central SCS. Meanwhile, the low-salinity water lens move northeastward gradually until mix with NPIW in September-October. In November-December, NPIW withdraws eastward and shrinks to the northwest of the Luzon Strait. In the past 140 years, the salinity of intermediate water in the northern SCS has been rising periodically, reaching the peak in 1985, which may be related to the early and strong summer monsoon. From 1963 to 2002, a strong 16-21–year periodicity is shown, and the years of the maximum (minimum) values of the real part of wavelet coefficients well agree with corresponding ENSO cold events. The first EOF analysis shows that the intermediate water salinity in the northern SCS is in a monopole and consistent variation pattern. The positive and negative phase centers identified by the second mode is symmetrical in distribution in NW-SE strike. The annual net salt fluxes of the intermediate water in the northern SCS flow eastward from the South China Sea to the Pacific Ocean, the least in summer and the most in winter. During the 140 years from 1871 to 2010, the salt fluxes in the intermediate water has been decreasing constantly and slowly.