Low Frequency Variability of Surface Circulation in the South China Sea

The role of ocean variability is vital in long-term changes of the earth’s climate. The low-frequency changes of the ocean circulation and air-sea heat fluxes contribute significantly to the long-term variability of this ocean-atmosphere coupled system. Low-frequency variability of the South China Sea (SCS) surface circulation over the period 1871-2008 is investigated using the SODA ocean reanalysis datasets. The South China Sea (SCS) surface circulation is mainly forced by seasonally varying monsoon winds and flow through the Luzon Strait. Surface winds over the SCS are southwesterly during the southwest/summer monsoon period (June-September) and northeasterly in the northeast/winter monsoon (November-February). Seasonal reversal of the winds leads to a basin-wide cyclonic and anticyclonic upper layer circulation pattern in the SCS during the northeast and southwest monsoons respectively. The surface circulation over the SCS shows a weakening trend over the past century. This weakening trend is evident in both the winter and summer monsoon circulation patterns. EOF analysis has been employed to identify the dominant modes of SCS surface circulation variability. The first and second dominant EOF modes explain about 65% of the total surface circulation changes over the SCS. The EOF1 of surface circulation variability exhibits significant low-frequency oscillations in the inter-decadal (15 years) time scale. The possible influences of remote forcings on the circulation variability are also investigated.