Simulated Sea Surface Salinity Variability in the Tropical Pacific

Sea Surface Salinity (SSS) variability simulated by an oceanic general circulation model forced by daily NCEP-NCAR reanalysis from 1990 to 2001 is analyzed in the research. With daily forcing, the model can reproduce SSS change of the tropical Pacific on different time scales. The model-simulated SSS compares well with the Tropical Atmosphere Ocean (TAO) mooring observation. Correlation coefficients between the model output and observation are around 0.6 and significant at a level of 99% for selected sites that have relatively long observations. The western tropical Pacific is a large variability center on different time scales. On the interannual time scale, the standard deviation of SSS in the region could reach 0.4 practical salinity unit (psu). The 1997-1998 ENSO event is associated with a freshening of the whole equatorial Pacific with an amplitude as large as 0.6 psu. However, the eastern equatorial Pacific shows relatively weak SSS variability (0.1 psu), implying that the western tropical Pacific and the eastern tropical Pacific have quite different mechanisms of SSS changes. On the seasonal time scale, the western tropical Pacific has variability around 0.3 psu. On the intraseasonal time scale (time scale shorter than 50 days), SSS variability in the equatorial Pacific has an amplitude of 0.1 psu. There is pronounced SSS variability associated with the Tropical Instability Waves (TIWs) in the eastern equatorial Pacific on intraseasonal time scale. The activity of TIWs has seasonal and interannual modulation. TIWs tend to be weak during boreal spring and early summer and strong during fall and winter. During the 1997-1998 ENSO event the westward propagating TIWs disappear completely. Relevance to the upcoming Aquarius satellite mission to measure SSS from space is discussed.