Precipitation and sea-surface salinity in the tropical Pacific Ocean

Monthly sea-surface salinity (SSS) and precipitation (P) in the tropical Pacific region are examined for the 1974–1989 period. The SSS data are derived mainly from water sample measurements obtained from a ship-of-opportunity program, and the rainfall data are derived from satellite observations of outgoing longwave radiation. The mean and standard deviation patterns of SSS and P exhibit good correspondence in the heavy-rainfall regions characterising the Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ) and part of the western Pacific warm pool. An Empirical Orthogonal Function (EOF) analysis indicates two dominant modes of variation linking P and SSS changes, one mode at the seasonal timescale in both convergence zones, and the other at the ENSO timescale in the central-western equatorial Pacific (165°E–160°W) and in the SPCZ. The inferences derived from the EOF analysis are used in a simple linear regression model in order to try to specify P changes from known SSS changes. A comparison between hindcast and observed P changes suggests that, at seasonal and ENSO timescales, SSS changes could be used to infer the timing, but not the magnitude, of P in the central-western equatorial Pacific (165°E–160°W) and in the SPCZ mean area. The effects of evaporation, salt advection and mixed-layer depth on the results are discussed.