Decadal variations and trends in tropical Pacific sea surface salinity since 1970

The dominant mode of variability in the tropical Pacific is the interannual El Nino-Southern Oscillation phenomenon. More subtle variations are also evident on decadal timescales reminiscent of the Pacific Decadal Oscillation (PDO). These variations are superimposed on trends that are possibly linked to global climate change. A major impediment for assessing this decadal and longer timescale variability is the shortness of instrumental records. Near-surface salinity has not yet been systematically analyzed for this reason. This article documents and interprets 1970-2003 observed tropical Pacific sea surface salinity (SSS) changes. Given the time/space SSS data availability, the analysis focuses on changes in three key regions, the western Pacific Warm Pool (WP), the South Pacific Convergence Zone (SPCZ), and the Equatorial Cold Tongue (ECT). A PDO-like signal shows up in these three regions, with some shifts in SSS appearing around the mid-1970s, mid-1990s, and to a lesser extent in 1989-1990. In addition, a clear freshening trend of the order of 0.1 to 0.3 psu per 30 years appears in the WP, SPCZ, and ECT, together with an extension of the low-salinity water surface in the WP and SPCZ. On the basis of available precipitation (P) and evaporation (E) products, and on a survey of the published literature, the PDO-like SSS changes are found to be qualitatively consistent with P and E and/or horizontal and vertical salt advection in the WP and the SPCZ. The repercussions and unsolved issues regarding the SSS decadal changes and freshening trends are discussed.