Variations of Upper-Ocean Salinity Associated with ENSO from PEODAS Reanalyses

Abstract

nterannual variations of upper-ocean salinity in the tropical Pacific and relationships with ENSO are investigated using the Bureau of Meteorology (Australia) POAMA Ensemble Ocean Data Assimilation System (PEODAS) reanalyses. Empirical orthogonal function (EOF) analysis reveals the systematic evolution of salinity and temperature during ENSO. EOF1 and EOF2 of both temperature and salinity capture the mature phase of El Niño and the discharge and recharge phase, respectively. Typical El Niño and La Niña evolution captured by the leading pair of EOFs depicts eastward or westward migration of the eastern edge of the warm/fresh pool in the western Pacific. Increased or decreased freshness in the western Pacific mixed layer occurs in the recharge/discharge phase. EOF3 captures extreme El Niño, when the strong positive temperature anomaly extends to the South American coast and the fresh pool detaches from the western Pacific and shifts into the central Pacific. Large loadings on EOF3 occurred only during 1982/83 and 1997/98, which suggests that eastern Pacific El Niño is actually the exception, whereas moderate central Pacific El Niño and La Niña are more typical. The eastward expansion of the warm/fresh pool during El Niño is also associated with a continuous eastward displacement of the barrier layer, indicating an active role of the barrier layer not just at the onset of an event. The barrier layer and fresh pool shift much farther eastward during strong El Niño, which could contribute to the eastward shift of strong events. The prior enhancement of the barrier layer in the western Pacific is also more concentrated and stronger, which might portend development of extreme El Niño.