Effects of interannual salinity variability and freshwater flux forcing on the development of the 2007/08 La Niña event diagnosed from Argo and satellite data

Abstract

Oceanic salinity and its related freshwater flux (FWF) forcing in the tropical Pacific have been of increased interest recently due to their roles in the El Niño-Southern Oscillation (ENSO), the global climate and water cycle. A comprehensive data analysis is performed to illustrate the significant effects of interannual salinity variability and FWF forcing during the 2007/08 La Niña event using three-dimensional temperature and salinity fields from Argo profiles, and some related fields derived from the Argo and satellite-based data, including the mixed layer depth (MLD), heat flux, freshwater flux, and buoyancy flux (QB). It is demonstrated that during the developing phase of 2007/08 La Niña, a negative FWF anomaly and its associated positive sea surface salinity (SSS) anomaly in the western-central basin act to increase oceanic density and de-stabilize the upper ocean. At the same time, the negative FWF anomaly tends to reduce a positive QB anomaly and deepen the mixed layer (ML). These related oceanic processes act to strengthen the vertical mixing and entrainment of subsurface water at the base of ML, which further enhance cold sea surface temperature (SST) anomalies associated with the La Niña event, a demonstration of a positive feedback induced by FWF forcing.