Can Oceanic Freshwater Flux Amplify Global Warming?

Abstract

The roles of freshwater flux (defined as evaporation minus precipitation) changes in global warming are studied using simulations of a climate model in which the freshwater flux changes are suppressed in the presence of a doubling of CO2 concentration. The model simulations demonstrate that the warm climate leads to an acceleration of the global water cycle, which causes freshening in the high latitudes and salinification in the subtropics and midlatitudes. It is found that the freshwater flux changes tend to amplify rather than suppress global warming. Over the global scale, this amplification is largely associated with high-latitude freshening in a warm climate, which leads to a shoaling of the mixed layer depth, a weakening of the vertical mixing, and thus a trapping of CO2-induced warming in the surface ocean. The latitudinal distribution of SST changes due to the effects of freshwater flux changes in a warm climate is complicated, involving anomalous advection induced by both salinity and wind stress changes. In addition, atmospheric feedbacks associated with global warming also amplify the SST warming.