Sensitivity of ENSO variability to Pacific freshwater flux adjustment in the Community Earth System Model

Abstract

The effects of freshwater flux (FWF) on modulating ENSO have been of great interest in recent years. Large FWF bias is evident in Coupled General Circulation Models (CGCMs), especially over the tropical Pacific where large precipitation bias exists due to the so-called "double ITCZ" problem. By applying an empirical correction to FWF over the tropical Pacific, the sensitivity of ENSO variability is investigated using the new version (version 1.0) of the NCAR's Community Earth System Model (CESM1.0), which tends to overestimate the interannual variability of ENSO accompanied by large FWF into the ocean. In response to a small adjustment of FWF, interannual variability in CESM1.0 is reduced significantly, with the amplitude of FWF being reduced due to the applied adjustment part whose sign is always opposite to that of the original FWF field. Furthermore, it is illustrated that the interannual variability of precipitation weakens as a response to the reduced interannual variability of SST. Process analysis indicates that the interannual variability of SST is damped through a reduced FWF-salt-density-mixing-SST feedback, and also through a reduced SST-wind-thermocline feedback. These results highlight the importance of FWF in modulating ENSO, and thus should be adequately taken into account to improve the simulation of FWF in order to reduce the bias of ENSO simulations by CESM.