Improvement of Thermocline Structure that Affect ENSO Performance in a Coupled GCM

Abstract

Many coupled atmosphere-ocean general circulation models (AOGCMs) have difficulty reproducing the El Nino/Southern Oscillation (ENSO) phenomenon. The AOGCM MIROC underestimates ENSO variability. This paper investigates the dependence of the simulated ENSO amplitude on the treatment of ocean subsurface processes.Excessive vertical diffusivity in the subsurface ocean interferes the reproduction of a tightly stratified thermocline because the MIROC model includes numerical instability in the vertical diffusion computation and uses a large background diffusivity. The introduction of a stable numerical scheme and small background diffusivity helped improve the stratification. The improved thermocline enhanced the ENSO amplitude because the vertical heat exchange between above and below the thermocline was enhanced during the El Nino phase.