ENSO Asymmetry and Subsurface Nonlinear Dynamical Heating in Reanalysis and CMIP5 Climate Models

A relationship between the El Niño-Southern Oscillation (ENSO) asymmetry and the nonlinear dynamical heating (NDH) in the subsurface ocean along the thermocline is delineated using ocean reanalysis datasets and outputs of climate models. The ENSO asymmetry is measured by the skewness of sea surface temperature (SST) anomalies in the Niño-3 region (90-150˚W, 5˚S-5˚N). The subsurface NDH, which is defined as the nonlinear advective terms, was calculated based on a heat budget analysis in the equatorial Pacific Ocean using ocean reanalysis products (ORAS3, SODA3.3.1, and GODAS) and preindustrial control simulations of CMIP5 climate models. The standard deviations of the Niño-3 SST simulated in CMIP5 models are comparable with the reanalysis. Nevertheless, the simulated levels of the Niño-3 SST skewness, in general, are much less than that in the reanalysis. The levels of skewness in both the Niño-3 SST and eastern-Pacific subsurface temperature are almost linearly related to the standard deviation of the eastern-Pacific subsurface NDH. This relation suggests that the ENSO asymmetry may be controlled by the subsurface NDH variability. As the latter is mainly contributed by its zonal component, weak variability in the equatorial undercurrent may contribute to weak ENSO asymmetries in CMIP5.