Factors that affect the amplitude of El Nino in global coupled climate models

Historically, El Nine-like events simulated in global coupled climate model s have had reduced amplitude compared to observations. Here, El Nino-like p henomena are compared in ten sensitivity experiments using two recent globa l coupled models. These models have various combinations of horizontal and vertical ocean resolution, ocean physics. and atmospheric model resolution. It is demonstrated that the lower the value of the ocean background vertic al diffusivity, the greater the amplitude of EI Nine variability which is r elated primarily to a sharper equatorial thermocline. Among models with low background vertical diffusivity, stronger equatorial zonal wind stress is associated with relatively higher amplitude El Nine variability along with more realistic east-west sea surface temperature (SST) gradient alone the e quator. The SST seasonal cycle in the eastern tropical Pacific has too much of a semiannual component with a double intertropical convergence zone (IT CZ) in all experiments. and thus does not affect. nor is it affected by, th e amplitude of El Nine variability. Systematic errors affecting the spatial variability of El Nine in the experiments are characterized by the eastern equatorial Pacific cold tongue regime extending too far westward into the warm pool. The time scales of interannual variability (as represented by ti me series of Nino3 SSTs) show significant power in the 3-4 year ENSO band a nd 2-2.5 year tropospheric biennial oscillation (TBO) band in the model exp eriments. The TBO periods in the models agree well with the observations, w hile the ENSO periods are near the short end of the range of 3-6 years obse rved during the period 1950-94. The close association between interannual v ariability of equatorial eastern Pacific SSTs and large-scale SST patterns is represented by significant correlations between Nino3 time series and th e PC time series of the first EOFs of near-global SSTs in the models and ob servations.