Cooperative Institute for Research in Environmental Sciences

Science Rendezvous > 2009 Posters
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How important is air-sea coupling in ENSO and MJO evolution?

M. Newman

The effect of air-sea coupling on weekly tropical climate variability is investigated in a linear inverse model (LIM) derived from the observed zero-lag and 6-day lag covariances. The model predicts the covariances and corresponding power spectra at all other lags. The predicted and observed lag-covariances and spectra are generally found to be in agreement, even at much longer lags. This validates the LIM’s basic premise that the dynamics of weekly averages are effectively linear and stochastically driven, and justifies further linear diagnosis of the system.

Analysis of interactions among the atmospheric and SST components of the LIM show that the main effect of air-sea coupling SST is to greatly increase the interannual variability of SST and lengthen the period of ENSO. In other words, if the ocean were forced only by noise and were otherwise uncoupled to the atmosphere, it might have a weak ENSO-like mode with a period near two years. On the other hand, coupling to SST has a very small effect on subseasonal heating (and atmospheric circulation) variability. In fact, the primary impact of coupling on the atmosphere is on interannual scales. These results suggest that errors in tropical atmosphere-ocean coupling in GCMs might cause substantial errors on interannual and longer timescales, but not on subseasonal time scales associated with the MJO.