DYNAMICS OF STATIONARY WAVE ANOMALIES DURING THE 1986/87 EL-NINO

The dynamics of the wintertime atmospheric response to the 1986/87 El Nino SST anomalies is studied. A GCM used for this purpose simulates a wave train over the Pacific/North American (PNA) region that agrees closely in amplitude with that observed, but phase shifted 30-degrees to the east. Linear baroclinic model experiments are performed in order to determine the origin of the GCM and observed stationary wave anomalies, with particular focus on the cause for GCM failure. Diagnostics with the linear model reveal that the GCM and observed wave train anomalies are maintained by very different processes. In the GCM, the forcing due to tropical diabatic heating and transient vorticity fluxes are equally important over the PNA region. In the observations, the transient vorticity fluxes assume the primary role. The cause for these discrepancies is traced to the different dynamic influences of suppressed rainfall near Indonesia. The associated diabatic cooling is found to excite a large amplitude wave train over the PNA region in the GCM, while no significant extratropical response to cooling is found in the observations. The combined effects of the diabatic cooling and the reorganization of the storm track transients by the remotely forced wave train acts to shift the GCM's wave train well to the east of that observed. Due to uncertainties in the observed diabatic forcing, however, it is not clear to what extent the GCM's failure is due to errors in the simulated anomalous forcing and/or to the GCM's mean climate error.