Decadal variability and the anomalous 1990s ENSO events

By using wavelet transform, the ENSO cycle (2-7 yr.) signal and decadal variability(8-20 yr.) are filtered out from the data sets in order to investigate the features of the decadal variability and its affect on ENSO cycle. There are two decadal SST modes------ horseshoe and horsesaddle patterns in the tropical Pacific. The former dominated the decadal change before 1990s, the later mainly did in 1990s. As the response of the ocean, there are two kinds of decadal wind patterns. One is prevailing by the zonal anomalous component, the other by the meridional anomalous component. These patterns can also be thought as different phases of the decadal oscillation. Evidences of both diagnose of observation data sets show that affect of the two decadal mode on ENSO cycle is diffierent. The horse saddle mode has stronger impact on ENSO cycle than the horseshoe does. Basis on the analysis, the possible mechanism of the decadal variability influence on ENSO cycle is figured out. The thermocline along the equator where is deep in West Pacific and shallow in East Pacific shifts up or down together when decadal variability is horseshoe pattern. But the see-saw gets curved oppositely in central and eastern Pacific where horse saddle pattern. Therefore it has stronger affect on ENSO cycle. At last, the numerical experiments are designed to identify the effect by running a hybrid tropical Pacific coupled model with the decadal changes. The results indicate that ENSO is rather sensitive to these change in the background conditions, coincided with the conclusions in diagnostic study.