Roles of initial ocean states on predicting the 2002/03 central Pacific El Niño

Abstract

The 2002/03 El Nino event, a new type of El Nino with maximum warm anomaly occurring in the central equatorial Pacific, is known as central-Pacific (CP) El Nino. In this study, on the basis of an El Nino prediction system, roles of the initial ocean surface and subsurface states on predicting the 2002/03 CP El Nino event are investigated to determine conditions favorable for predicting El Nino growth and are isolated in three sets of hindcast experiments. The hindcast is initialized through assimilation of only the sea surface temperature (SST) observations to optimize the initial surface condition (Assim_SST), only the sea level (SL) data to update the initial subsurface state (Assim_SL), or both the SST and SL data (Assim_SST+SL). Results highlight that the hindcasts with three different initial states all can successfully predict the 2002/03 El Nino event one year in advance and that the Assim_SST+SL hindcast performs best. A comparison between the various sets of hindcast results further demonstrates that successful prediction is significantly affected by both of the initial surface and subsurface conditions, but in different developing phases of the 2002/03 El Nino event. The accurate initial surface state can easier trigger the prediction of the 2002/03 El Nino, whereas a more reasonable initial subsurface state can contribute to improving the prediction in the growth of the warm event.