Sensitivity of ENSO events to the decadal salinity changes and wind bursts of the SouthWest Pacific

This study is based on the ENSO simulations obtained with an Intermediate Coupled Model (ICM) of the tropical Pacific Ocean/Atmosphere system. Using FSU wind and Sea Level data to partially control the ICM, we have shown that among the events observed between 1980 and 1998, some were sensitive to Westerly Wind bursts, others were not, because in addition to WWB, the ICM forecasts are sensitive to decadal changes of the tropical Pacific sea level average. First, we examine the sea level variations observed by the TOPEX-Poseidon-Jason (TPJ) satellites since 1992. Using the ECCO model outputs, we determine the changes that are due to heat and those to salt content in the upper ocean. It is found that the dynamic topography of the South West Pacific above 750m has been rising monotously by as much as 4 cm in 15 years. By contrast the dynamic topography due to thermal changes is mostly affected by the ENSO events, the biggest signal being 16cm drop between 1996 and 1998. These values are averaged on the Pacific region west of the dateline and between 5S and 20S. They are largely explained by the changes of the wind stress curl of the SPCZ. The variations of the basin-averaged sea level are explained in terms of equatorial zonal wind stress, wind stress curls in the North and South, and the mass adjustments between the tropical Indian and Pacific Oceans. Second, we force the ICM by observed winds: the ICM reproduces well the ENSO events observed since 1992, the model Nino3 SST and Nino4 zonal wind indices are correlated with observations by more than 0.7. When in addition to the winds forcing the ICM, sea level data are added to the equations to modify the closed boundary conditions of the model and take into account the connection with the Indian ocean, we find that the model Sea Level is improved, mostly in the western Pacific, but also on average over the basin. Third, initialized with the various conditions described above, the ICM is used to deliver a series of forecasts centered on the 3 cases: 1997-1998, 2001-2002, 2006-2007. Only the 1997-98 event can be predicted after the addition of the observed WWBs in winter 1996-1997. Even though strong WWBs were observed prior to the other 2 events, the ICM fails to predict these events which were very unusual. It appears necessary to control the mass and salinity changes in the West Pacific during the forecast experiment to be able to make progress.