Importance of the salinity barrier layer for the buildup of El Nino

Several studies using sea level observations and coupled models have shown that heat buildup in the western equatorial Pacific is a necessary condition for a major El Nino to develop. However. none of these studies has considered the potential influence of the vertical salinity stratification on the heat buildup and thus on El Nino. In the warm pool, this stratification results in the presence of a barrier layer that controls the base of the ocean mixed layer. Analyses of in situ and TOPEX/Poseidon data, associated with indirect estimates of the vertical salinity stratification, reveal the concomitant presence of heat buildup and a significant barrier layer in the western equatorial Pacific. This relationship occurs during periods of about one year prior to the mature phase of El Nino events over the period 1993-2002. Analyses from a coupled ocean-atmosphere general circulation model suggest that this relationship is statistically robust. The ability of the coupled model to reproduce a realistic El Nino together with heat buildup, westerly wind bursts, and a salinity barrier layer suggests further investigations of the nature of this relationship. In order to remove the barrier layer, modifications to the vertical ocean mixing scheme are applied in the equatorial warm pool and during the 1-yr period of the heat buildup. At the bottom of the ocean mixed layer, the heat buildup is locally attenuated, as expected from switching on the entrainment cooling. At the surface, the coupled response over the warm pool increases the fetch of westerly winds and favors the displacement of the atmospheric deep convection toward the central equatorial Pacific. These westerly winds generate a series of downwelling equatorial Kelvin waves whose associated eastward currents drain the heat buildup toward the eastern Pacific Ocean. The overall reduction of the heat buildup before the onset of El Nino results in the failure of El Nino. These coupled model analyses confirm that the buildup is a necessary condition for El Nino development and show that the barrier layer in the western equatorial Pacific is important for maintaining the heat buildup.