The effect of the salinity barrier layer in the preconditioning and onset of El Nino simulated with a coupled ocean-atmosphere general circulation model.

Christophe Maes

Institut de Recherche pour le Développement Laboratoire d'Etudes en Géophysique et Océanographie Spatiales

It has been proposed that the salinity stratification, known as the barrier layer, in the western Pacific warm pool is important in the coupling between sea surface temperature (SST) and winds that leads to El Nino-Southern Oscillation events. A thick barrier layer maintains surface waters warmer than 28C (the threshold for organized atmospheric convection) by reducing the entrainment cooling from below the ocean mixed layer. This mechanism allows an accumulation of heat in the upper ocean layers prior to El Nino. It also confines the forcing of westerly wind burst (WWB), which are generally believed to be a trigger of El Nino, to a shallow mixed layer thus increasing the eastward displacement of the eastern edge of the warm pool. The importance of the salinity barrier layer in the preconditioning phase characterized by high ocean heat content and in the onset phase characterized by high WWB activity is investigated using a general circulation coupled model of the tropical Pacific. The Météo-France/ARPEGE global atmospheric model coupled to the LODYC/OPA ocean model is able to reproduce self-sustained El Nino events together with WWBs. The methodology consists of disabling the stratification effect of salinity in the vertical mixing parameterization scheme. This cutoff is restricted to the western side of the equatorial band (4N-4S) where SST is greater than 28C. The main effects of removing the barrier layer are to reduce the ocean heat content in the preconditioning period and to modify the ocean dynamics in response to WWBs in the onset period. Ensemble hindcasts, each with 5 members, of three El Nino events of different intensities show that interactions between the ocean and the atmosphere over the warm pool do not amplify and each El Nino is weakened or even aborted when the barrier layer is disabled. A detailed analysis confirms that the physics of the warm pool such as vertical diffusion and horizontal advection is essential to set up favorable conditions for the development of El Nino. In this model study salinity stratification plays a crucial and significant role during the initial phases of El Nino and this result provides a strong indication that care should be taken to get the salinity stratification right in coupled forecast models in order to improve El Nino predictions.