How useful are satellite-based ocean color observations to detect the eastern edge of the equatorial Pacific warm pool?

The processes controlling the warm sea surface temperatures (SSTs) permanently found in the western equatorial Pacific and known as the warm pool, are central to defining climate and determining the character of large scale and deep atmospheric convection. Detection of very high SST (around 30°C) within the warm pool is associated with the strong intraseasonal variability of hot events. Zonal migrations of the warm pool are also important for the onset of the El Niño Southern Oscillation (ENSO) phenomenon. The separation between the cold tongue in the eastern-central equatorial Pacific and the warm pool relies at the heart of a recent revised delayed oscillator theory of ENSO. The variability of the eastern edge of the warm pool is thus crucial to understand and to monitor within the context of seasonal-to-interannual climate variations. However, the eastern edge of the warm pool along the equator could not be properly defined by a front in SST but have distinct hydrological features and ecosystem dynamics. This feature will be exploited in order to demonstrate how satellite-based ocean color observations are useful to detect the eastern edge of the equatorial Pacific warm pool. The analysis of satellite-based ocean color data shows that low concentrations of surface chlorophyll-a (chl-a) found in the equatorial region of the Pacific Ocean varies in phase with the eastern edge of the warm pool. As is true for high sea surface temperatures, the existence and maintenance of these low concentrations are linked to the upper ocean stratification due to salinity. The present study also establishes the quasi permanence of a frontal zone in chlorophyll-a separating the regimes of the western region and the eastern-central cold tongue and, through the identification of this front in satellite-based ocean color data, it provides, for the first time, a reliable method for locating the eastern edge of the warm pool from surface observations only. Finally, the recognition of this front offers the opportunity to define a simple and robust index of the horizontal extension of the equatorial Pacific warm pool within the context of the ENSO variability.