The warm pool variability of the tropical northeast Pacific

The Eastern Pacific warm pool (EPWP) defined by the area enclosed within the 28.5 degrees C isotherm is examined for its seasonal cycle and interannual variability. The study characterizes the EPWP by its area and objectively defined onset, demise, and length of its seasonality. The onset of the EPWP season is defined by the day when the daily anomaly of the area of EPWP exceeds its climatological annual mean. Similarly, the demise of the EPWP season is defined when the daily anomaly of the area of the EPWP falls below its climatological annual mean, after the onset date is detected. We show that the seasonal evolution of the EPWP has a strong asymmetry, with the climatological peak of the EPWP area occurring approximately 41 days from the onset while the demise of the season occurs after nearly 106 days from the climatological peak. The EPWP is part of a larger Western Hemisphere warm pool (WHWP) that extends into the Intra-Americas Seas and parts of the tropical northwest Atlantic Ocean. This study finds that the EPWP is weakly related to the Atlantic counterpart of the WHWP. This is partly due to the fact that the EPWP season precedes the seasonal peak of the warm pool in the Atlantic by several months and the size of the former is much smaller than the latter; therefore, the EPWP does not have a strong remote forcing on the Atlantic warm pool. The interannual variability of the area of EPWP is closely related to the El Nino and the Southern Oscillation (ENSO) variations in the equatorial Pacific with large (small) EPWP years associated with warm (cold) ENSO years.