Interannual volume variability in the tropical Pacific

The question of whether or not the volume of the tropical Pacific changes over the course of an El Nino event has potentially important consequences for our understanding of the dynamical mechanisms responsible for El Nino-Southern Oscillation (ENSO) events. In this study, we examine the volume variability of the tropical Pacific, defined as the areal integral of the sea surface height variability, using TOPEX/Poseidon altimetric sea surface heights and the output of a numerical model. We find that volume gradually builds up in the tropical Pacific prior to a typical ENSO event and rapidly decreases during the event due to volume flux out of the tropics. However, our results confirm the findings of previous studies showing that volume change over the entire tropical Pacific does occur on ENSO timescales, but that this variability is small when compared to the volume redistribution within the tropical Pacific. In particular, there is a large volume flux into and out of the equatorial waveguide. Furthermore, we find that the volume variability of the tropical Pacific during ENSO can be almost entirely accounted for by variations in the temperature field.