Interannual volume changes and heat transport pathways in the tropical Pacific

The question of whether or not the volume of the tropical Pacific changes over the course of an El Niño event has important consequences for out understanding of the dynamical mechanisms responsible for the variability. Previous observational and numerical modeling studies have arrived at contradictory conclusions about meridional mass and heat transport during El Niño events. The present study uses TOPEX/Poseidon altimetric heights and a sigma-coordinate model to assess tropical Pacific volume changes during El Niño. We find that the volume of the tropical Pacific (20°S–20°N) increases gradually prior to El Niño events, and then falls sharply during the events. The volume drop within a few degrees of the equator is largely countered by a volume increase between 8° and 20°N, but there is still a decrease over the tropical Pacific as a whole, with fluxes between the tropics and the subtropics. The interannual redistribution of volume within the tropics involves large-scale zonal and meridional transport, and is dominated by the heat fluxes. In this study, we explore the three-dimensional pathways of heat transport in detail using Lagrangian trajectory analysis on the model fields. We find that El Niño is not an isolated phenomenon within the equatorial waveguide, but relies on off-equatorial wind effects and transport of heat from several degrees off of the equator.