The Distribution and Dynamics of Surface Layer Divergence in the Eastern Equatorial Pacific

Equatorial upwelling plays a key role in controlling the sea surface temperature (SST) in the eastern equatorial Pacific, yet quantifying its variability and the dynamics that controls its spatial and temporal distributions remains uncertain. Only a handful of observationally based estimates of equatorial upwelling in the tropical Pacific have been published over the last 35 years. Most of these have estimated upwelling via the divergence of mass over a control volume bounded by latitudes well away from the equator, typically +/- 5º latitude, where geostrophy and linear Ekman theory are expected to provide reliable estimates of horizontal velocities. Recent numerical simulations with high resolution (10 km or finer) ocean general circulation models suggest that there is considerable spatial heterogeneity of the vertical velocity within these large control volumes. For example, the maximum climatological annual mean vertical velocity in the surface layer of the simulations tends to be located off of the equator (+/- 1º), whereas classical theory suggests that it should be centered on the equator. We investigate the structure and dynamics of the diverging surface layer transport between the equator and latitudes where linear Ekman theory holds. We analyze momentum and heat budgets from multiple ocean general circulation model simulations (POP, MITgcm) using differing forcing products (CORE-II, JRA55-do). This allows determination of the the distribution, scales and dynamics of the surface layer divergence across hemispheres, longitudes, seasons and ENSO phase. The implications of this structure and dynamics for observing system design will also be discussed.