The dynamically consistent, property-conserving ocean state estimates are available on the website (see link in Data section). We have used these results to diagnose heat and salt fluxes in the equatorial region during an El Niño-La Niña cycle. We see changes to stratification, the Equatorial undercurrent, and upwelling structure during this period. Freshwater and heat budgets for the upper 300 m of the tropical Pacific between 5˚S and 5˚N show how the region cooled and freshened during the El Niño.
Vertical mixing across 300 m is negligible, meaning property changes are balanced by horizontal and vertical advective divergence and surface forcing. The overturning circulation across the northern boundary of the analysis domain collapses during the El Niño. However, the geostrophic input of heat reduces by a larger amount than the Ekman removal of heat, and the region cools. While these advective processes across the north wall act to make the region saltier, the eastward propagation of the western Pacific warm-fresh pool and enhanced precipitation result in a freshening of the region during the El Niño.
1. The state estimation procedure worked well even for the strong El Niño-La Niña cycle meaning that nonlinearity in the adjoint iteration was not a problem.
2. Doubling the resolution to 1/6˚ showed changes in many details of the circulation, so increasing the model resolution is a priority, particularly for flow interactions with topography.
3. The state estimate reproduces the tropical instability waves and the Kelvin and Rossby waves propagating along the equator. Comparing these waves to the observations is a measure of the skill of the atmospheric reanalysis used to force the model.
Verdy, A., M.R. Mazloff, B.D. Cornuelle, A.C. Subramanian. ENSO influence on heat and freshwater budgets in a tropical Pacific Ocean state estimate, In Preparation.
Model output is saved in NetCDF format.
The output is available at http://www.ecco.ucsd.edu/tropac.html.