Realizations of a Yearlong Eddy-Permitting Hindcast of the Tropical Pacific With the ECCO Assimilation System

The ECCO assimilation system has been used to estimate the state of an eddy-resolving model of the tropical Pacific Ocean for the year 2000. The initial guess for forcing is from the NCEP-NCAR re-analysis and the boundary conditions come from the ECCO global re-analysis. The model is fit to observations in the tropical Pacific region by controlling the initial temperature and salinity; temperature, salinity and horizontal velocities at the open boundaries; and surface fluxes of momentum, heat and freshwater. The model is constrained with most of the available datasets in the tropical Pacific, including climatologies, TAO, float, XBT, drifter, and satellite SST and SSH data. The iterated adjoint-based descent is able to significantly improve the model consistency with the multivariate data sets, providing a dynamically consistent realization of the tropical Pacific circulation that generally matches the observations to within (admittedly poorly-known) errors. The adjusted control fields are smoothed and applied in model runs without assimilation to check that small changes in the controls do not greatly change the model hindcast and to provide a small ensemble of acceptable model solutions. In addition, the original and smoothed controls are applied to a version of the model with doubled horizontal resolution resulting in a broadly similar "downscaled" hindcast, showing that the adjustments are not tuned to a single configuration. The time-evolving model state and the adjusted controls should be useful for analysis or to supply the forcing, initial, and boundary conditions for runs of other models.