Speaker: Leonardo Lima – CMCC Bologna

ABSTRACT
Ocean general circulation models even with realistic behavior still incorporate large uncertainties from external forcing. This study involves the realization of ensemble experiments using the Regional Ocean Modeling System (ROMS) configured for the Southwest Atlantic (SWA) Ocean to investigate uncertainties derived from the atmospheric forcing and bathymetry.
The investigation is based on perturbing atmospheric surface fluxes and bathymetry through a series of ensemble experiments. Next, the Local Ensemble Transform Kalman Filter (LETKF) is applied to assimilate observations into ROMS with the aim to evaluate the data assimilation impact on the representation of the western boundary current system in the SWA Ocean.

The results showed a strong influence of the South Atlantic Convergence Zone on the underlying ocean, seven days after the initialization of the ensemble experiments. In this ocean region, precipitation and radiation flux perturbations notably impacted the sea surface salinity and sea surface temperature (SST), by producing values of ensemble spread that exceeded 0.08 and 0.2 °C, respectively.
Wind perturbations extended the impact on currents at surface, with the spread exceeding 0.1 m/s. The ocean responded faster to the bathymetric perturbations especially in shallow waters, likely due to the nature of the model sigma vertical coordinate.

The LETKF experiments showed the importance of the TS profiles assimilation to correct the thermohaline representation, reducing the root mean square error of the T (S) in 39% (27%). The absolute dynamic topography (ADT) assimilation was important to produce a more accurate representation of the oceanic mesoscale in the Brazil-Malvinas Confluence region.
For the Brazil Current, the assimilation of T, S, ADT and SST produced time-averaged transport values in 22°S of 5.5 Sv and in 30°S of 17.07 Sv while a control experiment without data assimilation generated respectively 8.90 Sv and 34.07 Sv. The results support the potential for LETKF to improve the representation of the western boundary current system in the SWA Ocean.