Joint observation-model mixed-layer heat and salt budgets in the eastern tropical Atlantic

Abstract. In this study, we use a joint observation-model approach to investigate the mixed-layer heat and salt annual mean and seasonal budgets in the eastern tropical Atlantic. The regional PREFCLIM observational climatology provides the budget terms with a relatively low spatial and temporal resolution compared to the online NEMO model, and this later is then re-sampled as in PREFCLIM climatology. In addition, advection terms are recomputed offline from the model as PREFCLIM gridded advection computation. In Senegal, Angola and Benguela regions, the seasonal cycle of mixed-layer temperature is mainly governed by surface heat fluxes; however, it is essentially driven by vertical heat diffusion in Equatorial region. The seasonal cycle of mixed-layer salinity is largely controlled by freshwater flux in Senegal and Benguela regions; however, it follows the variability of zonal and meridional salt advection in Equatorial and Angola regions respectively. Our results show that the time-averaged spatial distribution of NEMO offline heat/salt advection terms compares much better to PREFCLIM horizontal advection terms than the online heat/salt advection terms. However, the seasonal cycle of horizontal advection in selected regions shows that NEMO offline terms do not always compare well with PREFCLIM, sometimes less than online terms. Despite this difference, these results suggest the important role of small scale variability in mixed-layer heat and salt budgets.