Adjustment of the marine atmospheric boundary-layer to the North Brazil Current during the EUREC4A-OA experiment

Abstract

The region of Barbados is characterized by the presence of the North Brazil Current (NBC), which transports warm water along the Brazilian coasts and creates a large-scale northeast–southwest SST-gradient of 3 °C 1300 km${}^{-1}$. This study examines how the marine atmospheric boundary-layer (MABL) adjusts to the NBC SST-front represented by the SST analyses produced by the operational forecasting systems ECMWF and Mercator Ocean International (MOI). The NBC SST-front imprints large-scale dipole-anomalies in the surface buoyancy flux, sea level pressure, sea surface wind intensity, vertical shear of zonal wind (main component of trade winds), liquid water content and precipitable water in the MABL ranging from $50\text{\%}$ to $100\text{\%}$, compared to no NBC. The mesoscale SST structures associated with the difference between MOI and ECMWF induce MABL anomalies that are an order of magnitude weaker than those associated with the NBC SST-front. This adjustment is only statistically significant in areas where SST differences are greater than 0.3 °C. The dynamics of the MABL adjustment is investigated through the precipitable water and temperature budgets. The NBC increases the water loss by advection ($\simeq 1{\mathrm{mm\; day}}^{-1}$) and entrainment ($\simeq 2{\mathrm{mm\; day}}^{-1}$) and increases the gain of water by surface evaporation ($\simeq 2{\mathrm{mm\; day}}^{-1}$), while the SST difference between MOI and ECMWF increases the magnitudes of advection, entrainment and surface evaporation around $10\text{\%}$. Regarding the energy budget, the NBC forces warming by surface sensible heat flux, water vapor and cloud-induced infrared radiation flux ($\simeq 1{\mathrm{K\; day}}^{-1}$), which are partially balanced by negative horizontal advection. The SST difference between MOI and ECMWF alters the heat budget components by $10\text{\%}$.