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-sale northeast-southwest SST-gradient of 3 °C 1300 km−1. authors]This study examines how the marine atmospheric boundary-layer (MABL) properties and the processes involved adjust to the NBC and the superimposed SST diurnal cycle during the EUREC4A-OA experiment (January-February 2020; Bony et al., 2017; Stevens et al., 2021). [authors]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). authors]In reference to the absence of NBC and SST-diurnal cycle, the NBC alters the surface buoyancy flux by 40Wm−2 (40%), the sea level pressure of 0.5hPa (0.012%), the sea surface wind intensity of 1ms−1 (7.1%), the vertical shear of zonal wind (main component of trade winds) of 10day−1 (9.7%), the liquid water content of 1kgm−2 (26%) and the precipitable water of 5kgm−2 (8.4%) in the MABL. The SST diurnal cycle impacts the MABL properties less than the NBC by a factor 4. [authors]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% to 100%, 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. authors]Dynamics of the MABL adjustment is investigated through precipitable water and temperature budgets. The NBC increases the water loss by advection and entrainment and increases the gain of water by surface evaporation (≃2mmday−1, ≃25% of the reference) with a positive net contribution of 0.1mmday−1 off shore the Brazilian coasts, while the SST-diurnal cycle rectifies the water budget by up to 0.01mmday−1 (≃10% of the reference) across the domain. Regarding the energy budget, the NBC forces warming by surface sensible heat flux and cloud-induced infrared radiation flux (≃1Kday−1), which are partially balanced by negative horizontal advection. The heat budget is positive around 0.02Kday−1 off shore the Brazilian coasts, and the diurnal cycle rectifies the temperature budget up to 0.002Kday−1 across the domain. [authors]The dynamics of the MABL adjustment is investigated through the precipitable water and temperature budgets. The NBC increases the water loss by advection (≃1mm day−1) and entrainment (≃2mm day−1) and increases the gain of water by surface evaporation (≃2mm day−1), while the SST difference between MOI and ECMWF increases the magnitudes of advection, entrainment and surface evaporation around 10%. Regarding the energy budget, the NBC forces warming by surface sensible heat flux and water vapor and cloud-induced infrared radiation flux (≃1K day−1), which are partially balanced by negative horizontal advection. authors]The heat budget is positive around 0.02K day−1 off shore the Brazilian coasts, and the (MOI-ECMWF) SST difference rectifies the temperature budget up to 0.002K day−1 across the domain. [authors]The SST difference between MOI and ECMWF alters the heat budget components by 10%.