FY 2024

On the mechanisms driving latent heat flux variations in the northwest tropical Atlantic

Fernández, P., S. Speich, H. Bellenger, D. Lange Vega, J. Karstensen, D. Zhang, and C. Barbedo Rocha

J. Geophys. Res., 129(5), e2023JC020658, doi: 10.1029/2023JC020658, View open access article at AGU/Wiley (external link) (2024)

The Northwest Tropical Atlantic (NWTA) is a region of complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8°N, which leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km2. We show that these two processes influence the spatial variability of the region's surface latent heat flux (LHF). On the one hand, the presence of surface freshwater modifies the vertical stratification of the ocean, the mixed layer heat budget, and thus the air-sea heat exchanges. On the other hand, NBC rings create a highly heterogeneous mesoscale sea surface temperature (SST) field that directly influences the near-surface atmospheric circulation. These effects are illustrated by observations from the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte - Ocean Atmosphere (EUREC4A-OA) and Atlantic Tradewind Ocean-Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W m2, of which 100 W m2 are associated with wind speed changes and 40 W m2 with SST variations. Surface currents or heat release associated with stratification changes remain as second-order contributions with LHF variations of less than 10 W m2 each. This study highlights the importance of considering these three components to properly characterize LHF variability at different spatial scales, although it is limited by the scarcity of collocated observations.

Plain Language Summary. The Northwest Tropical Atlantic (NWTA) is a region with a complex ocean circulation. It is dominated by the North Brazil Current (NBC), which parallel to the South American coast and changes its direction at 8°N. This leads to the formation of closed swirling circulations known as NBC rings. The NWTA also receives the outflow of the Amazon River. These two features affect the heat exchange between the ocean and the atmosphere associated with water evaporation (latent heat flux, LHF) as they modify sea surface temperature, salinity and the near-surface atmospheric circulation. Here, we use the observations collected from the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte - Ocean Atmosphere (EUREC4A-OA) and Atlantic Tradewind Ocean-Atmosphere Interaction Campaign (ATOMIC) experiments, satellite data and combined observations with models to identify the key mechanisms leading to such LHF variations. More of 60% of them are associated to surface winds whilst sea surface temperature is behind a 25%. The Amazon outflow accounts for less than 10%. Although this study is limited by the paucity of oceanic, atmospheric and air-sea interface observations located at the same point in time and space, it highlights the importance of considering these three components to properly describe LHF variability.