Fernandez, Pablo, Speich, Sabrina, Bellenger, Hugo, Lange Vega, Diego, Karstensen, Johannes , Zhang, Dongxiao and Rocha, Cesar Barbedo (2024) On the Mechanisms Driving Latent Heat Flux Variations in the Northwest Tropical Atlantic. Journal of Geophysical Research: Oceans, 129 (5). Art.Nr. e2023JC020658. DOI 10.1029/2023JC020658.

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Abstract

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.

Key Points:
- Latent heat flux (LHF) presents strong spatial variations in the northwest tropical Atlantic (NWTA), which has a complex ocean circulation
- Surface winds and sea surface temperature are the major drivers of LHF changes. The Amazon plume remains as a second-order contributor
- It is necessary to distinguish between spatial scales (mesoscale and below vs. large-scale) when assessing the ocean's influence on LHF

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