On the Mechanisms Driving Latent Heat Flux Variations in the Northwest Tropical Atlantic: a Modeling Approach

Abstract. In this study, a high-resolution ocean–atmosphere coupled simulation is used to assess the effects of sea surface temperature (SST), surface currents, and ocean vertical stratification on the spatial variability of latent heat flux (LHF) and the stability of the marine atmospheric boundary layer (MABL) in the Northwest Tropical Atlantic during January and February 2020. The analysis focuses on the ocean mesoscale (O(50−250 km)) across the Northwest Tropical Atlantic (referred to as the EURECA region in this study) and within three sub-regions characterized by different ocean dynamical regimes: Amazon, Downstream, and Tradewind. Results indicate that the coupling between SST and wind speed (and specific humidity) is stronger (weaker) in the Amazon and Downstream regions (influenced by the warm coastal North Brazil Current eddy corridor and the Amazon river plume) than in the Tradewind region (representative of the open ocean), consistent with previous remote sensing studies. Overall, warmer SSTs are associated with increased wind speeds and variations in specific humidity, deviating from Clausius-Clapeyron expectations. We interpret this as the result of active ocean processes modifying the near-surface atmosphere, enhancing vertical motion in the MABL, and transporting momentum and drier air from the free troposphere toward the surface. This effect is particularly pronounced over waters influenced by the Amazon plume, where positive SST anomalies persist, primarily due to lateral advection in the mixed layer. To further investigate the impact of mesoscale SST features on LHF, we apply a linear, SST-based downscaling method. Results show that these mesoscale SST structures induce a substantial increase in LHF, 46.8 W m⁻² K⁻¹ on average in the Amazon and Downstream regions (warm eddy corridor). In the Tradewind region, the LHF sensitivity to SST is smaller, at about 35 W m⁻² K⁻¹. For the Amazon region, of the 46.7 W m⁻² K⁻¹ change in LHF associated with SST, approximately 7.8 W m⁻² K⁻¹ is attributed to direct mesoscale SST changes (thermodynamic contribution), while the remainder is linked to mesoscale SST-induced modifications in near-surface atmospheric circulation (dynamic contribution). Within the dynamic contribution, about 80 % (31.1 W m⁻² K⁻¹ out of 38.9 W m⁻² K⁻¹) is due to variations in specific humidity undersaturation, and the remaining 20 % (7.8 W m⁻² K⁻¹ out of 38.9 W m⁻² K⁻¹) is due to wind speed changes. Similar relative contributions are found in the other subregions and in the overall EURECA domain. Finally, the influence of surface currents on winds is weaker, with LHF deviations not exceeding 15 W m⁻². This study underscores the importance of a regionalized approach to mesoscale air–sea interaction studies in the Northwest Tropical Atlantic, as LHF sensitivity to SST and surface currents exhibits strong spatial variability driven by distinct oceanic dynamics. Submesoscale LHF sensitivity to SST and currents is not addressed here and will be the subject of future research.