Scale dependence of the marine atmospheric boundary‐layer response to ocean surface‐temperature variability in the EUREC4A region

Abstract

A large share of energy within the global oceans lies within the mesoscale range, (100 km), where very large dynamic and thermodynamic variability has been observed. We address how the marine atmospheric boundary layer (MABL) in a trade‐wind region adapts to fast and spatially varying sea‐surface temperature (SST) structures, with a focus on changes in behaviour as a function of spatial scales. High‐resolution atmospheric simulation data indicate that, at scales smaller than 1000 km, the effects of enhanced entrainment of dry free tropospheric air overcome those of surface evaporation over warm SST anomalies. This is supported by computations from a conceptual bulk model, which confirm two different responses in MABL temperature and specific humidity: an increase in the forcing SST warms the MABL and reduces its humidity content slightly. Locally, this behaviour suppresses the surface sensible heat flux (SHF) and enhances the surface latent heat flux (LHF), as observed recently with in situ, satellite, and numerical modelling data. At larger scales, instead, the MABL is more in equilibrium with the ocean surface and the sensitivity of turbulent fluxes to the underlying SST anomalies is significantly smaller. The scale dependence of the LHF variability is analysed with a linear scale decomposition method. SST is found to be the primary driver of LHF variability when scales smaller than about 1000 km are resolved, whereas atmospheric variability takes the lead for larger scales. Despite the effects on the mean LHF being small, the link between SST and LHF variability potentially has important implications for atmospheric shallow mesoscale circulations, which remain to be explored.