An extratropical driver of evaporation in the tropical and subtropical oceans

Ocean evaporation in the subtropics and tropics is the main supplier of moisture to the tropics and is of fundamental importance to the global hydrological cycle, influencing atmospheric and oceanic dynamics and thermodynamics. Ocean evaporation (turbulent latent heat flux) varies on a range of time scales, from the diurnal to longer climatic scales. While previous studies characterize its variability over seasonal-interannual to decadal time scales, here we focus on the subseasonal, and particularly daily-weekly scales, and aim to understand the drivers of this variability. We first show that latent heat flux variability entails transient evaporation hotspots, regionally exceeding 250 W/m², which build up the long-term climatology. We then show indications for the mechanisms governing this variability, and especially the extreme evaporation hotspots. Namely, it is dry air intrusions from the wake of midlatitude cyclones that reach the low latitudes. The relatively dry and cold airmass over the subtropical and tropical oceans dominates the occurrence and variability of intense latent heat flux in these regions. Our findings suggest, however, that the tropics are not zonally homogeneous, in the sense that such intrusions dominate tropical evaporation in particular regions more than others. For example, in the west of south Africa, southern South America and Mexico, evaporation hotspots co-occur with intrusions from the extratropics 70% of the time and more, while in the western South Pacific and Arabian Sea the co-occurrence reaches values of up to 30%. The fact that the variability and extremes of tropical latent heat flux are governed by extratropical dynamics on this scale, suggest that this mechanism should be viewed as a type of extratropical-tropical interaction, having implications for the predictability of tropical evaporation hotspots.