Decomposing the role of dry intrusions for ocean evaporation during mistral

The mistral is a northerly gap-wind regime blowing through the Rhone Valley in Southern France. It is held responsible for the sea-surface cooling necessary to produce deep convection in the Gulf of Lion through turbulent ocean heat loss. The mistral is tightly connected to lee-cyclogenesis in the Gulf of Genoa, where topography forces substantial downward motion. Dry intrusions (DIs) are airstreams forming the descending branch of extratropical cyclones. Known to induce cold and dry surface anomalies, DIs are potential contributors to enhanced surface evaporation during mistral. In this study, a climatological database (ERA-INTERIM, 1981-2016) of mistral-DI co-occurrence is constructed, allowing quantification of the impact of DIs on the mistral evaporative hot spot for the first time. We find that, on average, mistral-DI evaporation rates are doubled, compared to mistral without DIs. Moreover, cluster-composite analysis reveals amplifications exceeding 300% between dynamically similar mistral events, with response to DIs. Daily latent heat-flux anomalies in the Gulf of Lion are decomposed into contributions from the various parameters to analyse the mistral evaporation response to DIs. Mistral-DI events are shown to produce extreme evaporation rates through increased mistral wind speeds. The results highlight the downward momentum flux delivered by DIs to the mistral at the Gulf of Lion as the primary driver of the evaporation amplification mechanism. We further explore the variability between different mistral-DI events and conclude that extreme mistral-DI evaporation events are linked to descending air trajectories entering the Gulf of Lion at an early stage of their lifetimes. These DIs charge the mistral with maximum vertical momentum fluxes, which act to intensify surface winds and hence evaporation rates. Ocean evaporation in the Gulf of Lion is greatly enhanced when the mistral wind is embedded in large-scale dry intrusions at the rear of the Genoa low. Using a climatological set (1981-2016) of mistral events and contrasting it with mistral events occurring within dry intrusions, we find that downward momentum flux along the dry intrusions enhances mistral winds. The mistral wind speeds dominate the variability of ocean evaporation according to further decomposition of evaporation to its main drivers. image