Galernas: A history of coastally trapped disturbances (2003−2020) with hidden frontogenesis in the Bay of Biscay

Abstract

Galerna is the term accepted for an abrupt westerly change that affects the north coast of Spain. The wind surge travels from the mid-north coast of Spain to France, generally reaching their maximum intensity at the Basque Coast, and cuts off a period of hot weather, clear skies, and calm conditions at sea. The galernas have a large history of shipwrecks and fishermen deaths. They have been characterized as coastally trapped disturbances (CTD) and their propagation, enhanced with the local formation of a micro-front, was documented to behave like a density current. Alternatively, synoptic fronts have also been reported to cause galernas, considered to be more intense than those generated by a local micro-front. In this article we have generated the first climatology (2003–2020) of these events based on an objective identification methodology. The developed Event Identification Software (EIS), based on both 10-min surface station data and hourly ERA5 reanalysis fields, together with a new Front Identification Scheme (FIS) have enabled a deeper study into the origin and development of these micro-fronts, and a more comprehensive exploration of the interaction of the oceanic fronts entering the Bay of Biscay. Our results show that the area receives an average of four to five relatively intense galernas (Vmax > 50 km h−1) per year. Their number shows a great interannual variability (from one to seven) and a marked seasonality: May and June concentrate the largest fraction (almost one episode each year) and practically no episodes in winter. They occur more frequently between noon and the late afternoon, where the most intense wind records concentrate. Very strong galernas (Vmax > 72 km h−1) have occurred in all 18 years, can happen in any month from February to November, and their monthly distribution does not show the mentioned seasonality. On the contrary, the highest rates of temperature decrease across the galerna front in the coastal stations (−∆T/0.5 h > 4 °C) do have a stronger seasonality, with May and June concentrating a relatively large number of cases with a more abrupt temperature drop. The FIS shows that most of the galernas (83.5%) have a local origin inside the Bay of Biscay, and only a few ones (16.5%) are caused by oceanic fronts initiated out of the region. The local frontogenesis is more frequently initiated by the relatively cold marine southwesterly pre-frontals preceding a parent oceanic front and blowing against the warm continentals inside the Bay of Biscay, after being ducted along the north and northwestern coast of Spain. This hidden local frontogenesis, first revealed by the FIS, seems to be enhanced by the observed lee troughing, which could have both a thermal and dynamic origin, acting simultaneously after the intense Foehn at the coastal strip, preceding the formation of the galerna front. The local front enhancement appears to be the reason for the apparent jump of the primary front, which may eventually weaken, and even disappear, as the galerna front sharpens. Even during the more occasional frontal galernas, directly caused by the westerlies or north-westerlies behind the oceanic front, their eastward propagation is more rapid over the coastal area. The front deforms in shape and may cause its characteristic unexpected/abrupt irruption. All the EIS detected galernas, even the frontal ones, are wind reversals caused by a coastally trapped marine boundary layer. The upper-level ridge over Europe, observed in all of them, seems to be a synoptic ingredient for their development, preventing the eastward propagation of Atlantic depressions and enhancing at the same time the temperature and pressure gradients between the marine and continental air masses.