Intense wind-driven coastal upwelling in the Balearic Islands in response to storm Blas (November 2021)

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Abstract
This article analyzes the Balearic Islands wind-driven coastal upwelling in response to the intense and long-lasting storm Blas which affected the Western Mediterranean Sea in November 2021. The storm was associated with a pronounced pressure low, generating heavy rains and intense winds and showing some characteristics of a tropical cyclone. The Balearic Islands area was particularly affected since the core of the storm was moving over a one-week-long period from the south-west of this area to just above Menorca and Mallorca Islands. High-resolution regional forecast models indicated that the intense northeasterly winds blowing over the region during the first days of the storm led to the development of intense upwellings along the northwestern coasts of Mallorca and Ibiza Islands together with a reversal of the surface current. While the clouds associated with the storm prevented the radiometers onboard satellites to precisely observe the evolution of the sea surface signature of the upwelling, signals of enhanced chlorophyll concentration were still detected in the upwelling region. The high-resolution Western Mediterranean Operational model, which downscales the Copernicus Marine Service Mediterranean predictions, is used to describe the characteristics of this intense coastal upwelling event, as well as to analyze its singularity over the past 9-year time series through the comparison of different coastal upwelling indices. The upwelling event is found to have a duration of three days with a spatial off-shore extension of around 20 km. It was characterized by intense cold coastal sea surface anomalies of around 6 °C. While it was the most intense event over the past 9 years in terms of local cross-shore sea surface temperature gradients, it is ranked second in terms of the intensity of cross-shelf transports, following the Gloria episode in January 2020.
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