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[1] Recent observations of the outflowing Mediterranean water collected near the bottom in key points of the Strait of Gibraltar show the existence of a seasonal cycle with warmer and lighter waters leaving the Mediterranean Sea in winter and cooler and denser waters in spring early summer. The amplitude of the signal is around 5 10 À2°C for potential temperature and 1.5 10 À2 for potential density, salinity hardly showing seasonal fluctuations. The outflow also shows a seasonal cycle with maximum volume transport in April, in coincidence with the minimum of the signal of potential temperature. A simple analysis of the composition of the outflow in terms of the main water masses of the western Mediterranean basin and its comparison with climate indicators suggests that the seasonal cycle follows the annual process of the Western Mediterranean Deep Water formation that replenish the deep portion of the basin by the end of winter and rises the level of the deep water reservoir, facilitating the suction of cooler and denser water from the Mediterranean over the sills of the Strait. From this time onwards, the data show a smooth warming that would be explained by the progressive fall of the level of the Western Mediterranean Deep Water as it is drained out the Mediterranean, which would leave warmer water available for suction. The process is asymmetric in the sense that the transition from high to low temperature is completed in a short period while the progressive warming spans a longer period.

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[1] Four-year time series of observations in Espartel sill at the western part of the Strait of Gibraltar have been analyzed in order to investigate the variability of the Mediterranean outflow. It is assumed that the observed variability comes from the changing properties of the dense waters that are located at the maximum depth from where they can be uplifted in the upstream basin (Alborán Sea, inside the Mediterranean Sea) and evacuated through the strait. From this perspective, the following three mechanisms are investigated: (1) the replenishment of the deep basin by newly formed Western Mediterranean Deep Water that, depending on its density, can either uplift old resident waters or lay above them leaving in any case a cold signature in the temperature series; (2) the presence/absence of the energetic anticyclonic gyres in the Alborán Sea, particularly the western one, which can transfer momentum to the underlying Mediterranean vein and provide it with additional energy to ascend over the sills of the strait; and (3) the meteorologically enhanced flows that follow the rapid changes of atmospheric pressure over the western Mediterranean basin, which would be able to aspire deeper waters residing in the upstream basin. The three mechanisms act on different timescales, from annual in case (1) to monthly in case (2) to weekly in case (3) although these two latter are modulated annually by the seasonal prevalence of the western Alborán gyre in summer and of the strong meteorologically driven fluctuations in winter. The mechanisms overlap at annual timescales making it difficult to separate out the different contributions.