Abstract
The Mediterranean Sea is a hotspot for climate change, and recent studies have reported its intense warming and salinification. In this study, we use an outstanding dataset relying mostly on glider endurance lines but also on other platforms to track these trends in the northwestern Mediterranean where deep convection occurs. Thanks to a high spatial coverage and a high temporal resolution over the period 2007–2017, we observed the warming (+0.06 ^circC year^{-1}) and salinification (+0.012 year^{-1}) of Levantine Intermediate Water (LIW) in the Ligurian Sea. These rates are similar to those reported closer to its formation area in the Eastern Mediterranean Sea. Further downstream, in the Gulf of Lion, the intermediate heat and salt content were exported to the deep layers from 2009 to 2013 thanks to deep convection processes. In 2014, a LIW step of +0.3 ^circC and +0.08 in salinity could be observed concomitant with a weak winter convection. Warmer and more saline LIW subsequently accumulated in the northwestern basin in the absence of intense deep convective winters until 2018. Deep stratification below the LIW thus increased, which, together with the air–sea heat fluxes intensity, constrained the depth of convection. A key prognostic indicator of the intensity of deep convective events appears to be the convection depth of the previous year.
Highlights
The Mediterranean Sea is a semi-enclosed sea characterized by a basin-scale cyclonic circulation, and by ubiquitous and energetic meso- and submesosale d ynamics[1,2,3,4,5,6,7,8]
Deep water formation occurs in the northwestern Mediterranean Sea where it is driven by both intense winter atmospheric events (Mistral and Tramontane winds) and oceanic preconditioning
The combination of vertical ocean profiles data sets with mooring measurements from the deep convection zone has given a better understanding of the effects of deep convection events on intermediate layers at basin scale. This led to a reflection on the deep and bottom-reaching convection events that could occur in the future in the context of the global and Mediterranean changes. Thanks to this new set of high spatial and temporal resolution data in the northwestern Mediterranean, we have a better understanding of the warming and salinification trends
Summary
The Mediterranean Sea is a semi-enclosed sea characterized by a basin-scale cyclonic circulation, and by ubiquitous and energetic meso- and submesosale d ynamics[1,2,3,4,5,6,7,8] It is one of the rare hotspots for deep water formation in the global ocean[9], with several active sites of intermediate and deep water formation[10,11,12,13,14,15]. AW follows a cyclonic circulation along the northern continental slope of the western Mediterranean Sea and enters the Eastern basin through the Sicily C hannel[3] During this journey, it undergoes evaporation and mixing with resident waters, very likely due to vigorous mesoscale activity and vertical mixing during winter. This increase has been reported in the Balearic S ea[37,38], downstream of the LIW path through the deep convection area
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