Abrupt warming and salinification of intermediate waters interplays with decline of deep convection in the Northwestern Mediterranean Sea

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 $$^\circ$$ ∘ C year$$^{-1}$$ - 1 ) and salinification (+0.012 year$$^{-1}$$ - 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 $$^\circ$$ ∘ C 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.

Hotter and Weaker Mediterranean Outflow as a Response to Basin-Wide Alterations

“…They showed minima well below the mean, which makes the second half of the period be negatively anomalous in both, the Gulf of Lions and the whole basin. These results agree with Margirier et al (2020), who indicate the absence of intense deep convection in the Gulf of Lions between 2014 and 2017 (their study ends in year 2018). Interestingly, this period coincides with the second part of the oceanographic observations, when the strong trend of θ was detected in the monitoring station.…”

“…Since the former depends on the buoyancy losses of the MedSea, buoyancy fluxes B 0 were computed to get some insight on the variability of Q M production ( Figure 8A). The estimated B 0 agrees with the available data of annual deep water formation in the Gulf of Lions (Houpert et al, 2016;Margirier et al, 2020), which supports the use of B 0 as the proxy of Q M production postulated by equation [3]. It must be recalled that the steady-state nature of the model limits its application to long-term changes.…”

“…Even though no definitive pattern can be confirmed, there are reasonable indications that the two-part differentiation carried out in the oceanographic variables is still applicable to B 0 ( Table 2): a first part in which buoyancy fluxes tended to increase followed by a second one with the opposite trend. This second part sustained the three years of larger B 0 negative anomaly out of the sixteen year examined in the Gulf of Lions (Figure 8B), with the expectable result of no (or weak) WMDW formation those years, as stated in Margirier et al (2020).…”

“…All in all, the scenario that emerges is as follows. At the time of the beginning of the series, the outflow was submaximal (in terms of two-layer model) as a consequence of the weakness of winter buoyancy loss since 1988 in the northwestern MedSea (Herrmann et al, 2010;Margirier et al, 2020). It prevented strong convection and the formation of significant volumes of WMDW during the 1990s and early 2000s and enabled the increase of heat and salt content in the region.…”

“…Certainly, the trend could be partially due to the warming and salinification of LIW instead of WMDW. To this regard, a noticeable warming of LIW in the Gulf of Lions from 2007 to 2018 has been recently reported (Margirier et al, 2020). Even so, a consistent diminution of the proportion of WMDW in the Mediterranean outflow at the depth of the monitoring station is still necessary for that warming to manifest itself in the form of the observed trend in the Mediterranean outflow.…”