Thermohaline Evolution of the Western Mediterranean Deep Waters Since 2005: Diffusive Stages and Interannual Renewal Injections

Abstract

A large production of anomalous dense water in the northwestern Mediterranean Sea during winter 2005 led to a widespread abrupt shift in Western Mediterranean deep waters characteristics. This new configuration, the so‐called Western Mediterranean Transition (WMT), involved a complex thermohaline structure that was tracked over time through a deep hydrographic station located NE of Minorca Island, sampled 37 times between 2004 and 2017. In this study, the thermohaline evolution of the WMT signal is analyzed in detail. Using a 1‐D diffusion model sensitive to double‐diffusive mixing phenomena, the contribution to the heat and salt budgets of the deep Western Mediterranean in terms of ventilation and diffusive transference from the intermediate layers above is disentangled. Results show distinct stages in the evolution of the deep waters, driven by background diffusion and intermittent injections of new waters. The progression of a multilayered structure in the deep ocean is well represented through existing parameterizations of salt fingering and diffusive layering processes and makes it possible to infer an independent estimate of regional background diffusivity consistent with current knowledge. Overall, the deep layers of the Western Mediterranean underwent substantial warming (0.059 °C) and salt increase (0.021) between 2004 and 2017, mostly dominated by injections of dense waters in the 2005–2006 and 2011–2013 periods. Thus, within the WMT period, heat uptake rate in the deep Western Mediterranean was substantially higher than that of the intermediate levels in the global ocean.

Oxygen budget of the north-western Mediterranean deep- convection region

“…These water masses then mostly flowed towards the south of the western basin, while a smaller part was advected back in the convection area through mesoscale circulations counteracting the effect of the intrusions of low-oxygen LIW during the restratification period, in increasing the oxygen inventory of intermediate waters (not shown). A preferen-tial pathway to the south of the basin was the one along the eastern coast of Minorca in the Algerian sub-basin, in agreement with previous observational and modelling studies that examined the spreading of waters formed in winter in the NW region (Pinot and Ganachaud, 1999;Schroeder et al 2008b;Beuvier et al, 2012). Our simulated circulation of oxygen in the western basin is also consistent with the study of Piñeiro et al (2019), who reported the arrival of new dense water masses formed in the deep-convection area east of Minorca over the 2011-2013 period using temperature and salinity observations at the hydrographic stations RADMED.…”

Oxygen budget for the north-western Mediterranean deep convection region

“…Our modelling study indicates that, over the period September 2012 to September 2013, the upper layer of the NW deep (Pinot and Ganachaud, 1999;Schroeder et al 2008b;Beuvier et al, 2012). Our simulated circulation of oxygen in the western basin is also consistent with the study of Piñeiro et al (2019) who reported the arrival of new dense water masses formed in the deep convection area east off Minorca over the 2011-2013 period using temperature and salinity observations at the hydrographic stations RADMED. In our model outputs, the offshore Balearic Sea (bathymetry >1,000 m, surface area: 19,700 km 2 ) and Algerian sub-basin (bathymetry >1,000 m, surface area: 171,610 km 2 ) experienced an increase in their oxygen inventory, during and after the NW deep convection events, receiving oxygen through lateral transport (271 Gmol and 1,276 Gmol, respectively) while the amounts of oxygen captured at the air-sea interface during the period of intense vertical mixing were smaller in those areas than in the NW deep convection area by a factor of 10 and 3 respectively (i.e.…”

Persistent, Depth‐Intensified Mixing During The Western Mediterranean Transition's Initial Stages

“…Since this study focuses on the thermohaline evolution of the deep waters observed off Minorca during the 2005-2007 period, we consider as our observational record for the study period a subset (July 15, 2005-January 6, 2007 of the fortnightly hydrographic time series used in Piñeiro et al (2019). These time series were constructed by spatio-temporally smoothing of the raw profiles of the Minorca deep station during PIÑEIRO ET AL.…”

“…Since then, the original WMT signal has been eroded by diffusive mixing and subsequent instances of deep-water renewal, resulting in a denser, warmer, saltier, and more stratified deep water (relative to that present before 2005) that currently (as of 2020) fills the deepest layers of the WMED. The long-term thermohaline evolution of the deep layers during the WMT was tracked by means of a seasonally occupied deep hydrographic station located in the outer continental slope of Minorca Island (Figure 1), analyzed in detail by Piñeiro et al (2019). This comprehensive dataset enabled the study of deep-ocean mixing dynamics and their impact on the large-scale thermohaline evolution of the regional water masses.…”

“…This comprehensive dataset enabled the study of deep-ocean mixing dynamics and their impact on the large-scale thermohaline evolution of the regional water masses. A remarkable feature of the WMT hydrographic structure evolution shown by Piñeiro et al (2019) was the rapid erosion of the cWMDW-nWMDW portion of the water column during 2005-2007, concurrent with a prominent increase in the θ-S maximum associated with the nWMDW core ( Figure 2).…”