Abstract
Time series collected from 2004 to 2020 at an oceanographic station located at the westernmost sill of the Strait of Gibraltar to monitor the Mediterranean outflow into the North Atlantic have been used to give some insights on changes that have been taking place in the Mediterranean basin. Velocity data indicate that the exchange through the Strait is submaximal (that is, greater values of the exchanged flows are possible) with a mean value of −0.847 ± 0.129 Sv and a slight trend to decrease in magnitude (+0.017 ± 0.003 Sv decade−1). Submaximal exchange promotes footprints in the Mediterranean outflow with little or no-time delay with regards to changes occurring in the basin. An astonishing warming trend of 0.339 ± 0.008°C decade−1 in the deepest layer of the outflow from 2013 onwards stands out among these changes, a trend that is an order of magnitude greater than any other reported so far in the water masses of the Mediterranean Sea. Biogeochemical (pH) data display a negative trend indicating a gradual acidification of the outflow in the monitoring station. Data analysis suggests that these trends are compatible with a progressively larger participation of Levantine Intermediate Water (slightly warmer and characterized by a pH lower than that of Western Mediterranean Deep Water) in the outflow. Such interpretation is supported by climatic data analysis that indicate diminished buoyancy fluxes to the atmosphere during the seven last years of the analyzed series, which in turn would have reduced the rate of formation of Western Mediterranean Deep Water. The flow through the Strait has echoed this fact in a situation of submaximal exchange and, ultimately, reflects it in the shocking temperature trend recorded at the monitoring station.
Highlights
The Mediterranean Sea (MedSea hereinafter) is the only basin away from polar regions where open-ocean deep convection reaching the ocean bottom happens, despite its location in temperate latitudes
We focus on the WMDW and Levantine Intermediate Water (LIW), which are the main bulk of the outflow at the monitoring station, the rest being a very small fraction of Atlantic water, mainly North Atlantic Central Water (NACW) entrained by the Mediterranean flow a short distance upstream in the Tangier Basin (García Lafuente et al, 2007, García Lafuente et al, 2011)
Signatures of the pHT25 decrease and pCO2 rise can be detected in the LIW and WMDW separately (Figure 10) and it is plausible to assume that both water masses are contributing to the overall patterns detected in the outflow
Summary
The Mediterranean Sea (MedSea hereinafter) is the only basin away from polar regions where open-ocean deep convection reaching the ocean bottom happens, despite its location in temperate latitudes. The convection is the result of the net buoyancy flux to the atmosphere, mainly determined by the fresh water deficit (evaporation minus precipitation and river runoff, E − P) of the basin It drives an open thermohaline circulation cell starting and ending in the Strait of Hotter and Weaker Mediterranean Outflow. The cell starts with a surface inflow of Atlantic water that travels the whole Mediterranean Sea to be transformed into the very salty Levantine Intermediate Water (LIW) in the easternmost basin of the MedSea (Figure 1) There, it sinks and initiates the way back to the SoG and to the open ocean as a dense underflow, completing the thermohaline cell (see Hopkins (1985) or Tsimplis et al (2006), for instance, for a review on this general topic). Bernoulli aspiration of WMDW in the Alboran Sea by the energetic lower layer flow at the SoG (Bryden and Stommel, 1982; Naranjo et al, 2012) is responsible for the differentiated pattern of both basins, as the Strait of Sicily does not hold any comparable bottom current
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