Abstract

The presence of two large scale cyclonic gyres in the Algerian basin influences the general and eddy circulation, but their effect on water mass transfer remain poorly characterized. Our study has confirmed the presence of these gyres using the first direct current measurements of the whole water column collected during the SOMBA-GE2014 cruise, specifically designed to investigate these gyres. Using cruise sections and a climatology from 60 years of in situ measurements, we have also shown the effect of these gyres on the distribution at intermediate depth of Levantine Intermediate Water (LIW) with warmer (~0.15 °C) and saltier (~0.02 g.kg−1) characteristics in the Algerian basin than in the Provençal basin. The Algerian gyres also impact horizontal density gradients with sinking of the isopycnals at the gyres’ centres. Temporal cross-correlation of LIW potential temperature referenced to the signal observed south of Sardinia reveal timescale of transit of 4 months to get to the centre of the Algerian basin. The LIW temperature and salinity trends over various periods are estimated to: +0.0017 ± 0.0014 °C.year−1 and +0.0017 ± 0.0003 year−1 respectively over the 1960–2017 period, and accelerating to +0.059 ± 0.072 °C.year−1 and +0.013 ± 0.006 year−1 over the 2013–2017 period.

Highlights

  • The Mediterranean Sea is a semi-enclosed evaporation basin with water and heat deficits (Béthoux, 1979; Bryden and Kinder, 1991)

  • The signature of Levantine Intermediate Water (LIW) fades away to the west as the distance from the source location, the Sardinia channel, increases, but one can identify a marked patch of LIW at about 400 km during each cruise (Fig. 3)

  • Our study provides additional evidence that the Algerian Gyres represent an important circulation feature in the basin

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Summary

Introduction

The Mediterranean Sea is a semi-enclosed evaporation basin with water and heat deficits (Béthoux, 1979; Bryden and Kinder, 1991). The dynamics of the Mediterranean Sea is characterized by an active thermohaline circulation resulting from a strong air/sea coupling and preconditionning to deep vertical mixing (Robinson et al, 2001). The difference in water density and sea level at Gibraltar Strait forces a surface inflow of warm and fresh Atlantic Waters (AW). Flowing cyclonically along the continental slope of the different sub-basins (Millot, 1999; The MerMex Group: Durrieu de Madron et al, 2011).

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