Atlantic Inflow Trends (2012)

J. Soto-Navarro, F. Criado-Aldeanueva, J.C. Sanchez-Garrido and J. Garcia-Lafuente (1) 

Corresponding author : Javier Soto-Navarro

(1) : Physical Oceanography Group, Department of Applied Physics II, University of Malaga, Spain

Full paper: Soto-Navarro, J., F. Criado-Aldeanueva, J. C. Sánchez-Garrido, and J. García-Lafuente, 2012: Recent thermohaline trends of the Atlantic waters inflowing to the Mediterranean Sea, Geophys. Res. Lett. 39: L01604, doi:10.1029/ 2011GL049907.

Temperature and Salinity from Argo data collected between 2002 to mid-2010 are used to analyze the trends in the atlantic waters adjacent to the Strait of Gibraltar in order to identify the source of recent changes observed in the inflow of the Mediterranean Sea. Positive salinity trends of 0.038±0.009 year-1 and 0.013±0.003 year-1 have been found for the Surface Altantic Water and the Eastern North Atlantic Central Water respectively. Temperature trends are less significant, but we can detect a positive trend for the thermocline waters of 0.05±0.02°C/year. The Med Water does not show a significant trend for the entire period, but a switch from positive to negative trends is observed in year 2006. These thermohaline variations are driven by intrinsic water masses changes, instead of isopycnal vertical displacements, probably related to an enhancement of the net freshwater losses in the area.

Data and Method

The Mediterranean Sea is a concentration basin, where evaporation exceeds precipitation and run-off : the salinity increase due to evaporation over the Mediterranean surface is compensated by an outflow of salty water in the Atlantic Ocean. To supply this water deficit, Inflowing Atlantic Water (AW), mostly composed of parts of the Azores Current, the Iberian Current, and the Canary Current, flows across the western and eastern basins.

The inflow is composed by the near surface layers of the adjacent area that comprise the Surface Atlantic Water (SAW) between 0-100m ; the Eastern North Atlantic Central Water (ENACW) stretching down from 100 to 600m ; the Mediterranean Waters (MW) extending vertically from 600 to 1200m ; and further down, the North Atlantic Deep Water (NADW).

The area of study [28°N-42°N lat ; 5°W-24°W long] is divided into three zones (figure 1), in order to separate out the influence of the Iberian Current and MW (zone 1), the Azores Current (zone 2) and the Canary Current (zone 3). A total of 5077 Argo quality-controlled (T,S) profiles are collected between january 2002 to may 2010. They are separated by zones, monthly averaged and vertically interpolated in 22 pressure levels between the surface and 2000 meters.

 

Figure 1. (left panel) : area of study, divided in 3 subzones - (right panel) : monthly distribution of the Argo profiles for (top left) complete area, (top right) : zone 1, (bottom left) : zone 2, (bottom right) : zone3.

Results

The times series for the three zones and the three layers (0-100m, 100-600m, 600-1200m), in addition with the complete area and the complete vertical profile, are computed and the fitting results summarized in the table 1 below.

Table 1. Estimated Linear Trends for the Complete Area and for the 3 subzones. The significance of the fitting and the 95% confidence intervals are computed using a t-student test.

  • In the SAW layer (0-100m) : no significant temperature trend can be observed, but clear positive salinity trends are observed both in the complete area (0.038±0.009 year-1) and for zone 1 and 2 (0.010±0.005 year-1 and 0.04±0.01 year-1 respectively).
  • In the ENACW layer (100-600m) : positive trends are observed both for temperature (0.05°C/year) and salinity (values ranging 0.005-0.013 year-1)
  • In the MW layer (600-1200m) : the computed trends are not significant for the entire period, but a switch in 2006, in temperature and salinity, between a positive trend (observed between 2002 to 2006) to a negative trend (observed between 2006 to 2010) is well observed in the whole area and for zones 1 and 2.

Figure 2. Results of the application of the Bindoff and Mc Dougall (1994) decomposition for salinity and temperature. (a) Complete area, (b) zone 1, (c) zone 2, and (d) zone 3. On each panel the red line represents the trend along isobaric surfaces, the blue line the contribution of changes along isopycnal surfaces and the green line the contribution of the vertical displacements. The grey line is the sum of these two latter contributions. The left graph of each panel represents salinity trends and the right graph temperature trends.

More analyzes on the mechanism controlling those thermohaline changes are done by an evaluation of each term of the decomposition equation detailed in Bindoff and McDougall (1994). The origin of the water properties changes may be due to intrinsic properties changes of the water masses (because of mixing or horizontal advection), or to the isopycnal heave (due to vertical displacements of the water column). Figure 2 for the complete area and for each of the subzones clearly identify changes in the intrinsic properties of the water masses being the source of the thermohaline trends observed in the Atlantic inflow from Argo data.

Conclusion

The evolution of the thermohaline properties of the Atlantic inflow to the Mediterranean Sea is analyzed from Argo data collected near the Strait of Gibraltar between 2002 to mid-2010. Positive trends, almost observed in salinity, are higher than those estimated by historical studies for the 90s (Boyer et al., 2005 ; Polyakov et al., 2005), but confirm the results of some recent studies concerning the last decades (Hosoda et al.2009 , Roemmich et al 2009). Nevertheless, in contrast to previous findings, these thermohaline variations are driven by intrinsic water masses changes, instead of isopycnal vertical displacements, probably related to an enhancement of the net freshwater losses in the area.

This implies a salinity input to the Mediterranean Sea that may influence the intermediate and deep water formation processes and, consequently, the hydrological characteristics of the Mediterranean Waters.

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