The seasonal cycle of the Atlantic Jet dynamics in the Alboran Sea: direct atmospheric forcing versus Mediterranean thermohaline circulation

Diego Macias,Elisa Garcia-Gorriz,Adolf Stips

doi:10.1007/s10236-015-0914-y

Abstract

The Atlantic Jet (AJ) is the inflow of Atlantic surface waters into the Mediterranean Sea. This geostrophically adjusted jet fluctuates in a wide range of temporal scales from tidal to subinertial, seasonal, and interannual modifying its velocity and direction within the Alboran Sea. At seasonal scale, a clearly defined cycle has been previously described, with the jet being stronger and flowing towards the northeast during the first half of the year and weakening and flowing more southwardly towards the end of the year. Different hypothesis have been proposed to explain this fluctuation pattern but, up to now, no quantitative assessment of the importance of the different forcings for this seasonality has been provided. Here, we use a 3D hydrodynamic model of the entire Mediterranean Sea forced at the surface with realistic atmospheric conditions to study and quantify the importance of the different meteorological forcings on the velocity and direction of the AJ at seasonal time scale. We find that the direct effects of local zonal wind variations are much more important to explain extreme collapse events when the jet dramatically veers southward than to the seasonal cycle itself while sea level pressure variations over the Mediterranean seem to have very little direct effect on the AJ behavior at monthly and longer time scales. Further model results indicate that the annual cycle of the thermohaline circulation is the main driver of the seasonality of the AJ dynamics in the model simulations. The annual cycles in local wind forcing and SLP variations over the Mediterranean have no causal relationship with the AJ seasonality.

Highlights

The Alboran Sea is the unique connection between the Mediterranean basin and the open Atlantic Ocean
The entrance of the Atlantic Jet (AJ) through the strait drives the main circulation of the Alboran Sea, with two anticyclonic gyres: the more permanent Western Anticyclonic Gyre (WAG) and the more elusive Eastern Anticyclonic Gyre (EAG) in the open-sea regions (e.g., Lacombe 1971; Lanoix 1974; Arnone et al 1990; La Violette 1984) and smaller cyclones along the northern and southern shores (e.g., Peliz et al 2013)
There are two southward migrations of the AJ simulated in the model in fall of years 1997 and 2008 that are not apparent in the satellite estimates (Fig. 2a), and it seems that the seasonal cycles are shifted by approximately 1 month (Fig. 2b)

Summary

Introduction

The Alboran Sea is the unique connection between the Mediterranean basin and the open Atlantic Ocean. Water and substance interchanges take place through the narrow Strait of Gibraltar where a typical anti-estuarine circulation is present; a surface inflow of relatively fresh and nutrient poor Atlantic waters and a deep outflow of salty and nutrient-rich Mediterranean waters (Armi and Farmer 1988) Such interchanges are fundamental for the matter budget of the semi-enclosed Mediterranean Sea (Macías et al 2007a; Huertas et al 2012) and for the hydrodynamics (García-Lafuente et al 2002) and biogeochemistry of the Alboran Sea itself (e.g., Ruiz et al 2001; Sarhan et al 2000; Macías et al 2008). The interannual variability of this fishery is strongly linked to the circulation conditions created by the AJ dynamics, as recently described by Ruiz et al (2013)

Methods

Results

Discussion

Conclusion