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Influence of large-scale atmospheric circulation patterns on nutrients dynamics in the Mediterranean Sea in the extended winter season (October-March) 1961-1999

M. Reale*, S. Salon, S. Somot, C. Solidoro, F. Giorgi, A. Crise, G. Cossarini, P. Lazzari, F. Sevault

*Corresponding author:

ABSTRACT: We investigate the effects of variations in the four primary mid-latitude large-scale atmospheric circulation patterns on nutrients potentially limiting plankton growth in the Mediterranean Sea (nitrate and phosphate), with a focus on the key deep convective areas of the basin (Gulf of Lions, Southern Adriatic Sea, Southern Aegean Sea and Rhodes gyre). Monthly indices of these four modes of variability, together with a high resolution hindcast of the Mediterranean Sea physics and biogeochemistry covering the period 1961-1999, are used to determine the physical mechanisms explaining the influence of these patterns on nutrient distribution and variability. We find a decrease in the concentration of phosphate and nitrate for each unit of positive index values of the East Atlantic and East Atlantic/Western Russian variability modes in the area of Gulf of Lions, while a signal of opposite sign is associated with the North Atlantic Oscillation in the Aegean Sea and Rhodes gyre. In both cases the variability observed is related to a significant variation of the mixed layer depth driven by heat losses and wind stress over the areas. The East Atlantic pattern plays a major role in driving the long-term dynamics of both phosphate and nitrate availability in the Gulf of Lions, with a particularly pronounced effect in December and January. For both the Aegean Sea and Rhodes gyre the most prominent correlations are found between the North Atlantic Oscillation and phosphate, with a highly consistent behavior in the two areas associated with common physical forcing and exchange of properties among them.