Coastal upwelling over the North Aegean Sea: Observations and simulations

Prevailing northerly winds influence the eastern coastal area of the North Aegean (N. Aegean) Sea and form a strong front due to occurring upwelling waters during the summer months. The upwelling processes over the Lesvos coastal region are investigated with the use of in situ measurements, numerical simulations, and satellite observations. The interaction of the major lateral buoyant input of Black Sea Waters (BSW) with the upwelling processes and waters is also investigated. The offshore propagation of the up-welled waters over the central N. Aegean region and their interactions with the permanent central cyclonic eddies, the major BSW patterns and the general Aegean cyclonic circulation determine their fate over the Aegean Sea. The upwelling depth is small over the Lesvos coastal region and is restricted in the subsurface ocean (<40 m). The shallow upwelling depth is also confirmed by the reduction of the Apparent Oxygen Utilization (AOU) levels during upwelling events while the pre-existing surface waters, affected by the BSW plume reveal higher AOU values. More importantly, the up-welled waters come from above the thermocline nutrient depleted layer and, therefore, no major blooming events may be triggered in the central and eastern Aegean during the summer months. Southerly winds on the other hand, have a double effect. They form upwelling unfavorable conditions and they may facilitate the BSW propagation towards the upwelling region, blocking the eastward Ekman current and strengthening the stratification due to the formation of a surface barrier layer.