Mechanisms of Eastern Mediterranean Rainfall Variability

Abstract

This paper presents a simple theory for the association between observed eastern Mediterranean (EM) rainfall anomalies and North Atlantic (NA) climate variability. Large-scale NA atmospheric mass rearrangements, primarily a modulation of the Icelandic low and the subtropical high pressure systems, tend to extend beyond the NA. A particularly strong such teleconnection exists between the northern NA and southern Europe and the Mediterranean Basin. Pressure anomalies over Greenland?Iceland are thus associated with reversed-polarity anomalies centered over the northern Adriatic, affecting the entire Mediterranean Basin; elevated Greenland pressure is accompanied by an anomalous cyclone over the Mediterranean, and a Mediterranean high pressure system is present when pressure over Greenland is reduced. In the EM, these anomalies result in anomalous southerlies during Greenland highs, and northerlies during Greenland lows. Eastern Mediterranean southerlies warm the EM, while northerlies cool locally. Because heat advection by horizontal and vertical motions dominate the EM thermodynamic equation (or, put differently, because thickness advection dominates the omega equation), cooling by northerly winds results in enhanced subsidence. Conversely, warming by anomalous EM southerlies produces enhanced ascent. These vertical motion anomalies modify the stability of the mean column, resulting in the observed EM rainfall anomalies.