A 6-year AMSU-based climatology of upper-level troughs and associated precipitation distribution in the Mediterranean region

Abstract

We present a novel climatology of upper-tropospheric troughs and associated precipitation in the Mediterranean region, using 6 years (2001-2006) of Advanced Microwave Sounding Unit data. Troughs are more frequent or persistent during November through March in the whole of the Mediterranean basin, with up to 25% of days in the northwestern part of the basin. The interannual variability of number of intrusions is stronger from September through June. During summer, the variability is reduced, except in the NW region, and no troughs were observed in the SE region. Moderate precipitation occurs in at least 60% of the cases when an upper-level trough is present with weak seasonal dependency. In the southern Mediterranean moderate precipitation is an ubiquitous feature and presents small year-to-year fluctuations. The occurrence of deep convective rain associated with trough is lower, around 50%, and reduces to 20% in the northern part of the basin during winter. This occurrence varies a lot with individual years, especially in the southern Mediterranean. Upper-level troughs have more often a N-S orientation but, while no particular dependency of frequency of precipitation (whether moderate or convective) on their inclination was found, the location of precipitation is generally favored downstream. The North Atlantic Oscillation and Scandinavia patterns were found to be the most important large-scale circulation modes influencing the variability of number of intrusions, but the East Atlantic/West Russia and East Atlantic patterns are also important in the southeast region. Copyright 2008 by the American Geophysical Union.