European rain rate modulation enhanced by changes in the NAO and atmospheric circulation regimes

• Authors: Oleg M. Pokrovsky
• Venue: Computers & Geosciences
• Year: 2009

The aim of this study is to classify the circulation patterns in the Atlantic-European sector and to reveal linkages between anomalies in the pressure field over the North Atlantic (e.g. North Atlantic Oscillation (NAO)) and its respective circulation pattern occurrence over continents on the one hand and rain fields on the other hand. Changes in atmospheric circulation over Europe during the past 50 years were examined using both objective (modes of low-frequency variability inferred by regression analysis and objective cluster classification of circulation types-fuzzy logic) and subjective (Hess-Brezowsky classification of weather types) methods. The grid monthly geopotential (H700), wind zonal and meridional velocity components (U850 and V850) as well as the surface atmosphere pressure (SAP) and precipitation fields acquired from the NCEP/NCAR reanalysis dataset (for 1948-1998) were employed in this study. Joint regression analysis and fuzzy logic classification of these fields was a basic tool for finding major circulation regimes. The fuzzy set analysis of these fields revealed that the major circulation regimes over eastern North Atlantic and Europe were determined in summer by three vorticity poles: (1) North-western (Scandinavia), (2) Western Mediterranean and (3) Caucasian. It is worth noting that an anticyclone occurred in the western part of the North Atlantic for both seasons. The Scandinavia cyclone area explains rain rate maximums located in the 50-60^o latitude European area and the lower rain rate in Southern Europe because of hot and dry African air inflow. In late fall and winter the vorticity system consists of three other poles: (1) North-western, (2) Northern Africa and (3) Northern Russia (Kara Sea). A zonal circulation type dominates in this case and more precipitation is delivered from the Atlantic. Rain rate is more uniformly distributed in the winter in various latitude belts across Europe than in summer, but more intensive precipitation occurred in Southern Europe because of strong moisture transport into this area from Atlantic NAO as well as the substantially larger their magnitude of Arctic Oscillation (AO) indexes in the late 1980s and 1990s during global warming. The atmospheric circulation patterns, which transported very wet Atlantic air, moved northward during last two decades. As a consequence, the climate in Southern Europe became drier and respective rain amounts reduced primarily in the warmer part of year. In contrast, the rain rate increased here in the colder part of the year. This leads to more frequent floods and a wetter climate in autumn/winter.