A climatology of the Northern Hemisphere winter anticyclones

A climatology of the Northern Hemisphere winter anticyclonic activity is presented based on the European Centre for Medium‐Range Weather Forecasts reanalysis data set for the 1957–2002 period (ERA‐40) and a system‐centered method of tracking diagnostics. The method tracks the anticyclonic systems and monitors their evolution along their paths thus allowing documentation of aspects that have been previously overlooked due to the common perception of anticyclones as semi‐stationary, relatively uniform, slowly evolving structures. The anticyclonic signal is examined in a wide range of meteorological fields and at multiple levels, over the continental masses, over the subtropical ocean basins, and over the sectors where blocking highs tend to develop. Structural features, thermal and vorticity properties, and mobility characteristics of the corresponding anticyclones are highlighted. Over continental regions an association with low level pools of cold air is found to be prevalent for the Eurasian anticyclones but less so for the North American anticyclones. In the subtropical zone the anticyclonic signal in the regions affected by the winter monsoon, including southeastern Asia and the western Pacific features prominently in the upper levels. In the eastern sectors of the subtropical ocean basins the anticyclonic signal is less deep. Intense anticyclonic development at low levels is also identified over the western sectors of the subtropical continents where it appears associated to the effect of orography and of low level cooling. The dominant type of anticyclone observed in the regions of blocking is characterized by 5–6 days timescales and considerable eastward mobility.