Cold air incursions into low-latitudes: global perspective and regional analysis over South America

Abstract

This dissertation addresses several aspects of synoptic-scale incursions of cold air (cold surges) propagating equatorward along the east side of the Andes cordillera in South America. Similar cold surges are also a well documented feature of the synoptic climatology of southeast Asia (to the east of the Himalayan Plateau) and North/Central America (to the east of the Rockies/Mexican Sierras). The imprints of the cold surges in these regions are evident to varying degrees in global maps of variance and skewness of key meteorological variables and meridional heat flux. South American cold surges exhibit the most clear signatures at subtropical latitudes, a consequence of the favorable continental geography for their northward advance and the lack of other important synoptic phenomena over this region.The synoptic-scale structure and dynamic of the cold surges was studied by a compositing analysis of episodes selected on the basis of the sharp surface pressure rise that characterize their passage. Instrumental in this task was the availability of 17 years of twice-daily NCEP/NCAR reanalysis and fields of outgoing longwave radiation measured by satellites. Further dynamical aspects and mesoscale details of this phenomenon were derived from a numerical simulation of a typical episode using the MM5 model with horizontal resolution down to 20 km.Wintertime and summertime composite episodes exhibit a similar evolution. The upper-level circulation is characterized by a mid-latitude wave crossing the southern tip of the continent, while a cold-core surface anticyclone moves from the Pacific into southern Argentina strengthening the meridional pressure gradient along the east side of the Andes. Because of the elevation of the Andes the low-level easterly wind on the northward flank of the anticyclone is unable to cross the mountains, leading to cold air damming and downgradient southerly wind. The terrain-parallel flow advects cold air northward maintaining the intensity of the baroclinic zone as it moves into subtropical latitudes. Wintertime cold surges have a substantial impact on the low-level temperature field, with the most intense episodes causing widespread freezing over subtropical regions. Summertime episodes are less pronounced in terms of temperature changes, but they organize subtropical and tropical deep convection in the form of synoptic-scale bands moving at the leading edge of the cold air.