South American Cold Surges: Types, Composites, and Case Studies

Abstract

This paper examines the climatological, large-scale, and synoptic-scale aspects of South American cold surges using NCEP?NCAR gridded reanalyses for the 1992?96 period. Three common cold surge types are identified on the basis of a thickness (1000?850 hPa) criteria: type 1?a transient surge associated with weak anticyclone development east of the Andes in the absence of ridging aloft, type 2?a strong and persistent surge associated with dynamic anticyclogenesis aloft and strong surface anticyclone development east of the Andes, and type 3?a surge east of the Brazilian coastal mountains. Cold surges are most common during the winter and spring (Jun?Nov), accounting for 189 of the 256 events (74%). Case studies of two events (19?22 Jul 1992 and 12?14 Apr 1993) are conducted from both a conventional isobaric and a potential vorticity (PV) perspective. The upper-air flow pattern in the July 1992 type 2 case is characterized by the presence of a strong ridge?trough couplet, which amplifies and becomes quasi-stationary, allowing for a deep layer of equatorward flow over South America. Dynamically, this flow pattern favors the development of a very strong surface anticyclone to the east of the Andes in response to a combination of differential anticyclonic vorticity advection, low-level cold advection, and, equivalently, positive PV advection. Because of the associated cold air damming east of the Andes, modified cool air is transported into the western part of Amazonia. Cold air damming east of the Brazilian coastal mountains is associated with the transition of the July 1992 type 2 surge into a type 3 surge. The cold surge of April 1993 is examined as a rare event that does not fit the above classification. It is characterized by explosive cyclogenesis close to the coast of Argentina. Unlike the representative type 2 cold surge of July 1992, which tends to occur in association with southwesterly flow aloft, the April 1993 cold surge occurs beneath westerly and northwesterly flow aloft. Cold air penetration into lower latitudes is restricted because the geostrophic wind has a component directed away from the Andes equatorward of the cyclone. The dynamical forcing mechanisms associated with the April 1993 event are of smaller scale than those of the much more common surges typified by the July 1992 event.