A Climatology and Compositing Study of Cold Surges in South America

A climatology of South American cold surges was conducted using NCEP/NCAR Reanalysis data of 1000 hPa heights and 1000-850 hPa thickness for a five year period, 1992-1996. The results of this climatological study suggest that South American cold surges can be broadly stratified into five types: Type 1 features an anticyclone in the Pacific but the main anticyclone center never establishes itself completely east of the Andes. Typically a ridge will extend from the anticyclone in the Pacific across South America and lead to equatorward flow to the north of its axis. This type doesn't produce a very strong surge of cold air since the ridge axis is perpendicular to the mountains instead of parallel. Southerly flow will only exist equatorward of the ridge axis and thus this type of surge can't tap the much colder air to the south. Type 2 features an anticyclone in the Pacific which moves eastward across the Andes and positions itself over southern South America. This is the quintessential case of cold air damming and results in a ridge parallel and to the east of the Andes with its nose pointing equatorward. This type produces the greatest equatorward push of cold air as the anticyclone is able to tap cold air from the south and transport it equatorward under the influence of mountain-parallel southerly winds.

Type 3 features a cyclone in the western Atlantic which creates an easterly flow to the south of its center over South America. This leads to an equatorward push of cold air. Type 3 events will be short unless the cyclone remains stationary for many days. Type 4 features an anticyclone positioned near southeastern Brazil which leads to an easterly flow against the coastal mountains and a cool surge up the east coast of Brazil. It is likely that many Type 2 cases are followed by Type 4 cases as an anticyclone moves eastward across the Andes and southern South America before turning northeastward. Type 5 cold surges feature little, if any, direct orographic influences. They occur when cold air moves equatorward between the Andes mountains to the west and the Brazilian coastal mountains to the east with little evidence for cold air damming and orographic channeling of cold air.

In this talk we will present composites of each type in order to discuss the life cycle, frequency, duration, and seasonal distribution of each type of cold surge. In particular, we will identify the characteristic synoptic-scale and mountain-modified flow signatures associated with the different surge types.

Oral presentation preferred Presenter: Nocera (nocera@atmos.albany.edu)