Sudestada (Southeast blow) is the Argentinian name for a climatic phenomenon common to the Río de la Plata (an estuary formed by the combination of the Uruguay River and the Paraná River on the southeastern coastline of South America) and its surrounding region. The phenomenon consists of a sudden rotation of cold southern winds to the south-east. This change, while moderating the cold temperatures, loads the air masses with oceanic humidity, bringing heavy rain and rough seas in the coastal regions. Also, there is an increase in the intensity of the winds. The Sudestada can occur at any time of the year but is most likely to happen between July and October.
Storm front over the sea, viewed from the SE with lightning and thunder during an episode of Sudestada. Villa Gesell, Province of Buenos Aires, Argentina. Source: Wikicommons (Fernando de Gorocica, 2011)
Types and causes of sudestada
A sudestada may take place with or (less commonly) without precipitation.
A sudestada with rain is generated by the combined effect of two systems: a high pressure system located on the Atlantic Ocean off the coast of central Patagonia, which via is counterclockwise circulation brings cold sea air to the east of Buenos Aires Province and the south of the Argentine littoral and Uruguay, and a clockwise rotating low pressure system, located over the center-south of Argentine Mesopotamia and western Uruguay, that brings hot, humid air to the same region. As the pressure in the latter system drops, winds from the southeast increase accompanied by persistent rain and drizzle, low cloud and occasionally thunderstorms. As the depression moves east into the Atlantic Ocean, the winds turn to the south west and become drier. Sudestadas typically last one to three days.
Formation conditions for a "wet" Sudestada
A dry sudestada is caused by a high pressure system, centered on the southwest of Buenos Aires Province in Argentina, bringing persistent dry winds to the mouth of the Río de la Plata. This usually occurs after the passage of a cold front.
Impact of the winds
These south-easterly winds can reach 45 knots and persist for several days, pushing upstream surface water at the exit of Río de la Plata. For a speed of 35 knots, the river level increases by one meter. If the phenomenon continues for several days, or if the winds are stronger, the surge can reach more than 4 meters. Rising waters then cause flooding.