Origin of Convectively Coupled Kelvin Waves over South America


Brant Liebmann
CIRES Climate Diagnostics Center, Boulder, Colorado

George N. Kiladis
Physical Sciences Division, NOAA/Earth System Research Laboratory, Boulder, Colorado

Leila M.V. Carvalho
Department of Atmospheric Sciences, Institute of Astronomy, Geophysics and Atmospheric Sciences, University of São Paulo, São Paulo, Brazil

Charles Jones
Institute for Computational Earth System Science, University of California, Santa Barbara, Santa Barbara, California

Carolina S. Vera
Department of Atmospheric and Ocean Sciences, CIMA/UBA-CONOCET, Buenos Aires, Argentina

Ileana Bladé
Departament d'Astronomia i Meteorologia, Facultat de Física, Universitat de Barcelona, Barcelona, Spain

Dave Allured
CIRES Climate Diagnostics Center, Boulder, Colorado


(Manuscript received 26 November 2007, in final form 23 June 2008)

Abstract

Convectively coupled Kelvin waves over the South American continent are examined through the use of temporal and spatial filtering of reanalysis, satellite, and gridded rainfall data. They are most prominent from November to April, the season analyzed herein. The following two types of events are isolated: those that result from preexisting Kelvin waves over the eastern Pacific Ocean propagating into the continent, and those that apparently originate over Amazonia, forced by disturbances propagating equatorward from central and southern South America.

The events with precursors in the Pacific are mainly upper-level disturbances, with almost no signal at the surface. Those events with precursors over South America, on the other hand, originate as upper-level synoptic wave trains that pass over the continent and resemble the "cold surges" documented by Garreaud and Wallace. As the wave train propagates over the Andes, it induces a southerly low-level wind that advects cold air to the north. Precipitation associated with a cold front reaches the equator a few days later and subsequently propagates eastward with the characteristics of a Kelvin wave. The structures of those waves originating over the Pacific are quite similar to those originating over South America as they propagate to eastern South America and into the Atlantic.

South America Kelvin waves that originate over neither the Pacific nor the midlatitudes of South America can also be identified. In a composite sense, these form over the eastern slope of the Andes Mountains, close to the equator. There are also cases of cold surges that reach the equator yet do not form Kelvin waves.

The interannual variability of the Pacific-originating events is related to sea surface temperatures in the central-eastern Pacific Ocean. When equatorial oceanic conditions are warm, there tends to be an increase in the number of disturbances that reach South America from the Pacific.