Alternating Wet and Dry Conditions over South America during Summer

Julia Nogués-Paegle Department of Meteorology, University of Utah, Salt Lake City, Utah

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Kingtse C. Mo NCEP/NOAA, Washington, D.C.

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Abstract

Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin.

This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry conditions over the western Pacific and the Maritime Continent. At the same time, convection is suppressed in the region of the South Pacific convergence zone, over the Gulf of Mexico, and in the ITCZ over the North Atlantic.

In the opposite phase there is a strong influx of moisture from the Tropics into central Argentina and southern Brazil. The moisture influx is enhanced by a strong low-level jet (LLJ) east of the Andes. The LLJ displays a marked diurnal oscillation and characteristics similar to the well-documented LLJs over the Great Plains of North America.

Corresponding author address: Dr. Julia Nogues-Paegle, Dept. of Meteorology, University of Utah, 819 Wm. C. Browning Bldg, Salt Lake City, UT 84112.

Email: jnpaegle@atmos.met.utah.edu

Abstract

Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin.

This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry conditions over the western Pacific and the Maritime Continent. At the same time, convection is suppressed in the region of the South Pacific convergence zone, over the Gulf of Mexico, and in the ITCZ over the North Atlantic.

In the opposite phase there is a strong influx of moisture from the Tropics into central Argentina and southern Brazil. The moisture influx is enhanced by a strong low-level jet (LLJ) east of the Andes. The LLJ displays a marked diurnal oscillation and characteristics similar to the well-documented LLJs over the Great Plains of North America.

Corresponding author address: Dr. Julia Nogues-Paegle, Dept. of Meteorology, University of Utah, 819 Wm. C. Browning Bldg, Salt Lake City, UT 84112.

Email: jnpaegle@atmos.met.utah.edu

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