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Jose Marengo
,
Angel Cornejo
,
Prakky Satyamurty
,
Carlos Nobre
, and
William Sea

Abstract

The authors investigate one case of surges of polar air that occasionally propagate into southeastern Brazil during wintertime and are harmful to coffee production because of the freezing conditions associated with them. The cooling is also observed in southern and, with less intensity, in western Amazonia. The event of 26 June 1994 is studied. The frost event caused a sharp drop in coffee production and similarly dramatic increases in coffee prices.

The event was characterized by the presence of an anticyclonic perturbation off the coast of Chile that enters South America and crosses the Andes Mountains south of 45°S hours later. The anticyclone center tracks equatorward until it reaches 20°–30°S and then shifts eastward toward the Atlantic.

Based on a diagnostic analysis using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis, a two-stage process can be distinguished. In the starting period, midtropospheric troughing is established east of the Andes, over central Argentina, due to stretching and/or vorticity advection. This troughing is responsible for cold advection along the eastern flank of the Andes, which produces substantial temperature drops and deepens the upper trough locally. The increase in the magnitude of the zonal temperature gradients east of the cold advection zone at levels near 700 hPa is another result of the cooling process. This feature would ensure that the cold advection be extended eastward, producing an upper-level trough local deepening there. This intensification would produce an increase in the cyclonic vorticity advection, which would tend to produce sea level pressure drops underneath. This near-surface low pressure area would eventually contribute to the southeastern Brazil (SB) cooling due to the associated southerly winds.

Another feature is the near-surface local anticyclonic generation due to midtropospheric cold advection associated with descending motions, low-level divergence, and anticyclonic growth. Eventually this high pressure would also contribute to produce southwesterlies to transport cold air to SB.

The paper is divided in two sections. The first part is a study of synoptic and climatic aspects of the cold surge episodes by using daily surface climatic observations. The purpose of the second part is to analyze the dynamic aspects of this cold episode and to study the cooling mechanisms by using the four-times-daily surface and upper-air NCEP–NCAR reanalysis, as well as to look for possible predictors.

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Jason Beringer
,
Jorg Hacker
,
Lindsay B. Hutley
,
Ray Leuning
,
Stefan K. Arndt
,
Reza Amiri
,
Lutz Bannehr
,
Lucas A. Cernusak
,
Samantha Grover
,
Carol Hensley
,
Darren Hocking
,
Peter Isaac
,
Hizbullah Jamali
,
Kasturi Kanniah
,
Stephen Livesley
,
Bruno Neininger
,
Kyaw Tha Paw U
,
William Sea
,
Dennis Straten
,
Nigel Tapper
,
Richard Weinmann
,
Stephen Wood
, and
Steve Zegelin

Savannas are highly significant global ecosystems that consist of a mix of trees and grasses and that are highly spatially varied in their physical structure, species composition, and physiological function (i.e., leaf area and function, stem density, albedo, and roughness). Variability in ecosystem characteristics alters biophysical and biogeochemical processes that can affect regional to global circulation patterns, which are not well characterized by land surface models. We initiated a multidisciplinary field campaign called Savanna Patterns of Energy and Carbon Integrated across the Landscape (SPECIAL) during the dry season in Australian savannas to understand the spatial patterns and processes of land surface–atmosphere exchanges (radiation, heat, moisture, CO2, and other trace gasses). We utilized a combination of multiscale measurements including fixed flux towers, aircraft-based flux transects, aircraft boundary layer budgets, and satellite remote sensing to quantify the spatial variability across a continental-scale rainfall gradient (transect). We found that the structure of vegetation changed along the transect in response to declining average rainfall. Tree basal area decreased from 9.6 m2 ha−1 in the coastal woodland savanna (annual rainfall 1,714 mm yr−1) to 0 m2 ha−1 at the grassland site (annual rainfall 535 mm yr−1), with dry-season green leaf area index (LAI) ranging from 1.04 to 0, respectively. Leaf-level measurements showed that photosynthetic properties were similar along the transect. Flux tower measurements showed that latent heat fluxes (LEs) decreased from north to south with resultant changes in the Bowen ratios (H/LE) from a minimum of 1.7 to a maximum of 15.8, respectively. Gross primary productivity, net carbon dioxide exchange, and LE showed similar declines along the transect and were well correlated with canopy LAI, and fluxes were more closely coupled to structure than floristic change.

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