OCTOBER 29, 1999

DOES A MONSOON CLIMATE EXIST OVER SOUTH AMERICA?

The climatology and the basic state of the summertime circulation
and rainfall over South America are studied using assimilation products
from the Data Assimilation System (DAS) of Goddard Earth Observing System-1
(GEOS-1) and satellite-derived rainfall. Results indicate the existence of
a regional summer monsoon circulation regime induced by strong diabatic
heating over the subtropical South American highland centered at the
Altiplano Plateau.

Sensitivity of the results to the assimilation scheme is tested by
comparing that with the National Center for Environmental Prediction
(NCEP) reanalysis and with satellite rainfall estimates. Results show
general agreement between the model produced rainfall anomaly and the
satellite estimates, as well as consistency between the basic circulation
features in the GEOS-1 and the NCEP reanalyses.

A case study of 1989-1990 South American summer monsoon (SASM)
reveals the following characteristics:

1. In late spring, the onset of SASM is signaled by an abrupt merging of
the upper tropospheric double westerly jets, one in the subtropics and the
other in the subpolar region, into a single jet in the mid-latitudes. This
is followed by the establishment of a vortex to the southeast of Altiplano
and occurrence of heavy precipitation over subtropical eastern Brazil.

2. During the mature phase of SASM, the heavy rainfall zone moves over the
Altiplano Plateau and the southernmost Brazilian highland. The fully
established SASM features: (a) an enhancement of equatorial North Atlantic
trade wind which emanates from the Sahara high and crosses the equator over
South America, (b) a buildup of strong northwesterlies along the eastern
side of the tropical Andes and (c) development of the South Atlantic
Convergence Zone (SACZ) in the southernmost position with strong convective
activity. Meanwhile, the upper tropospheric return flow emerges from an
anticyclone formed over the Altiplano Plateau, crosses the equator and
sinks over northwestern Africa.

3. The withdrawal of SASM in late summer is signaled by the resplitting of
the mid-latitude westerly jet. At the same time, the low-level northwest
monsoon flow diminishes, reducing the moisture supply and leading to the
termination of heavy precipitation over the subtropical highland.

Results also show the above-mentioned characteristics of SASM are clearly
linked to the tropospheric temperature changes over the central South
American highland. Sensible versus latent heating over the highland play
an important role in the evolution of SASM.

To provide further support of presence of a monsoon climate over South
America, SASM is compared and contrasted to the "classical" East Asian
summer monsoon (EASM). Many similar features, including evolution
characteristics between the two systems, have been identified. Contrasting
aspects of the SASM from the EASM are also discussed. It is pointed out
that a number of monsoonal characteristics of the climate of South America,
such as the seasonal reversal of the low-level wind, become apparent only
when the strong annual mean wind is removed. Based on the characteristic
features and their evolution, we conclude that a monsoon climate does exist
over South America.

Reference: This paper by Jiayu Zhou and K.-M. Lau was published in Journal
of Climate, 11, 1020-1040, 1998.