scholarly journals Coupled Air, Sea, and Land Interactions of the South American Monsoon

2008 ◽  
Vol 21 (23) ◽  
pp. 6389-6403 ◽  
Author(s):  
Vasubandhu Misra
Keyword(s):  
General Circulation Model ◽  
General Circulation ◽  
Southern Oscillation ◽  
Austral Summer ◽  
Circulation Model ◽  
Precipitation Variability ◽  
Core Region ◽  
Southeastern Brazil ◽  
South American ◽  
South American Monsoon

Abstract The dominant interannual variation of the austral summer South American monsoon season (SAM) is associated with El Niño–Southern Oscillation (ENSO). Although this teleconnection provides a basis for the seasonal predictability of SAM, it is shown that the conventional tier-2 modeling approach of prescribing observed sea surface temperature (SST) is inappropriate to capture this teleconnection. Furthermore, such a forced atmospheric general circulation model (AGCM) simulation leads to degradation of the SAM precipitation variability. However, when the same AGCM is coupled to an ocean general circulation model to allow for coupled air–sea interactions, then this ENSO–SAM teleconnection is reasonably well simulated. This is attributed to the role of air–sea coupling in modulating the large-scale east–west circulation, especially associated with Niño-3 SST anomalies. It is also shown that the land–atmosphere feedback in the SAM domain as a result of the inclusion of air–sea coupling is more robust. As a consequence of this stronger land–atmosphere feedback the decorrelation time of the daily rainfall in the SAM region is prolonged to match more closely with the observed behavior. A subtle difference in the austral summer seasonal precipitation anomalies between that over the Amazon River basin (ARB) and the SAM core region is also drawn from this study in reference to the influence of the air–sea interaction. It is shown that the dominant interannual precipitation variability over the ARB is simulated both by the uncoupled and coupled (to OGCM) AGCM in contrast to that over the SAM core region in southeastern Brazil.

Simulation of the Southern Oscillation in an atmospheric general circulation model

1989 ◽  
Vol 329 (1604) ◽  
pp. 179-188 ◽  
Keyword(s):  
General Circulation Model ◽  
Wind Stress ◽  
General Circulation ◽  
Atmospheric General Circulation Model ◽  
Southern Oscillation ◽  
Low Frequency ◽  
Circulation Model ◽  
Western Pacific ◽  
Atmospheric General Circulation ◽  
The Western Pacific

An atmospheric general circulation model (GCM) was forced with the observed near-global sea surface temperature (SST) pattern for the period January 1970-December 1985. Its response over the Pacific Ocean is compared with Tahiti and Darwin station sea-level pressure and wind stress analyses obtained from Florida State University. The time-dependent SST clearly induces in the model run a Southern Oscillation that is apparent in the time series of all considered variables. The phase of the GCM Southern Oscillation is as observed but its low-frequency variance is too low and the spatial pattern is confined mainly to the western Pacific. The model is successful in reproducing the warm events of 1972—73 and 1982—83 and the cold event 1970—71, but fails with the cold events 1973-74 and 1975-76 and with the warm event 1976-77. Because the GCM is used as the atmospheric component in a coupled model, the response of an equatorial oceanic primitive equation model to both the modelled and observed wind stress is examined. The ocean model responds in essentially the same way to forcing with the observed wind stress and to forcing that corresponds to the first two low-frequency empirical orthogonal functions (EOFS) of the wind variations. These first two EOFS describe a regular eastward propagation of the so signal from the western Pacific to the central Pacific within about one year. The ocean model’s response to the modelled wind stress is too weak. It is similar to the response to the first observed wind stress EOF only. That is, the observed Southern Oscillation appears as a sequence of propagating patterns but the simulated Southern Oscillation appears as one standing pattern. The nature of the deviation of simulated wind stress from observations is further analysed by means of model output statistics.

scholarly journals Mechanisms for Precipitation Variability of the Eastern Brazil/SACZ Convective Margin

2011 ◽  
Vol 24 (13) ◽  
pp. 3445-3456 ◽  
Author(s):  
H.-Y. Ma ◽  
X. Ji ◽  
J. D. Neelin ◽  
C. R. Mechoso
Keyword(s):  
Los Angeles ◽  
Atlantic Ocean ◽  
Time Scales ◽  
General Circulation ◽  
Austral Summer ◽  
Circulation Model ◽  
Precipitation Variability ◽  
Western Atlantic ◽  
Low Level ◽  
Eastern Brazil

Abstract The present study examines the mechanisms for the connection between the precipitation variability in eastern Brazil and the South Atlantic convergence zone (SACZ) convective margin (eastern Brazil/SACZ convective margin) and the variability of low-level inflow on interannual time scales during austral summer. The authors' methodology is based on the analysis of observational datasets and simulations by the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) coupled to the Simplified Simple Biosphere Model. It is demonstrated that the inflow variability is associated with the leading mode of wind variability over subtropical South America, and the connection is established through the mechanism of an analytic prototype for convective margin shifts proposed in previous studies. Over the eastern Brazil/SACZ convective margin, the weaker (stronger) convection tends to occur together with stronger (weaker) low-level inflows in reference to the mean easterly trades. By changing the “ventilation” effect, stronger (weaker) inflows with low moist static energy from the Atlantic Ocean suppress (promote) convection. The causal relationship is verified by AGCM mechanism-testing experiments performed in perpetual-February mode, in which low-level, nondivergent wind perturbations are imposed in a region overlapping eastern Brazil and the western Atlantic Ocean. With solely the imposed-wind perturbations acting on the moisture advection in the model equation, the AGCM can reproduce the precipitation variability in the eastern Brazil/SACZ convective margin. The capability of the AGCM in capturing such precipitation sensitivity to the low-level inflow variability also suggests that the mechanism can be applied to other regions of convective margins or to other time scales.

Sign in / Sign up

Export Citation Format

Share Document