Submonthly variability of the South American Monsoon System.

摘要

The objective of this research is to gain a better understanding of the variability and predictability of the South American Monsoon System (SAMS) on submonthly time scales. Our methodology is based on the analysis of observation and simulations by the UCLA Atmospheric General Circulation Model (AGCM) coupled to the Simplified Simple Biosphere Model (SSiB).;We examine the impact of land surface processes on SAMS simulations. The simulated precipitation and low-level circulations are more realistic after consideration of vegetation biophysical processes, i.e., with the AGCM coupled to SSiB than with a simple representation of surface processes. The precipitation in central Amazonia is improved, and the overestimation of convection is reduced. Low-level circulations over South America are improved, especially on the lee of the Andes. This is due to a better representation of the Chaco Low position, and more realistic surface heat fluxes.;In the second part, we study SAMS variability as represented by westerly wind regimes (WWRs) and easterly wind regimes (EWRs), which are defined as periods of 3 days or longer during which time the anomalous low-level zonal flow in central Amazonia is persistently from the west or east, respectively. The major feature of the precipitation variability is a dipole-type pattern with centers in northwestern Amazonia and central-southeastern Brazil. The center with positive (negative) precipitation anomalies corresponds to stronger (weak) convection, and anomalous moisture flux convergence (divergence). In central Amazonia, the change of low-level anomalous winds is key to the moisture budget of the dipole. We provide supporting evidence that the SAMS regimes are associated with the development of the Pacific-South American modes. This suggests a potential for improved predictability of SAMS submonthly variability.;We also find similar intensity and structures of the diurnal cycles of rainfall between the wind regimes in central Amazonia. The secondary nocturnal peak of rainfall during EWRs previous reported in the Large-scale Biosphere Atmosphere campaign is not a significant feature of SAMS climate. The diurnal cycles of rainfall in central-southeastern Brazil are also similar, except for larger values during WWRs. In northwestern Amazonia, rainfall is stronger and has a secondary peak in the early morning during EWRs.