10.8
Multiple biosphere-atmosphere equilibria in Brazil under present-day conditions: a GCM simulation study
Marcos D. Oyama, Center for Weather Forecasting and Climate Studies - CPTEC/INPE, Cachoeira Paulista, SP, Brazil; and C. A. Nobre
The existence of multiple biosphere-atmosphere equilibria in Brazil is investigated under present-day conditions. The atmospheric model is the CPTEC/COLA GCM (T62L28), and it is coupled to a potential vegetation model which shows a good skill in reproducing the current natural vegetation distribution patterns both globally and for South America. The atmospheric and vegetation models are coupled assyncronously; the global vegetation distribution is updated every 3 years (the procedure of running the model and updating the vegetation is called iteration). To avoid unrealistic biome placement due to the atmospheric model systematic errors, the 3-year model climatology is corrected using a procedure analogous to "flux-correction" in AOGCM. All forcings (SST, carbon dioxide concentration, solar radiation etc) except vegetation distribution follows the present-day climatology. In the control run, the present-day potential biomes (output of the potential vegetation model forced by the present-day climate) are kept unchanged during a 10-year integration. Two 15-year (5 iterations) integrations are performed (DES and FOR). In the initial condition, all biomes (except ice) are changed to desert in DES, and to tropical forest in FOR. After the third iteration, the degree of similarity (measured by the kappa statistics) between successive iterations reaches a steady value. Thus, 5 iterations seem to be enough to reach a equilibrium biosphere-atmosphere state. The vegetation distribution and the climate of the last iteration are analyzed.
The FOR simulation leads to the present-day potential biomes. The DES simulation leads to a different vegetation distribution and climate: the eastern part of the Amazonia rainforest is replaced by savannas (hereafter referred as region A), a semi-desert area appears in the Northeast of Brasil (region B) and the Atlantic rainforest (which covers the coast and southern part of Brasil) expands northward (region C). In regions A and B, there is decrease in evapotranspiration; in region C, increase.
In region A, the change of tropical forest to savanna comprises two distinct sub-regions. In the northern part of A (AN), there is a weakening of the hydrological cycle: precipitation, evapotranspiration and moisture convergence decrease. In the southern part of A (AS), on the other hand, there is increase in precipitation despite the decrease in evapotranspiration. Region B behavior is similar to AN. In region C, there is an enhancement of the hydrological cycle. There is a belt extending from B to AN where precipitation decreases; and from C to AS where it increases.
Currently, both Amazonia and Northeast of Brazil are undergoing a process of anthropogenic environmental degradation. If this process is not slowed down and reversed, the climate over these regions may shift to a drier biosphere-atmosphere equilibrium state.
Session 10, Climate and society in the Southern Hemisphere
Wednesday, 26 March 2003, 10:30 AM-4:45 PM
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