Papers by Guillermo Obregon
Este trabalho analisa a variabilidade e as teleconexoes especiais e temporais do campo de vortici... more Este trabalho analisa a variabilidade e as teleconexoes especiais e temporais do campo de vorticidade relativa durante o verao do Hemisferio Sul 1979/1980. Utilizaram-se ventos em pontos de grade analisados pelo NMC no nivel de 200 hPa, no dominio espacial limitado pelos meridianos 0 graus e 180 graus W e pelas latitudes 19,6 graus N e 44,6 graus S. Espectros de potencia e espectro cruzados das seris temporais de vorticidade relativa identificaram que os periodos de 30-60 dias, 12-15 dias, 7,5-8,5 dias e 4,8-5,7 sao significativos. As series temporais foram filtradas em 3 faixas espectrais com periodos de 2,5-6 dias, maiores que 10 dias e de 30 a 60 dias. Estas series indicam que a regiao sudeste do Brasil, associada a Zona de Convergencia do Atlantico Sul (ZCAS), apresenta a maior variabilidade. Na faixa espectral de 2,5-6 dias as perturbacoes extratropicais constituem os principais mecanismos que induzem ao padrao de comportamento da Alta da Bolivia (AB), da Zona de Convergencia d...
Bull. Amer. Meteor. Soc, 2003
Поиск в библиотеке, Расширенный поиск. ...
… . Eds. M. Keller, J. Gash, P. …, 2009
ABSTRACT This chapter summarizes our current knowledge on the mean climatological features of Ama... more ABSTRACT This chapter summarizes our current knowledge on the mean climatological features of Amazonia. Significant uncertainties remain in our understanding of the complex dynamics of climate and climate variability in that region, which are due, in part, to the lack of observational data. The strong seasonality of the rainfall and the relatively rapid transition between the wet and dry season associated with onset of the rainy season is related to the establishment of the South America Monsoon System (SAMS). The SAMS is controlled by large-scale thermodynamic conditions influenced by the near-equatorial sea surface temperature (SST). It has been suggested that land-surface dryness in the dry season is the main cause of the delay in the onset of the subsequent wet season. The 30- to 60-day oscillation is the major mode of intraseasonal variability. Interannual variability of the hydroclimatic system is strongly related to El Niño-Southern Oscillation. More generally, tropical Pacific and Atlantic SSTs control rainfall variability in Amazonia, and SW Atlantic SST anomalies influence the variability of the South Atlantic Convergence Zone (SACZ). Land surface-atmosphere interactions have been proposed as a possible dynamical mechanism for the unexplained variance at the annual and interannual timescales. At decadal and interdecadal timescales, rainfall variability is related to the Pacific Decadal Oscillation mainly over the southern portions, and linked to the North Atlantic Oscillation. At paleoclimate timescales, there is large uncertainty on major aspects of rainfall variability over tropical South America. For instance, there remains uncertainty on the basic character of rainfall anomalies over Amazonia, whether drier or wetter, during the Last Glacial Maximum, and paleoclimate reconstructions still suffer from lack of data.
Journal of Climate, 2012
This is an observational study of the large-scale moisture transport over South America, with som... more This is an observational study of the large-scale moisture transport over South America, with some analyses on its relation to subtropical rainfall. The concept of aerial rivers is proposed as a framework: it is an analogy between the main pathways of moisture flow in the atmosphere and surface rivers. Opposite to surface rivers, aerial rivers gain (lose) water through evaporation (precipitation). The magnitude of the vertically integrated moisture transport is discharge, and precipitable water is like the mass of the liquid column—multiplied by an equivalent speed it gives discharge. Trade wind flow into Amazonia, and the north/northwesterly flow to the subtropics, east of the Andes, are aerial rivers. Aerial lakes are the sections of a moisture pathway where the flow slows down and broadens, because of diffluence, and becomes deeper, with higher precipitable water. This is the case over Amazonia, downstream of the trade wind confluence. In the dry season, moisture from the aerial lake is transported northeastward, but weaker flow over southern Amazonia heads southward toward the sub-tropics. Southern Amazonia appears as a source of moisture to this flow. Aerial river discharge to the subtropics is comparable to that of the Amazon River. The variations of the amount of moisture coming from Amazonia have an important effect over the variability of discharge. Correlations between the flow from Amazonia and subtropical rainfall are not strong. However, some months within the set of dry seasons observed showed a strong increase (decrease) occurring together with an important increase (decrease) in subtropical rainfall.
Most fires in Amazonia result from the combination between climate and land-use factors. They occ... more Most fires in Amazonia result from the combination between climate and land-use factors. They occur mainly in the dry season and are used as an inexpensive tool for land clearing and management. However, their unintended consequences are of important concern. Fire emissions are the most important sources of greenhouse gases and aerosols in the region, accidental fires are a major threat to protected areas, and frequent fires may lead to permanent conversion of forest areas into savannas. Fire-activity models have thus become important tools for environmental analyses in Amazonia. They are used, for example, in warning systems for monitoring the risk of burnings in protected areas, to improve the description of biogeochemical cycles and vegetation composition in ecosystem models, and to help estimate the long-term potential for savannas in biome models. Previous modeling studies for the whole region were produced in units of satellite fire pixels, which complicate their direct use for environmental applications. By reinterpreting remote-sensing based data using a statistical approach, we were able to calibrate models for the whole region in units of probability, or chance of fires to occur. The application of these models for years 2005 and 2006 provided maps of fire potential at 3-month and 0.25-deg resolution as a function of precipitation and distance from main roads. In both years, the performance of the resulting maps was better for the period July-September. During these months, most of satellite-based fire observations were located in areas with relatively high chance of fire, as determined by the modeled probability maps. In addition to reproduce reasonably well the areas presenting maximum fire activity as detected by remote sensing, the new results in units of probability are easier to apply than previous estimates from fire-pixel models.
Books by Guillermo Obregon
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Papers by Guillermo Obregon
Books by Guillermo Obregon