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- Author or Editor: Carlos A. Nobre x
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Abstract
Meteorological observations from the Anglo–Brazilian Amazonian Climate Observation Study (ABRACOS), together with the global reanalysis from the National Center for Environmental Prediction (NCEP) and satellite images, have been used to study the spatial extent and intensity of cold surges (known locally as “friagens”) in the Amazon basin. Case studies are presented of two of the strongest events of the 1994 winter season: 26 June and 10 July 1994. In both events, daily minimum temperatures in southeastern Brazil dropped to near or below 0°C, while at the same time minimum temperatures in southern Amazonia (Ji-Paraná site) were almost 8°C below average. Air temperature and humidity also fell in central and western Amazonia (Manaus and Marabá sites, respectively), although the fact that these reductions were less substantial than those farther to the south indicates that the cold air is greatly modified as it moves across Amazonia.
In Ji-Paraná the largest drops in minimum temperature coincided with strong winds from the south, implying that cold advection was the main mechanism for the falling temperatures. In contrast, there was no increase in wind speed at Manaus and Marabá during the days with reduced temperatures. At these sites, cooling was due to a reduction of the maximum temperature caused, at least partially, by increased cloudiness rather than by a lowering of minimum temperatures as at Ji-Paraná. With regard to the depth of the atmospheric boundary layer (ABL), it is observed that during the passage of the cold air in southern Amazonia, the ABL was cooler and shallower than during the pre- and postfriagem days. The friagens presented are of 5- to 6-day duration, including the passage of the cold front, but the period with cold temperatures lasts between 2 and 3 days.
Abstract
Meteorological observations from the Anglo–Brazilian Amazonian Climate Observation Study (ABRACOS), together with the global reanalysis from the National Center for Environmental Prediction (NCEP) and satellite images, have been used to study the spatial extent and intensity of cold surges (known locally as “friagens”) in the Amazon basin. Case studies are presented of two of the strongest events of the 1994 winter season: 26 June and 10 July 1994. In both events, daily minimum temperatures in southeastern Brazil dropped to near or below 0°C, while at the same time minimum temperatures in southern Amazonia (Ji-Paraná site) were almost 8°C below average. Air temperature and humidity also fell in central and western Amazonia (Manaus and Marabá sites, respectively), although the fact that these reductions were less substantial than those farther to the south indicates that the cold air is greatly modified as it moves across Amazonia.
In Ji-Paraná the largest drops in minimum temperature coincided with strong winds from the south, implying that cold advection was the main mechanism for the falling temperatures. In contrast, there was no increase in wind speed at Manaus and Marabá during the days with reduced temperatures. At these sites, cooling was due to a reduction of the maximum temperature caused, at least partially, by increased cloudiness rather than by a lowering of minimum temperatures as at Ji-Paraná. With regard to the depth of the atmospheric boundary layer (ABL), it is observed that during the passage of the cold air in southern Amazonia, the ABL was cooler and shallower than during the pre- and postfriagem days. The friagens presented are of 5- to 6-day duration, including the passage of the cold front, but the period with cold temperatures lasts between 2 and 3 days.
Abstract
Large-scale conversion of tropical forests into pastures or annual crops could lead to changes in the climate. We have used a coupled numerical model of the global atmosphere and biosphere (Center for Ocean-Land- Atmosphere GCM) to assess the effects of Amazonian deforestation on the regional and global climate. We found that when the Amazonian tropical forests were replaced by degraded grass (pasture) in the model, there was a significant increase in the mean surface temperature (about 2.5°C) and a decrease in the annual evapo-transpiration (30% reduction), precipitation (25% reduction), and runoff (20% reduction) in the region. The differences between the two simulations were greatest during the dry season. The deforested case was associated with larger diurnal fluctuations of surface temperature and vapor pressure deficit; such effects have been observed in existing deforested arms in Amazonia. The calculated reduction in precipitation was larger than the calculated decrease in evapotranspiration, indicating a reduction in the regional moisture convergence. There was also an increase in the length of the dry season in the southern half of the Amazon Basin, which could have serious implications for the reetablishment of the tropical forests following massive deforestation since rainforests only occur where the dry season is very short or nonexistent. An empirical bioclimatic scheme based on an integrated soil moisture stress index was used to derive the movement of the savanna-forest boundary in response to the simulated climate change produced by large-scale deforestation. The implications of possible climate changes in adjacent regions are discussed.
Abstract
Large-scale conversion of tropical forests into pastures or annual crops could lead to changes in the climate. We have used a coupled numerical model of the global atmosphere and biosphere (Center for Ocean-Land- Atmosphere GCM) to assess the effects of Amazonian deforestation on the regional and global climate. We found that when the Amazonian tropical forests were replaced by degraded grass (pasture) in the model, there was a significant increase in the mean surface temperature (about 2.5°C) and a decrease in the annual evapo-transpiration (30% reduction), precipitation (25% reduction), and runoff (20% reduction) in the region. The differences between the two simulations were greatest during the dry season. The deforested case was associated with larger diurnal fluctuations of surface temperature and vapor pressure deficit; such effects have been observed in existing deforested arms in Amazonia. The calculated reduction in precipitation was larger than the calculated decrease in evapotranspiration, indicating a reduction in the regional moisture convergence. There was also an increase in the length of the dry season in the southern half of the Amazon Basin, which could have serious implications for the reetablishment of the tropical forests following massive deforestation since rainforests only occur where the dry season is very short or nonexistent. An empirical bioclimatic scheme based on an integrated soil moisture stress index was used to derive the movement of the savanna-forest boundary in response to the simulated climate change produced by large-scale deforestation. The implications of possible climate changes in adjacent regions are discussed.
Abstract
The environmental conditions associated with squall lines (SL) that were observed during the period of 13 April–13 May 1987 (GTE/ABLE-2B) originating at the northern coast of South America and propagating over the Amazon Basin are documented. The SL observed on 5–7 May are examined in more detail. The SL days had in common a stronger and deeper low-level jet when compared with the days without SL. Two possible explanations are found for the intensification of the low-level jet: propagating easterly waves in the tropical Atlantic, which eventually reach Manaus, and localized heat sources in the western Amazon. Both were observed on 5–6 May. It is suggested that numerical simulations should be performed to unravel the relative importance of each large-scale mechanism.
Abstract
The environmental conditions associated with squall lines (SL) that were observed during the period of 13 April–13 May 1987 (GTE/ABLE-2B) originating at the northern coast of South America and propagating over the Amazon Basin are documented. The SL observed on 5–7 May are examined in more detail. The SL days had in common a stronger and deeper low-level jet when compared with the days without SL. Two possible explanations are found for the intensification of the low-level jet: propagating easterly waves in the tropical Atlantic, which eventually reach Manaus, and localized heat sources in the western Amazon. Both were observed on 5–6 May. It is suggested that numerical simulations should be performed to unravel the relative importance of each large-scale mechanism.
Abstract
In 2005, large sections of southwestern Amazonia experienced one of the most intense droughts of the last hundred years. The drought severely affected human population along the main channel of the Amazon River and its western and southwestern tributaries, the Solimões (also known as the Amazon River in the other Amazon countries) and the Madeira Rivers, respectively. The river levels fell to historic low levels and navigation along these rivers had to be suspended. The drought did not affect central or eastern Amazonia, a pattern different from the El Niño–related droughts in 1926, 1983, and 1998. The choice of rainfall data used influenced the detection of the drought. While most datasets (station or gridded data) showed negative departures from mean rainfall, one dataset exhibited above-normal rainfall in western Amazonia.
The causes of the drought were not related to El Niño but to (i) the anomalously warm tropical North Atlantic, (ii) the reduced intensity in northeast trade wind moisture transport into southern Amazonia during the peak summertime season, and (iii) the weakened upward motion over this section of Amazonia, resulting in reduced convective development and rainfall. The drought conditions were intensified during the dry season into September 2005 when humidity was lower than normal and air temperatures were 3°–5°C warmer than normal. Because of the extended dry season in the region, forest fires affected part of southwestern Amazonia. Rains returned in October 2005 and generated flooding after February 2006.
Abstract
In 2005, large sections of southwestern Amazonia experienced one of the most intense droughts of the last hundred years. The drought severely affected human population along the main channel of the Amazon River and its western and southwestern tributaries, the Solimões (also known as the Amazon River in the other Amazon countries) and the Madeira Rivers, respectively. The river levels fell to historic low levels and navigation along these rivers had to be suspended. The drought did not affect central or eastern Amazonia, a pattern different from the El Niño–related droughts in 1926, 1983, and 1998. The choice of rainfall data used influenced the detection of the drought. While most datasets (station or gridded data) showed negative departures from mean rainfall, one dataset exhibited above-normal rainfall in western Amazonia.
The causes of the drought were not related to El Niño but to (i) the anomalously warm tropical North Atlantic, (ii) the reduced intensity in northeast trade wind moisture transport into southern Amazonia during the peak summertime season, and (iii) the weakened upward motion over this section of Amazonia, resulting in reduced convective development and rainfall. The drought conditions were intensified during the dry season into September 2005 when humidity was lower than normal and air temperatures were 3°–5°C warmer than normal. Because of the extended dry season in the region, forest fires affected part of southwestern Amazonia. Rains returned in October 2005 and generated flooding after February 2006.
This paper discusses the development of a prediction system that integrates physical, biogeochemical, and societal processes in a unified Earth system framework. Such development requires collaborations among physical and social scientists, and should include i) the development of global Earth system analysis and prediction models that account for physical, chemical, and biological processes in a coupled atmosphere–ocean–land–ice system; ii) the development of a systematic framework that links the global climate and regionally constrained weather systems and the interactions and associated feedbacks with biogeochemistry, biology, and socioeconomic drivers (e.g., demography, global policy constraints, technological innovations) across scales and disciplines; and iii) the exploration and development of methodologies and models that account for societal drivers (e.g., governance, institutional dynamics) and their impacts and feedbacks on environmental and climate systems.
This paper discusses the development of a prediction system that integrates physical, biogeochemical, and societal processes in a unified Earth system framework. Such development requires collaborations among physical and social scientists, and should include i) the development of global Earth system analysis and prediction models that account for physical, chemical, and biological processes in a coupled atmosphere–ocean–land–ice system; ii) the development of a systematic framework that links the global climate and regionally constrained weather systems and the interactions and associated feedbacks with biogeochemistry, biology, and socioeconomic drivers (e.g., demography, global policy constraints, technological innovations) across scales and disciplines; and iii) the exploration and development of methodologies and models that account for societal drivers (e.g., governance, institutional dynamics) and their impacts and feedbacks on environmental and climate systems.
Abstract
The Center for Weather Forecasting and Climate Studies–Center for Ocean–Land–Atmosphere Studies (CPTEC–COLA) atmospheric general circulation model (AGCM) is integrated with nine initial conditions for 10 yr to obtain the model climate in an ensemble mode. The global climatological characteristics simulated by the model are compared with observational data, and emphasis is given to the Southern Hemisphere and South America. Evaluation of the model's performance is presented by showing systematic errors of several variables, and anomaly correlation and reproducibility are applied to precipitation. The model is able to simulate the main features of the global climate, and the results are consistent with analyses of other AGCMs. The seasonal cycle is reproduced well in all analyzed variables, and systematic errors occur at the same regions in different seasons. The Southern Hemisphere convergence zones are simulated reasonably well, although the model overestimates precipitation in the southern portions and underestimates it in the northern portions of these systems. The high- and low-level main circulation features such as the subtropical highs, subtropical jet streams, and storm tracks are depicted well by the model, albeit with different intensities from the reanalysis. The stationary waves of the Northern and Southern Hemispheres are weaker in the model; however, the dominant wavenumbers are similar to the observations. The energy budget analysis shows values of some radiative fluxes that are close to observations, but the unbalanced fluxes in the atmosphere and at the surface indicate that the radiation and cloud scheme parameterizations need to be improved. Besides these improvements, changes in the convection scheme and higher horizontal resolution to represent orographic effects better are being planned to improve the model's performance.
Abstract
The Center for Weather Forecasting and Climate Studies–Center for Ocean–Land–Atmosphere Studies (CPTEC–COLA) atmospheric general circulation model (AGCM) is integrated with nine initial conditions for 10 yr to obtain the model climate in an ensemble mode. The global climatological characteristics simulated by the model are compared with observational data, and emphasis is given to the Southern Hemisphere and South America. Evaluation of the model's performance is presented by showing systematic errors of several variables, and anomaly correlation and reproducibility are applied to precipitation. The model is able to simulate the main features of the global climate, and the results are consistent with analyses of other AGCMs. The seasonal cycle is reproduced well in all analyzed variables, and systematic errors occur at the same regions in different seasons. The Southern Hemisphere convergence zones are simulated reasonably well, although the model overestimates precipitation in the southern portions and underestimates it in the northern portions of these systems. The high- and low-level main circulation features such as the subtropical highs, subtropical jet streams, and storm tracks are depicted well by the model, albeit with different intensities from the reanalysis. The stationary waves of the Northern and Southern Hemispheres are weaker in the model; however, the dominant wavenumbers are similar to the observations. The energy budget analysis shows values of some radiative fluxes that are close to observations, but the unbalanced fluxes in the atmosphere and at the surface indicate that the radiation and cloud scheme parameterizations need to be improved. Besides these improvements, changes in the convection scheme and higher horizontal resolution to represent orographic effects better are being planned to improve the model's performance.
