We have observed that the dry-season length (DSL) has increased over southern Amazonia since 1979, primarily owing to a delay of its ending dates (dry-season end, DSE), and is accompanied by a prolonged fire season. A poleward shift of the subtropical jet over South America and an increase of local convective inhibition energy in austral winter (June-August) seem to cause the delay of the DSE in austral spring (September-November). These changes cannot be simply linked to the variability of the tropical Pacific and Atlantic Oceans. Although they show some resemblance to the effects of anthropogenic forcings reported in the literature, we cannot attribute them to this cause because of inadequate representation of these processes in the global climate models that were presented in the Intergovernmental Panel on Climate Change's Fifth Assessment Report. These models significantly underestimate the variability of the DSE and DSL and their controlling processes. Such biases imply that the future change of the DSE and DSL may be underestimated by the climate projections provided by the Intergovernmental Panel on Climate Change's Fifth Assessment Report models. Although it is not clear whether the observed increase of the DSL will continue in the future, were it to continue at half the rate of that observed, the long DSL and fire season that contributed to the 2005 drought would become the new norm by the late 21st century. The large uncertainty shown in this study highlights the need for a focused effort to better understand and simulate these changes over southern Amazonia.climate variability | rainforests | climate model projection F ifteen percent of global photosynthesis occurs in the Amazon rainforest (1), where 25% of plant species are found (2). This rainforest ecosystem normally removes C from the atmosphere but released more than 1 Pg of C to the atmosphere in the 2005 drought (3). Consequently, even a partial loss of these forests would substantially increase global atmospheric CO 2 (4, 5) and reduce biodiversity. The dry-season length (DSL) is among the most important climate limitations for sustaining rainforests (6-9), especially in southern Amazonia, where rainforests are exposed to relatively long dry seasons and vulnerable to increasing conversion of native forests to cultivated crops (10-12). The extreme droughts in 2005 and 2010 had strong impacts on the rainforest and its C cycle (3,13,14). These unusual events, along with possible increase of drought severity and DSL during the past few decades (e.g., refs. 15 and 16) heighten the urgency of understanding what causes these dry anomalies and whether they will continue into the future. Contrary to the observed drying, some global climate models that previously projected strong drying over Amazonia now project much weaker drying by the end of the 21st century as these models evolve (17). Do these observed events represent the extremes of natural climate variability, or do climate projections underestimate potential future changes? This study explores ...

During boreal summer, much of the water vapor and CO entering the global tropical stratosphere is transported over the Asian monsoon͞Tibetan Plateau (TP) region. Studies have suggested that most of this transport is carried out either by tropical convection over the South Asian monsoon region or by extratropical convection over southern China. By using measurements from the newly available National Aeronautics and Space Administration Aura Microwave Limb Sounder, along with observations from the Aqua and Tropical Rainfall-Measuring Mission satellites, we establish that the TP provides the main pathway for cross-tropopause transport in this region. Tropospheric moist convection driven by elevated surface heating over the TP is deeper and detrains more water vapor, CO, and ice at the tropopause than over the monsoon area. Warmer tropopause temperatures and slower-falling, smaller cirrus cloud particles in less saturated ambient air at the tropopause also allow more water vapor to travel into the lower stratosphere over the TP, effectively short-circuiting the slower ascent of water vapor across the cold tropical tropopause over the monsoon area. Air that is high in water vapor and CO over the Asian monsoon͞TP region enters the lower stratosphere primarily over the TP, and it is then transported toward the Asian monsoon area and disperses into the large-scale upward motion of the global stratospheric circulation. Thus, hydration of the global stratosphere could be especially sensitive to changes of convection over the TP.climate ͉ CO ͉ stratosphere water vapor W ater vapor concentrations in the tropical lower stratosphere (LS) are 60% greater in boreal summer than in winter. This seasonal variation not only influences the radiation budget near the local tropopause but also propagates upward and toward the pole with the global stratospheric circulation (1, 2). Numerical simulations suggest that Ϸ75% of the total summer water vapor transport into the global tropical stratosphere may occur over the South Asian monsoon and Tibetan Plateau (TP) regions (3), contributing to Ͼ25% of the water vapor in the middle stratosphere (4).Studies have hypothesized that an increase in crosstropopause transport in the Asian monsoon͞TP region may have contributed to an increasing trend in stratospheric water vapor (5) during the 1980s and 1990s (6, 7). This trend probably increased the global greenhouse forcing (8) and enhanced ozone depletion in the Arctic (9). Any explanation of this trend or future trends would likely need to address how source regions for stratospheric water have changed. Recent studies have revealed high CO in the upper troposphere (UT) over the South Asian monsoon region (10). This CO is produced by biomass or fossil fuel burning, suggesting a human influence on transport of combustion pollutants and, perhaps, water vapor into the LS (11). Thus, a clarification of the mechanisms of water vapor and CO transport into the LS in this region is an important step toward understanding tropospheric influences on hydrati...

The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land-atmosphere feedbacks on regional drought. The runs were carried out with five different atmospheric general circulation models (AGCMs) and one coupled atmosphere-ocean model in which the model was continuously nudged to the imposed SST forcing. This paper provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Niñ o-Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic multidecadal oscillation (AMO), and a global trend pattern.One of the key findings is that all of the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific forcing pattern, with a cold Pacific leading to reduced precipitation and a warm Pacific leading to enhanced precipitation over most of the United States. While the response to the Atlantic pattern is less robust, there is general agreement among the models that the largest precipitation Further highlights of the response over the United States to the Pacific forcing include precipitation signal-to-noise ratios that peak in spring, and surface temperature signal-to-noise ratios that are both lower and show less agreement among the models than those found for the precipitation response. The response to the positive SST trend forcing pattern is an overall surface warming over the world's land areas, with substantial regional variations that are in part reproduced in runs forced with a globally uniform SST trend forcing. The precipitation response to the trend forcing is weak in all of the models. It is hoped that these early results, as well as those reported in the other contributions to this special issue on drought, will serve to stimulate further analysis of these simulations, as well as suggest new research on the physical mechanisms contributing to hydroclimatic variability and change throughout the world.

Although it is well established that transpiration contributes much of the water for rainfall over Amazonia, it remains unclear whether transpiration helps to drive or merely responds to the seasonal cycle of rainfall. Here, we use multiple independent satellite datasets to show that rainforest transpiration enables an increase of shallow convection that moistens and destabilizes the atmosphere during the initial stages of the dry-to-wet season transition. This shallow convection moisture pump (SCMP) preconditions the atmosphere at the regional scale for a rapid increase in rain-bearing deep convection, which in turn drives moisture convergence and wet season onset 2-3 mo before the arrival of the Intertropical Convergence Zone (ITCZ). Aerosols produced by late dry season biomass burning may alter the efficiency of the SCMP. Our results highlight the mechanisms by which interactions among land surface processes, atmospheric convection, and biomass burning may alter the timing of wet season onset and provide a mechanistic framework for understanding how deforestation extends the dry season and enhances regional vulnerability to drought.he southern Amazon, which covers ∼30-40% of Amazonia, is a transitional region between tropical rainforests to the north and west and subtropical savanna and agricultural lands to the south and east (Fig. 1). Rainforests in this region, which play an important role in the global carbon cycle (1), are vulnerable to slight decreases in annual rainfall or increases in dry season length (2). This vulnerability is exacerbated by large-scale agricultural land use. The southern Amazon dry season has lengthened in recent decades, primarily due to delays in wet season onset (3). Model simulations suggest that continuation of this trend could trigger an abrupt transition of rainforest to savanna (2, 4), which would substantially reduce dry season rainfall over the southern Amazon and downwind agricultural regions (5, 6).Rainforest vitality is known to depend on rainfall amount and dry season length (2, 7-9), but major knowledge gaps remain regarding rainforest influences on wet season onset. Rainforest evapotranspiration (ET) accounts for ∼30-50% of regional rainfall (10-13), but it is unclear whether ET actively modifies or merely responds to rainfall seasonality. Credible assessments of land use contributions to recent increases in dry season length and the frequency of extreme droughts in this region (14, 15) require these gaps to be filled. The Deep Convection Moisture PumpWet season onset in the tropics is generally associated with either monsoon reversals in the land-ocean temperature gradient or north-south migration of the Intertropical Convergence Zone (ITCZ), both of which are driven by seasonal changes in the distribution of solar radiation. However, wet season onset over the southern Amazon precedes the southward migration of the Atlantic ITCZ by ∼2-3 mo (16) and occurs without a reversal in the land-ocean surface temperature gradient (17, 18). Conventional mechanisms therefore c...

This study investigates the changes of the North Atlantic subtropical high (NASH) and its impact on summer precipitation over the southeastern (SE) United States using the 850-hPa geopotential height field in the National Centers for Environmental Prediction (NCEP) reanalysis, the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40), long-term rainfall data, and Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) model simulations during the past six decades . The results show that the NASH in the last 30 yr has become more intense, and its western ridge has displaced westward with an enhanced meridional movement compared to the previous 30 yr. When the NASH moved closer to the continental United States in the three most recent decades, the effect of the NASH on the interannual variation of SE U.S. precipitation is enhanced through the ridge's north-south movement. The study's attribution analysis suggested that the changes of the NASH are mainly due to anthropogenic warming. In the twenty-first century with an increase of the atmospheric CO 2 concentration, the center of the NASH would be intensified and the western ridge of the NASH would shift farther westward. These changes would increase the likelihood of both strong anomalously wet and dry summers over the SE United States in the future, as suggested by the IPCC AR4 models.