Abstract
The Hadley Centre coupled climate–carbon cycle model (HadCM3LC) predicts loss of the Amazon rainforest in response to future anthropogenic greenhouse gas emissions. In this study, the atmospheric component of HadCM3LC is used to assess the role of simulated changes in mid-twenty-first century sea surface temperature (SST) in Amazon Basin climate change.When the full HadCM3LC SST anomalies (SSTAs) are used, the atmosphere model reproduces the Amazon Basin climate change exhibited by HadCM3LC, including much of the reduction in Amazon Basin rainfall. This rainfall change is shown to be the combined effect of SSTAs in both the tropical Atlantic and the Pacific, with roughly equal contributions from each basin. The greatest rainfall reduction occurs from May to October, outside of the mature South American monsoon (SAM) season. This dry season response is the combined effect of a more rapid warming of the tropical North Atlantic relative to the south, and warm SSTAs in the tropical east Pacific. Conversely, a weak enhancement of mature SAM season rainfall in response to Atlantic SST change is suppressed by the atmospheric response to Pacific SST. This net wet season response is sufficient to prevent dry season soil moisture deficits from being recharged through the SAM season, leading to a perennial soil moisture reduction and an associated 30% reduction in annual Amazon Basin net primary productivity (NPP). A further 23% NPP reduction occurs in response to a 3.5°C warmer air temperature associated with a global mean SST warming.
Highlights
First-generation coupled climate–carbon cycle models indicate that carbon cycle feedbacks may accelerate anthropogenic climate change through the twenty-first century ( Friedlingstein et al 2006)
While future drying of the Amazon Basin is not common to all general circulation models (GCMs) contributing to the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4; Christensen et al 2007), it remains useful to study the processes that contribute to the strong Amazon climate change exhibited by Cox et al (2000, 2004)
A scenario as striking as the predicted loss of Amazon forest by Cox et al (2000), along with the associated social and economic implications for the region, warrants further investigation. This is important because regional climate projections for tropical South America are not consistent across different GCMs contributing to the IPCC AR4
Summary
First-generation coupled climate–carbon cycle models indicate that carbon cycle feedbacks may accelerate anthropogenic climate change through the twenty-first century ( Friedlingstein et al 2006). Betts et al (2004) estimated that global carbon cycle feedbacks and vegetation structural changes in HadCM3LC contribute to approximately 40% of the simulated Amazon Basin precipitation reduction This indicates that the majority of the regional drying is part of a wider atmosphere–ocean response to greenhouse. The HadCM3LC trend in the twenty-first century Pacific sea surface temperature (SST) has been described as ‘El Nino like’ (Collins 2005), and Cox et al (2004) showed that a future weakening of Amazon Basin rainfall during December to February (DJF) is correlated with a progressive weakening of the equatorial Pacific east–west SST gradient They suggest that Amazon Basin drying is related to this SST change through Walker circulation perturbations, similar to those thought to relate observed South American rainfall and El Nino-Southern Oscillation (Aceituno 1998; Grimm 2003).
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