Abstract
AbstractObservational studies demonstrate a poleward shift in the South Atlantic Convergence Zone (SACZ) in recent decades with regional impacts on extreme precipitation. However, the relative contribution of anthropogenic forcing and natural variability to the observed trends is presently unknown. To evaluate the main forcings associated with the recent trends in precipitation rate consistent with the poleward shift of the SACZ, this study examines the last 40 years of the historical scenario of 20 global climate models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The relative contribution of natural variability and anthropogenic‐related forcings to precipitation in the 20th century is assessed based on historical, natural, anthropogenic, and pre‐industrial simulations from CMIP5 models. Over the study area, precipitation trends simulated by the CMIP5 historical scenario are small, with large disagreement among models and members. Despite the small magnitude of the trends, the spatial pattern emerging is coherent with a poleward displacement of the SACZ, as indicated by a reduction (increase) in the precipitation rate over Southeastern Brazil (Southeastern South America). Even with large discrepancies among models, the attribution analysis suggests that precipitation trends could partially result from land‐use change, followed by changes in ozone concentrations, with more significant influence over Southeastern Brazil. The large uncertainty in the simulated precipitation suggests that not all mechanisms related to the position and intensity of the SACZ events are well captured by the CMIP5 models considered here.
Highlights
| INTRODUCTIONThe austral summer (December–February, DJF) precipitation over tropical South America (SA) is strongly linked to the South American Monsoon System (SAMS) and the South Atlantic Convergence Zone (SACZ; Zhou and Lau, 1998; Barros et al, 2000; Zhou and Lau, 2001; Carvalho et al, 2004; 2011; Vera et al, 2006a; Muza et al, 2009)
The objective of this study is to evaluate the contribution of natural variability and anthropogenic-related forcings to precipitation trends in the South Atlantic Convergence Zone (SACZ)-related dipole, associated with a poleward shift of the convergence zone in the 20th century
The present analysis indicates that precipitation trends observed over the SACZ and Southeastern South America (SESA) regions over the last 40 years consistent with Zilli et al (2019) are detectable in some of the historical simulations of the CMIP5 climate models
Summary
The austral summer (December–February, DJF) precipitation over tropical South America (SA) is strongly linked to the South American Monsoon System (SAMS) and the South Atlantic Convergence Zone (SACZ; Zhou and Lau, 1998; Barros et al, 2000; Zhou and Lau, 2001; Carvalho et al, 2004; 2011; Vera et al, 2006a; Muza et al, 2009). Zilli et al (2019) related the reduction in precipitation rate over EBr to the weakening of the northerly winds and decrease in the available moisture at the mid-to-lower troposphere (700 hPa) colocated with the climatological position of the SASH These changes in circulation and low-level moisture have reduced convergence and moisture advected to EBr. Using an Atmospheric Global Climate Model (AGCM), Talento and Barreiro (2017) identified negative precipitation trends and related circulation changes over tropical Brazil as a response to extratropical warming over the northern hemisphere, similar to the atmospheric warming observed over the last decades. To analyse how CMIP5 models simulate the location and intensity of the SACZ, we compare the austral summer (December to February—DJF) average of daily precipitation rate (PP) and its trend (τ) from GPCP with respective
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