Journal
CLIMATE DYNAMICS
Volume 45, Issue 11-12, Pages 3183-3203Publisher
SPRINGER
DOI: 10.1007/s00382-015-2533-1
Keywords
Southern Amazon wet season; North American monsoon; South American monsoon; North Atlantic subtropical high; Cross-equatorial flow; Global warming
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Funding
- Comision Nacional de Investigacion Cientifica y Tecnologica de Chile grant FONDECYT [3140570]
- Program Estrategia de Sostenibilidad at Universidad de Antioquia
- National Science Foundation [AGS-0937400]
- NOAA Climate Program Office Climate Prediction Program for the Americas (CPPA) Grant [NA10OAR4310157]
- CONICET/PIP [112-20120100626CO]
- UBACyT [20020130100489BA]
- FONDAP-CONICYT [15110009, NC120066]
- Div Atmospheric & Geospace Sciences
- Directorate For Geosciences [0937400] Funding Source: National Science Foundation
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Our observational analysis shows that the wet seasons of the American monsoon systems have shortened since 1978 due to correlated earlier retreats of the North American monsoon (NAM) and late onsets of the southern Amazon wet season, an important part of the South American monsoon (SAM). These changes are related to the combination of the global sea surface temperature (SST) warming mode, the El Nio-Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO), the westward shift of the North Atlantic subtropical high (NASH), and the enhancement of Pacific South American and Pacific North American wave train patterns, which induces variations of the regional circulation at interannual and decadal scales. The joint contributions from these forcing factors are associated with a stronger and more equatorward regional Hadley cell, which enhances convergence towards the equator, strengthening and possibly delaying the retreat of the tropical part of the NAM. This in turn accelerates the demise of the northern NAM and delays the reversal of the cross-equatorial flow over South America, reducing moisture transport to the SAM and delaying its onset. In addition, the thermodynamic response to warming appears to cause local drier land conditions over both regions, reinforcing the observed changes in these monsoons. Although previous studies have identified the isolated influence of the regional Hadley cell, ENSO, AMO, global SST warming, and NASH on the NAM, the correlated changes between NAM and SAM through variations of the cross-equatorial flow had not been established before.
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