A correlated shortening of the North and South American monsoon seasons in the past few decades

• Main Authors: Arias, P.A., Fu, R., Vera, C., Rojas, M.
• Format: Journal/Newspaper
• Language: unknown
• Subjects: Cross-equatorial flow; Global warming; North American monsoon; North Atlantic subtropical high; South American monsoon; Southern Amazon wet season; annual variation; Atlantic Multidecadal Oscillation; decadal variation; El Nino-Southern Oscillation; Hadley cell; monsoon; sea surface temperature; wet season; Amazonia; North Atlantic
• Online Access: https://hdl.handle.net/20.500.12110/paper_09307575_v45_n11-12_p3183_Arias

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 Niño-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. © 2015, Springer-Verlag Berlin Heidelberg. Fil:Vera, C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.