Are the American Monsoons Becoming Shorter?: Evaluation of CMIP5 Simulations and Projections

Our previous observational work evidences a shortening of the American monsoons and a lengthening of their transition season during the last few decades (1978-2009). This lengthening appears to be explained by a stronger regional Hadley Cell in the Intra-American sector, which induces enhanced convergence over the equatorial Americas (see Figure 1), causing an earlier retreat of the North American monsoon (NAMS) and a late onset of the South American monsoon (SAMS). In addition, the enhanced convergence over the equatorial Americas is accompanied by a delayed reversal of the cross-equatorial flow in South America and a westward shift of the North Atlantic subtropical anticyclone (see Figure 1). This work aims to explore if the mechanisms involved in the lengthening of the transition season between the American monsoon systems evidenced in the last three decades are still observed under climate change conditions by the end of the century (2080-2099), using CMIP5 projections. Results indicate that CMIP5 models exhibit strong biases simulating NAMS retreat. In addition, RCP8.5 projections show a longer transition from NAMS to SAMS by the end of the century, with transition season lengths larger than those observed in present climate. Such lengthening is associated to both an earlier retreat of the NAMS and a late onset of the SAMS. The projected regional circulation patterns are also analyzed in order to give a coherent picture of this lengthening of the transition season between the American monsoons. This analysis would allow us to identify more clearly the role of anthropogenic forcing on the changes of these monsoon systems in recent past climate. Figure 1: Scheme of the mechanisms and modulators involved in the recent shortening of the American monsoons. From Arias et al. (2015). Arias, P.A., R. Fu, C.S. Vera, and M. Rojas, 2015: A correlated shortening of the North and South American monsoon seasons in the past few decades. Climate Dynamics, 10.1007/s00382-015-2533-1.