An Interhemispheric View of the American Monsoons: Vegetation and Moisture Transport Processes (Invited Talk)

The American monsoons are among the most relevant climate features in the tropical-subtropical Americas, affecting highly populated regions. Several studies have addressed the dynamics and processes related to each individual monsoon, but literature regarding possible interhemispheric links between these systems is relatively scarce. Consequently, it remains unknown whether there are such connections among the American monsoons, and if so, what mechanisms are responsible for them and how important are they in determining the variability of these monsoon systems, especially in a changing climate. Previous research have highlighted the more frequent occurrence of longer dry seasons in the Amazon during the past two decades, in association with delayed onsets of the South American monsoon system (SAMS). The delayed onsets of the SAMS correlate with earlier retreats of the North American monsoon system (NAMS), explaining a longer transition season between the American monsoons. Changes in both systems appear to relate to a stronger regional Hadley Cell in the Intra-American sector with enhanced convergence over the equatorial Americas. This enhanced convergence in the equatorial region is accompanied by a delayed reversal of the cross-equatorial flow in South America and a westward shift of the North Atlantic subtropical anticyclone, which in turn would affect atmospheric moisture transport toward the monsoons.

Our study identifies that these changes in the timing of the American monsoons are linked to atmospheric moisture transport anomalies not only in the monsoon regions but also in the entire Intra-American sector. During the late onsets of the SAMS (i.e longer dry seasons in the Amazon), water vapor content over the southern and eastern Amazon basin decreases, due to significant reductions of evaporation and recycled precipitation rates in these regions, especially during the transition from dry to wet conditions in the Amazon. Such reductions appear to relate to changes of surface conditions due to vegetation activity. On the other hand, late onsets of the SAMS also relate to enhanced atmospheric moisture content over the Caribbean and northern South America regions, mainly due to increased contributions of water vapor from oceanic regions (Caribbean and north Atlantic). Furthermore, our analysis of CMIP5 simulations and projections suggests that the pattern of water vapor transport expected by the end of the 21^st century in the region could resemble that observed in association with longer transition seasons between the American monsoons, as occurred more frequently during the last few decades. The latter would have key implications in atmospheric moisture transport processes and future water availability in the Mesoamerican sector under a global warming scenario.