Multiscale Interactions in Determining the Hydrological Extremes in the American Monsoon Regions

Both the North and South American monsoon regions are prone to hydrological extremes. Often, these extreme events cannot be fully explained by the well-known oceanic forcings, such as sea surface temperature anomalies (SSTA) in the tropical Pacific and Atlantic. In searching for additional sources of rainfall variability and predictability, we present observational evidence to suggest that, on the large scales, the equatorial contraction of the Atlantic Inter-tropical convergence zone and increasing westward displacement of the North Atlantic Subtropical High reduce atmospheric moisture transport to the American monsoon regions, further enhancing the earlier demise of the North American Monsoon, and the delaying onset of the South American monsoon, associated with the warm SSTA in the north tropical Atlantic. At regional scale, land surface/vegetation memory can prolong rainfall anomalies, and increase fire and dust aerosols. Aerosols can further exacerbate the dry or wet rainfall anomalies by invigorating clouds and rainfall under wet conditions, while decreasing rainfall under dry conditions. The resultant change in vertical latent heating profiles re-enforces the original anomalies of large-scale moisture transport. These interactions between the large and small-scales processes can work in concert to amplify rainfall variability, and so contribute to hydrological extremes in the American monsoon regions.