Recent Changes in North and South America Monsoon Rainfall

The North American Monsoon (NAM) and South American Monsoon (SAM) are the two dominant seasonal precipitation systems of the Western Hemisphere, yet a systematic comparison of their recent changes over the satellite era remains limited. This study investigates recent changes in NAM and SAM rainfall over the period 2001–2024 using NASA's Integrated Multi-satellitE Retrievals for GPM (IMERG) daily precipitation product, interpolated to a 1x1deg grid. Monsoon onset and demise dates are defined objectively from the minimum and maximum of the 31-day smoothed cumulative daily rainfall anomaly, with the SAM calculated on a time-converted annual cycle, and monsoon length is defined as the difference between the demise and onset dates. The results show that the recent trends of the two systems are opposite, with annual mean precipitation increasing for the NAM (0.22 mm day-1 per decade) and decreasing for the SAM (-0.19 mm day-1 per decade). The seasonal evolution is likewise distinct: the NAM onset shows an advance trend (-2.4 days per decade) while the SAM onset shows a slight delay (0.29 days per decade), and although both demise dates are delayed, the NAM extension (5.43 days per decade) far exceeds that of the SAM (1.13 days per decade). Consequently, the monsoon length shows an obvious increasing trend for the NAM (7.83 days per decade) and only a slight increasing trend for the SAM (0.84 days per decade). These contrasting changes are likely attributable to differential responses to the recent northward shift of the tropical Atlantic Intertropical Convergence Zone and to asymmetric teleconnections from Atlantic warming.