Diminishing seasonality of subtropical water availability in a warmer world dominated by soil moisture-atmosphere feedbacks.

Global warming is expected to cause wet seasons to get wetter and dry seasons to get drier, which would have broad social and ecological implications. However, the extent to which this seasonal paradigm holds over land remains unclear. Here we examine seasonal changes in surface water availability (precipitation minus evaporation, P-E) from CMIP5 and CMIP6 projections. While the P-E seasonal cycle does broadly intensify over much of the land surface, ~20% of land area experiences a diminished seasonal cycle, mostly over subtropical regions and the Amazon. Using land-atmosphere coupling experiments, we demonstrate that 63% of the seasonality reduction is driven by seasonally varying soil moisture (SM) feedbacks on P-E. Declining SM reduces evapotranspiration and modulates circulation to enhance moisture convergence and increase P-E in the dry season but not in the wet season. Our results underscore the importance of SM-atmosphere feedbacks for seasonal water availability changes in a warmer climate.