Emergence of seasonal delay of tropical rainfall during 1979–2019
Study key findings & significance
- Humidity has declined in summers in the Southwest since the mid-20th century due to declines in soil moisture in winter and spring associated with rising temperatures.
- The declines have accelerated since 2000, a period of persistent drought in the region.
- The decrease in humidity is contrary to the global trend of warm air holding more moisture.
- In most of the world, evaporation from the ocean is responsible for the majority of the humidity, but in the Southwest, soil moisture is the dominant source.
- As winter and spring soils have dried out, there’s less moisture in the ground to evaporate.
- The findings also link climate change to worsening wildfires in the region.
There’s this base expectation of greater humidity in some average sense. That makes it all the more surprising that the Southwest pops out as the region where you see declines in summertime humidity.
It’s clear that the declines of summer soil moisture are due to declines in soil moisture in winter and spring.
Karen McKinnon, lead author and climate scientist at the University of California, Los Angeles
Tropical rainfall exhibits a prominent annual cycle, with characteristic amplitude and phase representing the range between wet and dry seasons and their onset timing, respectively. Previous studies note enhanced amplitude over ocean and delayed phase over land in model projections of global warming, underpinned by first-order physical principles. However, it is unclear whether these changes have emerged in observations. Here we use gridded precipitation datasets to report a seasonal delay of 4.1 ± 1.1 and 4.2 ± 0.9 days (P < 0.05) during 1979–2019 over the northern tropical land and Sahel, respectively. Most of the delay is driven by external forcings, dominated by greenhouse gases (GHG) and anthropogenic aerosols (AER). Increasing GHG and decreasing AER in the recent decades delay rainfall by producing a moister atmosphere, thus increasing its lag in response to seasonal solar forcing. As GHG increase and AER decrease, these seasonal delays are projected to further amplify in the future.