PNNL

The Science

The tropical rain belt moves seasonally between the southern and northern hemispheres, providing water resources for ~40% of the world’s population. Previous studies found that global warming delayed this seasonal shift in tropical rainfall. By carefully examining multiple global warming simulations, scientists discovered that seasonal precipitation behaves differently over the land and ocean under global warming. They saw consistent precipitation delays over land, but uncertain changes over the ocean.

The Impact

The robust delay of tropical rainfall over land may be a detectable footprint of global warming, as observational records are more abundant and reliable over land. Changes in land temperature and rainfall have important implications for agriculture and water management. As a large fraction of the world’s population lives in the tropics, the delay of seasonal rainfall and reduced differences in seasonal temperatures over tropical land discovered in this study could have far-reaching social and economic impacts for this critical and populous area of the globe.

Summary

This study analyzed 37 climate models from the fifth phase of the Coupled Model Intercomparison Project and five large ensemble simulations to reveal contrasting seasonal transitions in the precipitation annual cycle between land and ocean, with a robust delay over land and uncertain changes over the ocean. Researchers primarily attributed the seasonal delay over land to the increased effective heat capacity of the atmosphere (C_A) as the atmosphere becomes moister under global warming. However, another mechanism competes against and counters the impact of increased C_A over the ocean. It manifests in a shift of tropical precipitation from over land to the ocean during peak rainy season. This leads to uncertain phase changes over the ocean between 25°S–25°N and primarily a phase advance over the ocean between 40°S-40°N. The shift in tropical precipitation from land to ocean can be linked to the enhanced annual range of sea surface temperature and the reduced land surface temperature annual range. The former is related to the wind-evaporation-sea surface temperature feedback. Disabling this feedback mechanism in a coupled climate model resulted in the suppression of the seasonal advance over the ocean under global warming, verifying its effects. The reduced land surface temperature amplitude can be rationalized as due to the increased C_A and surface cooling feedback under a warmer climate.

PNNL Contact

Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov 

Funding

All authors were supported by the U.S. Department of Energy’s Office of Science Biological and Environmental Research program’s Regional and Global Model Analysis program area.