Is There a Winner in the Atmospheric Tug of War?
Predicting environmental conditions several weeks to months in advance has significant socioeconomic value. However, a gap exists between current short-term weather forecasts and subseasonal forecasts made two weeks to three months in advance, respectively. Further complicating the latter challenge is the potential for subseasonal precipitation variability to change in a warming climate. Scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory led a study to explore the mechanisms for future changes in subseasonal precipitation variability in North America during winter. They found that in a warmer climate, subseasonal precipitation variability consistently increased over most of North America—with an intensified swing between wet and dry extremes—but declined with weakened extremes over Mexico. The explanation for these contrasting changes was a tug of war between the dominant effects of increasing background moisture in the United States and weakening of subseasonal circulation variability in Mexico.
Precipitation variability manifests in a swing between extreme floods and droughts, posing significant risks to life, property, and the environment, and challenging water management. Remarkably, compared to mean and extreme precipitation changes, few studies have explored the changes in precipitation variability with warming. This study investigated the response of subseasonal precipitation variability to thermodynamic and dynamic changes associated with changes in atmospheric moisture and circulation, respectively, in a warming world. It provides a mechanistic understanding of competing processes that control the changes in subseasonal precipitation variability, with implications for the swings between wet and dry extremes.
In this study, researchers explored the response of subseasonal precipitation variability to a warming climate in North America during winter. They focused on the relative contributions of thermodynamic and dynamic effects, analyzing Representative Concentration Pathway 8.5 experiments and historical simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) archive. The team found a consistent increase in subseasonal precipitation variability over the middle to high latitudes with an intensified swing between wet and dry extremes. However, they found a decline in subseasonal precipitation variability in Mexico with weakened extremes. Enhanced variability in surface-specific humidity mainly contributed to the increased subseasonal precipitation variability in the United States. The reduced variability in vertical motion of air mainly contributed to the weakened subseasonal precipitation variability in Mexico.
Scientists also explored the mechanisms that lead to changes in subseasonal precipitation variability, with the help of a moisture budget decomposition framework. Results indicated that the thermodynamic changes from increasing background moisture intensified the subseasonal precipitation variability. In contrast, the dynamic changes from the weakening of subseasonal rainfall-related circulation variability had the opposite effect. The thermodynamic and dynamic changes produced a widespread tug of war not only in the tropics, but over the extratropics. Because the dynamic changes were much stronger over the subtropics than the midlatitudes, the thermodynamic changes won the tug of war and dominated the enhanced variability in the United States. The dynamic component dominated the weakened variability in Mexico.