Hot weather events shift from dry to wet over polar ice sheets
This paradigm shift of compound extremes may contribute to accelerating polar ice sheet melting, researchers report
image: Regarding the ex-polar lands, compound hot-dry extremes prevail due to active land–atmosphere interactions. Specifically, a mid-tropospheric ridge initiates descending motions, creating clear-sky conditions downstream. The resultant excessive short-wave heating of the land surface depletes soil moisture through enhanced evaporation. A dryer land heats more quickly, and the associated excessive sensible heating warms the air above and further strengthens the existing mid-tropospheric ridge, establishing a positive feedback during an extreme hot-dry event. While over polar ice sheet, compound warm-wet extremes are mostly “externally” driven by intruding lower latitude moist air through long-range advective transport. The brought-in moisture creates clouds and precipitation, and the clouds, together with moisture, lead to much enhanced downward thermal radiative energy fluxes that cause substantial warming at the surface.
Credit: Ran Yang
While compound extreme events across the bulk of the planet are characterized by hot and dry conditions, those over polar ice sheets are predominantly associated with hot and wet conditions, according to a new study by an international research team. Analyzing 42 years of global weather, the researchers found that extreme warm events, or heatwaves, and extreme precipitation events, such as heavy rainfall, overlapped far more in Antarctica than in lower latitudes. This co-occurrence may also accelerate polar ice sheet melting, the team said.
“Our findings indicate that the mechanisms responsible for extreme warm events and extreme precipitations events in polar regions are interconnected,” said Ran Yang, first author on the paper and doctoral candidate in the School of Atmospheric Sciences at Sun Yat-sen University in China. “We analyzed the global pattern of the concurrent likelihood of extreme warm events and extreme precipitation events, revealing a paradigm shift of compound extremes: predominantly hot-dry conditions across most ex-polar lands — or the lands below Greenland and above Antarctica — transition to warm-wet conditions over the polar ice sheets.”
The team published their results in Ocean-Land-Atmosphere Research.
The researchers examined warm-wet synchrony — quantified by the number of days that were both warm and wet in comparison to the total number of days that were just wet — around the world from 1979 to 2021. For this study, the researchers defined extreme warm events as days with temperatures higher than 90% of recorded temperatures for that day since 1979. They defined extreme precipitation events as days exceeding 90% of recorded precipitation for that day since 1979.
“Globally, warm-wet synchrony at the 90th percentile is approximately 20%, with most regions exhibiting values less than 25%,” said Xiaoming Hu, co-corresponding author on the paper and professor in the School of Atmospheric Sciences at Sun Yat-sen University. “The region that exhibits the most striking warm-wet synchrony is Antarctica, where the average synchrony is greater than 50% across the entire plateau.”
The team also found that high synchrony is mainly found across inland regions, which they said indicates that the likelihood of compound extremes increases with surface elevation.
“This warm-wet synchrony is likely driven by intrusions of warm-moist air from lower latitudes, and under global warming, its net effect could destabilize the polar ice sheets and accelerate global sea level rise,” said Qinghua Yang, co-corresponding author on the paper and professor in the School of Atmospheric Sciences at Sun Yat-sen University.
The reason for the diverse nature of compound extremes, according to the researchers, is that they are climate-zone dependent. Active land-atmosphere interactions in ex-polar lands can create positive feedback loops where a mid-tropospheric ridge initiates descending motions, the resulting clear-sky conditions increase incident solar radiation, which dries the land, warms the air and strengthens the ridge. This self-reinforcing ridge further insulates the region from precipitation-producing synoptic disturbances.
At the polar regions, however, such efficient local feedback processes do not exist, Hu said, so extreme warm events are mostly driven by “warm-moist air intrusions.” These intrusions are made by traveling low-pressure systems originating in the mid-latitude zones.
“As climate models project an increase in extreme events worldwide, understanding the dynamic origins and multifaceted implications of these events is becoming increasingly important,” Ran Yang said. “Under global warming, intruding air masses are likely to become warmer and moister, leading to more intense compound warm-wet extremes over the polar ice sheets comprising heavy precipitation — in the form of both rain and snow — and surface melt. This increase in intensity could destabilize buttressing coastal ice shelves, leading to dynamic ice sheet mass loss that could accelerate global sea level rise.”
Other authors on the paper are Song Yang and Lianlian Xu, School of Atmospheric Sciences and the Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies at Sun Yat-sen University; Ming Cai, Department of Earth, Ocean and Atmospheric Sciences, Florida State University, United States; Yi Deng, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, United States; and Kyle R. Clem, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, New Zealand. Hu, Qinghua Yang, Song Yang, and Xu are also affiliated with the Southern Marine Science and Engineering Guangdong Laboratory.
The National Natural Science Foundation of China, the U.S. National Science Foundation, the U.S. National Oceanic Atmospheric Administration, the Royal Society of New Zealand Marsden Fund, and the Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies supported this work.