‘To expect that rainfall patterns would stay the same is very naïve’
By Summit Voice
SUMMIT COUNTY — A growing temperature disparity between the southern and northern hemispheres could have significant long-term effects on tropical rainfall patterns, potentially shifting monsoons in some areas, or leading to drought in other regions.
Climate scientists aren’t exactly sure how that will play out, but they are starting to measure the temperature differences between the two hemispheres to create an index that might help forecast some of the changes.
“Tropical rainfall likes the warmer hemisphere,” University of California Berkeley geographer John Chiang, who is also part of the Berkeley Atmospheric Sciences Center. “As a result, tropical rainfall cares a lot about the temperature difference between the two hemispheres.”
The greater amount of land mass in the north warms up faster than the ocean-dominated south, Chiang said
“A key finding is a tendency to shift tropical rainfall northward, which could mean increases in monsoon weather systems in Asia or shifts of the wet season from south to north in Africa and South America,” said UC Berkeley graduate student Andrew R. Friedman, who led the analysis.
Chiang and Friedman, along with University of Washington colleagues Dargan M. W. Frierson and graduate student Yen-Ting Hwang, reported their findings in a paper now accepted by the Journal of Climate, a publication of the American Meteorological Society.
Generally, rainfall patterns fall into bands at specific latitudes, such as the Intertropical Convergence Zone. The researchers said that a warmer northern hemisphere causes atmospheric overturning to weaken in the north and strengthen in the south, shifting rain bands northward.
The regions most affected by this shift are likely to be on the bands’ north and south edges, Frierson said.
“It really is these borderline regions that will be most affected, which, not coincidentally, are some of the most vulnerable places: areas like the Sahel where rainfall is variable from year to year and the people tend to be dependent on subsistence agriculture,” said Frierson, associate professor of atmospheric sciences. “We are making major climate changes to the planet and to expect that rainfall patterns would stay the same is very naïve.”
The researchers were able to look back about 100 years to see how their model holds up. When they compared yearly average temperature difference between the Northern and Southern hemispheres with rainfall records throughout the 20th century, they noticed that abrupt changes coincided with rainfall disruptions in the equatorial tropics.
The largest was a drop of about one-quarter degree Celsius (about one-half degree Fahrenheit) in the temperature difference in the late 1960s, which coincided with a 30-year drought in the African Sahel that caused famines and increased desertification across North Africa, as well as decreases in the monsoons in East Asia and India.
“If what we see in the last century is true, even small changes in the temperature difference between the Northern and Southern hemispheres could cause measureable changes in tropical rainfall,” Chiang said.
This bodes ill for the future, he said. The team found that most computer models simulating past and future climate predict a steadily rising interhemispheric temperature difference through the end of the century. Even if humans begin to lower their greenhouse gas emissions, the models predict about a 1 degree Celsius (2° F) increase in this difference by 2099.
He and his colleagues argue that climate scientists should not only focus on the rising global mean temperature, but also the regional patterns of global warming. As their study shows, the interhemispheric temperature difference has an apparent impact on atmospheric circulation and rainfall in the tropics.
“Global mean temperature is great for detecting climate change, but it is not terribly useful if you want to know what is happening to rainfall over California, for example,” Chiang said. “We think this simple index, interhemispheric temperature, is very relevant on a hemispheric and perhaps regional level. It provides a different perspective on climate change and also highlights the effect of aerosols on weather patterns.”
The research was supported by the U.S. Department of Energy and the National Science Foundation.