The destructive duo of tropical precipitation and atmospheric winds whip up hurricanes in the Gulf of Mexico and Caribbean Sea every year. The dynamics of weather in the tropics, however, affect the entire world and are of great interest to COAS assistant professor Eric Maloney and other atmospheric scientists.
Understanding the coupling of atmospheric winds and precipitation processes not only allows better predictions of extreme weather, but also could help prepare for the consequences of climate change.
There’s still a lot to learn. Scientists know that along the tropics there are oscillations between westerly and easterly winds on time scales of about 40 to 50 days (called the Madden-Julian Oscillation, discovered in the 1970s).
“If you live in the tropics, you might have a period of westerly winds for about two to three weeks, followed by easterly winds for about the same amount of time,” Maloney said. “With the westerly winds, you tend to get a four-fold enhancement of tropical cyclone formation, compared to winds from the east. But we still don’t have a basic understanding as to why such oscillations occur.”
One of Maloney’s current studies could eventually help answer those questions. He uses satellite data to look at African atmospheric waves that form in the Sahel region of Western Africa, just south of the Sahara Desert, to understand how weather thousands of miles away is affected.
These atmospheric wave disturbances, called African easterly waves (AEWs), are periodic changes in atmospheric variables such as surface pressure, temperature or wind velocity. “A complex wind and temperature structure occurs in the Sahel region during northern hemisphere summer,” Maloney said. The contrasting hot, dry weather of the Sahara and slightly cooler, moist regions further south generate a strong atmospheric jet stream that produces AEWs.
These vigorous disturbances are important precursors to the formation of tropical cyclones. The AEWs “ripple” off Africa, moving westward out over the Atlantic, where they can eventually turn into hurricanes and other tropical storms. The AEWs are at maximum strength from June through October and in 1995 were found to have formed 17 of the 19 Atlantic tropical cyclones that year.
“What we’re trying to understand,” Maloney said, “is what regulates the formation of easterly waves on bi-weekly, monthly and longer time scales to determine if the formation of the waves might be predictable in advance.”
Maloney also studies the processes that regulate tropical precipitation in other parts of the world. Satellite data provide a crucial tool for the study of oceanic precipitation. Ocean winds from NASA’s Quick Scatterometer (QuikSCAT) satellite can be combined with precipitation measurements from satellites to determine the importance of variations in wind-driven surface evaporation in producing tropical precipitation.
In the Pacific Ocean, Maloney and many other researchers also rely on data from an array of 70 moored buoys sitting on the Equator. The buoys send real-time data about ocean and atmosphere conditions to the international Argos satellite system, which relays the information to the Pacific Marine Environmental Lab in Seattle, where it is distributed through the Internet. This Tropical Atmosphere Ocean (TAO) project took 10 years to develop and buoys were deployed in 1994 to detect, understand and predict the Pacific Ocean oscillations—El Niño and La Niña. The United States National Oceanic and Atmospheric Administration cooperates with France, Japan, Korea and Taiwan to monitor and maintain this global climate monitoring system.
Another motivation to study tropical precipitation is that it helps regulate the global response to climate change. For example, in El Niño conditions, easterly trade winds weaken and allow warmer waters of the western Pacific to migrate eastward. Precipitation is then enhanced in the central and eastern Pacific Ocean. This enhanced precipitation can influence mid-latitude weather, including that in Oregon, by changing the global atmospheric circulations patterns.
“In a climate change scenario, you might expect related shifts in tropical precipitation patterns, which would be expected to cause variations in the large-scale flow patterns and weather where we live,” Maloney said. “We already have evidence that changes in Indian Ocean precipitation are altering weather patterns in the North Atlantic.
“One of the goals in understanding the processes that help regulate tropical precipitation is to improve our climate models. If our simulations are more accurate, then we can have more confidence in how tropical precipitation will vary with climate change.”
