Tropical cyclones pack a punch, and their aftereffects can linger for weeks. As these tempests move over the ocean, they suppress cloud coverage and rainfall in their wakes, new research has revealed. Such changes, which might become more pronounced with climate change, could affect navigation and fishing, researchers have suggested.
Dredging from the Deep
The most devastating weather systems in the world are tropical cyclones, said Zhanhong Ma, a tropical cyclone researcher at the National University of Defense Technology in Nanjing, China. (“Hurricanes” and “typhoons” are regionally specific names for storms of similar intensity.) The strong winds of tropical cyclones routinely cause devastation on land, but they also churn up ocean water, dredging up deeper, colder water from depths of hundreds of meters. “Cold water is brought to the surface,” said Ma.
This movement, which can play a key role in nourishing ecosystems by transporting nutrient-laden waters, results in a “cold anomaly” in the wake of the tropical cyclone. Anywhere from 1°C to 10°C colder than the surrounding seawater, this anomaly is typically about 500 kilometers wide and 1,000–1,500 kilometers long. “It looks like a cold tail behind the tropical cyclone,” said Ma.
Using multiple publicly available databases, Ma and his colleagues measured the tracks of 1,317 tropical cyclones that occurred in the Northern Hemisphere from 1998 to 2016. They also mined sea surface temperatures before and after the passage of each storm and tabulated cloud fractions from NASA’s Aura satellite and rainfall records from NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite.
Less Rain, Fewer Clouds
The researchers found suppressed cloud coverage and rainfall in the wakes of tropical cyclones. On average, cloud coverage fell by about 7%, and rainfall decreased by about 17%, Ma and his colleagues noted. These effects persisted across all four ocean basins that the team studied: the western North Pacific, the eastern North Pacific, the North Atlantic, and the northern Indian. “The cold wakes left behind by tropical cyclones have a significant effect on the local atmosphere,” said Ma.
These results make sense from a physics perspective, said Ma. A cooler ocean surface means that there’s a smaller difference in temperature between the air and the water, which leads to less heat and moisture being transported from the sea surface into the atmosphere. As a result, the air is less saturated with moisture and is also more stable; both of these attributes contribute to wakes experiencing less rainfall and fewer clouds.
Ma and his collaborators also found that cloud coverage and rainfall were more strongly suppressed in the wakes of more intense or more slowly moving tropical cyclones. Given that the prevalence of intense storms has increased as the planet has warmed, this suppression of clouds and rainfall might continue to intensify in the future, the scientists suggested.
That’s an intriguing idea but one that needs more support, said Karthik Balaguru, an oceanographer at the Pacific Northwest National Laboratory in Seattle, Wash., not involved in the research. “This is too simple a study to make a broad-sweeping statement like that.”
Try a Simulation
There’s also the issue of Clausius-Clapeyron scaling, said Balaguru. The Clausis-Clapeyron equation, which dictates that warmer air can hold more moisture than cooler air—roughly 7% more per degree Celsius—means that storms in the future will tend to dump more rain. “You’re going to get more rain because there’s just more moisture in the atmosphere,” said Balaguru.
Determining which process will ultimately dominate—Clausius-Clapeyron-driven increases in rainfall or wake-driven decreases in rainfall—isn’t clear, Balaguru said. Ideally, you’d want to run a global climate simulation that models tropical cyclones in current and future climates. “I think that’s the only way to know the net effect,” he said.
Regardless of what the future holds, changes in rainfall and cloud fraction are likely to affect a wide variety of marine activities such as navigation, fishing, and atmospheric and oceanic observations, Ma and his colleagues suggested. These results were published last month in Geophysical Research Letters.
In the future, Ma and his collaborators plan to investigate how these variations in rainfall and cloud fraction—which will result in increased fluxes of solar radiation—might affect local climate.
—Katherine Kornei (@KatherineKornei), Science Writer