Tropical turbulence is driven by sunlight warming the ocean surface on calm days

CORVALLIS, Oregon — Hurricane Ida barreled through Louisiana with 150 mph winds after crossing over the shoreline, becoming tied for fifth-strongest storm ever to strike the mainland. Scientists continue to investigate the ways that tropical storms gain steam and are impacted by climate change. According to a study published earlier this year, turbulence in the atmosphere increases when sunlight and low-pressure winds increase the temperature of ocean surfaces in the afternoon.

Researchers at Oregon State University say this information could be used to model climate change and to help forecast the weather.

“The ocean warms in the afternoon by just a degree or two, but it is an effect that has largely been ignored. We would like to know more accurately how often this is occurring and what role it may play in global weather patterns,” explains study lead author Simon de Szoeke, a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences, in a statement.

Thunderstorms are formed when the land warms up in the afternoon and forms convection and turbulence in the atmosphere. When the ocean surface heats up, water vapor is pulled into the atmosphere by the convection which forms clouds. This warming does not occur as quickly and tends to increase in the presence of low-pressure winds.

De Szoeke took a trip to the Indian Ocean to study ocean warming in a boat that included Doppler lidar, which is similar to radar. The device measures the velocity of air by a laser pulse and was used to measure turbulence strength and height after the first warming. In previous studies, the only way to collect these measurements was by aircraft.

“With lidar, we have the ability to profile the turbulence 24 hours a day, which allowed us to capture how these small shifts in temperature lead to air turbulence. No one has done this kind of measurement over the ocean before,” says de Szoeke.

The study lasted for two months and the lidar was used to collect data at all times. Over the course of four days, the temperature of the ocean surface increased steadily under low wind speeds. This allowed for perfect conditions for the researchers to gather a turbulence profile. “It took a ‘perfect storm’ of conditions, including round-the-clock sampling by the lidar and a long ocean deployment, to capture these unprecedented observations,” notes de Szoeke.

In the afternoon the surface temperature increases by at least one degree Celsius. In the tropical oceans of the world, warming takes up about 5% of each day. Roughly 2% of the Earth is composed of these oceans, about the same size as the U.S.

Monsoons and tropical storms cause low winds and rising air temperatures over these areas. “To determine the role these changing temperatures play in weather conditions in the tropics, weather models need to include the effects of surface warming,” says de Szoeke.

“There are a lot of subtle effects that people are trying to get right in climate modeling. This research gives us a more precise understanding of what happens when winds are low,” added de Szoeke.

This study is published in Geophysical Research Letters.