Increasing atmospheric moisture could alter critical weather patterns over Africa, making it harder for precursors of many Atlantic hurricanes to form, according to a new study published this month.

The research team, led by scientists from the U.S. National Science Foundation (NSF) and the National Center for Atmospheric Research (NCAR), used an innovative model that allows high-resolution hurricane formation simulations. This enabled researchers to study the effects of increased regional moisture over Africa, which is the source of weather systems that later produce hurricanes over the Atlantic.

Previous research suggested that warmer ocean waters and a more humid atmosphere could cause more intense hurricanes with higher rainfall. However, how atmospheric moisture, predicted to increase in a warmer climate, might affect hurricane formation itself had not been studied in detail until now.

The researchers found that a wetter environment produces weaker and slower African easterly waves, or disturbances that are the primary precursors or "seeds" for hurricanes in the Atlantic. Adding moisture shifted the location of storms within the wave, making wave growth more difficult. Increased moisture also slowed the wave's movement, resulting in weaker and delayed hurricane seed formation by the time it reached the eastern Atlantic waters.

“Significant work over the last two decades has highlighted the role of deep moist convection in explaining the development of African easterly waves,” said NSF NCAR scientist and lead author Kelly Núñez Ocasio. “But the precise role of moisture has been somewhat elusive. With the development of new modeling capabilities, I was able to focus on the role of moisture in cyclogenesis arising from hurricane seeds.”

The study was funded by NSF NCAR and published in the Journal of Advances in Modeling Earth Systems. Núñez Ocasio conducted the research through the NSF NCAR Advanced Study Program, which allows graduates and post-doctoral researchers to focus on new areas of science.

New modeling methods
Hurricane and other tropical cyclone birth, known as cyclogenesis, is a complex process where small- and large-scale weather events occur simultaneously. This complexity makes studying and modeling tropical cyclone formation challenging. Most climate models provide only a rough picture of what happens with localized weather, making it difficult to learn about the role of individual ingredients, such as moisture, that mix to create cyclogenesis.

To solve this problem, the research team turned to the Model for Prediction Across Scales (MPAS). MPAS has the capability to model weather locally and globally. This capability enabled Núñez Ocasio and her colleagues to increase and simulate global moisture and then zoom in to see how that moisture affects localized weather events that lead to tropical cyclone formation.

The researchers began the experiment using MPAS to reproduce a moisture-driven African easterly wave that became Hurricane Helene in 2006. The team used this baseline to add or remove moisture and study what happened with those changes.

“When I increased the moisture, we saw more convection and storms, as expected; however, we found that the waves had difficulty pairing with the more intense and deeper convection,” said Núñez Ocasio. “With increased moisture, the energy source for tropical cyclone seeds moved farther north and away, reducing the kinetic energy available to the African easterly wave, leading to weak, energy-deprived tropical cyclone seeds.”

Studying the evolution of tropical cyclones after this initial stage was beyond the scope of this study. More research is needed to determine whether these weaker seeds will result in weaker tropical cyclones and hurricanes or if they will simply take more time to form.

The conditions leading to tropical cyclone formation are complex, but researchers hope these new modeling methods will lead to better predictions. For example, Núñez Ocasio is beginning to conduct simulations where she alters other atmospheric variables key to generating tropical cyclones.

“In addition to moisture, I am changing other variables in the model to more realistically reproduce a future climate scenario in collaboration with NSF NCAR project scientist Erin Dougherty,” she said. “For now, I see similarities with the results of this study even as I change those other significant parts.”

Source: University Corporation for Atmospheric Research