New research published this month finds that higher humidity can alter key weather patterns over Africa, making it harder for the precursors to many Atlantic hurricanes to form.
The research team, led by scientists from the U.S. National Science Foundation's National Center for Atmospheric Research (NSF NCAR), used an innovative model that allows for higher-resolution simulations of hurricane formation than ever before. This allowed researchers to study the effects of increased regional moisture over Africa, the birthplace of weather systems that later produce hurricanes over the Atlantic Ocean.
Previous research has suggested that warmer ocean waters and a more humid atmosphere can cause hurricanes to become more intense with greater amounts of rainfall. But how atmospheric moisture, which is predicted to increase in a warming climate, might affect hurricane formation itself has not been studied in detail until now.
The researchers found that a wetter environment produced weaker and slower-moving African easterly waves, or disturbances that are the primary precursors or “seeds” for Atlantic hurricanes. The addition of moisture shifted the location of thunderstorms within the wave, making it harder for the wave to grow. More moisture also slowed the wave’s movement, resulting in weaker and delayed hurricane seed formation by the time it reached eastern Atlantic waters.
“Significant work over the past two decades has emphasized 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 that emerges from the hurricane seed.”
The study is funded by NSF NCAR and published in the Journal for Advances in Earth System ModelingNúñez Ocasio continued the research through the NSF NCAR Advanced Study Program, which allows graduate and postdoctoral students to focus on emerging areas of science.
Next Generation Modeling
The formation of hurricanes and other tropical cyclones, known as cyclogenesis, is a complex process in which small-scale weather events and large-scale atmospheric events occur simultaneously. This complexity has made it difficult to study and model the formation of tropical cyclones. Most climate models provide only a granular picture of what is happening to local weather, making it difficult to learn about the role of individual ingredients, such as moisture, that combine to create cyclogenesis.
To address this, the research team turned to the Model for Prediction Across Scales (MPAS). MPAS has the ability to model weather both locally and globally. This capability allowed Núñez Ocasio and her colleagues to zoom out and simulate global moisture, and then zoom in to see how that would interact with local weather events that lead to the formation of tropical cyclones.
The researchers began the experiment by using MPAS to recreate a moisture-driven easterly wave in Africa that became Hurricane Helene in 2006. The team used that baseline to add or subtract moisture and study what happened to those changes.
“As I increased the humidity, we saw more convection and thunderstorms, which is to be expected; however, we found that the waves had difficulty coping with the more intense and deeper convection,” Núñez Ocasio said. “With the increased humidity, the energy source of tropical cyclone seeds moved north and farther away, reducing the kinetic energy available to the African easterly wave, leading to weak, low-energy tropical cyclone seeds.”
Studying the evolution of tropical cyclones after this initial phase was beyond the scope of this study. More research is needed to determine whether these weaker seeds lead to weaker tropical cyclones and hurricanes or whether they simply take longer to form.
The conditions that lead to tropical cyclones are complex, but researchers hope these newer modeling techniques will lead to better predictions. For example, Núñez Ocasio is starting to run simulations that alter other atmospheric variables that are 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 Erin Dougherty, NSF NCAR project scientist,” she said. “So far, I see similarities to the results of this study, even when I change those other key components.”
