Three main factors work together to make hurricanes stronger. The first is that as the world warms overall, the oceans are also warming. Water that evaporates at the surface rises, releasing heat that fuels the developing hurricane. The warmer a patch of seawater is, the more energy a cyclone has to use. When a hurricane like Lee forms off the coast of Africa, it can feed on a large amount of Atlantic Ocean as it makes its way to the East Coast of the United States. As we approach this year’s hurricane season, temperatures remain very high in the tropical Atlantic.
The second factor is humidity. As the atmosphere warms, it can hold more water vapor, making some parts of the world wetter. Hurricanes love this because drier air can cause cooling and downdrafts that counteract the updrafts that drive the storm. “As long as it remains humid, the storm may intensify or maintain its intensity,” Balaguru said. “However, once the core moves into a dry environment or becomes less humid, the storm will weaken.”
Finally, hurricanes hate wind shear, which are winds of different speeds and directions at different altitudes. (Think of it like layers of a cake made up only of air.) Instead, cyclones prefer a stable atmosphere that allows their winds to swirl and intensify. Wind shear can also blow drier air from outside the storm into the hurricane’s core, further weakening it. As the world warms, wind shear is decreasing along the East Coast of the United States and in East and South Asia, providing ideal atmospheric conditions for hurricane formation and intensification. “With climate change, the upper troposphere is expected to warm faster than the surface,” says Balaguru. “This can increase the stability of the atmosphere and also weaken the circulation in the tropics.”
In the short term, La Niña conditions in the Pacific could help hurricanes form and strengthen this summer. Even though La Niña is in a different ocean, it tends to suppress the winds over the Atlantic, meaning there is less wind shear, which hurricanes hate. As a result, the University of Arizona is predicting an extremely active hurricane season, combined with very high sea surface temperatures in the Atlantic that are fueling the storms. In contrast, last year’s El Niño created wind conditions in the Atlantic that prevented hurricanes from forming.
Even then, Hurricane Lee developed into a monster storm last September. A week earlier, Hurricane Idalia rapidly intensified just before hitting Florida. Such intensification near the coast is extremely dangerous. “If the storm is very close to the coast – say a day or two away – and then suddenly it intensifies quickly, it can take preparations by surprise,” Balaguru said. A city might have planned its evacuations with winds of 100 mph, and suddenly it’s closer to 130 mph.
Unfortunately, Balaguru’s new study finds that climatic conditions, particularly near the coast, are conducive to the intensification of storms. It’s up to teams like Zeng’s at the University of Arizona to further refine their forecasts to address this growing risk to coastal populations. “For scientists, seasonal forecasting is a reality check on our understanding,” Zeng says. “We have done pretty well over the last few years and this will give us more confidence.”