Three main factors converge to intensify hurricanes. The first is that as the world as a whole warms, so do the oceans. Water evaporating from the surface rises, releasing heat that fuels the developing hurricane. The warmer a portion of ocean water is, the more energy a cyclone has to explode. If a hurricane like Lee forms off the coast of Africa, it will have plenty of Atlantic Ocean to feed on as it moves toward the east coast of the United States. As we approach this year’s hurricane season, tropical Atlantic temperatures remain very high.
The second factor is humidity. As the atmosphere warms, it can hold more water vapor, which is why some parts of the world are becoming wetter. Hurricanes love that, as drier air can cause cooling and downdrafts that counteract the updrafts that drive the storm. “As long as it remains wet, the storm can strengthen or maintain its intensity,” says Balaguru. “However, once the core enters a dry environment or becomes less humid, the storm will begin to weaken.”
And lastly, hurricanes hate wind shear, or winds of different speeds and directions at different altitudes. (Think of it like layers of a cake, just made of air.) Instead, cyclones like a stable atmosphere, allowing their winds to swirl and intensify. Wind shear can also inject drier air from outside the storm into the hurricane’s core, weakening it further. As the world warms, wind shear is decreasing along the East Coast of the United States and East and South Asia. providing ideal atmospheric conditions for cyclones to form and intensify. “Under climate change, the upper troposphere is expected to warm at a faster rate than the surface,” says Balaguru. “This can improve 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 form and intensify hurricanes this summer. Although La Niña is in a different ocean, it tends to suppress winds over the Atlantic, meaning there is less of the wind shear that hurricanes hate. Hence the University of Arizona’s prediction of an extremely active hurricane season, combined with very high sea surface temperatures in the Atlantic to fuel the storms. By contrast, last year’s El Niño created windy conditions in the Atlantic that discouraged cyclone formation.
Even then, Hurricane Lee became a monster storm last September. A week before that, Hurricane Idalia rapidly intensified just before hitting Florida. That kind of intensification near the coast is extraordinarily dangerous. “When the storm is very close to the coast, let’s say it’s a day or two away, if it suddenly intensifies quickly, it can take you by surprise in terms of preparations,” Balaguru says. A city may have planned its evacuations expecting 100 mph winds and suddenly they are more like 130 mph.
Unfortunately, Balaguru’s new study finds that weather conditions, particularly near the coast, are becoming more conducive to storm intensification. It’s up to teams like Zeng’s at the University of Arizona to further refine their forecasts to manage that growing risk to coastal populations. “For scientists, seasonal forecasts are a test of the reality of our understanding,” Zeng says. “We’ve done quite well in recent years and that will give us more confidence.”