What caused Hurricane Hilary
Californians unaccustomed to facing the powerful winds and heavy rains of tropical storms — the last time was 84 years ago — may be wondering today why Hurricane Hilary became the rare tropical cyclone to make landfall in the Golden State.
To understand that, it’s helpful to know why the event is so uncommon in the first place. According to the Los Angeles Times, “California has been protected from the wrath of hurricanes by three robust natural defenders:” A cold current along the Pacific Coast, which reduces the strength of storms reliant on heat; a prevailing east-to-west wind pattern that pushes storms out to sea; and a downward flow of air over the state that suppresses storms.
But this year, those conditions have shifted.
Warm water
The cold current has been mitigated by record-setting air and water temperatures around the globe this summer, due to climate change and El Niño, a band of warm ocean water that develops in the Pacific Ocean. Southern California has recently experienced severe heat waves and Baja California, Mexico, where Hilary made landfall, has seen higher-than-average ocean temperatures this summer.
This is part of a long-term trend, driven by rising global average temperatures. Kim Wood, a Mississippi State University atmospheric scientist who studies tropical cyclones in the Eastern Pacific, told Inside Climate News that Southern California’s waters have been warming a half-degree Fahrenheit every 10 years for the last six decades.
Studies previously have found that hurricanes will land farther north in North America as a result of climate change, and forecasters have predicted a strong hurricane season this year due to warm ocean temperatures.
These conditions are also more common in El Niño years, especially in combination with global warming. In 1997, Hurricane Linda in the Pacific didn't make landfall in California, but it caused heavy rain in the state. In 2015, another year with El Niño, Hurricane Patricia hit southwestern Mexico.
Unusual wind patterns
Shifts in wind patterns have also played a role. Typically, tropical storms are shifted away from California by winds blowing east to west. But right now, an unusual high-pressure ridge in the central U.S. is blocking that pattern. Paul Miller, a Louisiana State University oceanographer, told Vox that a trough in a jet stream over California — essentially a low pressure zone to the west of the state — is also “grabbing Hilary and pulling it northward,” leading it to land farther north than usual.
With the convergence of warm water, a high-pressure zone to the east and a jet stream to the west, “You have to have the exact right atmospheric setup [for a hurricane to hit California,]” Dan DePodwin, AccuWeather’s director of forecasting operations, told the Wall Street Journal. “And that’s what we have.”
Storms are getting stronger
Scientists also say that tropical storms are growing larger and causing heavier rainfall because warmer air holds more moisture.
“In terms of temperature and precipitation extremes, they’re definitely getting more extreme,” Mathew Barlow, a professor of environmental, earth and atmospheric sciences at UMass-Lowell, told Yahoo News last year. “We see that in the observations, that’s predicted by theory and it’s confirmed by models.”
