Hurricane Patricia, which barreled across Mexico October 20-24, was the most intense hurricane ever formed in the western hemisphere, exceeding 200 miles per hour in sustained winds. This hurricane occurred in the eastern North Pacific Ocean (ENP).
Compared to North Atlantic hurricanes, tropical cyclones stemming from the ENP basin have received considerably less attention. However, an average of four more tropical storms per year are produced in the eastern North Pacific than better-known the North Atlantic basin.
Considerably fewer hurricanes originating in the Pacific achieve landfall compared to the Atlantic. Hurricane Patricia, though, inundated the Mexican Pacific coast. The previous record-holder for the strongest eastern Pacific hurricane was Hurricane Linda in 1997, with sustained winds of 185 miles per hour, and before that Hurricane John in 1994.
All three of these events occurred during El Niño years. Is there a causal linkage between ENP hurricanes and El Niño conditions?
El Niño fingerprints
Before answering this question, let’s review what El Niño is.
El Niño is part of a coupled interaction between the atmosphere and ocean in the Pacific Ocean, more broadly referred to as ENSO (El Niño - Southern Oscillation). El Niño refers to multi-year fluctuations in the ocean water temperatures at the surface, and Southern Oscillation describes the linked changes to atmospheric conditions.
During El Niño years, the waters off the tropical Pacific coast of the Americas are warmer than normal and there is a concurrent weakening of the equatorial trade winds. This more inactive atmosphere and the greater source of oceanic energy from the warmer waters allow for stronger tropical cyclones to form, which precisely what occurred to produce record-breaking Hurricane Patricia.
Hurricane Patricia grew dramatically from a tropical storm to a Category 5 hurricane (the highest hurricane strength in the Saffir-Simpson hurricane scale) in less than 36 hours. But the same dynamics are not at work in the Atlantic in El Niño years. Because the warmer ocean waters are not available to enhance these storms, El Niño events tend to provide a less conducive environment for the development of tropical cyclones in the North Atlantic basin.
In research published this year, we showed that El Niño years at the eastern North Pacific basin tend to be more active – generating more storms with stronger wind speeds and longer durations – than those that are produced during La Niña years (the opposite phase, in which sea surface temperatures are cooler than normal).
Our work concluded that the potential destructive power of eastern North Pacific storms is significantly greater during El Niño years.
Proxy for climate changes?
Although Hurricane Patricia had weakened from its peak intensity prior to landfall, it still encountered the coast of southwestern Mexico near Cuixmala with Category 5 strength, maintaining sustained winds of 165 mph. Reports from Mexico indicate the aftermath of Hurricane Patricia is fortunately not as catastrophic as expected.
Although the storm had quickly weakened upon reaching mountainous terrain, we should be mindful that Patricia’s maximum sustained wind speed of at least 200 miles per hour had far surpassed the minimum requirement (sustained winds of 157 miles per hour) for a Category 5 hurricane.
This leads to a second question, what will be the impact of a warming climate on the frequency and intensity of ENP hurricanes?
Although there are other factors affecting hurricane frequency and intensity, El Niño years, with their changes in atmospheric flow and sea surface temperature, can be used as analogues for a future warmer climate.
Projected environmental warming thus suggests that the strongest hurricanes could become even stronger, generating more superstorms such as Hurricane Patricia. Evidence in the western North Pacific, such as the 2013 Super-Typhoon Haiyan, suggests changes to hurricane intensity are not restricted to the ENP basin.
The current metrics for hurricane strength may need to be revisited. Hurricane Patricia, with sustained winds of 200 miles per hour, greatly exceeded the threshold value of 157 miles per hour for Category 5 storms. Category 4 storms are triggered at 130 miles per hour.
Thus, a Category 6 storm may be in order, perhaps with a threshold of 180 miles per hour, to account for the increasing strength of tropical storms.