Seasonal #Hurricane #Forecasts and ENSO

Like a bad remake of Groundhog Day, it’s that time of year when the various research groups emerge from their ivy covered lairs and issue forecasts for the upcoming Atlantic Hurricane season. I used to play that game, with the annual press conferences, media interviews, and associated tabulation of number of articles and citations to go into the next annual report and round of funding requests. But the last decade or so I have given up on the annual media circus as NOAA has started issuing its own estimates, and our research has moved on to site specific seasonal impact estimates rather than simply counting the number of storms. After all, having 20 storms doesn’t matter if none of them hit you; likewise, one storm can ruin your decade. So while this post does end with a suitably depressing outlook for 2021, it is more about the influence of the big driver of storm activity in the Atlantic: the ENSO or El Niño cycle.

CFS2 ENSO Forecast this fall from the NOAA Climate Prediction Center

The El Niño-Southern Oscillation (ENSO) has a huge impact on global weather. Although the usual way of measuring it is in terms of East Pacific sea surface temperatures, ENSO is a complex phenomena that changes both oceanic and atmospheric circulation patterns worldwide. The Wikipedia page has a nice overview of the system; what concerns us here is the impact on hurricanes. In the Atlantic, the warm phase of ENSO (El Niño) typically depresses hurricane activity; likewise, the cold phase (La Niña) tends to result in more and stronger storms. Periods in between are called “ENSO Neutral” or neutral. The reason ocean temperatures in the Pacific influences Atlantic hurricanes (in another ocean thousands of miles away) are complex, but mostly have to do with wind shear over the Atlantic and the so-called “steering currents” that push storms around as they move across the ocean. Don’t confuse these Pacific SST’s with ocean temperatures in the Atlantic – that is a separate cycle and phenomena. Normally we associate warmer oceans with more hurricanes, but in this case, a warmer Pacific during a El Niño phase means the Atlantic becomes less favorable (at least from a wind shear perspective)! The worst combination is a cold Pacific (La Niña) and a warm Atlantic: the cold Pacific is cold because winds are favorable over the Atlantic, and the warm Atlantic means more energy for Atlantic storms. Confused yet? Don’t be – in this case it’s simple, we’re just looking at how the three phases of ENSO correlate with hurricane landfalls and damage since it is such a big factor.

Let’s take a look at the peak of hurricane season, the month of September to see what impact the contrasting ENSO phases have on the number of storms, as well as on damage. Using data since 1871, it’s pretty even split between the two contrasting conditions. About 25% of Septembers are El Niño, 24% are La Niña, whereas 51% are neutral. But in terms of total numbers of storms, 30% occur in La Niña years, whereas 22% occur in El Niño. That doesn’t seem like much on the surface, but it actually translates in to a significant difference in the number of storms between the two years – an El Niño September typically has one third fewer storms than either a neutral or La Niña year. The number of people impacted by hurricane conditions also reflects this difference in a similar way. About a third fewer people are impacted by hurricane conditions in an average El Niño September than in other kinds of years. Damage basin-wide is not quite so dramatic, with damage during La Niña years only about 25% higher than in El Niño. But … there are regional twists to this story.

The biggest driver of the economic impact of hurricanes in the Atlantic is of course the mainland United States. The US experiences nearly DOUBLE the economic impact of hurricanes in an average La Niña September as compared to El Niño years (33% vs 17%)! This is partly due to the higher intensity, but more importantly due to the landfalls in La Niña years being closer to high value exposures (cities, or targets if you prefer 😮 ) in the Northeast and Atlantic coasts. Looking at the other two active months, La Niña Augusts tend to have more storms – but those storms stay offshore, so the damage actually tends to be less than in El Niño years, but in October the impacts are dramatic. A La Niña October tends to generate three times the damage as an El Niño year.

If you look at individual states there are also dramatic differences. In Florida, La Niña Septembers have generated FOUR TIMES as much damage as El Niño years! In New York, New Jersey, and Massachusetts there have been so few El Niño losses the computer said “Fuhgettaboutit” when I asked, they are so rarely damaged during those years, and La Niña years are so bad. The Gulf Coast isn’t quite so dramatic, but still Louisiana has had just under twice as much damage during La Niña.

Another interesting area is the Georgia/South Carolina Lowcountry coast (south of Edisto Isl). The unusual shape of the coastline, combined with being at the latitude where storms begin to recurve to the northeast means that for hurricane damage, the difference between La Niña and El Niño isn’t quite so dramatic – only about a 30% difference. However, unusually, activity is depressed in ENSO Neutral years, and elevated in both La Niña and El Niño Septembers.

What can we expect for this year? The current ENSO forecast for August to October is that we will almost certainly be at least neutral, and there is around a 50% chance of being back into La Niña conditions by September, and a higher chance for that in October. While not as favorable as last year, that’s Not Good for hurricane season, as it means higher than average activity. Therefore, I expect the hurricane forecasts coming out over the next few weeks to reflect that. Not likely as many storms as last year, but very likely to have multiple threats over the season. But there are a lot of other factors that go in to how many storm form in a given year, much less where any individual storms goes once it forms. The atmosphere is a very complex beastie.

So let the scare mongering begin … or, you could just enjoy the beautiful spring weather, the flowers blooming, and consider that as bad as hurricanes are, and unlike tornadoes that give you little warning, or earthquakes that give you almost none at all, you can see them coming days away, and have time to get out of the way. So as a reminder, this is the time of year to revisit your hurricane plans, especially insurance. There is a “lock out” period for changes prior to a storm and if you wait until one is headed your way, it’s too late. Check out Ready.gov for checklists and advice.

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