Now that the 2024 Atlantic hurricane season seems to be coming to a close, it is worth reflecting on what transpired and what we might learn from it.
By most measures, it was an active, destructive and--unfortunately--deadly storm season, with 11 tropical cyclones reaching hurricane strength, 5 of them making landfall on the U.S., two of them--Helene and Milton--as major hurricanes. One particularly notable feature was the extreme rapid intensification of several storms, including Beryl, which intensified from a tropical storm to a hurricane in under 24 hours, Helene which intensified from a weak tropical storm to a cat 4 major hurricane in 48 hours, becoming the strongest storm on record to make landfall on the Florida "Big Bend" region, and Milton, which went from cat 1 to a 185 mph monster cat 5 in under 24 hours. Most remarkable of all, however, was Oscar which the National Hurricane Center identified as a small tropical disturbance off the coast of the Dominican Republic at 8 AM EDT on October 19th, only to upgrade it to a Hurricane 5 hours later. It left a trail of destruction after making landfall in eastern Cuba, resulting in a half dozen fatalities. Unfortunately, we shouldn't be surprised--this is part of a steady trend toward rapid intensification and was predicted a number of years ago by leading hurricane scientist Kerry Emanuel of MIT as a consequence of warming oceans.
The increased intensity and destructive potential of these storms too can be attributed to human-caused warming. A recent study found that human greenhouse warming substantially boosted the intensity of the 2024 storms, concluding that the two category 5 storms--Beryl and Milton--likely would not have reached that status in the absent of human-caused warming. Indeed Milton nearly breached the threshold of 192 mph sustained winds argued by one recent study to constitute a whole new "category six" caliber of hurricanes that has emerged in an era of unprecedented ocean warmth. A separate study estimated that the deadly flooding in the southeastern U.S. from hurricane Helene was increased by 50% by human-caused warming.
While the 2024 hurricane season was unprecedented in a number of ways, with impacts that have clearly been exacerbated by climate change, there was one piece of the puzzle that didn't quite fit. Yes, it was an active season as measured by the number of tropical cyclones (i.e. the named storm count)--nominally 18 (though that number could still rise, as discussed below). That places it among the top 11 seasonal totals historically going back to 1851. But the season was not as active as one might have expected.
Our team was among several groups that predicted an extremely active season, with named storm counts in the mid 20s to low 30s. These forecasts were driven by the favorable climate factors that were at play, i.e. record tropical Atlantic warmth and a transition underway from El Niño toward La Niña conditions. Both factors favor active Atlantic hurricane seasons, as they are associated with a thermodynamically favorable, low-shear environment that is conducive to tropical cyclogenesis. Previous recent years with this combination of factors, 2005 and 2020, were associated with record numbers of named storms (28 and 30 respectively). With record warmth in the main development region ("MDR") for Atlantic tropical cyclones going into this year's season, our statistical modeling approach predicted between 27 and 39 storms, with a most likely estimate of 33 named storms.
That forecast however was contingent upon the development of a moderate La Niña during boreal autumn as models were predicting at the time. This did not come to pass, with current estimates showing neutral values of the "Nino3.4" index as of the El Niño phenomenon in late November. Our forecast in the alternative scenario of a neutral tropical Pacific state was instead for the slightly lower total of 25 – 36 storms, with a best guess of 30.5. The actual total of 18---as of the official end of the storm season--is clearly lower than the predicted range (we note here however that there is still a possibility that the final total will be 19 or even 20; during the 2005 season, for example, named storms continued on into January of the following year. Moreover, there is some subjectivity in the unofficial season total--if "potential cyclone eight" had been named by the National Hurricane Center, the total would already be 19. In some past years post-season analysis has added storms like this to the seasonal record, increasing the count total. That may well happen here). Nevertheless, the current count of 19 is roughly two standard errors below our mean forecast, which is almost certainly sufficient to declare our forecast "busted".
What might have gone wrong here? There are a couple confounding factors that are likely at work here. The hurricane season was unusually quiet during July and August when seasonal activity is typically ramping up. The season picked up considerably in September however. In fact, there were nearly as many named storms (13--or 14 if one includes "potential cyclone eight") during September-November as during the record-active years of 2005 and 2020 (16 named storms in both cases).
So there's no real discrepancy when it comes to the latter half of the season. It was basically as active as predicted. The puzzle is why July and August were so quiet despite clearly favorable seasonal large-scale climate conditions. This is where one runs into complications with intraseasonal variability. Of particular relevance is the so-called Madden-Julian oscillation or simply "MJO" to its friends. The MJO is a roughly 40-50 day oscillation in the tropical atmospheric circulation which influences the location of convection, which shifts east and west over the course of a single 40-50 day cycle. When the center of convection coincides with the tropical Atlantic, conditions are more favorable for tropical cyclogenesis. Conversely, if the center of convection shifts to e.g. the Pacific, conditions become unfavorable. This year, it happens that the unfavorable phase of the MJO coincided roughly with the peak of the storm season, inhibiting tropical cyclone formation at the very time it would typically be most prevalent. Secondarily, dry Saharan air outbreaks in July and August created unfavorable conditions for tropical cyclogenesis as well. The net effect was that unfavorable conditions related less to climate and more to just the vagaries of weather and intraseasonal variability conspired to make the 2024 storm season less active than it otherwise would have been. We should be thankful for that, given the devastating consequences of the storms that did form.
There is one other noteworthy detail here. Our group makes an alternative forecast in which tropical sea surface temperature (SST) in the main development region (MDR) is replaced with what we call "relative SST", defined as the difference between MDR SST and the average SST throughout the entire tropics, which some researchers have argued might be a better predictor of Atlantic hurricane activity. While our previous analyses have found that this alternative model yields less skillful predictions, it is notable that this year it yielded a much more accurate prediction of 19.9 +/- 4.5 total named storms that was remarkably close to the seasonal total.
So there are some interesting takeaways and a few conundrums to reflect upon as we look back at this unprecedented and unusual Atlantic hurricane season. With regard to our statistical model of Atlantic hurricane activity, it has generally yielded among the most accurate forecasts. In years where it's "missed" (i.e., the observed counts were outside the uncertainty range of the prediction), it has typically predicted too few storm counts. For example, in 2020, while we predicted the most active season of all forecasters (as many as 24 named storms), resulting from a similar combination of favorable factors that were observed heading into this season (i.e. very warm tropical Atlantic SSTs and a transition toward La Nina conditions), our prediction was too low--the actual count was a record 30 named storms. This is the first year where our prediction substantially exceeded the observed storm counts.
It's tempting to dismiss this as a one-off, i.e. the conspiring of unusual weather conditions at the height of the storm season, a bad roll of the dice. And that could be all it is. A more disturbing possibility is that the climate system is no longer behaving quite the way it used to, and some of the old rules and relationships no longer apply. Lest this sound like special pleading (and maybe it is), it is worth noting that respected colleagues of mine (climate scientists Gavin Schmidt and Zeke Hausfather) have argued precisely that in a recent New York Times op-ed entitled "We Study Climate Change. We Can’t Explain What We’re Seeing". Among other things, they argue that the progression of the latest El Niño episode simply doesn't match the pattern seen in past El Niño episodes. It is possible that climate change is altering the behavior of the phenomenon. And if that's the case, that it may also be altering the impact the phenomenon has on other attributes of the ocean-atmosphere system, including its influence on seasonal hurricane activity.
It's a disquieting possibility. And while in this case we're talking about impacts that were reduced relative to what had been predicted, there could well be far more unpleasant surprises in the greenhouse. It's unwise, in short, to tinker with a system you don't entirely understand. Particularly when our entire civilization is at stake.
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