How Luck and a Shift in Climate Explain the Recently Ended "Drought" in Major Hurricane Landfalls on the U.S. Mainland
Posted on 1/28/18 5:11 PM
Author: Dr. James Kossin
Hurricane Harvey made landfall in 2017 as a major hurricane (a major hurricane is Category 3, 4 or 5 on the Saffir–Simpson scale). The landfall occurred in Texas, thereby breaking what has sometimes been referred to as the “U.S. major hurricane drought”. Prior to Harvey’s landfall in Texas, it had been almost 12 years since a major hurricane made landfall in the continental United States, when Wilma struck Florida as a Category 3 hurricane in October 2005. Such a prolonged absence of U.S. major hurricane landfalls had never been observed before in a record that dates back to the mid-19th century. This is particularly remarkable given that the Atlantic ocean has been warmer than average and hurricane activity has been greater than average since 1995. This caused many to wonder whether this is just very lucky or whether the same shift in climate responsible for the increased hurricane activity may also be playing a role in keeping major hurricanes away from the U.S. coast.
Just 16 days after Harvey made landfall in Texas, Irma struck Florida as a major hurricane and 10 days after that, Maria struck the U.S. territory of Puerto Rico as a strong Category 4 hurricane. Needless to say, the U.S. major hurricane “drought” was considered to be well over at this point. But why did the drought happen in the first place? As noted above, speculation about the effects of climate shifts abounded and studies began to emerge that explored this question.
Before proceeding, it’s very important to note that the specification of U.S. major hurricane landfall is not at all relevant to regions outside of this defined perimeter, and is not necessarily even relevant to locations along the U.S. coast. For example, neither the Small Island Developing States of the Caribbean, nor Mexico and Central America have experienced an absence of major hurricane landfalls during the period of the U.S. major hurricane landfall drought, and Hurricane Sandy devastated the New York metropolitan region as a Category 1 hurricane in 2012. The frequency of U.S. major hurricane landfalls is of interest, but it is by no means a complete indicator of general hurricane risk and its societal relevance.
Questions about where hurricanes have made landfall over the years are very different than questions about how many hurricanes have formed per year or how strong they have been. This is because landfall depends very strongly, and somewhat obviously, on the track of the hurricane. A hurricane’s track is dictated by the “steering flow” that it’s embedded in, and this steering flow can be highly variable, particularly when a hurricane is moving out of the tropics and approaching the U.S. mainland coast (see the Box below for more detail on what controls hurricane tracks). For example, Hurricane Matthew in 2016 turned from moving northward to northwestward just after passing Cuba. If this turn had been only a couple of degrees more westward, Matthew would have struck Florida as a major hurricane instead of just moving along, but off, the coast. The takeaway is that there is a large element of luck to where, or if, a hurricane makes landfall, which makes it hard to predict in any systematic or longer term sense (Hall and Hereid 2015).
So, luck likely played a large role in the U.S. major hurricane landfall drought. But a shift in climate likely played some part too. For example, it’s been shown that Atlantic ocean temperature affects where hurricanes form as well as the steering flow they move within (Wang et al. 2011). Both of these factors affect track in such a way that hurricanes are less likely to strike the U.S. mainland coast when tropical ocean temperatures are warmer than normal. There is also a relationship whereby the same warmer-than-normal ocean temperatures that lead to greater hurricane activity in the tropics also make hurricanes much more likely to weaken as they approach the U.S. mainland coast (Kossin 2017). In this case, a major hurricane approaching the coast is more likely to weaken to a sub-major hurricane by the time it makes landfall. All of these factors have likely played some role in the 12-year absence of U.S. major hurricane landfalls during this period of heightened overall hurricane activity.
So the evidence points to a combination of luck and a shift in the climate as the explanation for the drought. Can we expect another similar period to occur anytime soon? The random luck aspect says most probably not, but it’s still possible just as a flipped coin can repeatedly and sequentially come up as heads. The climate shift aspect says that it may be a bit more likely as long as the Atlantic remains in this state of heightened overall activity. This last statement may seem a bit counter-intuitive (that is, reduced U.S. major hurricane landfall risk during more active periods), but there is evidence that this may be the case.
Whatever the answer, it would be irresponsible and dangerous to assume in any year that a major hurricane won’t strike the U.S. coast and as we’ve seen many times in the past, it only takes one such occurrence to cause severe damage and even loss of life.
The climate system that has been operating in the North Atlantic has been very fortuitous for residents along the U.S. coast. This situation, in which the same conditions that lead to increased overall hurricane activity also cause conditions that can reduce the chance of major hurricanes striking the U.S. coast, is a very lucky by-product of this climate system. At this point, however, we don’t yet know what effect climate change may have on this behavior in the future. If this naturally-offsetting relationship is reduced or even reversed by climate change, then U.S. hurricane risk could increase substantially. Such a change would come under the heading of a “climate surprise”, which describes potential effects of climate change that are outside the usual more obvious expectations, such as sea level rise or more frequent heat waves.
Extra: Atlantic Hurricane Tracks
In the tropics, hurricanes typically move westward following the trade winds, and their tracks have a small northward component due to their interaction with the spin of Earth (this is called the “beta effect”). The trade winds in the tropical Atlantic are part of a larger circulation pattern driven by an expansive swirl of clockwise wind-flow that sits to the north of the tropics (Figure 1). This pattern goes by a few names: the “Azores High”, the “Bermuda High”, or the more generic “Atlantic subtropical ridge” and it’s a fairly permanent large-scale feature during the Atlantic hurricane season. It does, however, vary in its shape and strength and these variations can have a profound effect on how any given hurricane will track. In particular, the shape and size of the pattern dictates when or if a hurricane will “recurve”.
Figure 1. The expansive clockwise wind-flow in the North Atlantic, which results from the fairly constant presence of the “Azores/Bermuda High” pressure system (also generically called the “Subtropical Ridge”) during the hurricane season. The southern part of the flow sits in the tropics and creates the “Trade Winds”, which carry hurricanes westward as they develop. Depending on the particular shape and strength of the pressure system on any given day, a westward traveling hurricane might follow the flow northward (called “recurvature”) or might continue on its westward path. Recurving hurricanes may stay out at sea their whole lives if they recurve early enough, or they may strike Caribbean Islands and/or the U.S. east coast. Non-recurving hurricanes may enter the Gulf of Mexico or continue more westward to threaten Mexico and/or Central America.
A hurricane recurves when the large-scale steering flow turns northward following the clockwise flow around the subtropical ridge and the exact point of recurvature dictates whether the hurricane will stay out at sea or threaten land, including along the U.S. coast. Even small day-to-day variations in the size or strength of the subtropical ridge can dramatically change the threat landscape. Additionally, once a hurricane begins to track northward out of the tropics, it leaves the fairly steady and predictable steering flow of the trade winds as it enters the sub-tropics and mid-latitudes. At these higher latitudes, the steering flow is much more erratic and unpredictable as eastward-moving weather systems interact transiently with the hurricane’s steering flow. This can cause abrupt track changes and makes prediction difficult (for example, when Hurricane Sandy’s track turned from northeastward, which kept it over open water, to northwestward, which brought it into New Jersey).
In summary, whether or not a hurricane makes landfall in any particular region is highly sensitive to the random vagaries of the larger-scale winds that it interacts with. That is, there is a large “luck” factor, good or bad.