As hurricane season approaches: The factors that could drive the North Atlantic’s risk

[EDITOR’S NOTE: This article originally appeared in the September 2022 issue of PIA Magazine. As we prepare for the 2023 hurricane season—which Colorado State University’s Department of Atmospheric Science has been predicted to be ‘slightly below-average’—it stands as a good reminder of all the factors that are involved in a hurricane season’s activity level. Hurricane season runs from June 1 through Nov. 30. For resources, PIA members can access PIA’s Hurricane Info Central.]

At the time of this writing, the 2022 North Atlantic hurricane season scorecard already has recorded three tropical storms with Tropical Storms Alex, Bonnie and Colin. With the peak of the season quickly approaching and with eight hurricane landfalls in the last two seasons, the question on everyone’s mind is: Will the 2022 North Atlantic hurricane season be as impactful and costly as both 2020 and 2021?

Even though 2021 was the third most-active North Atlantic hurricane season on record, in terms of the number of named storms, it will be remembered by insurers for just one event: Hurricane Ida. It looks likely that Ida will be one of the costliest hurricanes on record. Observed insured losses to date are in line with the RMS^® estimated range of between $31 billion and $44 billion. To put this into context, the total loss from Ida alone exceeds the total loss caused by the six hurricanes that made landfall in 2020 in the U.S.

The main meteorological forecast agencies and groups have issued their initial hurricane season forecasts, and RMS analysis of their predictions indicates overall that 2022 is likely to be another above-average hurricane season. If these current forecasts prove right, 2022 would be a record seventh consecutive above-normal season, extending the current ongoing record of six seasons dating back to 2016.

What does an average hurricane season look like? Using the U.S. National Oceanic and Atmospheric Administration 1991-2020 U.S. Climate Normals, an average hurricane season would produce 14 named storms, seven hurricanes, and three major hurricanes.

For the 2022 season, NOAA has forecast 14-21 named storms, of which 6-10 are expected to become hurricanes, and 3-6 of those are forecast to become major hurricanes (Category 3 or stronger). Outlooks from other meteorological forecast agencies and groups are broadly in line with the guidance issued by NOAA in calling for an above-average season.

Not all hurricanes make landfall, but long-term statistics indicate that the probability of a hurricane making landfall increases during more active seasons in the U.S. Colorado State University anticipates a 76% probability of at least one major hurricane making landfall in the U.S. this season, while Tropical Storm Risk forecasts two hurricanes and four tropical storms to make landfall over the contiguous U.S. in 2022.

So, what are the major factors driving hurricane risk for this season? These forecasts of an above-average season reflect the influence of several key seasonal oceanic and meteorological factors, including the El Niño-Southern Oscillation, sea-surface temperatures in the tropical Atlantic, and the Atlantic Multidecadal Oscillation/Variability. Let’s examine each of these factors.

El Niño-Southern Oscillation

A large proportion of the uncertainty associated with seasonal hurricane activity forecasts can be attributed to the uncertainty about which El Niño-Southern Oscillation phase will materialize during the peak months of the hurricane season.

ENSO is a coupled ocean-atmosphere climate phenomenon characterized by periodic fluctuations in sea-surface temperatures and sea-level pressure gradients across the equatorial Pacific Ocean. Over a two- to seven-year period, sea-surface temperatures in the equatorial Pacific transition between anomalously warm (El Niño) and anomalously cool (La Niña) phases. But ENSO’s influence extends far beyond the boundaries of the equatorial Pacific Ocean; it is regarded as the leading mode of climate variability at interannual timescales and recognized as a key influencer of North Atlantic hurricane activity.

Typically, El Niño conditions (anomalously warm sea-surface temperatures over +0.5°C) in the equatorial Pacific Ocean lead to stronger upper-level westerly winds and stronger lower-level easterly trade winds across the Atlantic Basin, enhancing vertical wind shear in the region. Generally, this suppresses hurricane formation, development, and intensification. The troposphere across the tropical Atlantic also is generally more stable during El Niño periods, deterring cyclogenesis.

Conversely, during La Niña phases (anomalously cool sea surface temperatures below −0.5°C), hurricane activity in the North Atlantic generally is marked by a reduction in the vertical wind shear and an increase in atmospheric instability in the Atlantic Basin, resulting in a more active hurricane season.

For this year’s hurricane season, most forecasts favor cooler La Niña conditions, and in the absence of influence from any other atmospheric or oceanic conditions and all other factors remaining equal, this generally would result in slightly above-average tropical activity in the North Atlantic Basin.

Atlantic sea-surface temperatures

Generally, there is low skill and high uncertainty in forecasting North Atlantic sea-surface temperatures several months in advance. Both the NOAA Climate Forecasting System and North American Multi-Model Ensemble models are forecasting average-to-above-average departures, with many areas forecast to experience anomalies of between +0.08°C to +1.2°C for the period covering the peak months of the hurricane season between August and October 2022. The average departure across the main development region is anticipated to be +0.17°C. Typically, warmer sea-surface temperatures enhance tropical activity by providing increased energy and moisture to the environment.

Another key factor is the Atlantic Multidecadal Oscillation—or Atlantic Multidecadal Variability as it has been more commonly referred to in recent literature. It is a mode of natural climate variability that results in 25-to-40-year periods of increased or suppressed hurricane activity in the North Atlantic. In its positive (warm) phase, sea-surface temperatures in the main development region are anomalously warm, and the tropical Atlantic is anomalously warm compared to the remainder of the global tropics—these conditions are conducive to increased hurricane activity in the basin. The opposite impacts are associated with its negative (cool) phase.

The forecast of an above-average sea-surface temperature pattern in the tropical Atlantic and a warmer pattern compared to the remainder of the global tropics is consistent with the ongoing positive (warm) phase of the Atlantic Multidecadal Oscillation/Variability. According to NOAA, the AMO/AMV has been in a positive (warm) phase since 1995, which is representative of a high-activity era.

During warm phases of the AMO/AMV the frequency of weak-category storms, such as tropical storms and low-end hurricanes, is not much affected. However, the number of storms that intensify into major hurricanes is noticeably greater than during cool phases—at least twice as many.

Atlantic wind shear

Vertical wind shear, which is defined as a change in wind speed or direction with height, is another key atmospheric driver that modulates tropical cyclone activity in the North Atlantic.

Changes in wind shear in the North Atlantic are strongly governed by which phase of ENSO is prevalent in the equatorial Pacific. During La Niña phases (anomalously cool), the Atlantic Basin typically experiences weaker vertical wind shear over the Caribbean and main development region, weaker trade winds across the tropics, and increased atmospheric instability. These conditions generally are conducive to hurricanes, and typically lead to an above-average number of storms and hurricanes forming during the season.

The latest wind shear forecast for the period between August and October 2022, which demarcates the peak months of the season, indicates that negative vertical wind shear anomalies are expected across parts of the tropical Atlantic and the Caribbean Sea. In the absence of other factors, this would indicate an increase in tropical activity compared to average.

There are other factors, such as the North Atlantic Oscillation, the Madden-Julian Oscillation, and the Saharan Air Layer, that can influence tropical cyclone activity on a weekly or monthly basis, but are difficult to forecast at seasonal timescales.

Mid-season forecasts

Climatologically, tropical cyclone activity in the North Atlantic Basin peaks during a period between mid-August and mid-October. NOAA states that the peak of the hurricane season is on Sept. 10, and this peak almost makes it a “season within a season.” Some 96% of category 3, 4, and 5 hurricane days are within this period.

As useful as the initial season forecasts are, with most of the hurricane activity during this peak, consequently, some forecasting groups (most notably NOAA, CSU and TSR) issue a revised forecast in early August. These updated forecasts better reflect the increased certainty in the meteorological and oceanic variables that will govern conditions in the North Atlantic during the peak months of the hurricane season, and generally provide a more valuable outlook than those issued in June. It pays to remember that even as all these main factors point to an above-average hurricane season, as 2021 shows with Hurricane Ida, it takes just one major event to turn a season.