{"id":23521,"date":"2021-04-02T11:39:26","date_gmt":"2021-04-02T16:39:26","guid":{"rendered":"https:\/\/www.realclimate.org\/?p=23521"},"modified":"2021-04-02T11:39:29","modified_gmt":"2021-04-02T16:39:29","slug":"should-the-official-atlantic-hurricane-season-be-lengthened","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2021\/04\/should-the-official-atlantic-hurricane-season-be-lengthened\/","title":{"rendered":"Should the official Atlantic hurricane season be lengthened?"},"content":{"rendered":"
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By Jim Kossin, Tim Hall, Mike Mann, and Stefan Rahmstorf<\/em>

The 2020 Atlantic hurricane season broke a number of records<\/a>, with the formation of an unprecedented 30 “named storms” (storms that reach wind-speed intensity of at least 18 m\/s and are then given an official name). The season also started earlier than normal. In fact, when ranked by their order in the season, the date of formation of every named storm, from Tropical Storm Arthur to Hurricane Iota was substantially earlier than normal (Fig. 1).<\/p>\n\n\n\n

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Fig.\u00a01<\/em><\/strong> Average number of named storms by day of the year in the historical record from 1851\u20132019 (dark blue line). The light blue shading denotes the range between the minimum and maximum number of storms observed by each day. The days of formation for the 2020 named storms are shown by the red squares.<\/em><\/figcaption><\/figure><\/div>\n\n\n\n\n\n\n\n

In 2008, after a series of Atlantic hurricane seasons that began earlier than normal, a study was published that explored whether the anomalously early starts may be part of a larger trend (Kossin 2008<\/a><\/span>). The results of that study identified increasing trends in the length of the active hurricane season, with storms forming earlier and later in the season than normal (Fig.\u00a02), but there was quite a bit of uncertainty in the statistical significance of the trends, and additional uncertainty in the century-scale trends (red lines in Fig.\u00a02) due to missing or uncertain data in the earlier part of the record. Still, regardless of statistical significance, there is clearly a very steep trend towards earlier storms in the past four decades (lower green line in Fig.\u00a02), which represent a period of reliable data.<\/p>\n\n\n\n

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Fig. 2 Time series of the first and last day of storm formation in each season from 1851 to 2020 (lower and upper black lines, respectively) and their linear trends over the periods 1851\u20132020 (red), 1950\u20132020 (blue), and 1980\u20132020 (green)<\/em>.<\/figcaption><\/figure><\/div>\n\n\n\n

The official start to the Atlantic hurricane season has been set at June 1st since the 1960s, but this steep trend toward earlier storm formation over the past four decades has spurred many discussions<\/a> about modifying this date, and the World Meteorological Organization has recently proposed<\/a> that the National Hurricane Center extend the official start of the hurricane season backwards by two weeks to May 15th. If this pronounced tendency toward earlier storms over the past four decades is defining a “new normal”, then extending the season backwards by two weeks seems entirely supportable. Furthermore, there\u2019s no question that the season length is linked to Atlantic sea-surface temperature (SST), with the season starting about 15\u201320 days earlier and about 15\u201320 days later per \u00b0C of anomalous Atlantic warming (Kossin 2008<\/a><\/span>).<\/p>\n\n\n\n

But this also raises other questions: 1) Can we expect the season to continue to lengthen as global warming from increasing concentration of globally well mixed greenhouse gas (GWM-GHG) continues to warm the Atlantic SSTs? and 2) Is there any process that might drive the new normal back to the old normal, thereby reducing the season length? Answering these two questions is the goal of this article.<\/p>\n\n\n\n

While season length is clearly linked to Atlantic SST, it\u2019s not so clear that this is part of a longer-term trend linked to global warming. In fact, there is quite a bit of uncertainty regarding the effect of GWM-GHG warming on tropical storm formation. Although some modeling studies find an increase in storm frequency related to GWM-GHG warming (e.g., Emanuel 2021<\/a><\/span>; Hall et al. 2021<\/a><\/span>; Knutson et al. 2020<\/a><\/span>), many others do not. And some studies actually predict a decrease in frequency of tropical storm formation, both globally and in the North Atlantic (all while allowing those storms that do<\/em> form to become more intense and produce more rain) (Knutson et al. 2019<\/a><\/span>). If SST warming due to GWM-GHG does reduce Atlantic formation rates, or does not increase them substantially, then there is no clear and obvious expectation that this warming should cause further lengthening of the season.<\/p>\n\n\n\n

This answers our first question, at least as far as we can, given the uncertainty in what we know about how formation rates respond to global warming. The answer is no. This doesn’t mean that it won’t happen; only that we don’t understand the physical relationships well enough and we don’t have the consistent numerical modeling results to form a clear expectation.<\/p>\n\n\n\n

The statement that GWM-GHG SST warming may not increase storm formation rates might appear incongruent with the earlier statement that the season is about 30\u201340 days longer on average per \u00b0C of anomalous SST warming. But it emphasizes an important point about the relationship between SST and hurricane behavior in general: The hurricane response to SST changes depends on what caused the SST changes (Emanuel and Sobel 2013<\/a><\/span>). In particular, Atlantic SST warming caused by local forcing does increase Atlantic tropical storm formation rates, while Atlantic SST warming caused by increased GWM-GHG concentration may not. The difference is due largely to the differences in the regional atmospheric response that occurs in concert with the SST warming.<\/p>\n\n\n\n

The key to understanding this lies in understanding what tropical storms respond to. In particular, they don\u2019t just respond to SST changes, but also how the atmosphere changes as the SSTs change. For example, warming caused by GWM-GHG can cause the atmosphere in some regions to become more stable and drier in the mid-levels (Knutson et al.\u00a02020), and can cause an increase in vertical wind shear over the Atlantic (Kossin 2017<\/a><\/span>; Ting et al. 2019<\/a><\/span>). All three of these factors have a powerful quelling effect on tropical storm formation. So it\u2019s okay to say that SST warming will increase storm formation rates, all other things equal, but all other things are not<\/em> equal. It depends on what\u2019s driving the warming because the atmosphere will respond differently depending on the driver. The local atmospheric response to warming can be strong enough to actually reduce formation rates in some regions even while that same warming is increasing the local SST.<\/p>\n\n\n\n

In the North Atlantic, examples of local SST warming mechanisms that can increase Atlantic storm formation rates are:<\/p>\n\n\n\n