Seasonal Influences upon and Long-Term Trends in the Length of the Atlantic Hurricane Season

The length of the Atlantic TC season varies from year to year with some seasons significantly shorter or longer than the average. Albeit only to between 80-90% confidence, Kossin (2008) suggested that the duration of the year over which TCs formed within an extended main development region of the Atlantic basin has increased by 1-2 days per year over the course of the last several decades. Herein, it was first determined whether this result is maintained when TC genesis events occurring over the full Atlantic basin are considered or when the period of record is extended to the present. Subsequently, monthly-mean synoptic- to planetary-scale conditions associated with early and late starting and ending Atlantic TC seasons were examined.

Quantile regression was applied to National Hurricane Center HURDAT2 data (Landsea and Franklin 2013) for the 36 year period between 1979 and 2014 to first replicate and subsequently expand upon Kossin (2008)’s findings. Extending the end of the analysis through 2014 eliminated the positive but non-statistically-significant trend toward longer Atlantic TC seasons identified by Kossin (2008). Rather, this trend appears to be a function of several abnormally-long Atlantic TC seasons – 2001, 2003, 2005, and 2007 – that occurred during the early-to-mid-2000s (Tables 1 and 2). Of these particular seasons, all but 2003 were associated with weak to moderate La Niña and negative-phase PDO conditions, each of which were found to be associated with later ends to the Atlantic TC season. However, the precise reasons as to why these seasons were considerably longer than normal remains to some extent unclear, and further study is planned to address this question. Furthermore, when TC formation events across the entire Atlantic basin were considered, no statistically-significant trend in Atlantic TC season length was identified for the period 1979-2007 and no trend whatsoever in Atlantic TC season length was identified for the period 1979-2014. Variations in season length therefore appear to be primarily controlled by interannual variability in the necessary conditions for TC formation, particularly early and late in the Atlantic TC season.

Utilizing linear regression applied to June and November monthly-mean reanalysis data, synoptic- to planetary-scale variability associated with early and late starting and ending Atlantic TC seasons was identified. Earlier starting Atlantic TC seasons are associated with increased June monthly-mean 850 hPa relative vorticity across the eastern Gulf of Mexico, indicative of a greater likelihood that a precursor disturbance for TC formation exists in the Gulf of Mexico in June in earlier-starting Atlantic TC seasons. Given that most early-forming Atlantic TCs do so via tropical transition, it is believed that a stationary frontal boundary along and ahead of the June climatological mean 500 hPa trough in the far western Atlantic is the likely source of such a disturbance. There also exist statistically-significant correlations between the 10^th percentile Atlantic TC formation date and large-scale variability within the Southern Hemisphere. However, whether these correlations are causal or merely associative is uncertain, as are the causes of such variability.

Late-ending Atlantic TC seasons primarily occur during La Niña and negative-phase PDO events. The former is consistent with several previous studies (e.g., Dunion 2011; Klotzbach 2011a,b) and is believed to be the primary influence upon late-forming TCs (independent of genesis pathway) in the western Caribbean Sea. Further investigation into the physical connection(s) associated with the latter is planned for further study. A later end to the Atlantic TC season is also associated with an eastward shift in the November monthly-mean 500 hPa longwave pattern across the United States and subtropical western North Atlantic Ocean. It is this modulation of the atmospheric pattern that is hypothesized to increase the likelihood of late-season TT events across the subtropical Atlantic Ocean, although it is uncertain as to whether this result can also be attributed to modulation of the mid-latitude atmospheric pattern by the ENSO and/or PDO.

While the linear regression analyses are to some extent representative of variability in monthly-mean fields observed with both early- and late-starting and -ending seasons, there exist multiple pathways to an early- or late-starting and/or -ending Atlantic TC season. Consequently, while associated with statistically-significant (to ≥ 90% confidence) linear relationships, these results only explain a small portion of the total variation in Atlantic TC season length. For instance, in order for TCs to form, environmental conditions must locally be favorable on shorter time scales of several hours to a few days, such as may be associated with a transient Madden-Julian Oscillation event (e.g., Klotzbach 2010) and/or a convectively-coupled atmospheric Kelvin wave (e.g., Schreck 2015). Thus, while the results elucidate variability in the large-scale conditions necessary to support TC formation associated with early- and late-starting and -ending Atlantic TC seasons, variability on smaller spatiotemporal scales must be known in order to more completely quantify variability in Atlantic TC season length.

The results presented in this manuscript motivate a number of future studies aimed both at better understanding past, present, and future variability in TC season length and at quantifying the large-scale conditions that promote early-starting and late-ending TC seasons. For instance, what results in multiple successive short or long Atlantic TC seasons, such as was observed in the early 2000s? Note that while no statistically-significant linear relationship exists between the 10^th percentile (R = -0.18) or 90^th percentile (R = 0.11) formation dates and annual Atlantic TC count, the 90^th minus 10^th percentile formation date (R = 0.43) is strongly linearly correlated to annual Atlantic TC count. Thus, to some extent, more (less) active Atlantic TC seasons are also longer (shorter) Atlantic TC seasons, consistent with the hypothesis of Dwyer et al. (2015). Case studies of both early- and late-starting and -ending seasons may provide further insight toward addressing this question. Furthermore, the methods utilized in this study can be readily adapted to understand long-term trends in season length and large-scale variability contributing to early- or late-starting and -ending seasons in other oceanic basins that feature well-defined TC seasons. These methods may also be readily adapted to subsets of cyclones – e.g., only TCs with maximum sustained surface winds of ≥ 33 m s^-1 – for any basin in which TCs occur. Finally, given appropriate downscaling and statistical sampling methods, these methods may be applied to climate model outputs to evaluate potential future changes in both TC season length and early- and late-season genesis pathways under a wide range of emissions scenarios.

The results presented herein also motivate an investigation into the predictability – or lack thereof – of the large-scale conditions that promote early- or late-starting and -ending TC seasons, whether in the Atlantic basin or elsewhere. In other words, are shorter- and longer-than-normal TC seasons primarily driven by synoptic-scale variability? Or, are such events primarily driven by sub-seasonal to climate-scale variability that evolves more slowly (and is thus more predictable) than that on the synoptic-scale? The strong linear correlations of Atlantic TC season length with the ENSO, PDO, and seasonal Atlantic-basin TC count argue in favor of sub-seasonal to climate-scale controls upon Atlantic TC season length. Likewise, we note that linear regression analyses conducted between the 10^th percentile formation date and May monthly-mean fields and between the 90^th percentile formation date and October monthly-mean fields bear some resemblance to those presented in Figs. 5-6 and 8-9, respectively, particularly for the slower-evolving SST and 600 hPa relative humidity fields (not shown). However, the limited overall extent to which the linear regression analyses are representative of observed variability in both early- and late-starting and -ending Atlantic TC seasons argues against dominant climate-scale controls, though it remains possible that such relationships may exist if combinations of multiple modes of climate variability are considered. Further investigation is planned to address the intrinsic predictability of comparatively short and long Atlantic TC seasons.

We acknowledge fruitful discussions with Rob Hodges, Sergey Kravtsov, and Kyle Swanson during the course of this research. We are indebted to Ron McTaggart-Cowan for making available to us the genesis pathway classification database described in McTaggart-Cowan et al. (2013). This manuscript benefitted greatly from constructive review comments provided by Phil Klotzbach and two anonymous reviewers. Monthly mean ERA-Interim data were obtained from the ECMWF. NOAA ERSST v3b SST data were obtained from the NOAA/OAR/ESRL Physical Sciences Division. Teleconnection index data were obtained from NOAA/NCEP/Climate Prediction Center (ONI, NAO, AO, PNA, QBO), the University of Washington (PDO, SPI), and NOAA/OAR/ESRL Physical Sciences Division (AMM, AMO). Quantile regression computations were carried out utilizing MATLAB code provided by Aslak Grinsted.

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