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Juliana M. Karloski
Clark Evans


Considering a subset of the North Atlantic Ocean south of 30°N and east of 75°W, Kossin found that the Atlantic tropical cyclone (TC) season increased in length, at 80%–90% confidence, by about 2 days per year between 1980 and 2007. It is uncertain, however, whether the same is true over the entire Atlantic basin or when the analysis is extended to 2014. Separately, it is unclear whether meaningful subseasonal variability in the environmental factors necessary for TC formation exists between early- and late-starting (ending) seasons. Quantile regression is used to evaluate long-term trends in Atlantic TC season length. No statistically significant trend in season length exists for the period 1979–2007 when considering the entire Atlantic basin or for the period 1979–2014 independent of the portion of the basin considered. Linear regression applied to June and November monthly mean reanalysis data is used to examine subseasonal environmental variability between early- and late-starting (ending) seasons. Within an otherwise favorable environment for genesis, early-starting seasons are associated with increased lower-tropospheric relative vorticity where most early-season TCs form. Late-ending seasons are associated with La Niña, negative-phase Pacific decadal oscillation events, and environmental conditions that promote an increased likelihood of TC development along the preferred genesis pathways for late-season TCs. While confidence in these results is relatively high, they explain only a small portion of the total variation in Atlantic TC season length. More research is needed to understand how variability on all time scales influences Atlantic TC season length and its predictability.

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