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Causes and Predictability of the Exceptionally Active 2017 Atlantic Hurricane Season



The 2017 North Atlantic hurricane season was unusually active, reaching twice the climatological levels of Accumulated Cyclone Energy (ACE) and US hurricane landfalls. The 2017 season ranks 7th in overall activity among all years since 1920, reaching an ACE total of 226 that far exceeds the mean level of 105 from 1980 to 2016. Over the same period, 1.5 hurricanes, on average, reached the US coastline each year, while three (Harvey, Irma and Nate) made landfall in 2017.

CFAN’s inaugural seasonal forecasts of Atlantic hurricanes, issued in June and August 2017, predicted above-normal ACE and US landfalls. However, CFAN and other seasonal forecasts did not foresee the exceptionally high levels of ACE.

ACE # US landfalls

Observed 226 3

Jun 2017 fcst 135 (100-170) 3 (2-4)

Aug 2017 fcst 130 (100-160) 2 (1.5-2.5)

CFAN has prepared a report [link] that is a ‘hindsight’ analysis of the causes of the exceptionally active 2017 Atlantic hurricane season. In particular, the report addresses the question of whether this high level of activity was predictable. Such analyses help CFAN improve its seasonal forecast methodology. Summary findings from the report

  • The rapid transition towards La Nina conditions during early summer contributed to the high level of Atlantic hurricane activity but accounted for only a modest fraction of the extreme hurricane activity that later developed.

  • Hurricane-favorable divergence anomalies in the lower atmosphere of August-October 2017 were already established in early summer (May-July). Recent analogue years for this circulation pattern were 2008 (ACE 146, 3 US landfalls) and 1999 (ACE 173, 3 US landfalls).

  • The combined effects of the Pacific and Atlantic Meridional mode feedbacks were likely important contributors to high US landfall totals and extreme ACE anomalies over the broader North Atlantic.

  • Spring conditions in the Arctic proved to be a reliable indicator of U.S. landfalls

Evidence from the ocean and atmosphere suggests that North Atlantic and US hurricanes vary in response to the combined influences of ENSO, the Atlantic and Pacific and Meridional Modes, and less-understood, but important extratropical processes. Late-summer hurricane activity responds to processes that tend to evolve slowly over months, allowing probabilistic forecasts to be made 1 to 2 seasons ahead, and perhaps even earlier.

CFAN is presently considering these factors and others in our ongoing assessment of the upcoming 2018 North Atlantic hurricane season. Recent history shows that highly active years have occurred consecutively (1998-99, 2003-05), warranting particular attention to developing changes in late 2017 and early 2018.

Contacts:

James Johnstone, jajstone@gmail.com

Judith Curry, curry.judith@cfanclimate.com


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