Counting Global Tropical Cyclones: It’s harder than it sounds!

Tropical cyclones are among the most destructive weather disasters. The associated headlines often raise questions about the role of climate change in their impacts, which generates significant interest in metrics of annual tropical cyclone activity. NOAA, CSU, and numerous other groups also issue seasonal outlooks of this activity before and during the season to raise awareness.

Despite all of this focus, climate monitoring of tropical cyclone metrics is surprisingly challenging, particularly at the global scale. NOAA NCEI monitors the number of named storms (winds ≥34 kt), hurricane-strength tropical cyclones (≥64 kt), and major hurricane-strength tropical cyclones (≥96 kt). They also tally accumulated cyclone energy (ACE), an integrated metric of the strength, frequency, and duration of tropical cyclones.

The near-real-time metrics are derived from preliminary data in IBTrACS (International Best Track Archive for Climate Stewardship). The data for the Atlantic and Eastern Pacific come from NHC’s working best track (b-decks). The other basins come from the U.S. Military’s Joint Typhoon Warning Center (JTWC). The preliminary JTWC data are not publicly archived, so they are obtained from NCEP’s “tcvitals” that support the operational models. As a result, the metrics sometimes have to be manually adjusted to account for changes in JTWC’s working best tracks that were not reflected in the tcvitals. Numerous international agencies also track tropical cyclones in their regions, but these tracks are generally not available for IBTrACS until the subsequent year.

NOAA NCEI also has to make several decisions about the details of the counting methods:

• Subtropical portions of tracks are counted but extratropical portions are not
• Only synoptic hours (00z, 06z, 12z, 18z) are used for ACE
• An individual storm is counted in every month and basin in which it occurs. As a result, the yearly count does not equal the sum of the months nor does the global count equal the sum of the basins. However, ACE is counted for the month/basin at that time, so it can be aggregated.
• IBTrACS “spurs” are included for ACE but not for other counts
• Northern Hemisphere counts are for January–December. Southern Hemisphere counts are for July–June. Depending on the report, global counts are either January–December or the sum of the two hemispheres