The 10th September is the official statistical peak of the Atlantic Hurricane Season as depicted in Fig 1 from NOAA. So far, it has been a busy 3 months with 14 named tropical storms of which five have reached hurricane status and three major hurricane status. As such it is well ahead of schedule and the second half of the season (Sept-Oct-Nov) is normally the busiest period, with more systems which are often also stronger than during the first three months.
Figure 1. Number of storms per 100 yrs in respect to the calendar year (NOAA)
One major driving force behind each Atlantic hurricane season is the phasing of the El Niño/La Niña Southern Oscillation (ENSO). So far we have had ENSO-neutral conditions with near-to-below average sea surface temperatures (SSTs) persisting in the central and eastern equatorial Pacific. While other factors do come in to play, such as the Atlantic Multi-Decadal Oscillation (which has been favouring more active hurricane seasons since 1995), the west African monsoon (enhanced in 2021) and Accumulated Cyclone Energy (ACE) trends (hyperactive 2021), it is useful to focus solely on ENSO as it has a major influence on seasonal weather patterns.
The combination of an ENSO-neutral state and a rather busy first half to the hurricane season raises the analog question of “how does this hurricane season match-up against previous hurricane seasons that took place during similar ENSO-neutral phases”?
Figure 2. ENSO phasing since 1950 in respect to global temperature changes (HadCrut)
As indicated in Figure 2 there have been 8 years since 2000 with ENSO-neutral characteristics within the range of ±0.5°C of average SST’s: 2001-2, 2004-5, 2006, 2009 and 2013-14. Within these 2005 and 2013 stand out as distinct seasons (see Fig 3). 2005 was a record breaking tropical season (at the time) for the Atlantic basin with 27 named storms by the NHC, of which 15 reached hurricane (record) status and 7 became major hurricanes. Those records stood for 15 years until last year when the 2020 Atlantic hurricane season set multiple all new time records. While the 2013 season was slightly above average for named storms, it was the first since 1968 to have no hurricanes reach higher than Cat 2 strength.
Figure 3. Table comparing # of occurrences for each ENSO-neutral hurricane season (2021 to 15 Sept only)
Since 2000, during an ENSO-neutral Atlantic hurricane season, the running average of named storms is 13.78 per season. The current 2021 Atlantic hurricane season surpassed this average on 12th September as indicated in Fig 4. Referring back to Figure 2, it would appear that an increase in the global average temperature is continuing to be the dominating trend since 1980. Of course, this graphic doesn’t localize specific temperature increases for the North or South Hemispheres however, we do get a good idea of how a warming world may be continuing to act on successive hurricane seasons.
Figure 4. Comparison of mid-2021 Atlantic hurricane season to previous ENSO-neutral years
Presently, the 2021 Atlantic hurricane season is on pace with the record breaking 2005 season which resulted in 27 named storms and where nearly half of all hurricanes were of Category 3 or higher. If this pace continues then the 2021 season could match last year’s record breaking season with 30 named storms. Either way, the data suggest that there is definitely a running trend of similarity between hurricane seasons that occur within ENSO-neutral phases.
Add La Niña
However, the latest forecasts from the WMO Global Producing Centres of Long-Range Forecasts (see Fig 5) suggest that either the cool side of ENSO-neutral conditions continue or that La Niña conditions return near the end of the year: with a 60% chance of ENSO-neutral and 40% for La Niña for September-November, and equal chances of ENSO-neutral and La-Nina re-emergence in October-December and November-January. If La Nina transpires then this mix of competing oceanic and atmospheric conditions generally favour above-average activity for the remainder of the Atlantic hurricane season.
Figure 5. WMO ENSO update September 2021 (Source: WMO)
It is worth noting that the all time record setting 2020 season took place during a mild La Niña event when conditions are more conducive for hurricane development. The difference is that the 2021 season is off to a similar fast start, but started with a neutral ENSO state and now has a 40% chance of becoming La Nina when conditions will be more favourable. We are certainly headed for an above average active season which pre-season was predicted by the main forecasting centres, but will it surpass 2020? This also begs the bigger question that will we continue to see a rise in activity during future Atlantic hurricane seasons due to global warming regardless of ENSO?
Interestingly, the World Meteorological Organization (WMO) have designated 21 tropical cyclone names to the Atlantic, so we have only 9 storm names left until the end of the season. Also, after the record-breaking 2020 season, the WMO decided to end the use of the Greek alphabet and instead have established a supplemental tropical cyclone names list which is highly likely to be used this season for the first time.
Tropical storm prediction is complex and requires very close monitoring 24/7 by experts when optimising ship routes. This is especially true in an above average active hurricane season with the potential for the presence of multiple systems, reduced response times, and adjacent formation and late intensification close to busy shipping regions.
Stay tuned for further updates.
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