Preliminary 2021 North Atlantic Tropical Cyclone Season Outlook And 2020 Review

Highlight: The 2021 season is more active than normal with 16 tropical storms, 9 hurricanes and 3 major hurricanes.

The preliminary 2021 outlook: The primary climate indicators of seasonal activity of tropical cyclones across the North Atlantic basin is phase of the El Nino southern oscillation (ENSO) and upper ocean heat of the North Atlantic basin. An optimum condition for an active tropical cyclone season is the presence of La Nina and above normal sea surface temperature anomalies (SSTA) across the tropical/subtropical North Atlantic basin both of which were present last year (Fig. 1).

Currently, La Nina is weakening (Fig. 2). Waters off the northwest coast of equatorial South America are warming. However, the tropical/subtropical North Atlantic basin remains warmer than normal which is unusual for January when due to seasonality waters in this region generally cool.

Reliable global SSTA forecasts extend to June 2021, the beginning of the tropical cyclone season. Two lead models IMME (International Multi-model Ensemble) and ECMWF (European Center for Medium-range Forecasts) indicate the La Nina is near or at an end while the tropical/subtropical North Atlantic basin is marginally warm (Fig. 3-4). A collection of all dynamic and statistical ENSO phase forecast models indicates neutral ENSO is most likely for the 2021 tropical cyclone season (Fig. 5).

A simple analog forecast using an ENSO trend from La Nina to neutral ENSO with marginally warm SSTA in the tropical subtropical North Atlantic basin (using Atlantic multi-decadal oscillation and tropical North Atlantic index) taken from the 25-year climatology is produced to estimate 2021 seasonal activity (Table 1).  The analog years are 2018, 2012 and 2001. The analogs reveal a slightly more active season ahead for number of tropical storms (16) and hurricanes (9) with near the 25-year normal for major hurricanes (3) and seasonal accumulated cyclone energy index (123). The 25-year normal is used due to the long-term change of ENSO and North Atlantic SSTA patterns in the mid-to-late 1990’s which clearly have produced on average a more active season compared to the older climatology.

Table 1: Using an ENSO and AMO/TNA based analog the projection of North Atlantic basin seasonal tropical cyclone activity for 2021 is indicated.

Fig. 1: At the climatology peak of the 2020 North Atlantic basin tropical cyclone season Al Nina was in-place and the North Atlantic basin was very warm.

Fig. 2: Current global SSTA analysis reveals La Nina 2020-21 is beginning to weaken while anomalous warmth continues to dominate the North Atlantic.

Fig. 3: The IMME global SSTA forecast for June 2021 reveals weak La Nina to neutral ENSO and near normal subtropical/tropical North Atlantic SSTA.

Fig. 4: The ECMWF global SSTA forecast for June 2021 reveals neutral ENSO and marginally warm subtropical/tropical North Atlantic waters.

Fig. 5: The latest collection of dynamic and statistical ENSO phase forecasts from the International Research Institute for Climate and Society.

Review of the 2020 season: The 2020 season produced a record number of tropical cyclones (30), the second-highest number of hurricanes (13) and 6 major hurricanes. The most hurricanes on record (15) occurred in 2005. On 9 occasions a total of 6 (or more) major hurricanes have occurred. The last time 6 major hurricanes occurred in one season was 2017. The most major hurricanes (7) occurred in 2005 and 1961. The accumulated cyclone energy (ACE index) was 180 in 2020 which is 9^th highest on record. The highest ACE on record is 250 observed in 2005. The 2020 season (Fig. 6) produced 6 land-falling hurricanes in the U.S. one of which was a major hurricane (Laura). An additional 6 tropical cyclones struck the U.S. coast. The total number of coastal strikes (12) is a record.

The 2020 season is also memorable for three hurricanes striking the northwest coast of the Gulf of Mexico – each producing different characteristics upon landfall.

On August 27^th Category-4 Major Hurricane Laura made landfall at 0600 GMT near Cameron, LA. Laura was the first category-4 major hurricane to strike southwest Louisiana. Famously, NOAA/NHC issued a life-threatening storm surge from Freeport, TX to the mouth of the Mississippi River for Laura…some 360 miles in straight-line distance. The maximum storm surge was 17.2 feet located at Rutherford Beach, LA.

On October 9^th Hurricane Delta made landfall as a category-2 hurricane near Creole, LA. Maximum wind speed was 100 mph. Wind speeds near 100 mph were observed as far west as the Port Arthur area. The high wind speeds at great distance west of landfall are unprecedented.

On October 28^th Hurricane Zeta made landfall as a strong category-2 hurricane at Cocodrie, LA.  Despite 110 mph wind at landfall, the near-immediate back side of the hurricane (west of the landfall track) received greatly diminished wind speeds.

The lowest pressure of the 2020 season was 917 mb by the 30^th storm of the season…Category-5 Major Hurricane Iota in the western Caribbean Sea.

The climatology peak of the North Atlantic tropical cyclone season is September 10-11. On September 14, 2020 there were 5 tropical cyclones in the North Atlantic basin at one time (Fig. 7).

The catalyst to the record-breaking activity of the 2020 North Atlantic basin tropical cyclone season was the widespread warmer-than-normal ocean surface (Fig. 8). Thanks to strengthening La Nina the upper-level atmosphere ventilation and wind shear conditions were ideal to utilize the potential energy represented by the anomalous warm ocean.

The early June 2020 forecasts were updated increasing seasonal amount significantly (Fig. 10) as confidence increased that La Nina would develop and the North Atlantic Ocean surface could be near-record warm. The Climate Impact Company forecast indicated 20 tropical cyclones, 10 hurricanes and 4 major hurricanes which at that time were the most of the major forecasters. The projected hurricane tracks indicated at least 4 Gulf of Mexico hurricanes (6 occurred).

Fig. 6: The NOAA/NHC North Atlantic basin depiction of the 2020 tropical cyclone season.

Fig. 7: Satellite view of the North Atlantic basin on September 14, 2021 featuring 5 tropical cyclones at one time.

Fig. 8: North Atlantic basin sea surface temperature anomaly analysis at the climatology peak of season (September 10) identifying vast anomalous warmth.

Fig. 9: The North Atlantic basin 2020 tropical cyclone activity forecast including projected hurricane tracks by Climate Impact Company.

A complete verification of forecasts for 1995-2020 will be issued with the early April update.