Highlight: Cooler North Atlantic tropics keeps seasonal activity closer to normal.
Executive summary: The Climate Impact Company North Atlantic tropical cyclone season forecast of seasonal activity for 2021 is updated. The seasonal forecast activity is lowered slightly although the number of intense hurricanes increases from 2 to 3. The ACE index lowers sharply to 106 (from 127). The (new) key to the forecast is above normal trade winds up-welling cooler waters to prevent the typical warming of the deep tropics observed the past 20+ years. ENSO is neutral and favors above normal seasonal activity. The projected upper air features combined with analog years imply only 1 or 2 hurricane strikes to the U.S. this season with most staying offshore the U.S. East Coast. The forecast will be updated around Aug. 1st at the onset of the most active part of the tropical cyclone season.
|Tropical Storms||Hurricanes||Major Hurricanes||ACE Index|
Table 1: The Climate Impact Company 2021 North Atlantic basin seasonal tropical cyclone activity forecast.
Climate discussion: Predictors for the 2021 North Atlantic tropical cyclone season are aligning a little differently from expectations earlier this year. The primary adjustment is a less warm North Atlantic SSTA pattern (Fig. 1). Sufficient anomalous warmth to support the prevailing decadal warm character of the Atlantic multi-decadal oscillation (+AMO) remains in the forecast (~+0.40 for AUG/SEP/OCT), but barely. Most of the North Atlantic warming is in the subtropics to the northwest basin. The anomalous warmth in that stretch suggests presence of the Bermuda High enabling sunlight to warm the water surface. South of the Bermuda High trade winds are at moderate or higher speeds and forecast to up-well enough cool water to keep the North Atlantic tropics near to slightly above normal. The tropical North Atlantic index is forecast around +0.15 for AUG/SEP/OCT 2021 rivaling 2018 and 2011 for the coolest of the past 10 years.
The ENSO regime is projected neutral for AUG/SEP/OCT 2021 with a potential transition toward weak La Nina for late tropical cyclone season. The global climate pattern is likely to favor weak La Nina lingering from the 2020-21 (La Nina) episode. Consequently, below normal upper shear across the tropical North Atlantic is likely which is supportive of above normal tropical cyclone activity and intensity.
Fig 1: The C3S/ECMWF global SSTA forecast for AUG/SEP/OCT 2021 and regions of influence on the North Atlantic 2021 tropical cyclone season.
Finally, the 2013-2021 “warm blob” in the Northeast Pacific and compensating North Atlantic warm hole (NAWH) south-southeast of Greenland persist during the core of the 2021 tropical cyclone season. The influence on U.S. climate is a weak upper trough in the Mid-South States and upper ridge pattern in the mid-to-high latitudes (Fig. 2). The high-latitude ridge will sustain east-to-west steering current for tropical cyclones toward the U.S. while the upper trough in the Mid-south States has a tendency to turn Gulf storms northward.
Fig. 2: The C3S/ECMWF global 500 MB anomaly forecast for AUG/SEP/OCT 2021 and related influence on the North Atlantic tropics.
Forecast methodology: Analog years are selected based on an approximation of similar SSTA conditions to the AUG/SEP/OCT 2021 forecast by C3S/ECMWF from the past 25-year climatology. The analog(s) are required to contain a weak ENSO pattern, moderate +AMO, weak +TNA and possibly the “warm blob” and NAWH for more recent years. The selected analog years are 2000, 2001, 2012, 2016 and 2018. Previous forecasts used only 2000, 2012 and 2018.
The Forecast: The analog years are amazingly consistent with seasonal activity. Each analog year is weighted evenly and the consensus projects 16 tropical storms, 8 hurricanes and 3 intense hurricanes for the 2021 season (Table 2). The accumulated cyclone energy (ACE) index is lowered to 106. The seasonal activity forecast is close to the 15-year climatology. Although the number of intense hurricanes is increased from 2 to 3, the overall character of the seasonal forecast is slightly less active than the April outlook. The slight descent in the number of tropical cyclones, hurricanes and especially ACE index is the cooler than previously expected deep tropics SSTA pattern.
|Year||Tropical Storms||Hurricanes||Major Hurricanes||ACE Index|
|FORECAST||15.8 (16)||8.4 (8)||2.8 (3)||105.8 (106)|
Table 2: The FINAL Climate Impact Company 2021 North Atlantic basin tropical cyclone season forecast.
Projected tracks: The projected upper air pattern by C3S/ECMWF indicating a high latitude upper ridge pattern suggests steering currents are available to push tropical cyclones westward toward the U.S. coastline. The projected weak upper trough in the Mid-South U.S. implies Gulf systems turn northward. The combination of the two upper air features suggests limited Gulf of Mexico landfalling hurricanes while the East Coast is threatened by nearby storms.
The prevailing hurricane tracks of the analog years include mostly well offshore paths. The Southeast States had encounters with a couple hurricanes in 2016 and 2018.
Combining the projected tracks based on the upper air forecast and analog tracks combined with the lower ACE index outlook the Climate Impact Company projected tracks outlook indicates only 1 or 2 hurricanes able to move inland the U.S. Coastline, possibly 1 each (Gulf landfall and East Coast landfall). The near normal deep tropics SSTA forecast implies strongest storms for this season are in the subtropics. In this pattern, long distance travelers from the eastern tropical North Atlantic are below normal risk for the 2021 season.
Other forecasts: All forecasts are slightly above to above normal activity (Table 3). The Climate Impact Company forecast is the least active of the major forecasters. The final update from CSU is issued Friday June 4. Climate Impact Company cites cooler North Atlantic tropical SSTA to lower their forecast slightly since April. Conversely, TSR indicates the North Atlantic tropics will warm and their updated late May forecast is slightly more robust than April.
Table 3: The 2021 North Atlantic basin tropical cyclone seasonal forecast activity by leading forecasters. All outlooks are current except CSU.