2021 North Atlantic Tropical Cyclone Season Forecast

Highlight: Above normal tropical cyclone activity expected (again) in 2021 across the North Atlantic.

Executive summary: The early April tropical cyclone season forecast for the North Atlantic basin is issued. The outlook indicates another active year although not as prohibitive as last year. The forecast indicates 17 tropical storms will form this season of which 9 become hurricanes and 2 become intense hurricanes. The accumulated cyclone energy index (ACE) forecast is 127. The outlook is similar to the preliminary forecast issued in January. The outlook is classified as moderately above normal. The forecast is based on a constructed analog yielding neutral ENSO and a warm North Atlantic basin except near normal in the tropics. The near normal tropical SSTA pattern held the major hurricane total down at 2. Analog year 2012 was prominent in the outlook. Climate Impact Company will provide our hurricane track projection and regional summaries next week.

Table 1: The Climate Impact Company 2021 North Atlantic basin seasonal tropical cyclone activity forecast.

Climate discussion: The 2021 Climate Impact Company North Atlantic tropical cyclone season forecast is based on a constructed analog. The constructed analog identifies important early-to-mid-spring sea surface temperature anomaly (SSTA) regimes pertinent to the North Atlantic tropical/subtropical environment and their expected trend (using the International Multi-Model Ensemble or IMME) through summer and into early autumn. The regional SSTA regimes help to project the North Atlantic tropical/subtropical environment by using past similar regimes to assimilate tropical cyclone activity. The specific regimes include El Nino southern oscillation (ENSO), tropical North Atlantic (TNA) index and Atlantic multi-decadal oscillaltion (AMO).

Added climate factors include the anticipated quasi-biennial oscillation (QBO) regime, tropical Africa precipitation forecast and available mid-atmosphere moisture in the North Atlantic tropics/subtropics. Also considered is the anticipated strength of the North Atlantic Warm Hole (NAWH) and projected presence and strength of the Madden Julian oscillation (MJO).

About the climate predictors: Common-knowledge is the relationship between ENSO and North Atlantic tropical cyclone activity. Presence of El Nino indicates persistent heavy convection in the tropical East Pacific. The latest heat release from this convection is released downwind and helps to create upper shear across the tropical North Atlantic defeating tropical cyclone activity. When La Nina is present just-the-opposite happens…a more active than normal seasonal activity regime develops (as observed in 2020). The latest climatology calculated by Climate Impact Company identifies the typical seasonal activity based on the ENSO regime (Fig. 1).

Fig. 1-2: Climatology of North Atlantic basin tropical cyclone season activity according to ENSO regime and tendency for big hurricane years when the tropical North Atlantic index is warmer than normal during the past 25 years.

The North Atlantic SSTA pattern can be more important than ENSO given the tendency in recent years for North Atlantic warming. The TNA index identifies the SSTA pattern in-between the Caribbean Sea and northwest coast of Africa. This region is also the main development region (MDR) for North Atlantic hurricanes. When the TNA index is warm and no El Nino is present, watch out! Hurricane activity is likely very active (Fig. 2).

The AMO pattern more broadly covers the North Atlantic basin. The long-term cycle of the AMO which typically lasts 2-4 decades is currently warm and initiated in the mid-to-late 1990’s. The +AMO regime is supportive of above normal seasonal activity and the reason for the increase in activity in the 30-year climatology (Table 2).

Table 2: North Atlantic basin tropical cyclone season climatology. Note the more recent climatology produces higher number of storms directly related to the warming trend of the North Atlantic basin.

The QBO represents the upper atmosphere ventilation required to allow tropical cyclones to flourish. When the QBO is strongly negative, deep tropics ventilation is suppressed and prevents tropical cyclones. However, during that regime the subtropical latitudes encounter strong ventilation in the upper atmosphere. The subtropics can be very active in a -QBO regime. A great example is in 2005 when 27 tropical cyclones formed and almost all of them did not intensify until after leaving the deep tropics. When QBO is strongly positive, the deep tropics can be quite active and produce above normal hurricanes.

When tropical North Africa is wet, tropical waves moving into the North Atlantic tropics are more energetic and increasingly capable of producing hurricanes. If tropical North Africa is dry outer tropical North Atlantic tropical cyclone activity is suppressed.

The amount of humidity in the mid-troposphere across the tropical/subtropical North Atlantic basin is a key predictor to seasonal activity. If moisture is abundant at approximately 600 MB seasonal activity can be excessive as observed last year.

The NAWH pattern is a persistent cool pool of water located south-southeast of Greenland during the warm season caused by fresh water runoff from Greenland of the ice sheet. The cooler waters in this region have become more prominent in recent years. The cool pool of water is increasingly blocking the northeast progress of warm waters associated with the Gulf Stream. The blockage of northeast travelling warm water has caused a large mass of anomalous warm water to pile-up off the Northeast U.S. Coast. The warm waters have become well-correlated with blocking high pressure aloft over or north of the warm SSTA region. As a result, some storms such as Harvey (2017) and Florence (2018) have stalled once reaching the coastline and caused extreme rainfall and flooding. The high pressure ridge weakens steering currents allowing tropical cyclones to stop moving. The warm waters alsso increase sea level implying inland-moving storms on the U.S. Northeast Corridor will encounter more water moving ashore pushed by strong wind.

Finally, an active presence of the MJO is important as a short-term predictor of tropical cyclone activity. When the MJO (an area of heavy convection in the deep tropics) is present in the tropical North Atlantic and/or Africa, tropical waves are stronger and more capable of becoming hurricanes.

Forecast methodology: In this report, the ENSO and AMO/TNA pattern(s) are used to project an analog. The analog years are weighted due to other factors during that year such as the West African rainfall, QBO, 600 MB moisture and NAWH intensity. All of the factors mentioned will be more accurately understood in the early June update. The MJO is considered most heavily for 2-3 week ahead operational forecasts during the season.

Currently, ENSO is in a weak La Nina classification (Fig. 3). La Nina is weakening and neutral ENSO is forecast by the IMME model as the peak of seasonal activity begins in August (Fig. 4). The North Atlantic has cooled in recent weeks, typical of springtime. The AMO index is marginally warm while the TNA regime is neutral. The AMO index will trend warmer by August. However, unlike last year when TNA became quite warm and supportive of increased tropical cyclone activity the August 2021 forecast keeps TNA neutral to very weak warm phase.

The base-analog forecast is projected representative of past SSTA ocean-climate patterns when ENSO was neutral, TNA was near neutral and AMO in the warm phase.

The QBO is in a weak positive phase likely to transition toward weak negative phase later in the tropical cyclone season. The QBO is not likely a discernable influence on the 2021 seasonal outlook.

Fig. 3-4: Current global SSTA conditions and regional SSTA pertinent to the tropical cyclone season ahead and the IMME (model) August 2021 forecast and trend of the SSTA predictor regions.

The tropical Africa forecast for JUL/AUG/SEP 2021 by the CIC CA Forecast indicates an aggressive wet forecast although (curiously) dry on the tropical West Africa Coast (Fig. 5). April is too early to project 600 MB moisture in the North Atlantic tropics/subtropics (Fig. 6). This feature becomes a leading predictor once into June and July.

Fig. 5-6: The CIC CA forecast for Africa rainfall during JUL/AUG/SEP 2021 and current precipitable water at 600 MB across the North Atlantic basin.  

Once again, the NAWH is likely present during summertime. The IMME Forecast for August 2021 indicates a slightly warm zone southeast of Greenland with much warmer than normal waters southwest and northeast of that region. Implied is increased potential for a blocking high-pressure ridge over New England or to the north which slows steering currents during the peak of season and may allow U.S. coastal threats to stall.

The North Atlantic basin climatology of activeness of tropical cyclone activity given presence of the MJO and during specific ENSO regimes indicate above normal risk of tropical cyclone activity this season due to presence of neutral ENSO when the MJO is in phase_1/phase_2 or phase_7/phase_8 (Fig. 7). When MJO is in P1/P2 the North Atlantic tropics generate strong tropical waves capable of becoming hurricanes. MJO P7/P8 supports similar above normal activity for the western North Atlantic basin.

Fig. 7: When the Madden Julian oscillation shifts across the tropical North Atlantic/Africa region during a La Nina regime hurricane risk peaks in the North Atlantic basin.  

Climate factors summary: Forecast factors are generally supportive of an above normal seasonal activity year in the North Atlantic basin. Some forecast favors are not clear yet as to their influence on seasonal activity. Forecast confidence is average and should increase to above average with the early June update. Table 3 identifies the influence on the 2021 North Atlantic tropical cyclone season forecast.

Table 3: Forecast factors and their projected influence for the 2021 North Atlantic tropical cyclone season.  

Seasonal forecast: The analog years are selected from the 25-year climatology. Since the middle 1990’s the ENSO and AMO long-term cycles have changed. Also unique to the past 25 years is the increased influence of CO2 on the global atmosphere specifically on high latitude warming and erosion of the polar ice cap. Analog years prior to the middle 1990’s are not considered.

The ENSO analog years are 2018, 2012 and 2001. The analog years are selected based on multivariate ENSO index (MEI). All other predictors are weak for each ENSO analog year except 2012. The AMO pattern and NAWH presence/intensity are very similar to the 2021 projection. Therefore the weighting of the analog years is once for 2018 and 2001 and 3 for 2012 (Table 4). The forecast indicates 17 tropical storms, 9 hurricanes and 2 major hurricanes with an accumulated cyclone energy index of 127. The forecast is similar to the preliminary forecast issued in January. The seasonal activity forecast is greater than each climatology.

Table 4: The Climate Impact Company 2021 North Atlantic basin tropical cyclone season forecast.

Table 5: The 2021 North Atlantic basin tropical cyclone seasonal forecast will be issued by Colorado State University on April 8^th while Tropical Storm Risk/U.K. issues their forecast April 13^th. The forecast by TSR from last December is indicated.

Verification: Although last year’s forecast produced a large error, the outlook was for well above average amount supporting more lenient grading as a record number of tropical cyclones were observed.

Table 6: Climate Impact Company North Atlantic basin pre-seasonal forecasts compared to observed and the attendant error. Red = El Nino year.