Executive Summary: The 2018 Climate Impact Company North Atlantic seasonal tropical cyclone outlook is issued. The next update will be available around June 1. The outlook projects 12 tropical storms, 7 hurricanes and 3 intense hurricanes in 2018. The accumulated cyclone energy index forecast is 93. The outlook is slightly less active than the 30-year climatology and very similar to neutral ENSO climatology forecast for this season. The activity is considerably lower than last year. A transition from weak La Nina toward weak El Nino coupled with only near normal warmth of the ocean surface in the deep North Atlantic tropics are lead contributors to the forecast.
Regional forecast discussions: Based on the analog forecast and adjusted for the quasi-biennial oscillation a projection of expected activity for the 2017 season is indicated.
ENSO tropical cyclone climatology for the North Atlantic basin: The climatology of North Atlantic basin tropical cyclones is recalculated and updated. Activity has generally increased compared to the long-term average during the past 2 decades (Table 1) due to better detection systems and primarily the persistent warming of the North Atlantic basin.
The primary predictor of seasonal activity is the El Nino southern oscillation (ENSO) phase. The convention is a low (high) shear environment across the North Atlantic tropics/subtropics associated with La Nina (El Nino) causes above (below) normal seasonal tropical cyclone activity. A look at intensity of ENSO further demonstrates the very active tropical cyclone seasons when La Nina is strong and suppressed activity when El Nino is strong (Table 2). Note that in the absence of moderate to strong El Nino seasonal activity, especially hurricanes, there is similar to long-term climatology.
Table 1: North Atlantic basin tropical cyclone season climatology.
|Tropical Storms||Hurricanes||Intense Hurricanes|
Strong La Nina
Weak La Nina
Weak El Nino
|Strong El Nino||7.8||3.5|
Table 2: North Atlantic basin tropical cyclone season climatology according to phase of the El Nino southern oscillation.
Accumulated cyclone energy (ACE) is a measure of tropical cyclone intensity which is useful evaluating a single storm or intensity of storminess over a given period such as one month or a season. Last year the ACE index was 226 which was the 3rd highest on record. The month of September set a record for ACE activity. ACE responds similarly to ENSO phase peaking when strong La Nina is present with a suppressed seasonal value during strong El Nino.
Fig. 1: North Atlantic basin tropical cyclone season climatology.
Fig. 2: North Atlantic basin ACE climatology according to phase of the El Nino southern oscillation.
The ENSO forecast: La Nina formed last autumn and entered the mature phase during early 2018. La Nina is lingering as mid-spring approaches. The 2017-18 La Nina was one of the weakest on record but stubbornly is hanging on despite warming of the deep subsurface in the eastern equatorial Pacific. The trend in subsurface temperatures indicates the cool water supply sustaining La Nina in the equatorial East Pacific is increasingly shallow but remains in-place while warming shifting eastward from the West Pacific is undercutting the cool surface water. The subsurface temperature trend is the best diagnostic to judge ENSO phase ahead. Based on recent trends the weak La Nina may hang on for longer than previously expected and the idea of El Nino later in 2018 previously stated by Climate Impact Company may be too aggressive,
A collection of dynamic and statistical ENSO phase forecast models by the International Research Institute for Climate and Society indicates the most likely ENSO phase for 2018 is neutral after La Nina’s demise this spring season (Fig. 3). Similarly, the Climate Impact Company updated analog forecast of ENSO phase also indicates neutral ENSO becomes dominant in 2018 (Fig. 4). There is agreement that La Nina will fade and not be followed by a major ENSO episode with slight favoritism (by IRI) for El Nino later this year.
Fig. 3: The ENSO phase forecast by all dynamic and statistical models collected by the International Research Institute for Climate and Society.
Fig. 4: The ENSO phase forecast using analog years collected by Climate Impact Company.
Analog years to generate the forecast: A notable increase on climate has been generated by mid-latitude ocean and atmosphere regimes best measured by the Pacific decadal oscillation (PDO) and Atlantic multi-decadal oscillation (AMO).
The PDO influence on tropical cyclone season is to increase (decrease) the ENSO signal dependent on whether PDO has a similar phase to ENSO. Most of the time (~75% since 1950) PDO phase runs parallel to ENSO and increases the ENSO influence on U.S. climate and the tropical cyclone season.
North Atlantic basin tropical cyclone activity has generally increased over the past 20 years due to the warming of the North Atlantic as indicated by the arrival of the long-term warm cycle of the Atlantic multi-decadal oscillation (AMO). +AMO increases the number and intensity of tropical cyclones in the North Atlantic basin in the absence of El Nino. This observation helps to explain why neutral ENSO phase is nearly as active as La Nina producing seasonal tropical cyclone activity I the North Atlantic.
Currently, the PDO is in neutral phase. The PDO signature is unique. Both cool and warm SSTA anomalies are located in the northeast North Pacific while anomalous warmth dominates near the Dateline (Fig. 5). The warmth in the subtropics given presence of cool La Nina waters in the equatorial region is very unusual and renders PDO as an uncertain influence on the North Atlantic tropical cyclone season. The warmth in the subtropical East Pacific is likely to spawn above normal tropical cyclone activity this season. Usually seasonal activity in the East Pacific and North Atlantic basin(s) oppose one another.
Fig. 5: Current global SSTA and regional SSTA affecting the North Atlantic environment for tropical cyclone activity.
In the North Atlantic a very warm signature dominated 2017 and was in-part responsible for the intense 2017 tropical cyclone season. However, the North Atlantic, particularly the tropics has cooled the past 2-3 months. The tropical North Atlantic (TNA) index which defines SSTA in the main development region (MDR) for hurricanes in-between the Caribbean Sea and northwest coast of Africa was cooler than normal in JAN and FEB recovering to near normal in MAR. The basin-wide measure defined by the AMO is marginally warm.
The lead model forecasting global SSTA for JUL/AUG/SEP 2018 when the North Atlantic tropics becomes most active reveals neutral ENSO with a trend toward weak El Nino, a warm North Pacific and moderately warm AMO although the TNA index in the tropics is neutral (Fig. 6). The Climate Impact Company 2018 seasonal forecast is based on the historical relationship of climate patterns caused by neutral ENSO and a warmer-than-normal North Atlantic except near normal SSTA in the deep tropics.
Fig. 6: Current global SSTA and regional SSTA affecting the North Atlantic environment for tropical cyclone activity. The Climate Impact Company 2018 North Atlantic tropical cyclone season forecast is based on neutral ENSO, warm AMO and neutral TNA.
Of the ENSO analog years previously identified in the Climate Impact Company ENSO analog forecast years in which ENSO was neutral with warm AMO but neutral to weak +TNA identify only 2001 and 2006 as a qualifiers.
The forecast: The seasonal activity forecast is based on a simple average of the 2001 and 2006 tropical cyclone seasons (Table 3). Each year best approximates the projected ENSO, AMO and TNA conditions. The outlook indicates 12 tropical storms, 7 hurricanes and 3 intense hurricanes and an ACE index of 93. The seasonal activity forecast is very close to neutral ENSO climatology and slightly below the 30-year climatology and much less than last year.
|Forecast||12.0 (12)||6.5 (7)|
Table 3: The tropical cyclone season forecast for 2018 is based on 2 analog years.
In 2018 the ENSO trend is away from La Nina and possibly toward El Nino which is the opposite of 2017 helping to explain the lower seasonal numbers. In addition, despite no El Nino the subtropical East Pacific is very warm. The warm SSTA in this region could produce an over-achieving season in the East Pacific. If so, the influence downstream on the tropical North Atlantic would be to suppress activity. This potential scenario coupled with lack of anomalous warmth in the MDA/TNA region as forecast by the NMME model supports the limited activity potential as observed in 2001.
The ACE index forecast of 93 implies greatly diminished risk to the U.S. as compared to last year. However, the 93 forecast is considerably higher than observed in 2013-2016.
In 2018 the quasi-biennial oscillation (QBO), a measure of ventilation in the upper atmosphere important to tropical cyclones is expected to move into the negative phase. During the negative phase tropical cyclone activity is enhanced in the subtropics where upper ventilation is enhanced but suppressed in the deep tropics. In 2018 the strongest activity will be in the subtropics.
Projected tracks: Based on the two analog years and making adjustments for the QBO Climate Impact Company projects the 7 hurricane paths for the 2018 season. 3 intense hurricanes are indicated. One intense hurricane moves across the Caribbean Sea to the Lower Yucatan Peninsula and across the Bay of Campeche. This system will need to occur early in the season to avoid the suppressing effects of the –QBO peaking mid-to-late season. A late season intense hurricane forming in a familiar spot (southwest Caribbean Sea) roars to high intensity drifting north toward Miami and then out-to-sea. A 3rd intense hurricane is well out-to-sea. A hurricane forming over the warm Gulf Stream is forecast in August. Otherwise the remaining hurricanes for in the central subtropical North Atlantic and drift north to northeast. One system may reach Nova Scotia. The U.S. risk is relatively minimal except for southern Florida and potentially the North Carolina coast, Tropical cyclones not reaching hurricane strength could certainly effect the Gulf of Mexico and U.S. East Coast.
Fig. 7: Climate Impact Company seasonal forecast and projected hurricane paths for the 2018 North Atlantic tropical cyclone season.
Verification: The early season (around April 1) North Atlantic basin seasonal tropical cyclone forecast has been issued back to 1999. The departure from observed to forecast tropical storms, hurricanes and intense hurricanes is indicated in Fig. 8.
Fig. 8: Climate Impact Company early April seasonal forecast verification 1999-2017 is indicated. (Observed versus forecast.)