Discussion: The projected tropical North Atlantic (TNA) index for May 2018 is -0.50C. The daily value of TNA index for late May is -0.95C. The cool values are unheard of in the long-term warm cycle of the AMO which began in the mid-to-late 1990’s. The TNA index is a measure of the tropical/subtropical North Atlantic sea surface temperature anomalies (SSTA) in-between the Caribbean Sea and northwest Africa which is also the main development region (MDR) for tropical cyclones. The last time the cool values projected for May 2018 occurred was in the long-term cool cycle of the AMO during the 1980’s into the 1990’s. The result of those cool AMO/TNA years was diminished tropical cyclone activity.
Meanwhile the subsurface equatorial Pacific Ocean is very warm. The trend is warmer. ENSO phase forecast models are becoming more agreeable for this subsurface warming reaching the ocean surface. The reluctant NCEP CFS V2 model (to warm the Nino34 SSTA region) has flipped warmer and now indicates moderate-strength El Nino could evolve by October.
The combination of the cool tropical North Atlantic and increasing risk of El Nino in the East Pacific is identified (aggressively) by the European (model) for AUG/SEP/OCT 2018. If these conditions occur the 2018 the North Atlantic tropical cyclone season will be very quiet.
Here is a comparison amongst major providers of seasonal tropical cyclone forecasts for 2018 across the North Atlantic. Note the accumulated cyclone energy index of 226 last year, 3rd highest on record and the greatly diminished ACE forecasts for 2018 by leading providers. The destructive force of the 2017 season will not be repeated in 2018. Interestingly, ACE index has been greatly diminished in 2013-2016 while 2012 and 2017 were quite high.
The forecast trends by CSU, TSR and CIC are diminished activity from initial forecasts released in early April. Note the ACE index forecasts especially by TSR and CIC at less than half the normal value.
|Tropical Storms||Hurricanes||Intense Hurricanes||ACE Index|
Table 1: Leading providers of seasonal tropical cyclone activity including Colorado State University, Tropical Storm Risk U.K., Climate Impact Company and NOAA/NHC updated outlooks for 2018 compared to normal and last year are indicated. CSU, TSR and CIC each trend less active than outlooks released in early April. 2018 will be nowhere near as active as 2017.
Fig. 2-3: The North Atlantic cool pool south of Greenland is strengthening. Meanwhile the North Atlantic tropics/subtropics are also unusually cool.
Fig. 4: Widening and strengthening subsurface equatorial Pacific warmth increases risk of El Nino later in 2018.
Fig. 5: Previously reluctant to project El Nino for later 2018 the NCEP CFS V2 forecast now indicates a weak El Nino later this summer possibly strengthening to moderate intensity by October.
Caution: The charts and explanation provided are climate diagnostics, i.e. descriptions of current conditions. The diagnostics by themselves do not necessarily indicate a future forecast. However, the warmer subsurface TREND in the subsurface equatorial East Pacific and cool North Atlantic tropical SSTA IMPLY increasing probability of negating factors for 2018 North Atlantic tropical cyclone activity.