08/02/2023, 11:59 am EDT

North Atlantic 2023 TC Outlook Stays More Active Than Typical El Nino Year Although Lowered Slightly

Executive summary: Climate Impact Company has updated the 2023 North Atlantic basin tropical cyclone season forecast. Including 4 previous storms, the 2023 seasonal forecast is now 17 tropical storms, 6 hurricanes, and 2 intense hurricanes with an ACE index of 114. The updated forecast is a moderately sharp decline from the early August forecast which featured 9 hurricanes, 4 intense hurricanes, and an ACE index of 171. Updated forecasts by Colorado State University (issued tomorrow) and Tropical Storm Risk/U.K. (issued early next week) are also projected to feature a slight decline. The seasonal amount is lowered in anticipation that prohibitive warming of the equatorial East Pacific will cause a slow-to-emerge El Nino climate to arrive mid-to-late season and create enough shear to hold down seasonal activity despite the record warm North Atlantic. Despite the lowered seasonal forecast, the activity amount remains well above El Nino climatology. The exceptionally warm SST of the Gulf of Mexico, northwest Caribbean Sea, Bahamas, and off the U.S. East Coast remain supportive of rapid intensification of any tropical cyclone tracking through these waters potentially to major hurricane status. Although the seasonal totals are lowered, 2023 remains a potentially dangerous year to the U.S. coastline due to the presence of a very warm ocean surface. Discussion: In July, factors affecting tropical cyclone development were mixed. One telling observation was the lack of expected development of Tropical Disturbance 96L well east-southeast of Bermuda. Stronger-than-normal westerly flow aloft across the U.S. extended into the subtropical North Atlantic and the resulting shear pattern prevented 96L from becoming a tropical cyclone. At one time, this system received a 70-80% chance of becoming a tropical cyclone forecast by NOAA/NHC. Another possible inhibiting factor to 96L development was a plume of Saharan Dust moving into this area in recent days. Meanwhile, mid-level relative humidity was abundant in this zone and ocean temperatures are much above normal. The supporting factors weighed heavily by NOAA/NHC to make the 70-80% tropical cyclone development forecast were over-shadowed by upper shear and Saharan Dust. Will the upper shear and Saharan Dust continue to inhibit tropical cyclone development? The tendency for above normal wind shear in the subtropics should ease as summer-long global atmospheric angular momentum (GLAAM) shifts from positive phase (above normal mid-latitude westerlies) to the negative phase based on the latest 15-day outlook. The exceptionally dry and hot North Africa climate of summer so far is expected to ease. A wetter than normal climate is projected to develop across this region based on the seasonal precipitation probability forecast from the International Research Institute for Climate and Society. The North Atlantic basin remains record warm with ample upper ocean heat to fuel tropical cyclone development. As of August 1st, the North Atlantic basin SSTA was +1.15C down slightly from a peak of +1.23C one week ago but still at record levels. The tropical North Atlantic (TNA) index which measures SSTA in the main development region (MDR) for North Atlantic tropical cyclones is +1.24C and down slightly from a (record) peak 2 weeks ago. The Gulf of Mexico SSTA is +0.98C, a robust warm signature. Presence of 31C/87F SST or warmer, areas well-known to cause major hurricane development when a tropical cyclone passes through, are located across most of the north and northeast Gulf of Mexico, parts of the Bahamas, and northwest Caribbean Sea. El Nino formed in early June and the SSTA pattern across the equatorial East Pacific features prohibitive warmth including +1.22C in the Nino34 ENSO monitoring region and record warm +3.29C in the Nino12 region off the northwest coast of South America. Oceanic El Nino is in the moderate-to-strong intensity category and comparable with other strong El Nino events (1982-83, 1997-98, and 2015-16) of the past 50 years. The upper ocean heat in the equatorial East Pacific indicates fuel for additional strengthening is present. However, unique to El Nino 2023 is the slow development of the attendant El Nino climate. The most recent multi-variate ENSO index (MEI) for MAY/JUN was +0.2 which represents neutral phase. The JUN/JUL value will be issued next week. A daily climate diagnostic contributing to MEI is the southern oscillation index (SOI) which measure the pressure differential between Darwin north of Australia and Tahiti in the east Pacific tropics. Negative SOI values are required to push MEI into (El Nino-like) negative phase. In July, SOI was -0.33 (neutral phase) although July 18-28 featured a -SOI spike. The 15-day SOI forecast is negative for the next week reversing positive for the following week. Consequently, the El Nino climate is developing very slowly. Given the aggressive warming of the eastern equatorial Pacific, a more persistent -SOI and the attendant El Nino climate is ahead but likely delayed until mid-to-late tropical cyclone season. In summary, the two leading influences on the updated North Atlantic basin tropical cyclone season forecast are conflicting weak El Nino climate which strengthens late in the season coupled with the prohibitive warmth of the tropical and subtropical North Atlantic basin which includes the Gulf of Mexico and waters off the U.S. East Coast. Upper shear is expected to weaken in the subtropics as GLAAM turns neutral or negative while light shear in the deep tropics remains in-place likely increasing after mid-tropical cyclone season as the El Nino climate develops. Years past most closely representing these conditions are used to generate the August 1st to November 30th forecast adding the 4 tropical cyclones already generated in the 2023 season. First, using MEI we list the slowly evolving El Nino climate patterns during mid-to-late tropical cyclone season in the 1950-2022 climatology biasing our selection to the past 30 years due to climate change. For climate prediction purposes, Climate Impact Company defines climate change as a change in the long-term natural cycles of ENSO, PDO, and AMO plus the erosion of polar ice due to high latitude warming and the more recent warming of the mid-latitude oceans. The analog years are 1986, 1991, and 2009. The more recent analog (2009) is weighted twice due to occurrence within the current climate cycle. Second, very warm AMO and TNA analogs are selected. Since the long-term warm cycle of AMO developed in the mid-to-late 1990’s, each year since that time during tropical cyclone season has typically been prohibitively warm. Selected are very warm +AMO years when TNA has also been very warm but not in La Nina regimes. The results identify 1998, 2003, 2005, 2006, 2010, 2012, 2016, 2017, 2021, and 2022 as the warmest AMO/TNA years although due to La Nina presence 1998, 2010, 2016, 2017, 2021, and 2022 are removed from the list leaving 2003, 2005, 2006, and 2012 as the North Atlantic analog component to the forecast. Marginal El Nino climate was present in 2006 therefore weighted twice in the analog forecast. The analog forecast consists of 7 years (1986, 1991, 2009, 2003, 2005, 2006, and 2012) with 2 years weighted twice (2009 and 2006). The range of possibilities is impressive with near record low activity (1986) to near record high activity (2005). Each near record analog year identifies the possibilities if El Nino dominates the climate pattern (1986) or if the warm North Atlantic dominates the pattern (2005). Obviously, a mix of these two extreme scenarios is likely. The average tropical cyclone activity, based on all analog years plus the 4 systems so far in 2023 renders and updated (and final) seasonal forecast of 17 tropical storms, 6 hurricanes, and 2 intense hurricanes with an accumulated cyclone energy index of 114 (Table 1). The new forecast is considerably less aggressive than the forecast issued one month ago. The analog suggests that the El Nino climate pattern will develop and hold down activity (and intensity) despite the very warm North Atlantic. While the number of tropical cyclones forecast for 2023 is above normal, the number of hurricanes, intense hurricanes, and ACE index shifts slightly below normal. The updated 2023 forecast is not as active as the recent (2016-2022) very active period. The outlook is compatible with previously forecast seasonal totals by NOAA. CSU and TSR/UK will update their forecasts Aug. 3 and Aug. 7 respectively. The lower adjustments by Climate Impact Company suggest CSU and TSR will also lower their seasonal amount slightly. Weight Tropical Storms Hurricanes Intense Hurricanes ACE Index 1986 1 6 4 0 36 1991 1 8 4 2 34 2009 2 9 3 2 53 2003 1 16 7 3 175 2005 1 28 15 7 250 2006 2 10 5 2 79 2012 1 19 10 2 133 Average 12.8 6.3 2.4 99.1 2023 4 0 0 14.7 Total 16.8 6.3 2.4 113.8 Forecast   17 6 2 114 Previous 20 9 4 171 30-Year 15.0 7.4 3.2 126.4 15-Year 16.3 7.4 3.2 122.5 50-Year 12.7 6.5 2.6 103.6 2016-2022 18.6 8.4 3.7 150.0 CSU 18 9 4 160 TSR/UK 17 8 3 125 NOAA 12-17 5-9 1-4 N/A Table 1: A comprehensive forecast table identifying the Climate Impact Company analog process, weighted forecast results, and comparison with various climatology, and forecasts from Colorado State University plus the Tropical Storm Risk/U.K.