Mixed predictions for 2015 Atlantic hurricane season
COLORADO – While long-range forecasts for 2015 Atlantic hurricane activity indicate another average to slightly below average season, there is some divergence of opinion so far in advance of the official start of the season on June 1.
The respected team of Klotzbach and Gray from Colorado State University’s Department of Atmospheric Science believes that we remain in an active era for Atlantic basin tropical cyclones since 1995 (despite the quiet seasons that occurred in 2013-2014), and say that they expect typical conditions associated with a positive Atlantic Multi-Decadal Oscillation (AMO) and strong thermohaline circulation (THC) will return in 2015.
In December, the Colorado State forecasters noted that it was challenging to forecast whether or not the then developing weak El Niño would persist through the 2015 hurricane season. While significant weakening of the Atlantic Multidecadal Oscillation (AMO) and thermohaline circulation (THC) was noted during the spring of 2014, North Atlantic SST and sea level pressure patterns have since rebounded to conditions characteristic of an active era.
Klotzbach and Gray anticipate that the 2015 Atlantic hurricane season activity will be primarily determined by the strength of the THC/AMO and by the state of ENSO.
For the 2015 hurricane season, four possible scenarios are anticipated with the probability of each as indicated below:
THC circulation becomes unusually strong in 2015 and no El Niño event occurs (resulting in a seasonal average net tropical cyclone (NTC) activity of ~ 180) – 10 percent chance.
THC continues in the above-average condition it has been in since 1995 and no El Niño develops (NTC ~ 140) – 40 percent chance.
THC continues in above-average condition it has been in since 1995 with the development of a significant El Niño (NTC ~ 75) – 40 percent chance.
THC becomes weaker and there is the development of a significant El Niño (NTC ~ 40) – 10 percent chance.
Hence a large range of uncertainty remains about the outlook, which appears to suggest anything from slightly above average to below average, based on the extended range qualitative outlook. Typically, hurricane seasons with those NTC values have the following tropical cyclone activity:
180 NTC – 14-17 named storms, 9-11 hurricanes, 4-5 major hurricanes
140 NTC – 12-15 named storms, 7-9 hurricanes, 3-4 major hurricanes
75 NTC – 8-11 named storms, 3-5 hurricanes, 1-2 major hurricanes
40 NTC – 5-7 named storms, 2-3 hurricanes, 0-1 major hurricane
The greatest probability envisioned so far in advance was consequently thought to be from 8 to 15 named tropical storms, with from 3 to 9 hurricanes and 1 to 4 major hurricanes.
An earlier prediction from British-based Tropical Storm Risk (TSR) said that their extended range forecast suggested another below average season in 2015.
Based on projected climate signals, TSR forecast Atlantic basin tropical cyclone activity to be around 20 percent below the 1950-2014 long-term norm and around 30 percent below the recent 2005-2014 10-year norm.
TSR said that its main predictor for this extended range forecast is expected July-September trade wind speed over the Caribbean Sea and tropical North Atlantic. TSR noted that this can influence cyclonic vorticity (how storms spin up) as well as vertical wind shear in the main hurricane track region.
While noting a low level of precision in its long range predictions, TSR forecast 13 tropical storms, 6 hurricanes and 2 intense hurricanes, with accumulated cyclone energy (ACE) of 79.
The 65-year climate norm is 11 tropical storms, 6 hurricanes, 3 intense hurricanes and ACE of 102, while the 10 year average is 15 tropical storms, 7 hurricanes, 3 intense hurricanes and ACE of 113.