Two of the leading Atlantic basin hurricane forecasters have both predicted higher-than-normal tropical cyclone activity during this year’s hurricane season, which runs from 1 June to 30 November.
On Thursday, scientists Phil Klotzbach and William Gray issued Colorado State University’s Tropical Meteorology Project’s hurricane forecast for the 30th year. The duo predicts 18 named storms, nine hurricanes and four major hurricanes of Category 3 strength or higher during the 2013 hurricane season. Those number are all significantly higher than the 1981-2010 median figures of 12 named storms, 6.5 hurricanes and two major hurricanes.
University College of London professors Mark Saunders and Adam Lea of the UK-based Tropical Storm Risk.com site also predicted a moderately active hurricane season in their update issued on 5 April, forecasting 15.2 named storms, 7.5 hurricanes and 3.4 major hurricanes, all more than the 63-year climate norm of 10.8 named storms, 6.3 hurricanes and 2.7 major hurricanes.
One of the key factors influencing Klotzbach and Gray’s forecast is the El Niño–Southern Oscillation – also know as ENSO – which is a cyclical anomalous warming of the sea surface temperatures in the equatorial eastern Pacific Ocean. This anomalous warming, known as an El Niño event, creates increased wind shear in the upper atmosphere of the Atlantic basin, something that is known to inhibit the formation and strengthening of tropical cyclones. Conversely, a La Niña is the anomalous cooling of the sea surface temperatures in the equatorial eastern Pacific Ocean, a phenomenon that allows for favourable upper atmospheric conditions for hurricane formation and strengthening in the Atlantic basin.
This year, the large majority of the forecast models predict an ENSO neutral condition, meaning the sea surface temperatures in the tropical Pacific Ocean will fall within 0.5 degrees Celsius of normal. ENSO neutral conditions don’t inhibit tropical cyclones in the Atlantic and tend to allow other climatic factors to dictate how active the hurricane season is in the basin.
“It appears that the chances of an El Niño event this summer and fall are unlikely,” Klotzbach and Gray’s report states, although it later admits that there is “considerable uncertainty as to what is going to happen with the current neutral ENSO”.
“The spring months are known for their ENSO predictability barrier,” the report states. “This is likely due to the fact that from a climatological perspective, trade winds across the Pacific are weakest during the late spring and early summer and therefore changes in phase of ENSO are often observed to occur during the April to June period.”
However, their report also states that the European Centre for Medium-Range Weather Forecasts generally shows the best prediction skill of the various ENSO models and that the average of its ensemble members is calling for an ENSO anomaly of only 0.2 degrees Celsius, well within ENSO neutral conditions.
Sea surface temperatures
Another key factor that affects the hurricane season is the sea surface temperatures in the tropical Atlantic Ocean north of the equator, also known as the Main Development Region. Higher sea surface temperatures in that region promote more tropical cyclone activity.
Tropical Storm Risk and the Colorado State University duo both pointed to higher sea surface temperatures as a factor in their forecasts.
“Significant anomalous warming has occurred across the tropical Atlantic during the past two months,” Klotzbach and Gray’s report states. “[Sea surface temperatures] in the western tropical Atlantic are at near-average values, while the eastern tropical Atlantic is now significantly above average.”
Klotzbach and Gray’s report states that the current negative phase of the North Atlantic Oscillation creates weakened trade winds across the tropical Atlantic, which reduces mixing of warmer surface waters with cooler deep waters, resulting in higher sea surface temperatures.
“Anomalously weak westerly winds in the mid-latitudes also promote anomalous ocean currents out of the south, which contributes to the general warming of [sea surface temperatures] throughout the North Atlantic basin,” the report states.
The combination of higher sea surface temperatures and lower wind shear are usually a precursor to an active hurricane season.
“The atmospheric state across the tropical Atlantic looks quite favourable for an active season, as wind shear anomalies across the basin have generally been well below average over the past two months,” the Klotzbach/Gray report states.
The report lists five hurricane seasons since 1900 with characteristics most similar to what was observed in February-March 2013. Those years include 1915, 1952, 1966, 1996 and most recently, 2004, the year that spawned 15 named storms, nine hurricanes and six major hurricanes, including Hurricane Ivan, which devastated Grand Cayman.