As tropical storm Nadine slowly winds down near the Azores islands, it appears that we’ll see a break in the action across the basin. The season is well ahead of climatology for this date, but will that trend continue?
We’ve already had 14 tropical storms, 8 hurricanes, and 1 major hurricane, but most signs point to a generally quieter period coming up compared to what we’ve seen so far.
Of course, a potent hurricane can still occur even during periods of below-average activity, so it’s definitely too early to dismiss the season!
The “Cape Verde season”, when long-lived storms form off the west coast of Africa and track across the deep Atlantic for 10-14 days, is nearing its climatological end. This year, that shouldn’t make a big difference, since we only had one storm briefly become a hurricane south of 25N (Ernesto in early August). All other hurricanes formed above 25N which is quite rare. In the 10 weeks or so remaining in the official hurricane season, most storms typically form in the western Caribbean, Gulf of Mexico, or off the U.S. East coast
The sea surface temperatures across the Caribbean and western Gulf of Mexico are just slightly above average, while the eastern Gulf of Mexico is slightly below average. Overall, these sea surface temperatures shouldn’t be a big factor.
In the diagram above, the phase (location around the globe) of the MJO is shown. The MJO can enhance or suppress disturbances over a large area for weeks at a time. It can be tracked around the world: the purple and red lines trace the observed location and strength of the MJO since August 10, and the green and yellow lines are forecasts starting on September 19. When the MJO is in phases 1-2, activity in the Atlantic typically picks up, and when it’s in phases 6-7, Atlantic activity is suppressed. In other phases, the effect is fairly neutral. As you can see, for at least the next two weeks, the MJO should act to suppress tropical activity in the Atlantic.
ENSO is another important ingredient in controlling relative activity levels in the Atlantic, and is governed by the warmth of the equatorial East Pacific ocean. When it’s warmer than normal, there’s an El Nino, and when it’s cooler than normal, there’s a La Nina.
La Nina is generally conducive for Atlantic activity, while El Nino is unfavorable (this is due to a complex feedback that increases the vertical shear over the deep tropical Atlantic during El Nino and decreases it during La Nina). In the figure shown above, a number of sophisticated models from around the world all predict El Nino conditions to dominate and persist through next spring (positive temperature anomalies in the equatorial East Pacific).
So, while we had an active streak since early August, it should slow down, but that doesn’t mean the season is over. Having slightly below average activity this time of year can still result in some powerful hurricanes, perhaps just not as many or maybe not as strong.
Season statistics so far
Accumulated Cyclone Energy (ACE) is a standard metric used for objectively measuring tropical cyclone activity. It’s doesn’t take into account exactly how many hurricanes form, or exactly how long a storm was a major hurricane, but rather it simply squares the wind speed of any storm exceeding 40mph at each 6-hour interval and adds them all up. Therefore, it’s possible that a very long-lived tropical storm could contribute more ACE than a short-lived low-end hurricane.
As of today, the ACE is 88.3, while the 1981-2010 average for this date is 69.9... that’s 126% of “normal”. By the end of the season, the median ACE wraps up at 92 (the mean is 104). So to even meet the median, we only need another 4 ACE units, which isn’t hard to do.
The seasonal forecast issued by Colorado State University predicts a total ACE of 99, while the nebulous seasonal forecast issued by NOAA predicts a 70% probability of the ACE falling in the 69–124 range. An ACE above 103 is considered to be “above normal”.
As far as Nadine goes, it has now been on the books for nine days, and could be around for at least another week, but probably not as a tropical cyclone. Given its current organization, it may only have 1-2 days left before it becomes an extratropical cyclone or even a low-level remnant low.
Being the only game in town, however, NASA’s big field program, HS3 (Hurricane and Severe Storm Sentinel), is investigating Nadine and its environment in details with the unmanned Global Hawk aircraft. It departed from Wallops Island, VA yesterday afternoon around 2:30 p.m., made the long flight out to the Azores, flew the prescribed “lawnmower pattern” over the storm, and is now heading back to base.
At 11 a.m. EDT today, Nadine is a 50 mph tropical storm, and has been maintaining that approximate intensity for over two days. The official forecast keeps it there for the next five days. Meanwhile, the steering flow is extremely weak and model guidance is all over the place (as it typically is when faced with weak forcing), so it won’t budge too far from the Azores through the weekend.
* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.