Average 20 year old weather nerd. Plymouth State University Meteorology, Class of 2018. NOAA Hollings scholar. Summer 2016 intern at NWS Boston.
By: MAweatherboy1, 5:58 PM GMT on September 07, 2015
Good afternoon, and Happy Labor Day to everyone! I hope you’re all enjoying this unofficial end of summer. It certainly feels like summer here in the Northeast, with a late season heat wave kicking off for some places today, and record or near-record heat in the low-mid 90s likely for the next three days. This time of year is also the climatological peak of Atlantic hurricane season, and my post today will go into how this season has performed relative to expectations so far, and a quick look at what’s going on right now and what to expect for the remainder of this season.
There’s been a good deal of discussion on the main blog about whether this has been the season it was billed to be. I made a comment on the matter a few days ago, but feel it is something worth going into a little deeper. Pre-season forecasts for the Atlantic were essentially unanimous in calling for a below average, perhaps well below average, hurricane season. The two main factors behind this were the expectations for a strong, near record El Nino, an anomalous warming of the equatorial Pacific waters that increases wind shear and subsidence across the Atlantic basin, and a cool phase of the Atlantic Multi-decadal Oscillation (AMO), which promotes cooler than normal waters and more stable than normal conditions over the Atlantic deep tropics, or “Main Development Region (MDR). Nearly all factors pointed towards very little activity in the MDR, with the subtropics seen as a better bet for tropical cyclone formation. Revised forecasts issued by Colorado State University and NOAA in early August continued to echo these sentiments.
So, how have we performed relative to expectations? Well, to some extent it depends on how you look at it. With the recent formation of Tropical Storm Grace, the Atlantic basin is actually about a week ahead of the typical date of formation of the seventh storm of the season. Furthermore, an impressive four storms (Danny, Erika, Fred, and Grace) so far have formed in the MDR, an area that was supposed to be very hostile for tropical cyclone formation this year. In addition, the subtropics have been quiet after a few weak early season storms in those regions. This idea of more MDR activity runs opposite to seasonal expectations. However, it is not all bad news for the seasonal forecasts. Accumulated Cyclone Energy (ACE), generally considered a better overall metric for measuring seasonal activity, is just over 50% of normal year-to-date so far in the Atlantic, which fits in nicely with predictions for a quiet season.
Why the disparity between named storms and ACE? Typically, storms forming in the MDR tend to be large ACE producers, since they often become powerful and have a lot of time to rack up ACE units as they move west or northwest. Not this year. Despite getting a handful of MDR storms, more than expected, the MDR has still been quite hostile. Of the four storms to form in the MDR, only two have become hurricanes, and only one (Danny) very briefly became a major hurricane, just crossing the Category 3 threshold. All of the MDR storms this year have been what I’ve taken to calling “opposite storms”. That is, they form in the MDR, but rather than strengthen as they move west as most do in a typical year, they’ve all weakened and fallen apart, reaching their peak intensities farther east. Grace is proving to be the latest example of this, with the NHC forecasting it to weaken to a depression as it approaches the Caribbean, with a distinct possibility that it doesn’t even survive that long. Not all storms that form in the MDR strengthen as they head west. Even in a favorable year, there are intra-seasonal variations that promote less favorable conditions. But to see all the MDR storms this year behave in a similar fashion is telling.
Tying it all together, we can now look at why we’ve seen the pattern of storm formation this year that we have. As expected, the dominating influence at work is El Nino. This year’s El Nino, one of the top 3 strongest on record by most measures, is causing record high wind shear across the Caribbean Sea and to its immediate north and east, and contributing to a severe drought in much of the Caribbean. Any storms or want-to-be storms that have approached that area this year have met their demise, and it is likely that pattern will hold for the rest of the season. The hostile conditions in a typically favorable area have prevented MDR storms from turning into the ACE producing machines they often are. Low instability levels and dry air in the Atlantic have also helped keep a lid on things.
Figure 1: Global SST anomalies. Note the significant warm anomalies in the equatorial Pacific, from the dateline all the way to the coast of South America. A classic strong El Nino, contributing to a record setting Pacific hurricane/typhoon season, but a below average Atlantic season. (Image credit: Levi Cowan)
Figure 2: Graph of vertical instability in the tropical Atlantic. Well below average instability has persisted again this year, hampering the prospects for strong cyclones in the tropics.
Still, something has to explain why we’ve seen so many MDR storms this year, even if they haven’t been very impressive. I think the reason is two-fold, although it’s certainly possible more factors are involved. First, sea surface temperatures in the central and eastern MDR (east of around 50W) have not been below average as forecast. Rather, they’ve turned generally above normal in this region for the peak of hurricane season. It’s still not an ideal setup, as anomalies in the subtropics to the north are warmer than anomalies in the tropics, but it’s much more conducive than neutral-cold anomalies in the MDR and warm anomalies to the north, which was what was expected, and what the sea-state was for much of the first couple months of the season.
Figure 3: Atlantic Ocean SST anomalies. Note how warm anomalies cover most of the basin south of 40N, and even most of the MDR south of 20N and east of ~60W. While not a strong +AMO signal by any means, it's not the -AMO that could've made the season even less active. (Image credit: Levi Cowan).
Second, this year has seen an unusually strong amount of African Easterly Wave (AEW) activity. These “tropical waves” as they’re called are large clusters of thunderstorms that move westward off the coast of west Africa into the east Atlantic, a seasonal occurrence coinciding with the northward migration of the ITCZ and the peak months of Atlantic hurricane season. We see tropical wave activity every year, but the “wave train” has been especially productive this year, evidenced by wet anomalies in the Sahel region of Africa. Consequently, several of these waves have emerged off Africa already bearing a significant amount of spin and convective activity. They’re ready-made tropical cyclones, with Fred being the best example, an extremely rare hurricane impact to the Cape Verde islands. These strong waves have been occurring frequently since the start of August, although they were inhibited through mid-August by frequent outbreaks of Saharan Air Layer (SAL) dust that blew off the Sahara and into the Atlantic. This dry, stable air inhibits tropical cyclone formation. For the past few weeks, however, as is typical this time of year, these SAL outbreaks have diminished, and while the mid-levels of the Atlantic atmosphere remain drier and more stable than normal, it’s been enough to allow these waves to become tropical cyclones, if only for fairly short durations for MDR storms. Shear has also been fairly low in the central-eastern MDR, with the arm of El Nino’s shear not quite long enough to reach all the way across the basin.
Figure 4: August 2015 precipitation anomalies (in mm) for north-central Africa. The strip of positive (wet) anomalies coincides well with Africa's "Sahel" region. Wet anomalies in this region during hurricane season are indicative of strong tropical waves, which can become tropical cyclones over the Atlantic. (Image credit: University of Washington JISAO).
All in all, I think the seasonal forecasts are doing a pretty good job representing this season’s activity. My original forecast from way back in mid-March called for 8 named storms, 4 hurricanes, and 1 major hurricane, with ACE 40-60% of normal. Looking back on that blog (note: if you go to it, the charts have automatically updated with time and may not match up well with all the text), most of what I said stands true, and forecasts by official agencies echoed many of the same thoughts. I recall a couple of speculative comments I made in early-mid August before Danny formed that my numbers may be too high, but as it turns out that will likely not be the case. As usual, however, there have been some surprises, namely the two primary factors I mentioned above. The MDR has been generally hostile, but the unexpectedly strong wave train, combined with warmer than expected central-eastern tropical Atlantic SSTs, have created a gap in the armor, and our last few cyclones have taken advantage of it, if only for short periods of time. While a moot point, it’s interesting to consider a couple of possibilities. One, as has been mentioned on the main blog, if we had not been in a strong El Nino, this could have been a very dangerous season, given the strong wave train and strong Atlantic ridging that has tended to steer storms westward, towards land. Conversely, given the lack of activity in the subtropics, if the wave train hadn’t been so impressive, this could’ve been a historically quiet season, at least in terms of ACE. As it stands, however, the season is really what it was expected to be: below average, even if not panning out exactly as expected.
Where do we go from here? Well, given the date, we’re just about to hit the typical peak of the mountain, and activity usually slides downward from here. We currently have Tropical Storm Grace, but it is already weakening, sooner than expected, and should become a remnant low in the next few days, meeting the same fate as its predecessors.
Figure 5: Tropical Storm Grace. Grace is weakening now, moving into a drier, higher shear environment, with only a small, disorganized area of thunderstorms. This has been a common theme for MDR-genesis storms moving westward this year.
The reliable global models (ECMWF and GFS) are showing signals of a healthy wave train continuing for the next couple weeks, and have occasionally shown some of these waves becoming tropical cyclones. However, there remains no indication that conditions will improve closer to the Caribbean and US for the rest of the season. In strong El Nino years, Atlantic wind shear typically increases even more towards October, and so El Nino years tend to “shut down” earlier than normal years as well, and this year will likely be no exception. Given the pattern we’re in I would expect another 1-3 storms to form in the MDR in the next month, and follow the general pattern of all the others there this year. As we get towards October, typically we look towards the Caribbean and Gulf of Mexico for late season hurricanes, which can be more threatening due to their genesis locations, with Wilma in 2005 perhaps the best example. However, given the wind shear, the odds of a late season Gulf or Caribbean storm appear lower than average this year.
That is about all I have for today. We’re still just over halfway through the climatological season, so the only sure way to figure out where we go from here is to watch it unfold. Labor Day and the changing of the seasons also means the return of the school year for me, and last week I began my sophomore year here at Plymouth State University, pursuing my degree in meteorology. Given the increased workload I probably won’t have time for any more full length blogs until winter break (unless something irresistible to write about hits me), but I’ll still be popping in on the main blog to discuss whatever may be going on in the world of weather. Thank you as always for reading, and feel free to leave questions/comments!
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.