MAweatherboy1's Blog

Is the Atlantic's Active Era Over?

By: MAweatherboy1, 1:26 PM GMT on August 18, 2014

Good morning. My topic today goes a bit outside of the usual individual storm forecasting I do here whenever I feel the urge to. It is now mid-August, and we are approaching the climatological peak of the 2014 North Atlantic hurricane season. We have so far had just two storms, Arthur and Bertha, both of which also became hurricanes, with Arthur by far the more impressive of the two, a strong early season Category 2 storm. However, we have been mired in a slow period lately, and Atlantic Accumulated Cyclone Energy (ACE), has now fallen below normal for the year. I find ACE to be the best way to measure seasonal activity, as opposed to total numbers of storms. For example, last year had an average to above average number of total named storms, but ACE was abysmal, less than a third of normal. This was puzzling, because last year was forecast to be an active year by all major seasonal forecasting agencies. This year, with expectations for El Nino to develop and an Atlantic basin that remained stable and featured a sea surface temperature pattern traditionally unfavorable for development, expectations were much lower, and this time the Atlantic appears to be behaving as forecast. Frequent outbreaks of Saharan dust over the Atlantic, combined with dry, stable air at all levels throughout the basin and high wind shear in the Caribbean, due largely to warm waters in the East Pacific producing convection there, which has sent shear into our basin, has slowed the Atlantic's pace to a crawl. The wave train has been at least average, perhaps better than average, so far this year, but time and again we have seen strong waves hit the water and fizzle due to the dry, stable air. Now, even with the convection-focusing MJO moving through our region of the world, the Atlantic still does not appear poised to produce a significant cyclone in the next 10 days, though a tropical storm threat will exist in the East and Central Atlantic over the next several days as a passing Kelvin wave interacts with a couple of tropical waves. As a whole, however, this year will likely end up well below average, probably with about 6-9 named storms, 3-4 hurricanes, and 0-1 major hurricanes, with ACE only 30-60% of normal. Combined with last year's inactivity and the fact that the preceding year, 2012, had only two storms briefly become major hurricanes despite being an overall active year, has many wondering if we have reached the end of the Atlantic's "active era". This active era began in 1995, as the 1994-1995 El Nino crashed into a La Nina, sparking an active 1995 season. Ever since then, the Atlantic has, on average, been more active than it had been previously observed to be, though advances in satellite technology may be partially to blame for the detection of more storms now. While there have been obvious exceptions, such as the 1997 Super El Nino year and the 2009 El Nino, the past two decades have wowed us with years like 1998, the remarkable 2003-2005 stretch, 2007-2008, and 2010. This is largely due to the Atlantic Multidecadal Oscillation (AMO) moving into its positive phase, which is favorable for Atlantic cyclone development. The AMO has showed signs of wavering back towards neutral and negative territory this year, a red flag that the Atlantic could head into an extended period of relative calm, though certainly with individual exceptions.




Figures 1 & 2: Extremely low vertical instability, as well as frequent outbreaks of Saharan dust, have hampered this hurricane season and last.

So here's my take on the original question, the title of the blog. My simple answer is "maybe." That probably isn't too satisfying though, so I'll dive in a little deeper. Assuming this year turns out quiet, and that is almost a sure bet, that leaves us with two quiet seasons in a row, and three if you count 2012, which featured limited Cape Verde hurricane activity despite having a high number of storms, including Hurricane Sandy. In an “active era”, a streak like that is certainly suspicious. For the past few seasons, particularly since the active 2010 season, we have seen vertical instability levels in the Atlantic fall below normal, which has prevented stronger hurricanes from developing. This year has continued and accelerated that trend of low instability, with instability this year being so low that I think a major hurricane in the open Atlantic is borderline out of the question this year, since the atmosphere just can’t support one. We will have to watch the Caribbean and Gulf of Mexico later this year, as waters there are very warm, and any disturbance that can form with favorable shear conditions will have high potential. But again, as a whole, it is going to be a quiet year, especially the Cape Verde season, because of the low instability. Why the low instability? I really don’t know, I’m not sure that anyone does. But it has literally put a lid on the Atlantic for the past few seasons. So how do I think we’ll know if we’re still in the active era or not? By watching the next two seasons. The key factor for me is El Nino. El Nino, warmer than average waters in the equatorial Pacific, is inherently detrimental to Atlantic tropical cyclone development because increased convection in the East Pacific tends to blow shear into our basin, hindering development. Ironically, however, an El Nino may be just what the Atlantic needs. This was supposed to be an El Nino year, and while the atmosphere has in some ways resembled it, the Pacific SST anomalies don’t lie; we’re not in an El Nino. However, with new warm pools forming beneath the surface in the Pacific, some climate models have started to trend back more aggressively with their El Nino forecasts, and there is a good chance that we will see a moderate El Nino peaking this winter. El Nino affects weather patterns worldwide, and it may be the shakeup needed to boost Atlantic instability at least back to normal levels. If this El Nino happens, the next two years will be telling. I have lost some of my optimism for next year, as the delayed El Nino means we may still be in a warm ENSO state come next season, which is unfavorable for development as previously discussed, and Atlantic instability may not have time to recover even if we do trend the ENSO towards neutral for the peak months of 2015. If we get more of a “flash crash” like we saw in 1995, then it is possible next year could be more active, especially if the instability begins to recover. This is getting very far out in time now, but it is possible that 2016 will be the judgment year for the Atlantic and its active era. Even if next year turns out slow because of a lingering warm ENSO, I wouldn’t be willing to call the active era over because El Nino is almost always going to lead to a below average Atlantic season no matter what. Projecting the ENSO out to 2016 is basically impossible, but if we assume we are heading downwards towards neutral by mid next year, it may be possible to infer that 2016 will be a neutral or perhaps La Nina year, both of which would be favorable, or at least non-detrimental, to Atlantic development. A year with those circumstances, coming off an El Nino, should produce huge numbers in an active era like we have been in. If 2016 turns out similar to this year or last with what should be favorable conditions, then I think we could say with some confidence that our active era is over, particularly if the AMO turns into its negative phase. So for now, it’s just watching and waiting. One thing is for sure- the Atlantic hurricane season isn’t going away. Even if the AMO turns negative and we enter a long term quieter period, tropical cyclones will always be a threat in our basin, and people should remember to always take them just as seriously. Any year can be “that year”, and any storm can be “that storm”.
Finally, I leave you with some personal news; On September 1, I will move into Plymouth State University to start my freshman year of college. I am going to be majoring in meteorology and attempting to earn my Bachelor’s Degree over 4 years (and probably try for a Master’s beyond that). So starting then you won’t be seeing as much of me on the blog, as I’ll be quite busy with schoolwork as well as acclimating to college life. This may be the last blog I post before then, unless I feel an urge to write a blog on one of the developing storms in the East Pacific.
Thank you as always for reading my blog; I’d be very interested in any comments regarding the main subject of the blog, specifically do you think what I have posted is reasonable and what are your thoughts on the state of our active period? Enjoy the week!

-MAweatherboy1


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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About MAweatherboy1

Average 20 year old weather nerd. Plymouth State University Meteorology, Class of 2018. NOAA Hollings scholar. Summer 2016 intern at NWS Boston.