Is 2018 an ominous indicator for the future?

[Shell Mound]

To begin, let's review several of the key indicators for the 2018 Atlantic hurricane season and how they have fared so far:


Figure 1. From 1 August to 20 September 2018, vertical wind shear at 850-200 mb over the MDR was mostly within the range of 12-20 knots above average. Over the same time frame, shear over the Caribbean averaged slightly higher, at up to 26 knots above average. Overall, shear across the basin has been near to somewhat above average. The most hostile conditions have prevailed over the Caribbean and western MDR.


Figure 2. Rainfall over the Sahel and tropical West Africa has persistently averaged above normal. However, rainfall over the MDR has been solidly below normal, except within the low-latitude ITCZ. The overall distribution indicates an active African monsoon, but with sinking air (suppressed convection) over the tropical Atlantic. The next graphic, concerning velocity potential, shall illustrate this condition.


Figure 3. Note the sinking air (subsidence) over the western North Atlantic basin, including the Caribbean, and the northwestern part of the MDR. Upward motion has been primarily centred over the low-latitude ITCZ, with an extension northward to near the Cabo Verde islands. The areas of subsidence and uplift coincide fairly nicely with areas of below- and above-average precipitation (cf. Figure 2).


Figure 4. Soil moisture confirms a wet African monsoon over the Sahel and parts of tropical West Africa.


Figure 5. A classic –AMO signature is evident over the North Atlantic basin: warmer-than-average subtropical SSTs and well-below-average SSTs in the MDR.

Summary

• Negatives: near-to-above-average vertical wind shear and subsidence (sinking air = suppressed convection) over the tropical Atlantic, combined with a strong –AMO
• Positives: active African monsoon and wet soils over the Sahel and West Africa

Results

• Five hurricanes in the North Atlantic basin thus far: Beryl, Chris, Florence, Helene, and Isaac
• Four attained hurricane status in the MDR close to West Africa: Beryl, Florence, Helene, and Isaac
• Five named storms have formed in the MDR thus far: Beryl, Florence, Helene, Isaac, and Kirk
• 2017: four storms (three of them hurricanes) formed in the MDR: Irma, Jose, Lee, and Maria

Given all the highly unfavourable indicators we have seen thus far in 2018, the MDR seems to have performed much better than one might have expected.

If 2019 merely features much less shear and warmer SSTs, the MDR could match or exceed 2017 next year, given what we have already seen this year.

Thoughts?

[NotSparta]

To begin, let's review several of the key indicators for the 2018 Atlantic hurricane season and how they have fared so far:

-snip-

Given all the highly unfavourable indicators we have seen thus far in 2018, the MDR seems to have performed much better than one might have expected.

If 2019 merely features much less shear and warmer SSTs, the MDR could match or exceed 2017 next year, given what we have already seen this year.

Thoughts?

Seems that the West African Monsoon (WAM) had a major effect, and the break in EP activity coupled w/ the failure of El Niño helped.

Back in July, I remember Eric Webb posting a graphic showing how strong the WAM was, comparable to even 2017. 2017 showed us what a yr w/ a very strong WAM & warm SSTs w/ low shear could do.

The WAM helped focus more rising air towards the east MDR, allowing waves to develop there, where shear is lower. The strong EP in August mostly shut down the ATL tropics, but as it quieted, Florence formed.

Your question depends on conditions. If there is less shear and warmer SSTs, the season will be more active (assuming the WAM remains extremely strong). It may be something to watch next spring.

[CyclonicFury]

The strength of the African monsoon definitely does seem to be playing a role in keeping the MDR quite active in recent years, even though aside from 2017 many of those storms have been weak. The past four seasons have all featured at least six tropical cyclones forming in the MDR. The African monsoon likely plays a role in the active/inactive era cycle, and even with a persistent -AMO SSTA configuration, the Atlantic has been a bit above normal so far this season.

As far as 2019 goes, it's possible El Nino could develop very late this year and strengthen into 2019, though this is far from a guaranteed scenario. There's also a chance this upcoming El Nino fails to materialize and ENSO actually cools somewhat next year, but I don't think that's particularly likely.

[Shell Mound]

Interestingly, the extended-range, trimonthly averaged CFSv2 suggests that the colder-than-average far North Atlantic will warm up nicely during the upcoming 2018-19 winter, suggesting a potential –NAO. If this were to verify and persist through spring and early summer, then it would tend to favour a warmer MDR and tropical Atlantic than we have seen thus far in 2018, that is, a +AMO instead of the ongoing –AMO dip. Additionally, note that the tropical Pacific north of the equator is forecast to cool down over time, while the tropical Atlantic is expected to warm significantly vs. other basins globally. This type of pattern would tend to favour upward motion (convection) and reduced vertical shear over the tropical Atlantic/Caribbean and MDR. Barring potential ENSO considerations, the overall pattern evolution, if verified and sustained, would tend to suggest a more active season in 2019 vs. 2018. Of course, take such a long-range outlook with a grain of salt. If the ENSO were to be biased cooler than suggested (as is quite possible), then 2019 might be even more favourable than suggested here.

http://www.cpc.ncep.noaa.gov/products/people/wwang/cfsv2fcst/images3/glbSSTSeaadj.gif

[Ntxw]

Interestingly, the extended-range, trimonthly averaged CFSv2 suggests that the colder-than-average far North Atlantic will warm up nicely during the upcoming 2018-19 winter, suggesting a potential –NAO. If this were to verify and persist through spring and early summer, then it would tend to favour a warmer MDR and tropical Atlantic than we have seen thus far in 2018, that is, a +AMO instead of the ongoing –AMO dip. Additionally, note that the tropical Pacific north of the equator is forecast to cool down over time, while the tropical Atlantic is expected to warm significantly vs. other basins globally. This type of pattern would tend to favour upward motion (convection) and reduced vertical shear over the tropical Atlantic/Caribbean and MDR. Barring potential ENSO considerations, the overall pattern evolution, if verified and sustained, would tend to suggest a more active season in 2019 vs. 2018. Of course, take such a long-range outlook with a grain of salt. If the ENSO were to be biased cooler than suggested (as is quite possible), then 2019 might be even more favourable than suggested here.

http://www.cpc.ncep.noaa.gov/products/people/wwang/cfsv2fcst/images3/glbSSTSeaadj.gif

Weak-Moderate Modoki El Nino winters tends to favor -NAO. I suspect if you did a composite map of years like 1963-1964, 1976-1977, 1977-1978, 1986-1987, 1987-1988, 2002-2003, 2009-2010 you may see a potent -NAO.

Notice I left out the 1990s and 2014-2015. These were events nested within very positive multiyear +NAO as we have been in since 2013. Curious to see if that changes.