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?