Excerpts. TL;DR: Strong Azores high imparting dry air, TUTT in the western Atlantic, and subsidence over West Africa are the culprits; but the first two are forecast to be gone in 1-2 weeks.
The National Hurricane Center currently is not expecting any TC formations in the next seven days. If that forecast verifies, it will be only the 2nd year since 1950 with no Atlantic named storm activity between 29 August and 16 September. The only other year was 1992.
Over the past ~2 weeks, this dryness has likely emanated from two sources. The Azores High and pressures across the eastern tropical Atlantic have been quite a bit higher than normal (Figure 4). Associated with the circulation around a high pressure system is anomalous northerly flow on its eastern flank (Figure 5). This anomalous flow brings down drier air from the subtropics into the tropics, inhibiting deep convection. Over the past ~2 weeks, the trade winds across the tropical Atlantic have generally been stronger than normal as well (Figure 6). The other large driver is likely the pronounced tropical upper tropospheric trough (TUTT) that has developed in the western subtropical Atlantic. TUTTs bring down dry air from the subtropics into the tropics, thereby also contributing to drying out the atmosphere. The development of the TUTT is discussed in the next sub-section.
As previously discussed, vertical wind shear has generally been below normal since early August across most of the Atlantic. However, in recent days, vertical wind shear in the western Atlantic has increased markedly, with above-average vertical wind shear across most of the western Atlantic in the past two weeks (Figure 8).
We believe that the next ~7–10 days are likely to remain quiet in the Atlantic, given largely TC-unfavorable vertical wind shear and moisture patterns (Figure 13). However, after that point, vertical wind shear is forecast by ECMWF to weaken substantially, potentially yielding much more Atlantic favorable TC conditions later this month. While the Madden-Julian oscillation (MJO) has generally been weak in recent days, there is the potential for the MJO to intensify over Africa and the Indian Ocean in ~2 weeks (Figure 14). If this were to occur, it would likely result in suppressed convection over the Pacific and rising motion over Africa/Indian Ocean, as forecast by ECMWF (Figure 9). This pattern is much more conducive for Atlantic TC activity than is the current upper-level pattern.
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Also, while sea level pressure is forecast to be remain quite high across the tropical and subtropical Atlantic for the next five days (Figure 15), ECMWF does forecast a marked weakening of the subtropical high by days 6–10 (Figure 16), likely reducing the source of dry air advection in the east Atlantic.
Current 30-day-averaged zonal vertical wind shear is the 2nd lowest on record across the western Atlantic (10–20°N, 8550°W) (Figure 22).
