Suppressed Tropics in August About to Change?

Suppressed Tropics in August About to Change?

Fig. 1-3: High-level and mid-level above normal westerly wind shear locations (top) and relative humidity levels (bottom) for August 1-25, 2022.

Discussion: The unusually quiet month of August for North Atlantic tropical cyclones is attributed to several factors lead by formation of a weak upper tropospheric trough (TUTT) during late spring that lingered into mid-summer. As a result, high-level westerly wind shear and suppressed upper ventilation inhibited tropical development across the Caribbean Sea and eastward during August (Fig. 1). The above normal westerly wind shear is observed as low as 500 MB (~18,000 feet) in the subtropics (Fig. 2). Note that in-between the above normal high-level westerlies in the subtropics and the tropical easterly jet (TEJ) off the West Coast of Africa an upper-level high pressure ridge is present. The upper-level ridge has caused a very dry mid-level atmosphere across the central North Atlantic tropics/subtropics (Fig. 3) leaving the atmosphere too dry for tropical cyclone development.

Due to seasonality and encouraged by a favorable climate pattern for tropical cyclone development thanks to La Nina, the early September tropical environment is improving. Tropical Disturbance 91L is in the central North Atlantic tropics and is expected to become a tropical storm in 4-5 days shifting north of the northeast Caribbean Sea by Friday (Fig. 4). Thunderstorm activity across the southwest Gulf of Mexico and Bay of Campeche is a concern. That general region is expected to gain a hurricane risk next weekend according to GFS. Consequently, an ALERT is issued. ECM does not have this risk. In the outer tropical North Atlantic, two tropical cyclones are possible beginning late this week into early next week. Each event is likely to stay out to sea.

Fig. 4: Weather satellite view of the North Atlantic basin with annotated important features.