Why is the North Atlantic tropical season so slow right now?
A review of tropical upper troposphere trough.
Discussion: All seasonal forecasts of North Atlantic tropical cyclone activity were revised higher to slightly above normal in early August. The primary reason was the finished 2018-19 El Nino which lowers upper shear in the tropical North Atlantic allowing tropical systems to develop and flourish.
While the eastern equatorial Pacific has cooled we have some evolving caveats. Research has shown that when the far eastern equatorial Pacific is cool and near the Dateline in the tropics surface water is warm there is a tendency in the upper air pattern over the subtropical western North Atlantic for a low pressure trough to form. This phenomenon is known as the tropical upper troposphere trough (TUTT).
TUTT has a tendency to form over the northern Caribbean Sea Islands. This high atmosphere phenomenon extends upper shear across the tropical Atlantic defeating tropical cyclone development. The TUTT has formed over the northern Caribbean Islands (Fig. 1) related to the atmospheric climate pattern driven by cool SSTA (Fig. 2) in the eastern equatorial Pacific (while central tropical Pacific is warm).
Other factors are slowing the North Atlantic tropics right now such as increased dust off the Saharan Desert and cool SSTA in the eastern North Atlantic tropics. However, the TUTT hasn’t been around for a few years and a quick review is necessary. Does the TUTT weaken? It can be a slow process but will need to if seasonal forecasts are to come close to verifying.
Fig. 1: High level wind (above 40, 000 feet and identified with light blue vectors) identify a tropical upper troposphere trough in the Caribbean Sea.
Fig. 2: The increased tropical convection in the central tropical Pacific caused by a warmer than normal ocean surface coupled with weaker convection patterns in the eastern Pacific tropics due to cooler surface water has an effect on the upper air pattern into the tropical North Atlantic favoring formation of a TUTT pattern.