What would it take to turn Atlantic tropics active?
Deep tropics could be quiet until middle third of September
Discussion: The Tropical North Atlantic (TNA) index is -0.18C on Aug. 15. The TNA index remains cool which represents below normal ocean heat for tropical cyclone development in the Main Development Region (MDR) for hurricanes located between the Caribbean Sea and Cape Verde Islands. TNA has warmed significantly, given seasonality in recent weeks BUT cooled again (slightly) the past week or so. A dramatic warm-up of this key ocean area for North Atlantic tropical cyclone activity seems unlikely for the 2018 season. This negating influence on seasonal activity has been noted by Climate Impact Company earlier this summer and more recently the National Hurricane Center whom lowered their seasonal activity outlook last week.
Additionally, the easterly phase of the quasi-biennial oscillation (QBO) is the strongest in the historical record. Implied is wind aloft is too fast from the east to allow tropical systems in the deep tropics too emerge.
So is there anything that can activate the deep tropics to produce tropical cyclones and possibly a hurricane?
The leading climate forecast diagnostic to project increasing risk of tropical cyclone development is the transient Madden Julian oscillation (MJO). The MJO is a large area of enhanced vertical motion in the deep tropics inspiring a large mass of thunderstorm activity which, during summertime enhances the wet monsoon and increases tropical cyclone risk. Currently, MJO is located in the tropical West Pacific (explaining presence of 3 tropical cyclones in the West Pacific) and forecast by most models to shift slightly west to Maritime Continent in the week 1-2 ahead time frame.
When the MJO is in this (Maritime Continent) location the subsidence phase of MJO is usually present in the tropical North Atlantic basin typically suppressing development. Therefore the environment to produce a tropical cyclone in the deep tropics of the North Atlantic basin is minimal to almost zero into early September.
The European Model forecasts MJO to 30 days and indicates a shift of the MJO into the tropical East Pacific the first 2 weeks of September. If so, the East Pacific should become more active producing tropical cyclones. However, the tropical North Atlantic remains in an unfavorable environment to produce a deep tropics system into mid-September if the MJO forecast is correct.
MJO should shift toward or at least influence the tropical North Atlantic basin more favorable by mid-September. However, the implication here is a wait until mid-September until a hurricane can generate in the deep tropics.
Seasonality can activate the intra-tropical convergence zone in the tropical North Atlantic and tropical cyclones can emerge without MJO support but it’s much more difficult.