Tropical Atlantic Quiet; Pacific Getting Active

The tropics are quiet in the Atlantic Ocean, where no tropical storm activity is likely for at least the next week. A moderate-strength El Niño event is underway in the Eastern Pacific, and the atmospheric circulation associated with the strong warming of the waters off the coast of Peru is creating strong upper-level winds over the Caribbean. These powerful winds were creating a very high 60 - 70 knots of wind shear over the Caribbean and Southern Gulf of Mexico on Tuesday, making tropical storm formation virtually impossible in these regions. In addition, the atmosphere over the tropical Atlantic, including the Caribbean Sea, has been dominated by high pressure and dry, sinking air since April, which has made it difficult for thunderstorms to develop. The high wind shear and low instability is forecast to persist in the Caribbean and tropical Atlantic for at least the next week. Wind shear is lower in the waters of the Gulf of Mexico and off the U.S. East Coast, so if we get any tropical storms forming during the first week of July, those would be the most likely locations. Tropical storms that form just off the U.S. coast typically get going along a cold front that moves off the coast and then stalls over the water. The models are currently showing no fronts active enough to promote such development through the first week of July.


Figure 1. Vertical instability over the Caribbean Sea in 2015. The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Normal instability is the black line, and this year's instability levels are in blue. The atmosphere has been dominated by high pressure and dry, sinking air since April, which has made it difficult for thunderstorms to develop. Instability has also been unusually low in the tropical Atlantic between the Lesser Antilles Islands and coast of Africa, but has been near average over the Gulf of Mexico and waters off the U.S. East Coast. Image credit: NOAA/CIRA.

Rare twin tropical cyclones form in the Pacific
The Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is currently located in the Western Pacific, and is forecast to grow to impressive strength by this weekend. In response to the MJO and the unusually warm waters from the current El Niño event, heavy thunderstorm activity is firing up along a large swath of the tropical Western Pacific Ocean, along what is called the "Monsoon Trough". On Tuesday morning, this activity spawned twin tropical cyclones just west of the Date Line, one storm on either side of the Equator. The Northern Hemisphere storm is Tropical Depression Chan-hom, which is likely to intensify into a typhoon this weekend and track northwest through the Northern Mariana Islands, north of Guam. Chan-hom is the ninth named storm in this very busy Northwest Pacific typhoon season. According to statistics from the Japan Meteorological Agency at Digital Typhoon, only two seasons since 1950 have had more named storms by the end of June--1971 (with 11) and 1965 (with 10.) The Southern Hemisphere twin storm is Tropical Cyclone Twenty-five, which is expected to slowly intensify and move southwards through the Solomon Islands. It is very rare to get a tropical cyclone in this portion of the South Pacific in late June and early July--winter in the Southern Hemisphere. According to statistics from NOAA's historical hurricane website, there have been only three July named storms in the waters of the South Pacific east of Australia since satellite data began in 1970. None of the these storms occurred in the waters north of the Solomon Islands, where Tropical Cyclone Twenty-five formed.


Figure 2. Surface wind flow over the equatorial Pacific as seen at 10 am EDT June 30, 2015. A series of four counter-clockwise rotating tropical disturbances (one of them being Tropical Depression Chan-hom) was in the Northern Hemisphere, and one tropical depression (Twenty-five) was in the Southern Hemisphere. Image credit: http://earth.nullschool.net/.

Twin tropical cyclones will aid El Niño
The counterclockwise flow around Tropical Depression Chan-hom in combination with the clockwise flow around Tropical Cyclone Twenty-five is generating a Westerly Wind Burst (WWB) near the equator, just west of the Date Line. The winds of this WWB are predicted to march eastwards towards South America during the coming weeks, pushing more warm water eastwards that will reinforce the on-going moderate-strength El Niño event. This El Niño event is already at the borderline of being categorized as "strong", and this new WWB could well push it past that threshold. This should make for an unusually active Eastern Pacific hurricane season, by bringing warmer waters and lower wind shear (next chance for a named storm there: in about ten days' time, when the MJO pushes eastwards into the Eastern Pacific.) Conversely, El Niño should bring a much less active than usual Atlantic hurricane season, thanks to the high levels of wind shear that typically occur there during an El Niño.

Jeff Masters