Satellites depict the Saharan air layer. on Tuesday. (NOAA/CIMMS)
After a record active start to hurricane season, the tropics have lain close to dormant for several weeks. Not only is the tropical Atlantic quiet, but a layer of desert air from the Sahara is also actively working to suppress tropical cyclone development. The only activity has been a weak tropical storm — Dolly — sputtering offshore of New England. But don’t count on that calm lasting for long.
[Record plume of Sahara Desert dust reduces visibility in Puerto Rico, aims at Southeast U.S.]
Hurricane season peaks in September, and experts across the board have predicted an active season. Already, the gears are slowly turning that could favor a rejuvenated bounce toward more typical activity, with a developing La Niña and other atmospheric parameters favoring a stormy season.
Tropical Storm Dolly as viewed by satellite Tuesday. (Tropical Tidbits)
The brief respite punctuates an already busy start to hurricane season, which has featured four named storms and included the earliest third named storm on record.
On Tuesday, Dolly abruptly formed over lukewarm waters in the northwest Atlantic but was expected to quickly dissipate as it moves over the open ocean. Only two other years — 2012 and 2016 — have reached the “D” name on the National Hurricane Center’s annual naming list before the start of July.
How the Sahara Desert could be suppressing storm growth
A satellite shows the dust plume over the Caribbean and a second one emerging off the coast of Africa on Tuesday. (NOAA)
A massive billowing cloud of desert dust from the Sahara stretched more than 4,000 miles across the Atlantic early Tuesday, bringing reduced visibility from the Leeward Islands to the Greater Antilles and the Caribbean. Before long, the hazy veil could cover the Yucatán Peninsula and western Gulf of Mexico, with even parts of the Mississippi Valley, Tennessee Valley, and the Southeast expected to encounter a dusty shroud over the weekend.
The dust heralded a layer of hot, dry desert air that has largely “capped” the lower atmosphere, putting a lid on thunderstorm development across the tropical Atlantic. The hot air prevents pockets of air below from rising, while the dry air works to shred any clouds that do manage to tower. Essentially, the atmosphere is squelching any attempts at storm genesis before it can even occur.
But that could change as we approach the end of June and head deeper into summer, the dust eventually thinning. At the same time, water temperatures will continue to warm, with other parameters becoming more supportive of eventual storm growth as well.
How the Madden-Julian oscillation could bolster hurricane season
Conditions associated with the Madden-Julian Oscillation are currently not favorable for lifting motion or storminess over the Atlantic. (NOAA/CPC)
Also working against storms has been the Madden-Julian oscillation, a large-scale overturning circulation in the tropical atmosphere. The feature shuffles across the tropics around the world. Thunderstorm growth and tropical development is enhanced when the lifting, upward motion of one side of the circulation passes overhead. But other times, fledgling storms are squashed when the pattern’s descending branch of sinking air passes through.
Right now, that inhibitive “subsidence,” or sinking air, is parked over the Atlantic. That looks to remain the case for a while, but models are indicating the potential for a brief period of enhanced rising motion during the first two weeks of July.
Rising motion is favored to approach the tropical Atlantic by August, with more rising motion and slightly enhanced odds of tropical cyclone development. (NOAA/CPC)
Beyond that, early indications — which yield considerable uncertainty — suggest that unfavorable conditions will persist for the remainder of July before the next wave of lift arrives in August. Given that the period from August into September features a general uptick in storminess anyway, any potential overlap could help abruptly swing the pendulum back toward active in short order.
La Niña may develop, boosting storm chances
A gentle swing toward La Niña during the fall could enhance hurricane activity over the Atlantic. This NOAA schematic depicts typical conditions associated with a La Niña weather pattern. (NOAA)
On Tuesday, the Australian Bureau of Meteorology posted a La Niña “watch,” its confidence increasing that a La Niña weather pattern could take hold later in the year into 2021. Already, sea surface temperatures across the Pacific have cooled during the past two months, with those anomalies extending deep into the ocean.
The Bureau of Meteorology noted that other indicators such as “cloudiness near the [International] Date Line” were more commensurate with a neutral state between El Niño and La Niña.
Sea surface temperature anomalies Monday depict cooling over the eastern Pacific, indicative of a trend toward La Niña. (NOAA)
The U.S. National Weather Service also agrees with the Bureau of Meteorology, predicting “roughly equal chances … of La Niña or [neutral conditions] during the autumn.”
“Most models predict either ENSO neutral or La Niña during the remainder of 2020,” they wrote.
The presence of an El Niño or La Niña pattern influences the position of several key weather systems and the jet stream, shaping how active hurricane season will be. During a neutral year or one that skews toward La Niña, conditions become more conducive for hurricane growth over the western Atlantic.
A NOAA schematic explains how reduced winds and wind shear aloft, a change of wind speed/direction with height, can prove beneficial for hurricane growth. (NOAA)
In a La Niña, high pressure generally results in weaker winds aloft over the tropical west Atlantic. If upper-level winds are too strong, they can tear apart a developing storm. That’s why the weak winds are instrumental in fostering the growth of hurricanes.
La Niña also reduces the amount of sinking air present in the tropics, making it easier for the upward motion of storm clouds to yield vigorous thunderstorms and tropical waves.
The bottom line
Hurricane Dorian on Sept. 1. (Colorado State University)
The ongoing dust event across the Atlantic is noteworthy and is putting a damper on the prospects of hurricane activity in the next couple of weeks. But late June into July is typically a quiet period anyway before things ramp up in August and into September.
An active hurricane season remains likely, and preparedness is vital for anyone living in vulnerable areas. That’s true not only near the coast, but also inland — where decaying tropical systems can frequently dump extreme rainfall.
[NOAA predicts Atlantic hurricane season will be unusually active]
