The eye of the butterfly

The difference between the tropical cyclone and mid-latitude cyclone

June’s arrival marked two significant weather periods, the beginning of “summer” and the June-November hurricane season, both raising the possibility of stormy weather and the more important question: What kind of storm? Nightly TV predictions of fronts, winds and rain — weather typical of the mid-latitudes — are increasingly appended by “what the tropics are doing,” with an eye kept on disturbances and depressions in the Atlantic, especially the belt between 5° N and 20° N that can include tropical misbehavior. So, let’s a look at both, starting with the tropics.

Senegal’s Cape Verde (Cap-Vert) is a peninsula at Dakar jutting into the North Atlantic at about 14.7° N by 17.5° W, claiming priority as the westernmost point of the mainland African continent. (Note that “Cape Verde” also applies to the independent island nation about 370 nm slightly northwest of its continental namesake.) Historically, Cape Verde has been associated with about 80 to 85 percent of the more destructive hurricanes (Category 3 to 5) that strike westward to the Antilles, the Gulf, Caribbean and U.S. East Coast, with September to October being statistically the most active period. But why would this African cape have such an outsized influence?

Imagine a rectangle, its west flank running from Florida southeast to the Lesser Antilles, crossing the North Atlantic, with its eastern flank not at Cape Verde but 2,500 miles further east across Africa to the Darfur Mountains and Ethiopian Highlands, their meridional ends linked by latitudes of about 5° N to 20° N. The trade winds flow west, prompted by Saharan hot, dry air to the north and warm, moist air from the Gulf of Guinea to the south; the African easterly jet results from this north vs. south contrast of temperature and density.

Tropical waves
During hurricane season, an easterly wave (EW) — undulations of low pressure within the trades assisted by the African easterly jet — departs westward across that 15-degree zone every four to seven days. It has carried multiple names: tropical wave, tropical easterly wave, African easterly wave, etc. They all refer, however, to an elongated trough of low pressure, its axes oriented roughly north-northeast/south-southwest with wavelengths averaging some 1,000 to 1,500 miles. Not every North Atlantic tropical cyclone has its start as an EW, but the fact that the most destructive ones do again invites the question: How is Cape Verde associated with tropical cyclones? The answer is location. Whether the coastal peninsula or the offshore islands, it is the rim of the northwest African coast that serves as an on-ramp for EWs launched not far from the Red Sea, hitching a ride west with the trades.

Somewhere between Cape Verde and the Antilles, a satellite view may show evidence of a tropical disturbance: clouds, hints of rotation about an indistinct center, not yet symmetrical. To continue on, several things have to come together. Along that trough, thunderstorms can form, each with their own bit of torque. Now if a couple or more get together, conserving and pooling their angular momentum, their united rotation increases — like a figure skater when they draw in their arms to reduce the radius of rotation as their spin becomes a blur. As the nascent circle tightens, more air is drawn in and rises, and the low deepens with winds up to 33 knots. A tropical depression develops as gaps in the isobars close and slowly tighten, lowering central pressure as increasing winds respond to the increasing pressure gradient. At 34 knots, it’s designated a tropical storm and will be named. Many other factors are critical: adequate distance away from the equator to have increased torque from the Coriolis effect; warm surface water reaching 80° F to at least 150 to 200 feet; absence of wind sheer at high altitudes; increased moisture at mid-levels in order to ignite evaporation and convection; and so on. At 64 knots, a Category 1 storm has developed and, given the right conditions, it may ascend to a Category 5 at 137 knots.

The jet stream over Africa affects tropical cyclone formation.

Zone and rotation
The term “tropical cyclone” defines a meteorological storm by 1) its zone of origin/development, and 2) its sense of rotation. Its more limited mesoscale size distinguishes it from the wider synoptic-scale mid-latitude cyclone storms that frequent the 30° N to 60° N latitudes. The tropical cyclone takes on local names in different parts of the world: In the North Atlantic, eastern Pacific and eastern South Atlantic (though rare), it’s a “hurricane”; in the northwestern Pacific, it’s a “typhoon.” In the Indian Ocean it is called a “cyclone,” a “baguio” in the Philippines, and a “willy-willy” in Australian waters. The word “cyclone” derives from “cyclonic,” defining rotation — counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere. If that rotation is about a low, it is a cyclone.

The tropical cyclone is a non-frontal homogeneous air mass rotating cyclonically in a closed circulation around a low-pressure center with a warm core. Its energy is primarily derived from latent heat stored by evaporation at the surface, raised by convection and released aloft by condensation. This release of energy further supplements and boosts its warm core and sustains the low surface pressure. Hurricane season brings attention to predictions of track and landfall, intensity, duration, etc., and the “spaghetti tracks” that attempt to show probabilities. The difficulty of prediction is best told in this story.

Using methods developed in the early 20th century by the Bergen School of Meteorology under Vilhelm Bjerknes and son Jacob, Lewis Fry Richardson pioneered the development of numerical weather prediction in the early 1920s. This task, however, remained a daunting one until the birth of the computer. One day, in 1961, MIT professor Edward Lorenz was working on numerical weather predictions, running a computer program to simulate weather patterns, when he decided to take a break. Having entered the critical data on initial conditions, he halted the run and recorded its status at that point so that on return he could restart with the same data — rather than starting the program over — expecting that the computations would continue where they left off. Murphy’s Law, however, was looming overhead.

Not long after he resumed the run, things went awry; the unexpected result was chaotic. Lorenz later realized that this wasn’t due to a mechanical glitch, but instead a result of how the program handled the data he re-entered. While Lorenz plugged in data that he had rounded off to three decimal places (0.506), the computer had calculated that same data at six decimal places (0.506127). This resulted in an entirely different weather pattern from what the program had previously been predicting.

Had the program simply ferreted out and built on something whose initial condition significance lay somewhere in that XYZ gap between 0.506 and 0.506127? Was it that theoretical butterfly in Brazil, whose wing flaps promoted chaos by flying through the abbreviated decimal, representing an unknown factor? Perhaps one such flap for the 2020 season might be appropriate; the past few months have raised predictions of a weakening El Nino factor and the edging to La Nina, neutral or slightly positive. This would favor Atlantic tropical cyclone development. Could this be within the unknown “XYZ” portion of initial conditions for 2020? 

A NOAA weather product showing the predicted development of a tropical circulation near Cape Verde on the west coast of Africa, which later formed into Hurricane Irma and struck Cuba, the Bahamas and the southeastern U.S.

Courtesy NOAA

Mid-latitude cyclones
One way to look at a tropical cyclone is to compare it with a mid-latitude (30° N to 60° N) cyclone. The “middle latitudes” are where dense, dry polar air moving south and warm, moist tropic air moving north — air masses with differing densities (temperature, moisture content, etc.) — meet at their fronts. These masses, essentially moving horizontally (advection), carry potential energy by virtue of their density gradient, ready to be transformed to kinetic energy when they meet one of differing clout. Horizontally moving fluids wedge cold air beneath the warm layer, cooling its core as the fronts gradually occlude and quiet down (the old adage to “lead, follow or get out of the way” comes to mind). This contrast is underscored by Jeffrey Rosenfeld in his 1999 book, Eye of the Storm:

“Tropical cyclones, driven by moist rising air, transfer solar energy directly to the upper troposphere for delivery to mid-latitudes. As heat engines, they sort out vertical temperature contrasts but thrive in warm uniformity. Extratropical cyclones, on the other hand, sort out the horizontal temperature contrast, between polar and tropical air. They are asymmetric and exploit the clash of air from opposite sides of fronts.”
And finally, some other terms that are seen frequently: the post-tropical cyclone is a tropical cyclone that has left the tropics and assumed the characteristics of a mid-level cyclone (cold core, fronts, etc.); the subtropical cyclone has attributes of both tropical and mid-latitude storms, its energy derived from both convection within an air mass of uniform density (as in the tropical cyclone) and the energy generated by advective colliding air masses (as in the mid-latitude cyclone); lastly, the extratropical cyclone is simply the same as the mid-latitude cyclone term used in this article.

Jim Austin, a Naval Academy graduate who served on a USN destroyer and cruiser, is a freelance writer and retired physician in Burlington, Vt.

Categories: Weather