Saharan dust strongest in decade, but will it help keep tropics quiet?

By Kimberly Miller From The Palm Beach Post

Plumes of Saharan dust wafting off the west coast of Africa this summer have been some of the strongest in a decade, sending a dry, sandy blast through a prime birthplace for tropical cyclones.

While it’s not entirely understood how Saharan dust impacts the formation of hurricanes, it’s generally recognized as a contributing factor during years when the number of storms is below average.

A July 18 forecast shows Saharan dust (in green) spreading over South Florida. This aerosol analysis from the Naval Research Laboratory … read more

Jason Dunion, a research meteorologist for the National Oceanic and Atmospheric Administration’s hurricane division in Miami, said satellite images reflect Saharan dust storms boiling up at a “fast and furious” pace.

Because hurricanes like very moist environments, adding Saharan dust to already dry air caused by El Nino can crush fledgling tropical waves, he said.

“Easily, this is the most dust we’ve had in the past 10 years,” Dunion said. “About a mile up into the surface atmosphere, what is usually really tropical and juicy, has about half the moisture, which makes it difficult for thunderstorms to form.”

Saharan dust as seen from a NOAA jet flying at 45,000 feet northeast of Barbados. Cumulus clouds can be seen poking … read more

The presence of Saharan dust was mentioned multiple times in the National Hurricane Center’s forecast discussion Monday morning, which predicted a week free of tropical disturbances.

Also, the Florida Department of Health identified Saharan dust in its air quality testing filters during the past couple of days, triggering a “moderate” rating for a swath of the state that includes Palm Beach County. A moderate rating means people who are unusually sensitive to air pollution may be affected. The dust was first identified in Palm Beach County this year in May.

“If it’s high enough we would put out a warning to those who are susceptible to dust, kind of like the smoke from wildfires,” said health department spokesman Tim O’Connor. “But so far we haven’t seen it in that quantity or density.”

A Saharan dust layer is made of sand and mineral particles that are swept up in air currents that carry them from 3.5 million square miles of desert and 4,000 miles west over the Atlantic.

The top of the dust layer is 16,500 feet above the Earth’s surface, according to NASA.

Dunion said recent waves of Saharan dust storms have been the size of the continental U.S. They usually pick up in July.

Conceptual image of how Saharan dust is transported from Africa. (NASA)

“It’s very hard to say specifically what suppresses the hurricane season because all the elements are very strongly correlated,” said Philip Klotzbach, a hurricane researcher for Colorado State University.

Klotzbach noted that wind shear — a known hurricane killer — is particularly strong this year. The shear cuts down thunderstorms and carries the Saharan dust.

Still, he agreed the dust in itself can cause environmental shifts that weaken storm formation.

More dust means less sunlight reaching the ocean’s surface, resulting in cooler surface temperatures. Also, Saharan dust absorbs solar radiation, heating the air around it and inhibiting thunderstorms.

Chris Landsea, a National Hurricane Center Science and Operations Officer, said Saharan dust was not specifically included in this year’s forecast of a lower than average hurricane season because it’s unpredictable.

While dust may be prevalent now, it could dissipate during the height of hurricane season — August through October.

“It’s pulses of dust, so it’s now always there, and it can be very discreet,” Landsea said. “It would be nice if we could continue the rest of the hurricane season being slow, but all we can say is the next five days is expected to be nice and quiet.”

What’s happening: Saharan dust plumes coming off the west coast of Africa are the strongest in at least a decade. The dust, which triggered the Florida health department to elevate air quality alerts in Palm Beach County to “moderate” in the past few days, may also be responsible for reducing hurricane activity in the Atlantic.

The details: How Saharan dust directly impacts hurricanes is unclear, but it is associated with years where fewer storms are predicted. Some researchers believe the dust blocks the sun, helping keep ocean surface temperatures cooler, which inhibits hurricane formation. Dust reflecting solar radiation also heats the air around it, creating hotter temperatures higher in the atmosphere, which would keep thunderstorms from strengthening.

What’s next: Saharan dust storms are unpredictable. Whether they will continue through the height of hurricane season — August through October — is unknown. Scientists hope to use drones in the near future to study hurricanes and Saharan dust.

IMAGES:
A July 18 forecast shows Saharan dust (in green) spreading over South Florida
Saharan dust as seen from a NOAA jet flying at 45,000 feet northeast of Barbados
NASA’s Aqua satellite observed some of that dust starting a trans-Atlantic journey
Conceptual image of how Saharan dust is transported from Africa. (NASA)

For more on this story go to: http://www.mypalmbeachpost.com/news/weather/hurricanes/saharan-dust-strongest-in-decade-but-will-it-help-/nm3gg/#1d5ae6b7.257747.735800

Related story:

Hurricane forecasters watching tropical wave in Atlantic

By Kimberly Miller From The Palm Beach Post

The National Hurricane Center is tracking a tropical wave that has developed a couple hundred miles southeast of the Cape Verde Islands.

While the system was given no chance of development over the next 48 hours, conditions may be more conducive for a longer-term development. Chances of development over the next five days are 20 percent.

So far, no storms that have formed off the coast of Africa have amounted to anything. The three named tropical storms were all products of the Gulf of Mexico or off the coast north of Florida.

For more on this story go to: http://www.mypalmbeachpost.com/news/weather/hurricane-forecasters-watching-tropical-wave-atlan/nm8W9/