Along much of the North Carolina coast, the first
week of March was ushered in by intermittent rain and fog, with a
stiff, offshore breeze. But on the other side of the world, in the
Pacific Ocean, the trade winds were dying. When they revived, instead
of blowing east to west, their usual direction, they reversed in strong
This about-face and a confluence of other
meteorological events have triggered one of the strongest El Niņos
seasons since 1950. It is likely to peak in late fall and early winter
before ending next spring. As a result, the Pacific has spawned eight
hurricanes and 11 typhoons, while the Atlantic has experienced a quiet
season. In fact, Currituck, Camden, Pasquotank and Perquimans counties
are classified as abnormally dry by the N.C. Drought Monitor.
It’s unclear whether natural forces alone have
contributed to the intensity of the 2015 El Niņo. The atmosphere in the
Pacific may naturally vary, or the system could be destabilized by
other external forces.
“There’s not a consensus on how human impacts
affect an El Niņo,” says Phil Klotzbach of the Tropical Meteorology
Project at Colorado State University, who writes the seasonal hurricane
forecasts. “That’s a huge question. We don’t fully understand the
physics of what drives an El Niņo.”
We do know they tend to occur every two to seven
years, and last from nine to 12 months. An El Niņo begins when trade
winds, having weakened or reversed course, generate a Kelvin wave, a
deep sloshing beneath the ocean’s surface. In the last nine months,
three Kelvin waves have crossed the Pacific. Each one has lumbered
eastward along the equator on a three-month journey from Indonesia to
South America. It has dragged warm water with it, increasing sea
surface temperatures in the central and eastern Pacific—as much as 3.6
degrees this year.
These warm waters release more heat into the
atmosphere, causing the air to rise and sparking storms. Larger
circulation patterns in the atmosphere alter the jet stream, calming
the weather patterns in the Atlantic basin more than 5,000 miles away.
“When the air rises one place, it sinks in another,” Klotzbach says. “Rarely does the entire globe go crazy.”
In a typical Atlantic hurricane season, the
30-year average calls for 12 named storms, including
six hurricanes, — two of them major—although these systems may not
affect the U.S. mainland.
Historical data, though, points to a correlation
between El Niņo events and a lower number of Atlantic hurricanes and
- In 1982, considered a strong El Niņo year,
meteorologists recorded less activity: just six tropical storms, two
hurricanes and one major hurricane in the Atlantic.
- In 1997, also a strong El Niņo season, there were eight tropical storms, three hurricanes and one major hurricane.
- The last El Niņo, classified as moderate,
occurred in 2009. There were nine tropical storms, three hurricanes and
two major hurricanes.
So far, this year’s season has logged four tropical storms:
- Ana, which made landfall as a tropical storm near Myrtle Beach, S.C.;
- Bill, which came ashore in Texas, causing coastal and inland flooding;
- Claudette, which did not affected the United States and brought only showers and wind to eastern Nova Scotia and Newfoundland;
- Erika, which dissipated before it reached Florida, where it dropped heavy rain.
- And Grace, Henri, and Ida out in the Atlantic.
As for hurricanes, strong wind shear quashed two
storms before they approached the East Coast. A Category 1, Hurricane
Fred died after moving through the Cape Verde Islands; and Hurricane
Danny peaked at Category 3 before reaching the Leeward Islands.
“They died a glorious death in the middle of the ocean,” Klotzbach says.
This season, hurricane forecasters have estimated
the chance for a hurricane to affect North Carolina at 14 percent,
compared to the average probability of 28 percent. For a major
hurricane, the chances drop to 3 percent, compared to the average of 8
Hurricane season officially ends Nov. 30.
However, just because chances are lower than
average does not mean that a hurricane cannot strike North Carolina.
State climatologist Ryan Boyles notes that 1992 was also an El Niņo
year, and only one hurricane hit the East Coast. That storm was
Hurricane Andrew, which destroyed 63,000 homes and damaged more than
100,000 others in Miami-Dade County, Fla. At least 65 people died. At
the time, Hurricane Andrew was the costliest in history, causing $2.6
billion in damage.
“Very few people make planning decisions based on seasonal forecasts,” Boyles says.
“It only takes one, and that’s what we prepare for,” says Julia Jarema, communications officer for N.C. Emergency Management.
Gabe Vecchi is the head of the Climate Variations
and Predictability Group at NOAA’s Geophysical Fluid Dynamics
Laboratory at Princeton University. He says several factors could
influence an active El Niņo season, and thus a calm Atlantic basin.
“It’s hard to point to one thing,” he said. “There’s more than one
One factor is the Atlantic Multi-decadal
Oscillation, the AMO for short. This circulation pattern runs in
25-to-30-year cycles, and affects sea surface temperatures and sea
level pressure—and by extension, hurricane formation. From the 1960s to
the first half of the 1990s, the AMO phase cooled the oceans, and there
was comparatively less intense hurricane activity. Then in 1995, Boyles
says, “things flipped” and we’ve had warmer ocean waters—and more
hurricanes—since, although that pattern could be changing.
Scientists are still trying to understand how the
AMO behaves. It appears to be linked to regional and global climate
trends, according to NOAA. It is driven by swings in temperatures in
the “Atlantic conveyer belt” or major ocean currents like the Gulf
Stream, off the N.C. coast that move warm surface water north to higher
latitudes or cold northern waters south.
The conveyor belt, though, is sensitive to
salinity levels in the ocean, a NOAA study reports. Those salinity
levels can vary depending on water evaporation—which increases the
ocean’s saltiness—or “freshening,” which decreases it. Lower salinity
equals cooler temperatures and less frequent hurricanes.
What causes the ocean to lose its salt? A melting
of the ice pack, ocean circulation patterns and rain can all dilute
salinity. This, what NOAA called the “Great Salinity Anomaly” occurred
in the mid-1960s and lasted for roughly 25 to 30 years.
For the past 20 years until recently, the pattern
seems to have reversed, and the waters near Greenland have become
saltier. Salinity levels appear to be decreasing again. This
contributes to a cooling of the waters in the North Atlantic and a
warming in the South—a pattern that began last November.
That cooling, plus higher air pressure, stronger
wind shear, volcanic ash and even dust blowing off Africa, dampens
“We’re not sure the AMO is fully natural in its occurrence,” Vecchi says.
Over the last century, greenhouse gases have
warmed the planet, which could affect the strength of the AMO.
Deforestation and farming practices can produce more dust and
pollution. “All these ingredients: how much does each one do?” Vecchi
An anomaly has also appeared in the Pacific Ocean
that has contributed to a stormy Hawaiian summer. This is the second
consecutive year that warmer waters have approached Hawaii, which
usually is insulated from hurricanes by cooler waters around the
island. However, on July 12, satellite imagery showed five named
tropical cyclones queued up from Mexico to Japan.
Scientists are studying an unusual formation—what
Klotzbach calls a “previously unobserved” band of extremely warm
water—north of the equator, stretching from western Mexico to near
El Niņo conditions likely contributed to the
band’s formation, but Vecchi says, “It’s not part of the El Niņo; it’s
a neighbor of El Niņo. It’s not typical. This hasn’t occurred with
other El Niņos.”
By next March, the El Niņo will likely begin to
lose steam. Warmer waters, carried from the western Pacific, will
spread east and toward the poles. When this happens, deeper, cooler
ocean waters move closer to the surface.
However, it’s difficult to predict how the end of
the El Niņo will affect next summer’s Atlantic hurricane season. “There
have been big advances, but there is always going to be inherent
uncertainty,” Vecchi says.
Boyles, the state climatologist, says the science
still needs better observation data and more powerful computers. “We
still don’t understand how hurricanes develop and intensify,” he says.
“We don’t know the state of the atmosphere.”
article is provided by Coastal Review Online, an online news service
covering North Carolina's coast. For more news, features, and
information about the coast, go to www.coastalreview.org.)