All eyes were on forecasters at NOAA’s Climate Prediction Center (CPC), as well as the Tropical Meteorology Project of Colorado State University (CSU), this August when they updated their 2010 hurricane season predictions. Would they backtrack from their forecasts of an unusually active season? After all, over a third of it was already over, and there had been only two named storms, Hurricane Alex (Category 2) and Tropical Storm Bonnie, both in the Gulf of Mexico. The Atlantic tropical basin was totally quiet.
Prediction becomes fact
Both teams stood by their earlier forecast. In fact, the CPC raised the odds a bit. Back in May, they had described an 85% chance of an above-normal hurricane season this year; now, they warned, the likelihood was 90%, with a high chance for 14-20 named storms, 8-12 hurricanes, and 4-6 major hurricanes (Category 3, 4, or 5). The CSU scientists maintained their earlier forecast, calling for 18 named storms ( there are 9.6 in an average season), 10 hurricanes (5.9 on average), and 5 major hurricanes (2.3 on average).
The scientists explained their reasoning in technical terms. CSU mentioned La Niña and SST anomalies among other things. The CPC described these as well as ENSO and the multi-decadal signal. This meant something to meteorologists, but most of us just tuned in to something like the Weather Channel to see how it all worked out.
Mother Nature soon verified the forecast: Tropical Storm Colin appeared in the Atlantic the very day the CPC issued its update; later that August came Category 4 hurricanes Danielle and Earl, as well as tropical storms Fiona, Gaston, and Hermine; and so far this September we have seen two Category 4 hurricanes (Igor and Julia), one Category 3 hurricane (Karl), and two tropical storms—Lisa (which occasionally has intensified into a Category 1 hurricane) and Matthew.
With 67 days still left in the season (as of this writing), there have already been 13 named storms, 7 hurricanes, and 5 major hurricanes; more will be on the way. How did forecasters see this coming?
Tropical cyclone formation – a love story?
Setting aside the technical jargon for a moment and limiting the discussion to the Atlantic basin (which includes the Caribbean and the Gulf of Mexico), a tropical cyclone happens when water, wind, and heat meet in the right circumstances and develop a complex and powerful relationship together.
First you need very warm water, about 80 degrees Fahrenheit or more, to warm and moisten the air just above it. As that humid, warm air starts rising, the air pressure down at sea level drops a little bit. Air from further away that is under higher pressures starts to move in as a gentle breeze.
At this point, it helps to have some extra wind at the surface to encourage that air flow in toward the center, as well as some winds higher up traveling in the opposite direction, to pull more of the surface air upwards. This is called shear, and a little of it is a very good thing; too much, though, can kill even a major hurricane.
Water is also at work here. The thermodynamics are complicated, but basically, as the rising air cools, its water condenses into clouds. Thunderstorms form and start organizing into bands around the central low pressure area.
Eventually a closed circulation of wind moving at 20 to 40 m.p.h. forms, and a tropical depression has been born. This process can intensify until the system takes on a more circular shape and starts packing some strong winds, from around 40 up to a little over 70 m.p.h., at which point it is now called a tropical storm. The storm is called a hurricane when its winds reach the mid-70s.
Record hurricane winds
There is no formal upper limit to hurricane wind speed. The strongest gust ever measured was 253 m.p.h. (Olivia, southern hemisphere, 1996); the highest sustained winds in a hurricane are now considered to have been the 190 m.p.h. winds of Camille (1969) and Allen (1980), although this can only be estimated because such intense sustained winds damage and destroy recording instruments.
Predicting a season not easy
Today meteorologists are able to make a pretty accurate forecast for individual storms, using several different computer models of how the ocean and atmosphere are going to be over the next few days. It is something else, though, to attempt a forecast for the whole season. Global variables are involved in complex interactions. As well, long-term modeling often involves big errors unless the data is complete and entered correctly.
SSTs
Since the 1950s, researchers have attempted to get a handle on the global patterns that affect the hurricane season. Some factors that make a hurricane season more or less active were recognized. For example, if there was more warm water to start with (technically, higher sea surface temperatures, or SSTs), then more tropical cyclones could form. However, the ability to make accurate seasonal forecasts for hurricane activity eluded researchers.
ENSO
Things changed in the early 1980s, when a world-renowned hurricane expert, Dr. William Gray (now a member of the CSU hurricane prediction team and formerly its head), noticed that there were fewer Atlantic hurricanes during years where there were unusually warm SSTs in the eastern Pacific Ocean. This warm water increased the harvest for South American offshore fisherman, who called the phenomenon El Niño, or “The Child,” because it occurred around Christmas.
Today, El Niño is seen as part of a wider pattern of see-sawing, or oscillating, air pressure changes between the eastern and western sides of the tropical Pacific. When pressure is unusually high in the eastern Pacific, it is unusually low in the western Pacific, and vice versa. No one is quite sure what causes this, but it has been found that sea surface temperatures change at the same time as the air pressure, and that is why scientists call it the ENSO, or El Niño-Southern Oscillation.
The ENSO has many effects worldwide. Its effect on the Atlantic hurricane season is believed to be through increasing or decreasing strong vertical shear over the Atlantic. That shear is very strong during an El Niño, inhibiting tropical cyclone formation. The opposite happens, too: in a La Niña phase, when the eastern Pacific SSTs are unusually cold, the vertical shear over the Atlantic weakens considerably, to the point where it can actually assist in tropical cyclone formation.
The multi-decadal signal
The Atlantic also has its own oscillation in SSTs and atmospheric parameters. Researchers first noticed it in 2001 and called it the tropical multi-decadal signal. Its sources are complex. In one phase, for example, from 1971 to 1994, it causes conditions that hinder tropical cyclone formation; in the opposite phase, like the one we have been in since 1995, those conditions reverse and are favorable for an active season.
Things are not as cut and dried as they might sound here—for example, the 2005 hurricane season was so busy, they ran out of names and had to start using letters in the Greek alphabet, but in 2009, there were hardly any hurricanes at all. All the while, the tropical multi-decadal signal was favorable. Still, it has been correlated overall with patterns of hurricane season activity since the 19th century and is a reliable indicator of tropical cyclone activity, all other factors being equal.
It all comes together in 2010
This year, tropical meteorology experts at NOAA and Colorado State University recognized conditions were right for an unusually active hurricane season:
• The tropical multi-decadal signal was still in effect.
• The El Nino in the Pacific from last year turned into a much more favorable La Niña this July.
• Sea surface temperatures in the tropical Atlantic were at record high levels.
In spite of the slow start to the 2010 season, scientists at NOAA and Colorado State University stood by their earlier forecast of a highly active hurricane season. Time, and the planet itself, proved them right.
Sources:
NOAA 2010 Updated Atlantic Hurricane Season Outlook, NOAA Climate Prediction Center, August 5, 2010 (see link at this site to the outlook issued earlier in the year)
Tropical Meteorology Project Page, Colorado State University (see links at this site to 2010 forecasts and information on the CSU team).
Weather Underground Hurricane Archive, 2010.
FAQ, Hurricanes, Typhoons, and Tropical Cyclones, NOAA.
El Nino: An Introduction. NASA.
Atlantic Seasonal Hurricane Frequency. Part II: Forecasting Its Variability. Dr. William Gray, 1984.
