Masters student in tropical meteorology at FSU. Raised in Alaskan blizzards, but drawn toward tropical cyclones by their superior PGF.
By: Levi32 , 3:16 AM GMT on February 22, 2007
Hints at what hurricane season might hold in store for us this year keep popping up everywhere lately. The ENSO has officially plunged into neutral, and I believe a coming La Nina is a strong possibility this spring. The latest ENSO report from Australia came out today. As always it's full of good information so I encourage you to read it. Also skyepony found an interesting link to an experimental product, the ESPI (ENSO Precipitation Index). It's just like the SOI (Southern Oscillation Index), but it measures precipitation instead of air pressure. The numbers are similar to most ENSO indices. The 30-day average ESPI has plunged from +0.61 to -0.14 in only two weeks, which supports the current trend towards La Nina.
Another thing that I'm concerned about is the wind shear values in the Atlantic. The SSD Tropical Genesis data page shows that the wind shear in the eastern Atlantic, and both the western and eastern Caribbean have all been decreasing rapidly over the last 3 weeks, and are now well below normal for this time of year. In fact the wind shear in these areas is so low that if it were summer they would be favorable for tropical development. This is a bad sign to me, as it is once again supporting a developing La Nina, and could mean an early start to the hurricane season due to more favorable conditions in the upper atmosphere. We'll have to see if the SST anomalies fall in line as well this spring, but right now it looks like we are going to have a more active season than last year at any rate.
On another note, a potentially significant severe weather outbreak could be in the works as early as Friday night for Texas, Oklahoma, and Kansas. A low coming down out of the pacific northwest will deepen over the plains with the support of a strong (110+ knot) jetstream aloft. A classic setup with good, moist, low-level inflow from the gulf of Mexico, strong upper level dynamics, and plenty of instability is what the models are forecasting at this time. The storms will begin to decrease in intensity as the low loses upper-level support by the time they get past the Mississippi River, but some potent storms could still threaten the areas of Tennessee, Mississippi, and Georgia on Saturday and the early part of Sunday. The SPC 3-day outlook already has a slight risk area posted for the plains.
However all this could change as the low which will spawn this outbreak is still in its formation stages off the California coast, and the models usually don't handle these storms well until they're inland and crossing the Rockies. Right now I think the SPC is on the right track putting a slight risk area down for the plains on Friday. I think the ingredients will be there for a bad outbreak, but how it actually turns out could be much different considering the last two outbreaks which looked like they might be bad but in reality turned out not to be so awful.
We shall see what happens!
What is El Nino?
El Nino is a reversal of the normal trade wind flow over the equatorial Pacific. When conditions are normal, trade winds flow from east to west. This usually sets up high pressure over northwestern South America, and low pressure in the western Pacific near Australia. A normal rainfall pattern with moist in Australia and dry in the eastern Pacific and South America is the result.
However when El Nino pops up, those trade winds can be reversed or greatly slowed down. The flow is then from west to east, which sets up the low pressure over NW South America and the high pressure over Australia. This is the total opposite of normal conditions. If El Nino sticks around for several months, Australia experiences severe droughts, and NW South America experiences heavy tropical rains. SSTs in the eastern equatorial Pacific are warmer than normal because no upwelling is occurring due to the reversed trade winds. Upwelling is brining up colder water from the deeper ocean to the surface. When the trade winds are from the east like normal, they "push" the ocean water westward from the coast of South America. As the warm surface water is pushed westward, the cold water from deep down moves up to replace it in response. But when the trade winds are reversed, upwelling is shut down, and the SSTs near the South American coast are warmed greatly. Warming of the SSTs near the equator is one of the first signs of an El nino, and is an easy signature to recognize on an SST anomaly map. El Nino is also known to cause global weather pattern changes which can be very severe. Where I live in Alaska, El Nino causes winters to be extremely mild and rainy, and summers to be very hot and dry. The affects are different for different parts of the world.
El Nino also has a large impact on the Atlantic and eastern Pacific Hurricane seasons. Warming of the SSTs in the eastern equatorial Pacific tends to be counter-acted by a cooling of the SSTs in the Gulf of Mexico, the Caribbean, and the western Atlantic. It also tends to increase wind shear. This lowers the average intensity and number of hurricanes. A classic example is last year, when even a weak El Nino greatly reduced the number and strength of hurricanes. The affects of El Nino can reduce the number of U.S. hurricane landfalls as well. When low pressure sets up over NW South America, high pressure sets up over the Caribbean, which directs tropical waves south and west over South America. These waves then pop out on the other side in the eastern Pacific, where they have a much better chance to develop. This results in a much more active eastern Pacific hurricane season. Therefore El Nino decreases hurricane activity in the Atlantic, and increases activity in the eastern Pacific.
La Nina is the opposite of El Nino, and is simply an intensification of normal conditions. Easterly trade winds are stronger, which causes more upwelling of colder deeper water in the eastern equatorial Pacific. SSTs are colder than normal, which is the signature of a La Nina on an SST anomaly map. Rainfall in Australia is increased, and NW South America experiences very dry conditions. La Nina, like El Nino, also has major impacts on weather patterns across the globe, though usually the opposite of El Nino. Likewise the affects on the eastern Pacific and Atlantic hurricane seasons are opposite. Strong high pressure builds in the eastern equatorial Pacific, which results in low pressure over the Caribbean. Tropical waves in the Atlantic are steered northwest towards the U.S. and Mexican coasts, increasing the number of storm landfalls. The cooling of the SSTs in the equatorial Pacific also tends to warm the SSTs in the western Atlantic. Wind shear is also decreased. This results in a more active Atlantic hurricane season. On the other end the eastern Pacific sees very little tropical activity, as SSTs are lowered and few tropical waves make it across into the pacific basin.
El Nino and La Nina are some of the toughest pieces of the weather to forecast. Either one could pop up any time without us foreseeing it. However lots of research is being put into these phenomenon, and hopefully some day we will be able to accurately predict these climate-changing events.
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