Masters student in tropical meteorology at FSU. Raised in Alaskan blizzards, but drawn toward tropical cyclones by their superior PGF.
By: Levi32 , 9:08 PM GMT on February 11, 2007
Hurricane Season 2007 is rapidly approaching, and there are many things to keep track of when trying to forecast how active a season we are to have in the Atlantic. Probably the most important variable to consider is the ENSO (El Nino Southern Oscillation).
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.
Right now there are strong signs that El Nino is weakening, and we are currently on our way into a neutral phase of the ENSO. I believe that a La Nina is possible by the time hurricane season starts. Evidence of this is shown in these two charts:
In the top image you can see the cold water bulging upward towards the surface. This is further shown in the bottom anomaly image by a large cold area near 125w at 100 meters. This spot has been migrating towards the surface over the last month or so. If this were to surface the SST anomalies would go down dramatically.
Model analogs have been showing many La Nina years and very few El Nino years lately. This could also be evidence of a developing La Nina. Also this January was very warm in the eastern U.S., unlike the cool fall season. This is also evidence of a weakening El Nino. Further more, if this pattern continues to develop, a ridge in the east would mean more hurricane landfalls in the U.S. this season.
MJO (Madden-Julian Oscillation)
Not much is known about this fascinating oscillation, but there is a lot being learned about it all the time. What it is basically, is a fluctuation of upward motion, convection, and precipitation near the equator all around the globe. Areas of more or less convection migrate west to east across the tropics, and can enhance or decrease tropical development. A chart of the MJO phase over the last 90 days can be found here. Each phase is a different part of the world, and this chart shows which phase the MJO is in right now, where it has been over the past 3 months, and its strength. For example right now the MJO isn't very strong, but it appears to be in phase 6. This means that you can probably find above normal convection in the Western Pacific. Eventually this convection will move eastward. A good site for monitoring the MJO is here. The MJO won't become very useful until hurricane season is actually in progress, but we can still draw patterns from it in correlation with the upper air patterns and El Nino.
The above image shows the vertical velocity anomalies at 200mb for the world. Areas under green lines have above normal convection or convection potential. Areas under brown lines have below normal convection. When the MJO is pronounced (right now it is very weak), you can see a main area of convection which migrates west to east with time. This becomes very important during hurricane season because when that area moves over the Atlantic chances for hurricane development are greatly increased.
Global models are forecasting above normal precipitation in the ITCZ over the Atlantic this summer, along with very dry conditions over the eastern equatorial Pacific. The dry Pacific indicates La Nina, and the wet ITCZ obviously points to more TC formations.
This is the average forecast of the 6 models. The same models are forecasting at least a small La Nina during the summer.
The first image is the average of the 6 models, where you can see a weak La Nina is forecast. The second image is an individual model which is forecasting a very strong La Nina to develop. Other models are also forecasting the same thing, along with above normal heights over the western Atlantic, indicating a more western position of the Bermuda High.
Right now I see a more active year than last year shaping up, with a La Nina instead of El Nino. The big key this year will be where the Bermuda High sets up. Right now global models are showing a chance that it will set up pretty far west close the eastern seaboard, which wouldn't be good news. If that does happen there will be a corridor leading every storm towards the US. The SSTs in the Caribbean and off the eastern Seaboard should also be warmer this year than they were last season. In short: Be ready for a ride!
We shall see what happens!
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