Masters student in tropical meteorology at FSU. Raised in Alaskan blizzards, but drawn toward tropical cyclones by their superior PGF.
By: Levi32 , 4:52 PM GMT on February 08, 2007
Hurricane season is only 3 months away, and like any year, the SSTs in the equatorial Pacific play a huge role in what the season will be like. Last year El Nino was partially responsible for the unexpected low number and intensity of storms. There has been an El Nino since last summer and through this winter, but there are signs of it weakening. The latest official update from Australia reflects this, and I encourage you to read it as it has some interesting information. Here are some maps and charts:
30-Day SOI (Southern Oscillation Index):
Here is a link to a 3-year chart with a brief explanation of the SOI.
SSTs and anomalies in the equatorial Pacific from the surface to 500 meters deep:
Near 125w at about 100 meters you can see a good-sized ball of cold water moving closer and closer to the surface in the anomaly map(bottom chart) over the last couple months. If this cold water does make it to the surface SST anomalies will go down dramatically. Also notice the cold water bulging upward towards the surface in the SST map (top chart). This is the area of cold water shown in the anomaly map above. This could be evidence of upwelling, which is bringing colder water up from the deep ocean closer to the surface.
Global SST Anomaly Map:
Experimental seasonal forecast models are all showing some sort of trend toward La Nina in their SST forecasts, some more dramatic that others. These same models are also forecasting precipitation patterns this summer which correspond to a La Nina pattern.
All these signs point to a weakening El Nino, and I think a La Nina could be in the making by the beginning of the hurricane season. Of course there's no way to tell how strong it will be, or exactly what impacts it will have. Predicting El Nino and La Nina is one of the toughest aspects of forecasting.
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.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.