Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2:04 PM GMT on September 08, 2006
An El Niño is on the way, according to the latest El Niño discussion posted September 7 by NOAA's Climate Prediction Center. A steady warming of the waters in the Equatorial Pacific near the coast of South America, combined with stronger than usual westerly winds over the Equatorial Pacific, point toward the emergence of a weak El Niño episode beginning in October or November. Certainly the above-normal hurricane activity in the Eastern Pacific and the appearance of record-breaking Hurricane Ioke in the Central Pacific were signs of a coming El Niño; intense hurricanes in those regions are highly correlated with the above normal ocean temperatures of a developing El Niño event. When the Equatorial Eastern Pacific waters warm to above 0.5ºC above normal for three consecutive months, an official El Niño is at hand. The ocean temperatures in this region are already at that level, and forecast to increase further over the next few months. An El Niño event can have far-reaching effects on global climate and Atlantic hurricane season activity.
Figure 1. Departure of Sea Surface Temperature (SST) from normal for September 2006 (top) and May 2006 (bottom). Note the departure of SST from normal affecting the Equatorial Eastern Pacific waters. In May, these waters were much cooler than normal, thanks to the lingering effects of the La Niña episode that ended in May. Now in September, these water have warmed dramatically, and may signal the beginning of an El Niño episode.
El Niño and climate change
A trend to El Niño at this time of year is unusual; May or June are the typical months that El Niño starts to develop. While the Climate Prediction Center expects that this will be a weak El Niño, the unusual timing of this event puts us in relatively uncharted territory. Since 1950, only one El Niño has started in the Fall, the El Niño of 1968. This event was an average El Niño, with a peak SST warming in the East Pacific of 1.0º C. For comparison, the warming was 2.3-2.5º C in the record El Niño events of 1997-98 and 1982-83. The unusual timing of the 2006 El Niño event comes on the heels of the unusual timing of the La Niña event that ended in May. The 2006 La Niña started very late--no La Niña of similar magnitude had ever formed in the middle of winter, as this one did. One may legitimately ask if these events might be linked to human-caused climate change. I am concerned that this might be the case, but we don't have a long enough record of historical El Niño events to know. Up until 1975, La Niña events and El Niño events used to alternate fairly regularly with a period of 2-7 years. Between 1950 and 1976 there were seven El Niño events and seven La Niña events. Since 1976, El Niño events have been approximately twice as frequent as La Niña events, with ten El Niño events and only six La Niñas. Some researchers have speculated that this is due to the effects of global warming causing a new "resonance" in the climate system. If so, this is one way in which global warming may end up causing a decrease in Atlantic hurricane activity over the coming decades, since the increased wind shear over the Atlantic during El Niño events greatly reduces the number and intensity of these storms.
Effect of El Niño on hurricane season
As most of you know, El Niño conditions put a major damper on both the number and intensity of Atlantic tropical cyclones. This is primarily due to increased wind shear. The upper air winds that develop when one heats the Equatorial Eastern Pacific waters tend to blow from west to east over the Atlantic at high speed. Since the tropical Atlantic trade winds near the surface typically blow the opposite direction, this creates a lot of shear that makes it difficult for a tropical cyclone to survive. Thus far in September, wind shear over the tropical Atlantic has been about normal, so there is no sign that a developing El Niño is suppressing hurricane activity yet. However, if the Climate Prediction Center is right, we can expect an earlier than usual end to hurricane season in the Atlantic, and a quiet November and December--unlike last year! El Niños can be long lived, and if the forecast El Niño for this year develops as expected, it will probably last through the hurricane season of 2007, suppressing hurricane activity next year.
Effect of El Niño on the coming winter
In the U.S., El Niño winters typically have above average rainfall across the southern tier of states, particularly along the Gulf of Mexico and California coasts. Temperatures tend to be warmer than average across the northern tier of states. Temperatures are typically cooler across the southern tier of states, due to increased cloud cover. For more info on El Niño's typical wintertime effects, see the Wikipedia El Niño page or the Climate Prediction Center winter precipitation and temperature impacts page.
I can basically repeat my blog for the past four days on Florence. Florence is a huge but disorganized tropical storm. Despite the fact that wind shear has decreased to 5-10 knots, Florence shows little sign of intensification. QuikSCAT data from this morning at 5:02am showed top winds of about 45 mph in some widely scattered pockets to the north of the center. There is still some dry air for the storm to contend with, but SSTs are a very warm 29º C and the models are still insisting the storm should intensify. It is a mystery why the tropical atmosphere has been so resistant to tropical cyclone intensification this year. It's a happy mystery for Bermuda, which figures to have a very close encounter with the core of the storm on Monday. Given Florence's continued refusal to intensify despite our expectations, I'd be surprised if the storm affected Bermuda as anything stronger than a Category 1 hurricane. Newfoundland also needs to keep an eye on Florence; some of the models are predicting she could brush that island later next week. The Hurricane Hunters are due to fly their first mission into Florence at 2am EDT Saturday.
Elsewhere in the tropics
There are no other threat areas to discuss. The stationary front off the Carolina coast may spawn another low pressure system that will try to develop into a tropical depression over the next two days, but any storm here will move quickly northeast out to sea and not affect North Carolina. The long-range GFS is predicting that two more tropical storms may develop off the coast of Africa over the next two weeks, but any storms that develop here are likely to recurve out to sea and never affect land.
My next update will be Saturday morning.
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