El Nino May Develop By This Fall - What Does It Mean For Our Area?

The Climate Prediction Center (CPC) has posted an El Niño Watch, meaning that El Niño conditions may develop in the next few months. Specifically, on the August 13th CPC update, they said that "El Niño conditions are likely to develop during August or September 2012". "El Niño refers to the periodic warming of sea-surface temperatures across the central and east-central equatorial Pacific. It represents the warm phase of the ENSO cycle, and is sometimes referred to as a Pacific warm episode" 1.

"El Niño, and its cool phase counterpart La Niña, are naturally occurring phenomena that result from interactions between the ocean surface and the atmosphere over the tropical Pacific. Changes in the ocean surface temperatures affect tropical rainfall patterns and atmospheric winds over the Pacific ocean, which in turn impact the ocean temperatures and currents. The El Niño and La Niña related patterns of tropical rainfall cause changes in the weather patterns around the globe". Therefore, even though these changes occur over the Pacific Ocean, they can have an impact on the weather here locally - in the Florida Panhandle, southeast Alabama, and southwest Georgia.

Here is a slide from the CPC Weekly ENSO Briefing (last edited July 5 2012) showing climate model probabilities of El Niño, La Niña, or neutral conditions for this winter. As you can see, El Niño appears to be the most likely ENSO phase this winter. Click for a larger version.

Local Impacts from El Niño

Summary

  • El Niño winters tend to have below normal temperatures on average, especially below normal maximum temperatures (daily highs).
  • El Niño winters will tend to have above normal precipitation over the Gulf Coast region.
  • Impacts will vary with specific El Niños.

Details

As we just mentioned, El Niño can impact the global circulation pattern, and thereby the weather patterns that affect our local area. Below are departures from normal temperatures and precipitation for October through March for the last 10 El Niños (or basically all El Niños since 1981). The El Niño winters we considered were: 1982-1983, 1986-1987, 1987-1988, 1991-1992, 1994-1995, 1997-1998, 2002-2003, 2004-2005, 2006-2007, and 2009-2010.

We considered the October to March period as an El Niño for these calculations when at least four of the 6 months were classified in the warm phase of ENSO.  Normal values were calculated on the period 1981-2010. We calculated the departures from normal at three locations: the Tallahassee airport, which is a climate site, and two cooperative observer sites in Geneva, Alabama, and Albany, Georgia.

Temperature Departures From Normal
Location October November December January February March
Tallahassee Airport (TLH) +0.2 +0.6 +1.4 -1.0 -1.6
-0.8
Geneva, Alabama (COOP) +0.9 +0.2 +1.1
-1.1
-1.7
-1.0
Albany, Georgia (COOP) -0.4 -0.2 +1.1
-1.4
-1.8
-0.3

In general, the temperatures during recent El Niños have been near or above normal from October through December, and then below normal for January through March. The net effect for the entire winter is, on average, below normal temperatures. However, temperatures can be strongly modulated by other influences and teleconnection indices such as the "North Atlantic Oscillation" (NAO) and the "Arctic Oscillation" (AO). These indices are not very predictable beyond several weeks, but can have a large influence on temperatures in the eastern United States. Therefore, the above temperature correlations are not necessarily experienced in every El Niño.

Precipitation Departures From Normal
Location Oct-Mar Total October November December January February March
Tallahassee Airport (TLH) +3.15
+0.27
+0.33
+1.28
-0.19
+0.79
+0.67
Geneva, Alabama (COOP) +2.25
-0.47
+2.06
-0.02
+0.36 +0.36
-0.05
Albany, Georgia (COOP) +4.08
-0.44
+1.53
+1.25
+0.43
+0.64
+0.66

When you consider the entire October to March period in our area, El Niño winters tend to produce above normal precipitation - on average between 2 and 4 inches above normal. However, precipitation departures vary from month-to-month and can vary substantially between individual El Niño events. At Tallahassee and Albany, 60% of the El Niño winters (Oct-Mar) had above normal precipitation. At Geneva, 50% of those winters had above normal precipitation.

Therefore, while El Niño winters on average have greater than normal precipitation, about 30-40% of the previous 10 El Niño winters had below normal precipitation (over 0.50" below normal).

Maps and Graphics

Here are national maps of temperature and precipitation anomalies (how far above or below normal) for the United States in the 10 El Niño winters we listed above. The numbers are calculated and plotted by climate divisions. Temperature is on the left, and precipitation is on the right.

 

Below are national maps, also broken down by climate divisions. It shows the risk, relative to climatological normals, of extreme temperature or precipitation events during El Niños in the December to February time frame. "Extreme" is defined as being in the highest or lowest 20% of the 100 year record. The maps show that the risk of experiencing significant precipitation events in the Gulf Coast region is more than double the climatological normal during El Niño winters.

 

Related to the above normal precipitation over the eastern Gulf Coast states, below is a graph of significant river flood events in the state of Florida from 1975-2006 from the Southeast River Forecast Center (SERFC). The red check marks are above years that were classified as moderate to strong El Niños.

Essentially, significant river flooding from synoptic systems does not exclusively occur in El Niños, but a fair amount of those river flood events in Florida do occur in an El Niño.

The Jet Stream and Severe Weather

One of the major pattern changes that usually occurs during an El Niño is a southward shift in the mean position of the jet stream relative to normal. Therefore, the strongest winds in the upper levels of the atmosphere tend to be displaced much closer to the Gulf Coast region. This is illustrated by the maps below. On the left is the mean vector wind at 250mb from October to March for the last ten winter El Niños. On the right is a map that is the same as the left, except it is the anomaly for those same El Niños. You can see that the westerly winds over the Gulf were, on average, about 3-5 m/s (7-11 mph) above normal.

 

One of the main ingredients for severe thunderstorms is wind shear, which is the change in wind speed or direction with height. Everything else being held equal, stronger upper-level winds will generally result in stronger wind shear. Therefore, as a result of the southward shift in the jet stream in most El Niño winters, the storm track also shifts south, as does the typical threat of severe thunderstorms.

OCT-MAR Severe Weather Statistics since Winter 1997-1998
Description Tornadoes Wind
Hail
Average # of reports 11.2 34.5 12.8
Average reports in El Niño 15.2 36.6 19.0
Departure for El Niño +4.0 +2.1 +6.2

In the table above, we have examined the average number of severe weather reports (by type) for our county warning area during October to March periods from winter 1997-1998 to winter 2009-2010. Therefore, these statistics are only applicable to southeast Alabama, southwest Georgia, and the majority of the Florida Panhandle and Big Bend. As you can see from the statistics, the number of severe weather reports generally come in above normal during El Niño winters.

Despite our local area (including north Florida) typically experiencing slightly above normal totals of severe weather reports during El Niño winters, the most notable difference is typically felt across the Florida Peninsula to the south of our area.2

Reference Information

1 "ENSO FAQ." Climate Prediction Center. Web. 19 Oct. 2011. <http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensofaq.shtml>.
2 Graphic from Kelly Godsey and Irv Watson, from a presentation about the effects of El Nino in Florida.



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