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The Weather Classroom


Your Alaskan guide through the murky waters of El Nino and La Nina -- Section 3
John Papineau -- NWS Anchorage

Alaska Impact
During most EN's there is a tendency during the winter for a higher frequency of ridges (when compared to the long-term average frequency) to form over western Canada and the eastern Gulf of Alaska, with fewer ridges centered over the Alaska Peninsula. This means that a higher frequency of storms move up the west side of the ridge into Southcentral Alaska, generating as you can imagine warmer temperatures. As a result, there is often an increase in cloud cover and possibly in precipitation (rain and snow) as well. In addition, we also find that in some locations, Anchorage for example, there is a higher number of windstorms (southeast winds) during El Nino winters.

During a LN winter ridging is favored between 170°W and 150°W (eastern Bering Sea to western Gulf of Alaska). The actual position of the ridge is important because it can mean the difference between either warmer or cooler than normal temperatures. For example, with a ridge centered near 160°W, western Alaska is often warm while the area east of the ridge center is cool. This weather pattern also tends to produce cloud free conditions over the region (one exception would be marine fog and stratus).

Keep in mind that the magnitude or impact of air temperature or precipitation anomalies is different from one location in the state to the next. The largest air temperature anomalies occur in the Interior while the smallest ones occur in the marine zones. Precipitation anomalies on the other hand are not as clear-cut in terms of percent of normal rain or snowfall- there is a considerable variation around the state. In terms of actual amounts, the coastal zone along the Gulf of Alaska usually experiences the largest anomalies. For any given EN or LN, the impact that it has on weather may or may not extend over the entire state. For example, there have been El Nino's which produce very strong anomalies over Southeast, Southcentral, and the Interior, with little or no impact in western Alaska and the arctic slope. In a similar manor, some ENSO events produce anomalies that have a significant impact on western Alaska and the arctic slope.

An additional point that needs to be stressed is that when an EN or LN does occur, the anomalies produced by that event are not constant throughout the life of the event. By way of example we will use monthly mean air temperatures at Juneau as seen in the following table. The weather during the winters of 1994-1995 and 2002-2003 were under the influence of an EN. There are a couple of things to notice from this data: there can be some large 'jumps' from one month to the next. Secondly, as illustrated by the 1994-1995 data, on occasions an EN does not produce the 'expected' warm temperatures. Thirdly, even during a strong EN, such as occurred in 2002-2003, there is considerable difference in the average monthly temperature anomalies.

Juneau average monthly temperature anomalies (° F)
Year Nov. Dec. Jan. Feb. Mar.
1994:1995 -3.8° -1.4° +1.5° +0.1° -3.3°
2002:2003 +6.9° +3.1° +6.1° +3.0° -3.1°


These types of temperature patterns are of course location dependent. The main point is that during a given LN or EN, even though the preferred temperature anomaly may be for cooler or warmer than normal conditions, daily, weekly, and monthly averaged temperatures may deviate significantly from the expected anomaly. A similar argument exists for rain and snowfall as well.

Summary
  • The largest impact on Alaska's weather from ENSO events occurs during the winter months.
  • El Nino's usually produce warmer than normal conditions over most of the state, while La Nina's frequently produce cooler temperatures. However, there is considerable variation in temperature (and precipitation) at all timescales.
  • The magnitude of a given anomaly is location dependent. Weather in western Alaska and along the arctic coast is not as influenced by ENSO events as the remainder of the state.

Additional Reading
There are several papers on this website that go into greater detail on this subject matter. Under the CLIMATE section: "Understanding Alaska's Climate Variation". Additionally, under RESEARCH PAPERS: "Winter Temperature Variability Across Alaska During El Nino Events".

Contact Information
If you have any questions or comments about this piece, please contact John Papineau at john.papineau@noaa.gov

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