El Niño Winter Temperatures

With a potentially strong El Niño taking shape, let's take a quick look at how temperatures in Alaska have responded to strong El Niños in the past. To define a strong El Niño, I used the Value of the Niño 3.4 region. A chart of historical values is shown in Figure 1. There are 8 events that crossed the 'Strong' threshold. Those 8 events were used in my subsequent analysis.

Figure 1. Oceanic Niño Index since 1950. Image source: http://ggweather.com/enso/oni.htm 

The first pass at looking for temperature patterns was the ESRL reanalysis site. I queried the December, January, February temperature anomalies for each of the years with a strong El Niño. Figure 2 shows the results.

Figure 2. Winter temperature departure (°C) from 1981-2010 normal during 8 strong El Niño events since 1950.

A pattern is clearly apparent. El Niño winters are markedly cooler in the northwest part of the state and slightly above normal in the Panhandle. End of story, right? Well, not exactly.

Remember that many stations in Alaska have seen their temperatures rise over the course of several decades. In some cases quite substantially. There are a myriad of reasons for this (climate change, station relocations, urbanization, sea ice reduction, etc.) that are not relevant to the current discussion. The ESRL reanalysis always uses the 1981-2010 climate normal period when calculating anomalies; whether it is wind, pressure, heights, or temperatures. Since the 1981-2010 temperature normals are higher (warmer) than previous decades, the data shown in Figure 2 actually overstates the magnitude (but not the pattern) of El Niño's effect on temperatures.

Since I have calculated normal temperatures for the three major stations in Alaska using the trailing 30-year climate normal periods (1921-1950, 1931-1960, etc.), we can compare the wintertime difference from the current normal period and also the wintertime difference from the temporally relevant normal period. Figure 3 shows the result of that analysis.

Figure 3. Winter temperature departure (°F) during 8 strong El Niño events since 1950 for Juneau, Fairbanks, and Anchorage using the current normal period and the temporally relevant normal period.

The chart in Figure 3 is a little busy but basically it gives a side-by-side comparison of how each of the 8 winter temperatures compares to the wintertime normal period at the time and the current wintertime normal. The solid fill pattern is the temperature departure from the relevant 30-year climate normal period and the stippled pattern is the temperature departure from the 1981-2010 normal period. What stands out, particularly in the rightmost (Overall) category is how the strong El Niño temperatures are actually warmer than normal in comparison to the normals from the relevant time period. Using the current climate normal period gives a false impression of how the temperatures actually respond to El Niño conditions. Using the appropriate climate normal period changes the relationship from negative to positive.

Additional information:

Here is a paper by John Papineau, formerly with the Anchorage NWS Office, with a description of the effect of El Niño in Anchorage.

John was also interviewed for an article in the Alaska Dispatch in 2012 discussing the effects of El Niño.