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The longer and deadlier the heat wave in western Russia becomes, the more frequently it is being linked to anthropogenic global warming.

But global warming theory doesn’t come anywhere close to explaining why it’s so darn hot this summer in Moscow.

Long-term observations suggest a more basic cause—an unusual and unprecedented (at least since 1950) confluence of several naturally-occurring atmospheric circulation patterns that together combined to set the stage for extreme warmth. Add to that urbanizationchanging forestry practices, and perhaps throw in a dash of global warming for good measure, and you take a situation that would otherwise be “very hot” and up it a notch to “record hot.”

The driving force of the 2010 heat wave has been a stationary weather system that has remainedlocked in place over western Russia since mid-June. The atmospheric is termed to be “blocked” when atmospheric circulation patterns remained fixed in place, instead of being progressive. The prolonged snow and cold in the eastern half of the U.S. last winter was caused by an atmospheric block which locked in a pattern which allowed arctic air to slide southward and storm systems to track up the east coast. The heat in Russia is caused by a blocking pattern which has locked in high pressure over Moscow and environs which favors southerly (warm) flow, a lot of sunshine, and little rain.

Atmospheric blocking is not unique to today’s climate. It is associated with atmospheric teleconnection patterns, described the National Weather Services Climate Prediction Center (CPC) as “and persistent, large-scale patterns of pressure and circulation anomalies that span vast geographical areas.” The CPC ‘s description continues:

Teleconnection patterns are also referred to as preferred modes of low-frequency (or long time scale) variability. Although these patterns typically last for several weeks to several months, they can sometimes be prominent for several consecutive years, thus reflecting an important part of both the interannual and interdecadal variability of the atmospheric circulation….

All teleconnection patterns are a naturally occurring aspect of our chaotic atmospheric system, and can arise primarily a reflection of internal atmospheric dynamics….

Teleconnection patterns reflect large-scale changes in the atmospheric wave and jet stream patterns, and influence temperature, rainfall, storm tracks, and jet stream location/ intensity over vast areas. Thus, they are often the culprit responsible for abnormal weather patterns occurring simultaneously over seemingly vast distances.

The CPC identifies and monitors about 10 different teleconnection indices, each with specific patterns of atmospheric circulation and associated impacts on surface temperatures and precipitation. The CPC has compiled the history of these teleconnection indices back to January 1950 and up through the present.

Several of the teleconnection patterns impact temperatures in the vicinity of western Russia, these include teleconnections named “West Pacific Pattern” (WP), “Polar/Eurasia Pattern” (POL), and the “East Atlantic/Western Russia Pattern” (EA/WR).

Figure 1 shows the July pattern of surface temperature anomalies when each of these three teleconnections is in their positive mode. When the teleconnections are in their negative mode, the temperature anomaly patterns are reversed. Notice that all three have “hot spots” in an around western Russia. The value of the WP, POL, and EA/WR indices for July, respectively were -2.93, 1.7, and -1.55. This means that each of these teleconnections was in a configuration that leads to higher than normal temperatures in western Russia.

Figure 1. Pattern of temperature anomalies that is associated with the positive mode of three teleconnections influencing western Russia (figures from the Climate Prediction Center).

Figure 2 shows the observed surface temperature anomalies for July, which include the big bull’s eye of high temperatures over western Russia—an expected result of the chance combination of the three teleconnection patterns.

Figure 2. Surface temperature anomalies (°C) for July, 2010 (figure from the Climate Prediction Center).

Figure 3 shows the timeseries of the combination (sum) of the July values for the WP, POL, and EA/WR teleconnection indices (we first flipped the sign of the WP and EA/WR indices so that the combined index reflects the same sign of the temperature anomalies over western Russia).

Figure 3. Combined index of the WP, POL, and EA/WR telectonnection values for the month of July, 1950-2010 (data from the CDC).

There are several things of note:

1) The July 2010 combined value is the highest since 1950—nearly 50% greater than the second highest value which occurred in 1952.

2) The combined index has been mostly positive since 1981, and mostly negative from 1955 through 1980. This behavior imparts an overall positive trend since 1950.

3) The 2010 value is about 3 times greater than the value expected based on the trend alone.

Is anthropogenic global warming behind any of this behavior?

It is hard to know for sure, but one thing that is certain is that if global warming does have a hand in the game (perhaps through the trend term), what it’s holding is pretty weak.

Figure 4 is a map of temperature anomalies for July 1936—another very hot month in western Russia. Notice how the general pattern of temperature anomalies looks a lot like those from July 2010 (in Figure 2). This would suggest that the combined teleconnection index for July 1936 was also quite high—we don’t know how high, because the compiled data from the CPC only goes back to 1950. But clearly, it can get hot in Moscow and western Russia without our help.

Figure 4. Surface temperature anomalies (°C) for July, 1936 (figure from the Goddard Institute for Space Studies).

The lack of data on teleconnections prior to 1950 limits the historical setting within which we can place the 2010 event. Would the increasing trend exhibited in Figure 3 look the same if we had that included the known warm period of the 1930s? Just how high was the combined teleconnection index in July of 1936?

We don’t know. And lacking that knowledge, it is hard to ascertain what role other factors such as urbanization, smoke, or global warming may have had in making this summer a record summer in western Russia.

On its own, the summer of 2010 would have gone down in history as an extremely hot one in Moscow, with or without any influence from enhanced greenhouse gas concentrations.

Did our changes to the environment, locally, regionally, or globally add even a bit more to what nature had in store already?

Possibly? Probably?

The jury is still way out on this one.

World Climate Report, 12 August 2010