Tuesday, January 7, 2014

Polar vortex and global warming

I've had several people to use the current cold weather in the US as "proof" that global warming isn't happening. Unfortunately for those arguments, they're pure bunk.

First, the Eastern US and Canada do not represent the entire planet. The December 2013 global map of average temperature anomalies show this:

While the eastern US and Canada are cold, the rest of the planet is relatively warm, with only a few areas (e.g. the Middle East) colder than the 1951-1980 average.  This same pattern has been in place since November 2013 and I expect that the January 2014 map won't change much.   [As expected, it did not.  Here's the January 2014 map:]

Again, the Eastern US was far below average whereas the rest of the planet, especially much of the Arctic was far above average.]  This demonstrates the importance of looking at the global temperature data, rather than just the weather in your backyard when contemplating global warming.

Second, the current weather pattern fits a trend that has been tied to global warming.  The rapid warming in the Arctic and loss of Arctic sea ice has been tied to negative phases of  the Arctic Oscillation (i.e. Francis and Vavrus 2012, Jaiser et al. 2012, Liu et al. 2012, Tang et al. 2013).  The Arctic has warmed at a rate of 0.4345ºC per decade since 1980 whereas September sea ice has melted at a rate of -850,560 km2 per decade.

The Arctic Oscillation (AO) measures the pressure difference between the Azores High and the Icelandic Low.  When the AO is negative, the pressure difference is low.  That low pressure difference weakens and slows the polar jet stream, allowing waves to develop.  The more negative the AO, the larger those waves become, allowing cold polar air to spill southward in the troughs and drawing warm air northward along the peaks.  When the AO is positive, the polar jet stream is faster and straighter, bottling cold polar air further north.  Large waves in the jet stream have become more frequent as the ice has melted (Cohen et al. 2013, Francis and Vavrus 2015), making the wintertime pattern of a warm Arctic coupled with cold temperatures over the continents much more frequent.  What does that mean for us?  Toss in the increase in water vapor in the atmosphere (i.e. Santer et al. 2007) and we get colder, snowier winters if in a trough of the jet stream and warm, snow-less winters if in a peak.  Looking at the temperature anomalies map I posted above, it's pretty obvious where the trough in the jet stream is this winter.

One of the other effects of a warming Actic is also evident.  Because the jet stream move slower, storm systems and weather patterns move slower.  Once a particular pattern is in place, it tends to stay that way.  And the more extreme the pattern, the more likely it is to get stuck in place (see Petoukhov et al. 2013).  So buckle up, USA.  It's going to stay colder than normal for a while this winter.  While we're shivering in -20ºC temperatures, just think of Siberia and Europe enjoying their warmer-than-average winter.

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