There is a more recent and better post on Polar Amplification on this blog: Notes On Polar Amplification.
The statement made by climatologists that always tweaks me, and not in a good way, is the one to the effect of: “This (fill in the blank) is consistent with climate models.” This mantra is repeated in most, if not all, news articles on the current bout of Arctic warming. What the climatologists or authors of the articles fail to acknowledge is that polar amplification is part of the warming process. If the globe warms, mid latitudes of the Northern Hemisphere warm more than the tropics, and the Arctic warms more than the mid latitudes. They also fail to acknowledge that there are multiple climate forcings, anthropogenic and natural, that cause polar amplification in the ethereal world of GCMs and in the real world. These include increases in CO2, increases in total solar irradiance (TSI), El Ninos, and, contrary to usual the usual line of thought, explosive volcanic eruptions.
There are many other contributors to Arctic temperature anomalies, but for now, let’s limit this discussion to the following, since it illustrates the cause of the recent high Arctic temperatures without muddying this thread with forcings that have poorly established and documented data sets.
A DEFINITION OF POLAR AMPLIFICATION
From Wikipedia: “Polar amplification is defined by International Arctic Science Committee on page 23 of the Arctic Climate Impact Assessment ‘Polar amplification (greater temperature increases in the Arctic compared to the earth as a whole) is a result of the collective effect of these feedbacks and other processes.’ It does not apply to the Antarctic because the Southern Ocean acts as a heat sink. It is common to see it stated that ‘Climate models generally predict amplified warming in polar regions’, e.g. Doran et al. However, climate models predict amplified warming for the Arctic but only modest warming for Antarctica.”
A DISCUSSION ON TROPICAL TROPSHERIC TRENDS AT REALCLIMATE
How does this relate to Arctic warming and polar amplification? Read on.
Gavin Schmidt of GISS: “Whether the warming is from greenhouse gases, El Nino’s, or solar forcing, trends aloft are enhanced. For instance, the GISS model equilibrium runs with 2xCO2 or a 2% increase in solar forcing both show a maximum around 20N to 20S around 300mb (10 km):”
I’ve added the two illustrations from the RealClimate thread as Figures 1 and 2. Figure 1 shows the tropical enhancement and polar amplification for a doubling of CO2 and Figure 2 illustrates the same effects for a 2% increase in solar irradiance.
“If the pictures are very similar despite the different forcings that implies that the pattern really has nothing to do with greenhouse gas changes, but is a more fundamental response to warming (however caused). Indeed, there is a clear physical reason why this is the case – the increase in water vapour as surface air temperature rises causes a change in the moist-adiabatic lapse rate (the decrease of temperature with height) such that the surface to mid-tropospheric gradient decreases with increasing temperature (i.e. it warms faster aloft). This is something seen in many observations and over many timescales, and is not something unique to climate models.”
There, I wasn’t making this up. To rewrite the pertinent point in the above RealClimate quote: The pattern (the same enhancement in the tropics and similar amplification in the Arctic) really has nothing to do with greenhouse gas changes, but is a more fundamental response to warming (however caused). He couldn’t have been more forthcoming than that. In the first of the quoted paragraphs, he also inferred an El Nino has the same impact on the Arctic as a 2% increase in solar forcing and a doubling of CO2.
The link to the webpage:
In it, on page 204, under the subheading of “5.3. Stratospheric Heating”, Alan Robock explains the impact explosive volcanoes have on winters in years after an eruption: “After the 1982 El Chichon and 1991 Pinatubo eruptions the tropical bands (30S–30N) warmed more than the 30N–90N band…producing an enhanced pole-to-equator temperature gradient. The resulting stronger polar vortex produces the tropospheric winter warming…”
In Chapter 2 of AR4, (page 195), the IPCC describes this winter warming further: “Anomalies in the volcanic-aerosol induced global radiative heating distribution can force significant changes in atmospheric circulation, for example, perturbing the equator-to-pole heating gradient…and forcing a positive phase of the Arctic Oscillation that in turn causes a counterintuitive boreal winter warming at middle and high latitudes over Eurasia and North America…”
Volcanic eruptions create short-term anomalies in Arctic temperature.
This link to a discussion on the Arctic Oscillation (AO) is being provided for reference purposes, but will not enter into the following discussion. JISOA does a great job of defining and documenting the effects of the AO at a basic level. No reason for me to repeat it. Refer to:
ILLUSTRATIONS OF POLAR AMPLIFICATION
The typical alarmist view of Arctic temperature is illustrated in Figure 3, which is a graph of Arctic and Global temperature anomalies from January 1975 to March 2008. Both signals are smoothed with an 84-month filter.
Figure 4 illustrates the same Arctic and Global Combined surface temperature anomalies, but in this graph the time scale is expanded to almost 130 years, from January 1880 to March 2008. From 1920 until the early 1940s, Arctic temperatures more than doubled the rise in global temperature. Arctic and global temperatures decreased from the mid-40s till the early seventies. Then Arctic temperatures began to rise and global temperatures soon follow. The Arctic rise again doubles the pace of the globe from the mid-1970s to present.
Subtracting global temperature anomaly from the Arctic anomaly creates to a graph that reinforces the opinion that the present Arctic warming is unprecedented. Refer to Figure 5. The current residual Arctic anomaly is much higher than the peak of the 1940s. The same data unfiltered, Figure 6, doesn’t help illustrate the cause of the recent bout of high Arctic temperatures.
Using the MSU data from UAH clarifies matters. In Figure 7, Global, Northern Hemisphere, and Arctic temperature anomalies from December 1978 to April 2008 are shown. The data has been smoothed with a 5-month running-average filter. The graph is a wonderful illustration of polar amplification. Note how, prior to 1978, Arctic temperatures reacts to perturbations at an amplified rate, but it follows Northern Hemisphere and Global temperature anomalies. Then, after the 97/98 El Nino, there is a step change in Arctic temperature that shifts it more than 0.5 degrees C.
Did TSI or CO2 rise significantly between the late 90s and now? No. That appears to leave only one climate forcing capable of creating the sudden rise in Arctic temperature: the 97/98 El Nino.