Arctic amplification metrics

One of the defining features of both recent and historical cases of global climate change is Arctic amplification (AA). This is the more rapid change in the surface air temperature (SAT) in the Arctic compared to some wider reference region, such as the Northern Hemisphere (NH) mean. Many different metrics have been developed to quantify the degree of AA based on SAT anomalies, trends and variability. The use of different metrics, as well as the choice of data set to use, can affect conclusions about the magnitude and temporal variability of AA. Here we review the established metrics of AA to see how well they agree upon the temporal signature of AA, such as the multi-decadal variability, and assess the consistency in these metrics across different commonly used data sets which cover both the early and late 20th century warming in the Arctic. We find the NOAA 20th century reanalysis most closely matches the observations when using metrics based upon SAT trends (A(2)), variability (A(3)) and regression (A(4)) of the SAT anomalies, and the ERA 20th century reanalysis is closest to the observations in the SAT anomalies (A(1)) and variability of SAT anomalies (A(3)). However, there are large seasonal differences in the consistency between data sets. Moreover, the largest differences between the century-long reanalysis products and observations are during the early warming period, likely due to the sparseness of the observations in the Arctic at that time. In the modern warming period, the high density of observations strongly constrains all the reanalysis products, whether they include satellite observations or only surface observations. Thus, all the reanalysis and observation products produce very similar magnitudes and temporal variability in the degree of AA during the recent warming period.