Blocking variability: Arctic Amplification versus Arctic Oscillation

To predict future changes in blocking and the resulting weather extremes, some studies have proposed the negative phase of Arctic Oscillation (−AO) as an analogue for Arctic amplification because of similarities between their mean states: reduced midlatitude-to-pole temperature gradients and weakened, equatorward shifted jet streams. Using well-controlled modeling experiments, we show that blocking variations associated with mean state anomalies are opposite depending on whether these anomalies are driven by the internal dynamics as in AO or forced externally as in Arctic amplification. While blocking increases and its latitudinal-distribution shifts poleward in −AO, we find opposite responses when a mean state identical to the −AO mean state is externally forced. Findings suggest that the observed blocking-AO relationship is a correlation which does not imply that the −AO mean state causes increased blocking and should not be employed as a prototype for Arctic amplification. Furthermore, results urge for a careful consideration of causality before using internal variability to predict low-frequency response to external forcings.