Arctic Oscillation cannot be used as analogue to predict the response of atmospheric blocking to Arctic Amplification

Abstract:

To predict future changes in atmospheric blocks and the resulting weather extremes, some studies have proposed the negative phase of Arctic Oscillation (AO), an internal mode of climate variability, as an analogue for Arctic Amplification because of similarities between their atmospheric mean-states anomalies: reduction in midlatitude-to-pole temperature gradient and weakening and equatorward-shift of midlatitude jets. Observations show an increase and poleward-shift in blocking activity in the negative phase of AO; hence, this analogy suggests increased blocking activity under Arctic Amplification.

Using well-controlled experiments with an idealized GCM, we show that blocking activity variations associated with mean-state anomalies are opposite depending on whether these anomalies are driven by the internal atmospheric dynamics as in AO, or forced externally as under Arctic Amplification. We find that blocking activity increases and its latitudinal-distribution shifts poleward in the negative phase of AO (consistent with observation), but decreases and shifts equatorward when the high-latitudes are forced to warm (as shown previously in Hassanzadeh et al. 2014 GRL), and even when a mean-state anomaly almost identical to that of the negative phase of AO is externally forced. The latter is achieved using the linear response function of the idealized model.

We conclude that the observed blocking-AO relationship is a correlation which does not imply that the negative phase of AO mean-state causes increased blocking, and should not be used as a prototype for Arctic Amplification. To understand the contrasting variations of blocking activity with the negative phase of AO and under Arctic Amplification, we further investigate the synoptic eddies, which are known to play critical roles in the dynamics of both AO and blocking.

Moreover, given that the atmospheric mean responses to external forcings other than Arctic Amplification also project onto Arctic/Antarctic-Oscillation, these results show that a careful consideration of causality is required before using internal variability as analogues to predict response to external forcings.