Exploring recent trends in Northern Hemisphere blocking
Recent studies have suggested that Arctic warming and sea ice loss over the past 15 years has led to an increase in the occurrence of blocking over the Northern Hemisphere [e.g. Liu et al., 2012; Francis and Vavrus, 2012; Tang et al., 2013]. We address whether blocking frequencies have exhibited robust trends in recent decades by applying three different blocking identification methods to four different reanalyses. No clear hemispheric increase in blocking is evident in any season for any blocking index, although robust seasonal increases and decreases are found for isolated regions. Compositing winter blocking frequencies on high and low September sea ice years yields opposite signed differences depending on the years analyzed, while summer blocking yields positive differences over the North Atlantic and negative over the North Pacific. We strongly caution, however, that these composite differences can be explained by many different dynamical mechanisms, and should not be simply viewed as evidence of the response of blocking to sea ice loss.
These conclusions support those of Barnes , namely, that the link between recent Arctic warming and increased Northern Hemisphere blocking is currently not supported by observations. While Arctic sea ice experienced unprecedented losses in recent years, blocking frequencies in these years do not appear exceptional, falling well within their historically observed range. The large variability of blocking occurrence, on both inter-annual and decadal time scales, underscores the difficulty in separating any potentially forced response from natural variability.