No pattern found in Northern Hemisphere atmospheric blocking and weather extremes

A new paper in press at GRL (Barnes et al 2014) suggests that the idea of “blocking” patterns being on the increase due to climate change factors such as sea ice have no statistically significant  component. As readers may know, such blocks are blamed for causing things like the Russian Heat Wave of 2010, which has been shown to be due to natural variability. The authors report “No clear hemispheric increase in blocking is found for any blocking index…”.

They go on to say that “…an increase in blocking could mean an increase in weather extremes as Arctic sea ice continues to decline. However, both observational and modeling studies suggest that any potential link between sea ice and midlatitude weather may be masked by internal variability.” and,  “…the link between recent Arctic warming and increased Northern Hemisphere blocking is currently not supported by observations.”.

Excerpts from the paper (link to full paper follows):

Exploring recent trends in Northern Hemisphere blocking

Elizabeth A. Barnes , Etienne Dunn-Sigouin , Giacomo Masato and Tim Woollings

Abstract

Observed blocking trends are diagnosed to test the hypothesis that recent Arctic warming and sea ice loss has increased the likelihood of blocking over the Northern Hemisphere.

To ensure robust results, we diagnose blocking using three unique blocking identification

methods from the literature, each applied to four different reanalyses. No clear hemispheric increase in blocking is found for any blocking index, and while seasonal increases and decreases are found for specific isolated regions and time periods, there is no instance where all three methods agree on a significant trend. Blocking is shown to exhibit large interannual and decadal variability, highlighting the difficulty in separating any potentially forced response from natural variability.

Introduction

Over the past two decades, the Arctic has experienced unprecedented sea ice loss [NSIDC, 2013]. Recent studies suggest that the recent decline in sea ice has led to an increase in blocking over North America and Europe in all seasons but spring [e.g. Liu  et al., 2012; Francis and Vavrus, 2012] due to a slow-down of the large-scale flow in response to the reduced lower-tropospheric temperature gradient. Blocking is strongly tied to weather extremes in the midlatitudes (e.g. cold snaps, heat waves), and can persist for days to weeks [e.g. Black et al., 2004; Dole et al., 2011], so an increase in blocking could mean an increase in weather extremes as Arctic sea ice continues to decline. However, both observational and modeling studies suggest that any potential link between sea ice and midlatitude weather may be masked by internal variability [e.g. Barnes, 2013; Screen et al., 2013]. Here, we address whether robust trends in blocking have been observed over the past few decades in response to the hypothesis that recent Arctic sea ice loss has increased the likelihood of blocking over the Northern Hemisphere.

While a handful of previous studies have published trends in blocking [e.g. Barnes, 2013; Croci-Maspoli et al., 2007], there is concern over whether the results are sensitive to the specific blocking detection algorithm employed, the data set used, or the season and time period over which the trend is defined. For this reason, we analyze blocking occurrence with three different detection methods, four different reanalyses, over all four seasons and three different year ranges. Our goal is to quantify whether blocking frequencies have in-fact changed over the satellite era in an effort to (1) advance the discussion of the effects of Arctic change on midlatitude weather and (2) evaluate whether trends in blocking have been observed, regardless of the sea ice hypothesis, e.g., associated with the Atlantic Meridional Overturning circulation, or global temperature increases.

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Figure 1. Climatological seasonal blocking frequency for the three indices using the MERRA reanalysis from 1980-2012. Red (blue) boxes denote regions where robust increases (decreases) in blocking frequency are found over the 1990-2012 period, as shown in Fig. 2b.

Conclusions

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 [2013], 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.

Full paper here: http://barnes.atmos.colostate.edu/FILES/MANUSCRIPTS/Barnes_DunnSigouin_etal_2014_GRL_wsupp.pdf