Abstract
ABSTRACTWe present an extended monthly and seasonal Greenland Blocking Index (GBI) from January 1851 to December 2015, which more than doubles the length of the existing published GBI series. We achieve this by homogenizing the Twentieth Century Reanalysis version 2c‐based GBI and splicing it with the NCEP/NCAR Reanalysis‐based GBI. For the whole time period, there are significant decreases in GBI in autumn, October and November, and no significant monthly, seasonal or annual increases. More recently, since 1981 there are significant GBI increases in all seasons and annually, with the strongest monthly increases in July and August. A recent clustering of high GBI values is evident in summer, when 7 of the top 11 values in the last 165 years – including the two latest years 2014 and 2015 – occurred since 2007. Also, 2010 is the highest GBI year in the annual, spring, winter and December series but 2011 is the record low GBI value in the spring and April series. Moreover, since 1851 there have been significant increases in GBI variability in May and especially December. December has also shown a significant clustering of extreme high and low GBI values since 2001, mirroring a similar, recently identified phenomenon in the December North Atlantic Oscillation index, suggesting a related driving mechanism. We discuss changes in hemispheric circulation that are associated with high compared with low GBI conditions. Our GBI time series should be useful for climatologists and other scientists interested in aspects and impacts of Arctic variability and change.
Highlights
High-pressure blocking over Greenland has been of great interest to weather forecasters and climatologists for many years
We evaluate the strength and changes of correlation between monthly/seasonal Greenland Blocking Index (GBI) and (1) North Atlantic Oscillation (NAO) indices and (2) an updated Composite Greenland Temperature 3, and discuss whether these comparisons yield possible insights into (1) the connection of Greenland blocking with wider-scale changes in atmospheric circulation over the North Atlantic, extending down to mid-latitudes, and (2) local-regional scale temperature changes around Greenland
Once the GBI data have been filtered, periods of around 8 years become apparent in most months (Figure 3): filtering time series naturally emphasizes longer-term periods, this dominant feature is robust to whether a 5, 7- or 11-point binomial filter is used and appears in the filtered NAO data wavelets (Figure S3)
Summary
High-pressure blocking over Greenland has been of great interest to weather forecasters and climatologists for many years. This blocking has traditionally been discussed in quite descriptive terms but more recently a Greenland Blocking Index (GBI) has been defined by Fang (2004) and popularized by Hanna et al (2013, 2014, 2015), where GBI is the mean 500 hPa geopotential height for the 60–80∘N, 20–80∘W region (Figure 1). We extend the GBI back to 1851 using the newly available Twentieth Century Reanalysis version 2c (20CRv2c; Compo et al, 2011, 2015) and are able to place previously published studies on observed GBI changes in the last 67 years in a much longer-term (165-year) climatic context. Recent work (Ferguson and Villarini, 2012, 2014; Wang et al, 2013) has highlighted homogeneity issues in the 20CR data, some of which are likely to be non-climatic in origin, that we attempt to remove through homogeneity testing
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