Greenland blocking index daily series 1851–2015: Analysis of changes in extremes and links with North Atlantic and UK climate variability and change

Abstract

We present a homogenized Greenland blocking index (GBI) daily record from 1851 to 2015, therefore significantly extending our previously published monthly/seasonal GBI analysis. This new time series is analysed for evidence of changes in extreme events, and we investigate the underlying thermodynamic and dynamic precursors. We compare occurrences and changes in extreme events between our GBI record and a recently published, temporally similar daily North Atlantic Oscillation (NAO) series, and use this comparison to test dynamic meteorology hypotheses relating negative NAO to Greenland blocking. We also compare daily GBI changes and extreme events with long‐running indices of England and Wales temperature and precipitation, to assess potential downstream effects of Greenland blocking on UK extreme weather events and climate change. In this extended analysis we show that there have been sustained periods of positive GBI during 1870–1900 and from the late 1990s to present. A clustering of extreme high GBI events since 2000 is not consistently reflected by a similar grouping of extreme low NAO events. Case studies of North Atlantic atmospheric circulation changes linked with extreme high and low daily GBI episodes are used to shed light on potential linkages between Greenland blocking and jet‐stream changes. Particularly noteworthy is a clustering of extreme high GBI events during mid‐October in 4 out of 5 years during 2002–2006, which we investigate from both cryospheric and dynamic meteorology perspectives. Supporting evidence suggests that these autumn extreme GBI episodes may have been influenced by regional sea‐ice anomalies off west Greenland but were probably largely forced by increases in Rossby‐wave train activity originating from the tropical Pacific. However, more generally our results indicate that high GBI winter anomalies are co‐located with sea‐ice anomalies, while there seems to be minimal influence of sea‐ice anomalies on the recent significant increase in summer GBI.

Atmospheric River Impacts on Greenland Ice Sheet Surface Mass Balance

“…Previous studies have shown that anomalous GrIS melt episodes during the warm season often occur under slow‐moving high‐pressure regimes known as “Greenland blocks,” with these blocking anticyclones favored during negative NAO conditions and often preceded by extratropical cyclones tracking to the west of Greenland (Ahlstrøm et al, ; Hanna, Jones, et al, ; Lim et al, ; McLeod & Mote, ). Greenland blocking has significantly increased in summer over the past few decades, and is an important contributor to recently enhanced GrIS melt rates (Hanna et al, , ). Another, possibly related, type of synoptic atmospheric circulation feature that may exert an important influence on GrIS SMB is the transport of water vapor by atmospheric rivers (ARs).…”

Recent changes in summer Greenland blocking captured by none of the CMIP5 models

“…• W and follows previous work 15 (Hanna et al, , 2014(Hanna et al, , 2018. We define a second GBI series, GB2, by subtracting the area-weighted mean GPH500 over the 60-80…”

“…Hanna et al (2018)). Further recent work (Overland et al, 2012(Overland et al, , 2015Hanna et al, 2016;Hanna et al , 2017) highlights Greenland as a key region linking the Arctic Amplification of global warming ) with mid-latitude extreme weather, although such links are intermittent, itinerant and state-dependent, competing with a multitude of other climate forcings Overland et al, 2016).…”

“…Since we are currently near the peak in the ( 80-year) AMO cycle, this effect could reverse in the next few decades, although -given the other drivers mentioned above -we consider this more likely to slow down the rate of GBI increase rather than result in decreased Greenland Blocking. Also, intrinsic atmospheric dynamics (internal variability) may have contributed to the recent GB2 increase, although there is likely to be a significant external forcing element too arising through Arctic/Greenland temperature feedbacks (Hanna et al, , 2018. There is an issue of how well 10 climate models capture internal variability in the GBI and NAO (Deser et al, 2017), and internal variability may result in different GBI trends in model output and observations for any given period of up to a few decades.…”

Summertime midlatitude weather and climate extremes induced by moisture intrusions to the west of Greenland

“…It was found that this pattern occurred more frequently during the second half of this time period. Not surprisingly, this pattern closely resembles the P2 minus P1 upper tropospheric circulation pattern shown in Figure b, which is characterized by an anticyclonic circulation centre located over Baffin Bay–Greenland, reflecting a record peak in Greenland blocking observed during recent summers (Hanna et al ., ; ; ). Moreover, there is a prominent cyclonic circulation centre located downstream over the eastern North Atlantic to the west of the United Kingdom followed by two more anticyclonic circulation centres located over eastern Europe–western Russia and northern China–eastern Russia.…”