Summer Greenland Blocking Diversity and Its Impact on the Surface Mass Balance of the Greenland Ice Sheet

Abstract

The increase in Greenland Ice Sheet (GrIS) surface runoff since the turn of the century has been linked to a rise in Greenland blocking frequency. However, a range of synoptic patterns can be considered blocked flow and efforts that summarize all blocking types indiscriminately likely fail to capture consequential differences in GrIS response. To account for these differences, we employ ERA5 reanalysis to identify summer blocking using two independent blocking metrics: the Greenland Blocking Index (GBI) and the blocking index of Pelly and Hoskins (2003, https://doi.org/10.1175/1520-0469(2003)060%3c0743:ANPOB%3e2.0.CO;2). We then conduct a self‐organizing map analysis to objectively classify synoptic conditions during Greenland blocking episodes and identify three primary blocking types: (a) a high‐amplitude Omega block, (b) a lower‐amplitude, stationary summer ridge, and (c) a cyclonic wave breaking pattern. Using Modèle Atmosphérique Régional output, we document the spatiotemporal progression of the surface energy and mass balance for each blocking type. Relative to all blocking episodes, summer ridge patterns produce more melt over the southern ice sheet, Omega blocks produce more melt across the northern ice sheet, and cyclonic wave breaking patterns produce more melt in northeast Greenland. Our results indicate that the recent trend in summer Greenland blocking was largely driven by an increase in Omega patterns and suggest that Omega blocks have played a central role in the recent acceleration of GrIS mass loss. Furthermore, the GBI exhibited a relative bias toward Omega patterns, which may help explain why it has measured stronger trends in summer Greenland blocking than other blocking metrics.

Feedback attribution to recent Arctic temperature extremes and the role of natural variability

“…The PTC and atmospheric variables regressed onto the GBI feature similar spatial pattern to the NAO case but with opposite signs (Figures 4 and 5). Due to a weakening and southward displacement of the westerly jet linked to a blocking high-pressure system in Z500 in positive GBI (negative NAO) years, the meridional wind component weakened (Figure 5c and 5g), resulting in warm and moist air advection towards southwest Greenland and the Labrador Sea 20,21,24 . The resulting changes in LWCS, Q700 and cloud cover lead to negative CRE in summer and positive CRE in winter.…”

“…The enhanced warming over the Arctic compared to lower latitudes weakened the westerlies, slowed-down Rossby waves, and decreased the meridional gradient of PV resulting in a longerlasting of blocking systems over eastern Europe 43 . Also, the GBI was reported to have a positive long-term trend 21,22,24 , which was most significant in the summer months [20][21][22]44 . As Z500 anomalies trended to intensify northwestward of Iceland, the long-term trends resulted in a northward displacement of blocking systems 30 .…”

“…A second mode, which is anti-correlated with the NAO, is the Greenland Blocking Index (GBI). The GBI, which indicates the strength and frequency of high-pressure blocking conditions over Greenland, is positively correlated with Greenland temperature 16,18,[20][21][22][23][24] . Both modes have exerted a substantial impact on the strength and position of the jet stream by increasing the westerly component, reducing the advection of warm and moist air towards Greenland in a positive NAO (negative GBI) year 11,15,16 .…”

“…For instance, the extreme summer 2010 in Russia was related to blocking associated with anticyclonic circulation generating a long-lasting, impact full heatwave 33 . Further, Preece et al 24 elaborate that heat extremes associated with Omega blocking systems favor the decline of the Greenland ice sheet. Also, climate variability on interannual or decadal timescale can to some extent be related to blocking, with regional effects depending on the location of the blocking system along the jet-stream 34 .…”

“…Previous studies showed that blocking systems can develop over various regions in the North Atlantic and Europe, including east Greenland. The blocking anti-cyclones over east Greenland have been most relevant for warm anomalies over Greenland 24,28,35 .…”