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

Abstract

Abstract. Recent studies note a significant increase in high-pressure blocking over the Greenland region (Greenland Blocking Index, GBI) in summer since the 1990s. Such a general circulation change, indicated by a negative trend in the North Atlantic Oscillation (NAO) index, is generally highlighted as a major driver of recent surface melt records observed on the Greenland Ice Sheet (GrIS). Here we compare reanalysis-based GBI records with those from the Coupled Model Intercomparison Project 5 (CMIP5) suite of global climate models over 1950-2100. We find that the recent summer GBI increase lies well outside the 5 range of modelled past reconstructions (Historical scenario) and future GBI projections (RCP4.5 and RCP8.5). The models consistently project a future decrease in GBI (linked to an increase in NAO), which highlights a likely key deficiency of current climate models if the recently-observed circulation changes continue to persist. Given well-established connections between atmospheric pressure over the Greenland region and air temperature and precipitation extremes downstream, e.g. over Northwest Europe, this brings into question the accuracy of simulated North Atlantic jet stream changes and resulting 10 climatological anomalies over densely populated regions of northern Europe as well as of future projections of GrIS mass balance produced using global and regional climate models.

North Atlantic Natural Variability Modulates Emergence of Widespread Greenland Melt in a Warming Climate

“…While the sign of the summer NAO does not look significantly different for 2070–2099 in comparison to past periods in CESM1‐LE (Figure b), the CMIP5 ensemble mean projects a slightly more positive summer NAO under future greenhouse gas forcing (Fettweis, Hanna, et al, ). However, the CMIP5 models are known to generally underestimate observed blocking in the high‐latitude North Atlantic and Europe (Anstey et al, ; Dunn‐Sigouin & Son, ; Masato et al, ; Hanna, Fettweis, & Hall, ), calling into question their ability to project future NAO changes and highlighting a need for better model representation of blocking and further study into the impacts of climate change on natural variability. An additional open question concerns the relative importance of different mechanisms for connecting high‐pressure conditions to extreme Greenland melt in different periods, as even for recent decades the mechanisms of warm air advection, cloud and radiative impacts, and large‐scale sinking and adiabatic warming have all been implicated (Ding et al, ; Fettweis et al, ; Hofer et al, ; Lim et al, ).…”

Brief communication: CMIP6 does not suggest any atmospheric blocking increase in summer over Greenland by 2100

“…Greenland blocking events are evaluated using the 500 hPa geopotential height (Z500) averaged and area‐weighted over the region 60–80°N, 20–80°W, defined as the Greenland Blocking Index (GBI, Fang, 2004; Hanna et al ., 2013, 2014, 2015, 2016, 2018a, 2020) which is used here to assess the representation in CMIP5 and CMIP6 models of the recent summer blocking events observed over Greenland. The GBI and thus summer Z500 increase reported over these last few decades, is variously and indeterminately influenced by two factors: (1) global climate warming and, (2) atmospheric dynamical changes linked to a more meridional configuration of the polar jet stream that favours anticyclonic conditions over Greenland (Overland et al ., 2012).…”

“…Despite the importance of such dynamical atmospheric changes, they are not represented by any of the CMIP5 Earth‐system models (regardless of their complexity level, general models are called ESMs hereafter) for present‐day climate (Fettweis et al ., 2013b; Hanna et al ., 2018a). None of the models suggests any increase in Greenland blocking events by 2100, while some models even suggest moderate decreases in blocking.…”

Rapid expansion of Greenland’s low-permeability ice slabs

“…Most ice-slab simulations forced by GCMs underestimate the current extent of ice slabs when compared to reanalysis-forced RCMs (Fig. 3a), in part because present-day GCMs do not capture atmospheric circulation changes over Greenland that have contributed to recent summer melt increases 17 . Under the RCP 8.5 scenario, the formation of new ice slabs accelerates from their 1990-2050 growth (1,240-4,160 km 2 yr −1 ) to approximately double that rate (2,890-7,130 km 2 yr −1 ) in the latter half of the century (Fig.…”