Unprecedented atmospheric conditions (1948–2019) drive the 2019 exceptional melting season over the Greenland ice sheet

Abstract

Abstract. Understanding the role of atmospheric circulation anomalies on the surface mass balance of the Greenland ice sheet (GrIS) is fundamental for improving estimates of its current and future contributions to sea level rise. Here, we show, using a combination of remote sensing observations, regional climate model outputs, reanalysis data, and artificial neural networks, that unprecedented atmospheric conditions (1948–2019) occurring in the summer of 2019 over Greenland promoted new record or close-to-record values of surface mass balance (SMB), runoff, and snowfall. Specifically, runoff in 2019 ranked second within the 1948–2019 period (after 2012) and first in terms of surface mass balance negative anomaly for the hydrological year 1 September 2018–31 August 2019. The summer of 2019 was characterized by an exceptional persistence of anticyclonic conditions that, in conjunction with low albedo associated with reduced snowfall in summer, enhanced the melt–albedo feedback by promoting the absorption of solar radiation and favored advection of warm, moist air along the western portion of the ice sheet towards the north, where the surface melt has been the highest since 1948. The analysis of the frequency of daily 500 hPa geopotential heights obtained from artificial neural networks shows that the total number of days with the five most frequent atmospheric patterns that characterized the summer of 2019 was 5 standard deviations above the 1981–2010 mean, confirming the exceptional nature of the 2019 season over Greenland.

Glacier Forecast: Jakobshavn Isbrae Primed for Thinning and Acceleration

“…A modest amount of thinning and acceleration at the glacier front has also been observed between March 2019 and spring 2020. However, our analysis shows that this thinning and acceleration occurred during the summer melt season of 2019, during a record year of ice-sheet surface melt (Velicogna et al, 2020;Tedesco and Fettweis, 2020). By the end of August 2019, surface melt on the ice sheet had largely subsided, but observations collected in the fjord at that time show the temperatures remained cold.…”

Surface melting over the Greenland ice sheet from enhanced resolution passive microwave brightness temperatures (1979–2019)

“…This offers the opportunity to improve our understanding of the spatial scale of the processes driving melting and potentially paves the way for using this dataset in statistically downscaling model outputs. In this regard, as a future work, we plan to extend the analysis of spatial scales to the atmospheric drivers of surface melting, such as incoming solar radiation, surface temperature and longwave radiation and complement this analysis with our previous work focusing on understanding the changes in atmospheric patterns that have been promoting enhanced melting in Greenland over the recent decades (Tedesco and Fettweis, 2020). Assessed the capability of this dataset and method in observing temporal trends, a further development can include a combination of the enhanced PMW product with higher resolution satellite data (optical sensors or lower frequencies) in order to investigate the evolution of the surface meltwater networks and the application of similar tools to other regions, such as the Canadian Arctic Archipelago, the Himalayan Plateau and the Antarctic Peninsula, where the enhancement in spatial resolution can be fully exploited.…”

“…Because of the large difference between dry and wet snow emissivity, even relatively small amounts of liquid water have a dramatic effect on the Tb values (e.g., Tedesco, 2009), making PMW data extremely suitable for mapping the extent and duration of melting at large spatial scales and high temporal resolution (in view of their insensitivity to atmospheric conditions at the low frequencies of the microwave spectrum). Consequently, PMW data have been widely adopted in melt detection studies and different remote sensing techniques have been proposed in the literature (e.g., Steffen et al, 1993;Joshi et al, 2001;Liu et al, 2005;Aschraft and Long, 2006;Macelloni et al, 2007;Tedesco et al, 2007;Kouki et al, 2019;Tedesco and Fettweis, 2020).…”

GrSMBMIP: intercomparison of the modelled 1980–2012 surface mass balance over the Greenland Ice Sheet

“…The polar (p) version of the Regional Atmospheric Climate Model (RACMO2.3p2) is run at 5.5 km horizontal resolution for the period 1958-2018 (Noël et al, 2019). The model incorporates the dynamical core of the High-Resolution Limited Area Model (HIRLAM; Undèn et al, 2002) and the physics from the European Centre for Medium-range Weather Forecasts-Integrated Forecast System (ECMWF-IFS cycle CY33r1; ECMWF-IFS, 2008). RACMO2.3p2 includes a multi-layer snow module that simulates melt, water percolation and retention in snow, refreezing and runoff (Ettema et al, 2010).…”

“…Since the end of the 1990s, the models suggest that the surface melt has almost doubled, reaching record melt volume in the summers of 2012 and 2019, while the snowfall accumulation has remained approximately constant (Noël et al, 2019;Lenaerts et al, 2019;Tedesco and Fettweis, 2020). This recent GrIS SMB decrease -largely driven by the increase in meltwater runoff Fettweis et al, 2017;Lenaerts et al, 2019; IPCC, 2019) -has been caused by Arctic amplification, a state change in the North Atlantic Oscillation and increased Greenland Blocking events in summer (Fettweis et al, 2013b;Delhasse et al, 2018;Hanna et al, 2018;Hahn et al, 2020), which raise the average temperatures (Screen and Simmonds, 2010), reduce the cloudiness (Hofer et al, 2017) and enhance the melt-albedo feedback (Box et al, 2012;Ryan et al, 2019;Noël et al, 2019).…”