Melt duration in 2019 (Fig. 1a) estimated from PMW data exceeded the long-term (1981–2010) mean by up to 40 d along the west portion of the ice sheet where dark, bare ice is exposed (Fig. 1b). Over the rest of the ice sheet, the anomaly of the number of melting days during the summer of 2019 from PMW data was around 20 d. Negative anomalies were rare and geographically concentrated over a small area in the southern portion of the ice sheet. Surface melting in 2019 started relatively early, around mid-April (Fig. 2a, day of the year, DOY 105), and exceeded the 1981–2010 mean for ∼82 % of the days during the period 1 June–31 August 2019 (DOY 152–244). A measure that is commonly used for quantifying melting from passive microwave observations is the so-called melting index (MI), defined as the number of melting days times the area undergoing melting and being a measure of the intensity of surface melting (i.e., Tedesco, 2007). In 2019, the MI ranked third, after 2012 and 2010. When looking at the different summer months separately, the MI values in 2019 ranked fifth in June, seventh in July, and ninth in August. The 2019 updated trends for MI and melt extent (here defined as the area subject to at least 1 d of melting) are, respectively, 78.836 km2 per decade (p≪0.01, MI) and 7.66 % per decade (p≪0.01; trend is here expressed as a percentage of the total area of the ice sheet). The maximum daily melt extent was reached on 31 July 2019, covering ∼73 % of the ice sheet surface. In comparison, the average daily maximum extent from PMW data for the same day for the 1981–2010 period is 39.8 %. Notably, the total area that at any time underwent melting was 95.8 % of the total ice sheet in 2019 (Fig. 2b), compared with the 1981–2010 averaged value of 64.3 %. Indeed, the persistency of the atmospheric conditions at the end of July that were responsible for promoting melting over 73 % of the ice sheet in a single day (31 July 2019) extended melting during the next few days over regions that were not originally involved in the melting on 31 July, with cumulative melt extent for the 3 d period (31 July–2 August) reaching up to ∼97 % of the ice sheet surface. We note that a similar value for the maximum melt extent was reached in 2012, though in this case it did happen in 1 d. As in 2019, the exceptional melt in 2012 was associated with the advection of very warm and wet air masses coming from the south and promoting the presence of liquid water clouds promoting surface melt in the dry snow zone (e.g., Tedesco et al., 2016b). However, in 2019, the air mass came from the east after promoting an exceptional heat wave in Europe, being warmer and drier than the air mass in 2012. Moreover, by crossing the relatively cold Atlantic Ocean from Scandinavia, in 2019 the lower atmospheric layers cooled down, increasing the stability of the air mass and then limiting the formation of liquid water clouds compared to July 2012, explaining why the melt extent was lower during this 2019 big melt event than in July 2012 while the temperature anomaly was higher in the free atmosphere in 2019 than in 2012.

We investigated the possibility that the sporadic melting detected at high elevations could have been due to a malfunctioning of the sensor or other issues related to data quality. Figure 3a shows a map of the number of melting days constrained to values ranging between 1 and 4 d to highlight those areas where melting occurred for a few days at high elevations. In the figure, we also show the time series of brightness temperatures for those pixels where melting occurred for only 1 d (Fig. 3b) or for 2 d (Fig. 3c). The sharp, sudden increase in brightness temperatures is not associated with data quality issues but rather with the insurgence of melting in both cases. Melting at high elevations is also confirmed from the analysis of in situ data. For example, Fig. 4a shows air temperature (2 m) recorded at the EGP PROMICE station (75.6247∘ N, 35.9748∘ W, 2660 m a.s.l., https://www.promice.dk/WeatherStations.html, last access: 31 March 2020) together with time series of spaceborne T bs at 19.35 GHz, horizontal polarization, recorded over the pixel containing the location of the EGP station (blue line). Air (2 m) pressure (hPa) recorded at the same station is also reported as a red line in the bottom plot. The figure shows that air temperature exceeded the value of 0 ∘C when Tb values sharply increased from ∼170 to ∼220 K. Concurrently, surface air pressure reached peak values of ∼749 hPa at EGP, likely as a consequence of the persistent anticyclonic conditions occurring during that period. We also note that air temperature exceeded the melting point at least twice in 2019 at the EGP station in addition to 31 July, according to the in situ data: the first time on day 163 (12 June) and the second time on day 201 (19 July). In both cases, however, the passive microwave data did not detect the presence of liquid water. This might be a consequence of the fact that air temperature can be exceeding the melting point when snow temperature is not and that the second event, when air temperatures exceed the melting point, was characterized by relatively low pressure. This suggests that the radiative forcing associated with the incoming solar radiation might not have been as strong as in the case of the end of July.

The spatial distribution of the anomaly of the number of melting days obtained from PMW observations is consistent with the one obtained from the MAR regional model, as shown in Fig. 5a. Here, we consider those cases when the integrated liquid water content in the top meter of the snowpack reaches or exceeds 1 mm w.e., following Fettweis et al. (2007). Meltwater runoff in JJA 2019 simulated by MAR and integrated over the whole ice sheet ranked second (consistently with the MI values obtained from the PMW data), reaching a total of 560 Gt in 2019 against an average value of 300±85 Gt yr−1 for the 1981–2010 period. As a reference, the value of runoff simulated by MAR for the JJA 2012 period (when the record was established) was 610 Gt. Despite ranking second in terms of surface runoff, September 2018–August 2019 (used to define the mass balance “year”) ranks first in terms of integrated SMB negative anomaly simulated by MAR, with a total surface mass loss anomaly of ∼320 Gt yr−1 with respect to the 1981–2010 SMB average, breaking the previous record established in 2011–2012 of ∼310 Gt yr−1 (Fig. 6, blue bars), though by only 10 Gt yr−1. It is however important to note that such a difference is below the uncertainty of the MAR model estimated to be 10 % of the mean SMB.

The SMB negative anomaly in 2018–2019 is larger than that in 2011–2012 mainly because the 2018–2019 snowfall negative anomaly ( ∼ - 50 Gt) is larger in magnitude than the one that occurred during the 2011–2012 SMB year ( ∼ - 20 Gt), with large negative summer snowfall anomalies in 2019 occurring along the southern and western portions of the ice sheet (Fig. 5b). The early melt onset and the negative snowfall anomaly promoted the exposure of bare ice prematurely, hence further enhancing melting and runoff through the melt–albedo positive feedback mechanism (i.e., Tedesco et al., 2016b). This is evident from the analysis of summer broadband albedo simulated by MAR (Fig. 5c), showing negative anomalies down to −0.2 along the western portion of the ice sheet. These results are also confirmed by albedo estimates obtained from MODIS (Fig. 7a), indicating a large, negative albedo anomaly occurring along the west coast where bare ice is exposed. Specifically, summer MODIS albedo ranked fourth (Fig. 8) within the 2000–2019 MODIS period, with −1.45 standard deviations (σ) below the mean (2000–2010 baseline period). The summer of 2019 precedes the ones of 2010 (−1.79σ), 2016 (−1.95σ), and 2012 (−3.33σ) in terms of MODIS albedo. When considering the summer months separately, June and July 2019 ranked, respectively, 10th (June) and seventh (July). A new record was, nevertheless, established in August 2019, with the absolute value averaged over the whole ice sheet reaching 77.51 % (−2.39σ) in 2019, followed by 2012 (77.86 %, −2.05σ) and 2016 (78.1 %, −1.81σ). The updated trend over 2000–2019 for summer broadband albedo is −0.4 % per decade, though it is not statistically significant (R2=0.04). Similarly, the trends for June (−0.1 %), July (−0.6 %), and August (−0.7 %) are also not statistically significant.

The analysis of the maps of the monthly averaged albedo (Fig. 7b through d) indicates that, as mentioned above, negative albedo anomalies occurred along the western portion of the ice sheet in June and July but, during the same period, albedo was within the average over most of the rest of the ice sheet. In June, only 23 % of the ice sheet surface was showing positive albedo anomalies. The value for July was 25 %, to be reduced to only 6 % in August. During this month, the negative albedo anomalies in the south are confined along a relatively small portion of the west margin of the ice sheet, but they extend further inland, reaching high elevations in the northern regions (Fig. 7c). The presence of negative albedo anomalies in August at higher elevations is consistent with the sporadic melting that occurred over the same region at the end of July and beginning of August 2019 (Fig. 3). The impact of such an event is, indeed, observable in the albedo changes of the pixel that underwent melting for 2 d at the end of July (Fig. 9, the same as the one whose Tb values are shown in Fig. 3b), showing a reduction from 87.4 % to 77.8 % due to the increase in grain size associated with the melting and refreezing cycle.