Atmospheric Rivers vis-à-vis the Summer Seasonal Cycle and Regional Greenland Surface Melt

Abstract

Recent analysis [Mattingly et al., 2023] suggests that Atmospheric Rivers (ARs) in combination with planetary scale dynamics and coupled orographic processes (e.g., foehn effect), could lead to enhanced melting in northeast Greenland and could, in turn, be linked to increasing mass loss from outflow glaciers there [Khan et al., 2022]. The importance of large-scale dynamics, which is supported by other studies too (e.g., Neff et al., 2014), led us to examine more generally the patterns of summer melt over the whole of Greenland as influenced by factors such as the seasonal cycle, the frequency of ARs, and general synoptic influences. Our AR detection method used ERA-5 reanalysis daily data at 65oN, 55oE and 850 hPa from 2000 through 2022, JJA, and for wind directions between 112.5o and 225.0o.  We carried out linear analysis correlation between integrated water vapor, IWV; tropospheric temperature, T850 hPa; tropospheric wind speed, WS 850 hPa; and melt fraction (MF) in an area over the southwest coast near where the typical AR track first encounters the ice sheet between 62-67oN and 50-47o E.  We found high correlation between high IWV and temperature; good correlation between IWV, coastal MF and T850 hPa; and  weak dependence of MF on southerly wind speed. A consideration in quantifying the effects of ARs on total surface melt is the fact that their influence can extend over multi-day periods. The effect continues along the west coast after the warm front has passed over the ice sheet at the end of the AR life cycle when residual moist, warm air remains trapped in the downstream low along the 3-km high ice sheet, affecting surface energy budgets and where smaller less-ordered mesoscale circulations remain. In addition, because the initial northward transport occurs in concert with a strong ridge centered just east of the center of the ice sheet.  In our analysis we will show results associated with four melt areas: 1) near coastal to the west, 2) over the lower accumulation region such as in the area of the old Dye-2 radar site, 3) at the Summit of Greenland where melt is historically low but of increasing frequency of late, and 4) in the far northeast which was of interest in Mattingly et al. (2023). ARs directly affect the southwest ice sheet and their frequency can modulate MF near the shoulder seasons. Secondary effects along the east coast as the ridge passes, which may include subsidence (Mattingly et al. 2023), are weak but detectable. The frequency of ARs is less influential in the southwest in mid-summer when mean temperatures are warmer throughout the region. Melting in the northeast is only weakly related to ARs and generally follows to the seasonal cycle of warming.   Neff, W., et al. (2014, JGR, doi:10.1002/2014JD021470). Khan, S. A., et al. (2022), E, Nature, doi:10.1038/s41586-022-05301-z. Mattingly, K. S.,  et al.(2023), , Nature Communications, 14(1), 1743, doi:10.1038/s41467-023-37434-8.