Impact of Snow Melt Date on Seasonal Energy Balance Across the North Slope of Alaska

B. Butterworth^1,2, C.J. Cox^1,2, G.D. Boer^1,2 and O. Persson^1,2

¹Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309; 208-596-7128, E-mail: christopher.j.cox@noaa.gov
²NOAA Physical Sciences Laboratory (PSL), Boulder, CO 80305

The date when the snow melts each year in Utqiaġvik, Alaska, has been documented annually since the early 20^th century. In recent decades, there have been a series of record late and early anomalies in this timing, spanning a range of approximately six weeks, which is linked to the state of the juxtaposed Aleutian Low and Beaufort High, in particular in May (Cox et al., 2019). Years with early spring melt result in additional radiative forcing contributing to the surface energy budget. Resulting ecological, biogeochemical, and physical responses in the environment have also been documented (Cox et al., 2017). The goal of this study is to understand the fate of the energy associated with these pulses; i.e., how much of the energy from early melt years is retained within the subsurface and for how long?

We begin with a case study from the NOAA Global Monitoring Laboratory (GML) Barrow Observatory near Utqiaġvik. Record early snowmelt there in 2016 resulted in +307.8 MJ absorbed by the surface relative to 2017 when the melt occurred 35 days later in the year. This pulse was 57% of the magnitude of the total net annual climatological radiation budget. We find that approximately 70% of the 2016 energy pulse was compensated by turbulent fluxes (52% sensible, 18% latent); thus, 30% of the pulse warmed the subsurface (93 MJ). This warming is observable in thermistor string measurements co-located with the radiometric observations. 

We expanded this case study climatologically using four AmeriFlux stations, two in the vicinity of Utqiaġvik, one at Oliktok Point (250 km east), and one at Ivotuk (315 km south). In the mean of the stations, we find interannual consistency with the Barrow case in the proportion of the radiative pulse that is retained in the subsurface relative to that returned to the atmosphere by the turbulent response. To first order, these proportions are constant regardless of magnitude. An intriguing preliminary indication is that the anomaly in the amount of absorbed energy is thus predictable if the anomaly in the melt date is known. Differences between sites are found, though, that could be explained by cloud and soil properties. We are also exploring how absorbed energy affects the depth of the active layer and whether the energy excess or deficit (depending on the sign of the melt date anomaly) is retained within the subsurface indefinitely (contributing to maintenance of the permafrost) or if the system tends toward climatology later in the year through slower processes such as conductive coupling with outgoing longwave radiation.

Cox, C.J., et al. (2017) Drivers and environmental responses to the changing annual snow cycle in northern Alaska. Bull. Amer. Meteorol. Soc., 98, 2559-2577, https://doi.org/10.1175/BAMS-D-16-0201.1

Cox, C.J., et al. (2019) The Aleutian Low - Beaufort Sea Anticyclone: A climate index correlated with the timing of springtime melt in the Pacific Arctic cryosphere. Geophys. Res. Lett., 46, 7464-7473, https://doi.org/10/1029/2019GL083306