Changes in the Length of the Snow-free Season in Barrow, Alaska

Abstract:

A forty-year climate record from the NOAA Global Monitoring Division’s (GMD) Barrow Observatory (BRW) in Alaska shows a trend toward earlier spring snowmelt and a lengthening snow-free season. The snow-free season is defined as the number of days between the date of snow disappearance in spring (snow-out date), and the first snow-in date in autumn. A longer snow-free season impacts biogeochemical cycles and habitats and influences permafrost temperatures. Vegetation appears to be thriving at BRW due an extended growing season, evidenced by recent higher peak NDVI values.

The timing of snow disappearance each year can influence the net energy budget for an entire season. Feedbacks involving the change in surface albedo may enhance or diminish any response, which may be manifested in the regional temperature regime. The most important factor determining snow-out date appears to be the amount of winter snow accumulation; however, changes in cloud properties, and adjacent sea ice and ground station conditions likely play a role in that date. Stone et al (2002) estimated an increase in total radiative energy of about 1% per day for each day the melt date advanced (became earlier) at BRW.

Although the 40-year record shows a trend toward warming and earlier snowmelt, analyzing the time series for just the last 20 years reveals a tendency towards later melt in the spring; since 1990 it has been essentially flat, although still a few days earlier than the first two decades. Despite a significantly warmer winter and spring in 2014 at BRW, the melt was 11 days later than in 2013, due mostly to differences in snow accumulation, with 2014 having about 80% more snow than the year prior.

At BRW and two other Arctic locations where GMD monitors snow albedo, Alert, Canada, and Tiksi, Russia, snow-in dates are now occurring later, resulting in an increased length of the snow-free season. This appears to be a significant Arctic-wide phenomenon. Preliminary analyses of the corresponding surface radiation and temperature at these sites indicate that the temperature-albedo feedback is at play, with longer snow-free seasons resulting in enhanced net energy, warming the boundary layer air and impacting permafrost. Key factors controlling the annual snow cycle at BRW will be identified and an estimate of the radiative impact of the observed trend presented.