Snow

C. Derksen and R. Brown

Climate Research Division, Environment Canada

November 7, 2012

Highlights

A new record low June snow cover extent (SCE) for the Northern Hemisphere (when snow cover is mainly located over the Arctic) was set in 2012. A new record low May SCE was also established over Eurasia.
2012 spring snow cover duration was the second shortest on record for both the North American and Eurasian sectors of the Arctic because of earlier than normal snow melt.
The rate of loss of June snow cover extent between 1979 and 2012 (-17.6% per decade relative to the 1979-2000 mean) is greater than the loss of September sea ice extent (-13.0% per decade) over the same period.

Snow covers the high latitude land surface for up to nine months of the year, and thereby plays a major role in the energy and freshwater budgets of the Arctic. Variability and change in snow cover extent (SCE) and snow cover duration (SCD) are of primary climatological importance, while estimates of the amount of water stored by the snowpack (snow depth or snow water equivalent) are important for hydrological purposes. While interannual variability in SCE and SCD during the snow melt period are controlled largely by surface temperature (warmer temperatures melt snow earlier), climate controls on the timing of snow cover onset in autumn and the seasonal accumulation of snow depth are more complicated, as they include influences by both temperature and precipitation. Recent reductions in Arctic spring snow cover have direct effects on many components of the Arctic physical environment, including the length of the growing season, the timing and dynamics of spring river runoff, the ground thermal regime, and wildlife population dynamics (Callaghan et al., 2011).

Northern Hemisphere spring SCE anomalies (relative to a 1988-2007 reference period) computed from the weekly NOAA snow chart Climate Data Record (CDR; maintained at Rutgers University and described in Brown and Robinson, 2011) for months when snow cover is confined largely to the Arctic showed a continued reduction from the historical mean during the spring 2012 (Fig. 5.1). New record lows for both May and June SCE were established over Eurasia in 2012 — the fifth consecutive year that a new record low June SCE was established for this region. Spring 2012 marked the third time in the past five years that a new record low June SCE was set for North America. The rate of snow cover loss over Northern Hemisphere land areas in June between 1979 and 2012 is -17.6% per decade (relative to the 1979-2000 mean), which exceeds the rate of September sea ice loss over the same time period (-13.0% per decade; Derksen and Brown, 2012). The rate of reduction in Arctic June SCE over the period of the NOAA data record exceeds the CMIP5 (Coupled Model Intercomparison Project Phase 5) model ensemble simulated and projected (historical + scenario rcp8.5 [rcp" representative concentration pathway of radiative forcing due to increasing greenhouse gas concentration) rate of decrease by more than a factor of three (Derksen and Brown, 2012).

Monthly Arctic snow cover extent standardized anomaly time series

Fig. 5.1. Monthly Arctic snow cover extent (SCE) standardized (and thus unitless) anomaly time series (with respect to 1988-2007) from the NOAA snow chart CDR for (a) April (b) May and (c) June. Solid black and red lines depict 5-yr running means for North America and Eurasia, respectively.

Spatial patterns of fall and spring SCD departures derived from the NOAA daily IMS snow cover product for 2011/12 show no fall SCD anomalies over the Canadian Arctic, earlier than normal snow onset across the eastern Siberian Arctic, and later than normal snow onset over northern Europe (Fig. 5.2a). There is an almost complete absence of longer than normal SCD during the Arctic spring, with the earliest snow melt departures occurring over the central Canadian Arctic and coastal regions across the Eurasian Arctic (Fig. 5.2b).

Fig. 5.2. Snow cover duration (SCD) departures (days; with respect to 1998-2010) from the NOAA IMS data record for the 2011-12 snow year: (a) fall; and (b) spring. Latitude circle denotes 60ºN.

During spring 2012 Arctic snow melt season was characterized by a strongly negative North Atlantic Oscillation (NAO) which reached a low of -2.25 in June (see also the essays on Air Temperature, Atmospheric Circulation and Clouds, and Greenland Ice Sheet). A negative NAO is associated with enhanced southerly air flow into the Arctic which contributes to warm temperature anomalies and rapid ablation of the snowpack. The only other year since 1950 to have a June NAO value lower than -2.0 was 1998, during which warm temperature anomalies were also present across Arctic land areas. The strongly negative NAO also played a key role in the extensive melting and mass losses observed in summer 2012 on the Greenland Ice Sheet (see the Greenland Ice Sheet essay).

A striking feature in the SCD anomaly time series (also computed from the NOAA snow chart CDR using a 1988-2007 reference period) is the seasonal asymmetry of the trends through the data record (Fig. 5.3). In contrast to the trend towards less snow in the spring period (as a result of earlier melt), the start date of snow cover over the Arctic appears to be unchanged during the satellite era. This is surprising because the in situ based air temperature record (CRUtem3v, Brohan et al., 2006) identifies significant warming trends over Arctic land areas (since 1980) in both the snow onset and melt periods. The seasonal asymmetry is consistent with a weaker coupling between snow cover and air temperatures in the autumn compared to the spring. The potential impact of increased Arctic atmospheric moisture availability (Serreze et al., 2012) on Arctic snow cover (e.g. snow onset date, annual maximum SWE, snow albedo) remains to be determined.

Arctic seasonal snow cover duration standardized anomaly time series

Fig. 5.3. Arctic seasonal snow cover duration (SCD) standardized (and thus unitless) anomaly time series (with respect to 1988-2007) from the NOAA record for (a) the first (fall) and (b) second (spring) halves of the snow season. Solid black and red lines depict 5-yr running means for North America and Eurasia, respectively.

Mean April snow depth from the Canadian Meteorological Centre (CMC) daily gridded global snow depth analysis (Brasnett, 1999) shows large regions of positive April snow depth anomalies over both the North American and Eurasian Arctic (Fig. 5.4). This means the record setting loss of spring snow cover in 2012 was driven more by rapid ablation rather than an anomalously low snow accumulation.

Fig. 5.4. April 2012 snow depth anomaly (% of 1999-2010 average) from the CMC snow depth analysis. Latitude circle denotes 60ºN.

References

Brasnett, B. 1999. A global analysis of snow depth for numerical weather prediction. J. Appl. Meteorol., 38, 726-740.

Brohan, P., J. J. Kennedy, I. Harris, S. F. B. Tett and P. D. Jones. 2006. Uncertainty estimates in regional and global observed temperature changes: A new data set from 1850. J. Geophys. Res.,111: D12106, doi:10.1029/2005JD006548.

Brown, R. and D. Robinson. 2011 Northern Hemisphere spring snow cover variability and change over 1922-2010 including an assessment of uncertainty. The Cryosphere, 5, 219-229.

Callaghan, T., M. Johansson, R. Brown, P. Groisman, N. Labba, V. Radionov, R. Barry, O. Bulygina, R. Essery, D Frolov, V. Golubev, T. Grenfell, M. Petrushina, V. Razuvaev, D. Robinson, P. Romanov, D. Shindell, A. Shmakin, S. Sokratov, S. Warren and D. Yang. 2011. The changing face of Arctic snow cover: A synthesis of observed and projected changes. Ambio, 40, 17-31.

Derksen, C. and R. Brown. 2012. Spring snow cover extent reductions in the 2008-2012 period exceeding climate model projections. Geophys. Res. Lett., doi:10.1029/2012GL053387 (in press).

Serreze, M., A. Barrett and J. Stroeve. 2012. Recent changes in tropospheric water vapor over the Arctic as assessed from radiosondes and atmospheric reanalyses. J. Geophys. Res., 117: D10104, doi:10.1029/2011JD017421.