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Snow

C. Derksen and R. Brown

Climate Research Division, Environment Canada

November 21, 2011

Highlights

  • Although snow water equivalent (SWE; the product of the snow depth and density, or the total water stored in the snowpack) was close to average for the Eurasian Arctic and well above average for the North American Arctic, spring snow cover extent and duration during 2011 were below average across the Arctic.
  • Persistent warm temperature anomalies in May and June created new record lows for snow extent and spring snow cover duration (for the satellite record dating from 1967) across the Eurasian sector of the Arctic.
  • The time series of snow cover duration anomalies (1966 through 2011) identifies a marked seasonal asymmetry: in contrast to a strong trend towards less snow in the spring period (as a result of earlier melt), the start date of snow cover over the Arctic has remained stable during the satellite era.

Arctic snow cover exhibits a high degree of spatial variability due to the pronounced topographic and vegetative controls on snow catchment, redistribution and metamorphosis in high latitude environments. In turn, this heterogeneity introduces uncertainty into the gridded datasets utilized to identify the response of Arctic snow cover to climate variability and change. The use of multiple datasets can mitigate this uncertainty by allowing the calculation of statistical uncertainty in time series data, and through comprehensive characterization of Arctic snow cover changes by considering snow cover extent (SCE), snow cover duration (SCD), and snow water equivalent (SWE).

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 reduction in SCE in 2011, continuing the trend identified from multiple datasets by Brown et al. (2010) (Fig. HTC1). This spring season trend to less snow cover becomes stronger from April through June because of the poleward amplification of SCE sensitivity to warming air temperatures (Dery and Brown, 2007). Variability in SCE across high latitudes is primarily controlled by surface temperature anomalies during the snow melt period (Brown et al., 2007). In April 2011, cold surface temperature anomalies extended over the Canadian high Arctic, while warm air temperature anomalies covered almost the entire Eurasian sector of the Arctic (see the essay on Temperature and Clouds). These warm anomalies persisted through May and June, driving a new record low June snow cover extent over Eurasia since satellite observations began in 1966. The cold anomaly over Arctic Canada subsided in May and June. Because these are the two primary months for snow melt in this region, SCE anomalies over the North American (NA) sector of the Arctic were strongly negative by June, in spite of the cold start to the spring season.

a. b.
Figure 1a -- Monthly Arctic snow cover extent (SCE) anomaly time series April Figure 1b -- Monthly Arctic snow cover extent (SCE) anomaly time series May
c.  
Figure 1c -- Monthly Arctic snow cover extent (SCE) anomaly time series June Fig. HTC1. Monthly Arctic snow cover extent (SCE) anomaly time series (with respect to 1988-2007) from the NOAA snow chart CDR for (a) April (b) May and (c) June. Anomalies for each year (the difference between the time series mean and each individual monthly value) were standardized by the monthly standard deviation through the time series and are, therefore, unitless. Solid line depicts 5-yr running mean.

Spatial patterns of fall, spring and seasonal SCD anomalies derived from the NOAA CDR for 2010/11 (Fig. HTC2) show little evidence of fall SCD anomalies over the Arctic, while the early spring melt anomaly in Eurasia located over the Ob and Yenisey basins contributed to the shortest spring SCD for Eurasia in the NOAA snow chart record (Fig. HTC3). A striking feature in the SCD anomaly time series is the seasonal asymmetry of the trends through the data record. 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 has remained stable during the satellite era. When Arctic and mid-latitude regions are compared, the SCD anomalies are consistent with the "warm Arctic-cold Continent" climate pattern noted in the Temperature and Clouds essay.

a. b.
Figure 2a -- snow cover duration departures fall Figure 2b -- snow cover duration departures spring
c.  
Figure 2c -- snow cover duration departures complete snow season Fig. HTC2. Snow cover duration (SCD) departures (with respect to 1998-2010) from the NOAA IMS data record for the 2010/11 snow year: (a) fall; (b) spring; (c) complete snow season.


a.
Figure 3a -- Arctic seasonal snow cover duration time series fall
b.
Figure 3b -- Arctic seasonal snow cover duration time series spring

Fig. HTC3. Arctic seasonal snow cover duration (SCD) standardized 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 lines denote 5-yr moving average.

Three pan-Arctic SWE datasets were compiled for 1980 through 2011: an assimilation of surface observations of snow depth and satellite passive microwave measurements recently developed within the European Space Agency GlobSnow project (www.globsnow.info) and described in Takala et al. (2011), the Canadian Meteorological Centre (CMC) daily gridded global snow depth analysis (Brasnett, 1999), and the ERA-interim atmospheric reanalysis (Dee et al., 2011). While the CMC analysis was utilized in previous Arctic Report Cards, the recent release of ERA-interim data back to 1980 combined with the first release of the GlobSnow data record (also available from 1980) allowed all 3 datasets to be available for each season since 1980, ten years longer than the previously considered time series. The multi-dataset Arctic SWE anomaly series (with respect to 1988-2007) for the month of April is shown in Fig. HTC4. Across the NA Arctic, 2011 was characterized by the third highest peak pre-melt SWE anomaly in the record. This is also reflected in mean April snow depth analysis from CMC (Fig. HTC5), which shows predominantly positive snow depth anomalies over the North American Arctic. While a negative SWE trend was evident between 1980 and 2003 (r=-0.56), the strong positive anomaly in 2011 continues a short-term positive trend apparent since 2004 (r=0.60). Across Eurasia, the 2011 April SWE anomaly was near normal, with the time series exhibiting a weak positive trend since 1980 (r=0.40).

a.
Figure 4a -- time series North America
b.
Figure 4b -- time series Eurasia

Fig. HTC4. Time series of multi-dataset average monthly April SWE standardized anomalies (+/- the standard error) relative to 1988-2007 for (a) North America and (b) Eurasia. Solid lines denote the linear trend. The break point in the NA best fit line is 2004.


Fig. 5 -- April 2011 snow depth anomaly

Fig. HTC5. April 2011 snow depth anomaly (% of 1999-2010 average) from the CMC snow depth analysis.

Variability in Arctic SWE is more difficult to interpret than SCE because it responds to both temperature and precipitation anomalies, and varies with snow climate regime and elevation (Brown and Mote, 2009). The overall positive pan-Arctic SWE trend observed over the past decade is consistent with documented trends of increasing precipitation over high latitudes (Zhang et al., 2007; Mekis and Vincent, 2011) and climate model predictions of increased high latitude precipitation across a warming Arctic (Min et al., 2008). The combination of continent-wide increases in SWE coupled with reduced spring SCD over the past 10 years is consistent with earlier peak stream flow, a more rapid recessional limb and higher peak runoff volume (Shiklomanov and Lammers, 2009). The recent reductions in Arctic spring snow cover extent and duration (due to earlier spring snow melt) are consistent with phenological changes in Arctic vegetation (Jia et al., 2009, and see the essay on Vegetation). Changing Arctic snow cover can also be linked to observed changes in wildlife (e.g., Drever et al., 2011).

References

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