Wednesday, October 13, 2010

Wither Northern Hemisphere winter?

There is an article by Choi et al. in this month's Journal of Climate examining changes in Northern Hemisphere snow cover from 1972/73 to 2007/08.

Using weekly snow-cover extent maps generated by NOAA and National Ice Center meteorologists (derived primarily from visible satellite imagery), they examine two measures of snow season: (1) the full snow season (FSS), which is the interval between the first appearance and last disappearance of snow cover and (2) the continuous snow season (CSS), which is the longest interval with an unbroken string of weeks with snow cover.

Curiously they find little hemispheric trend in CSS duration.  In other words, on a hemispheric scale, the period of continuous snow cover has changed little during the study period.  On the otherhand, they find that the FSS duration has decreased at a rate of about 5.3 days/decade.  This decrease is due primarily to an early offset (or end) of snow cover in the spring.  This trend is pronounced in the western United States.

I find these results to be broadly consistent with prior literature on western US climate.  Warming trends in recent decades are more pronounced in the spring than the fall (perhaps due to the snow-ice albedo effect) and one would expect precipitation and snowfall from spring storms to be sensitive to warming since temperatures are likely near freezing.

Given the warts in snow-cover datasets, it will be interesting to see how this study is scrutinized in coming weeks.  Also at issue is trend attribution, which was not addressed by the authors.  At issue is whether or not the hemispheric and regional changes are due to global warming from greenhouse gasses, decadal-scale variability, or other factors (e.g., black carbon, dust).


  1. This is probably a somewhat silly thought but ...

    Over the last few weeks I've been thinking about all of the unhealthy looking Aspen stands in the Wasatch. We are not alone in this nor is the Aspen tree alone. Wide spread beetle kill of conifers (and possibly aspens) comprises significant percentages of forested acres in the mountain west.

    As far as snow pack goes I would imagine that you might end up with higher albedos (fewer needles), less canopy catch, less sublimation and melt from the canopy. This might lead to a deeper snowpack in the kill zones.

    Once melt season rolls around I dont know who would win out - deeper snow or less shading=more insolation?

    Certainly isnt clear how significant any of this would be ....

  2. During my stint at the Mount Washington Observatory, one of the staff scientists did some work with the snowfall and snowdepth data (75 year record). It indicated that the range between the date of first and last snowfall was shrinking. I also believe that it showed that the establishment of a snowpack was delayed in the fall and it was melting out quicker in the spring. While not a robust study, it did confirm many of our suspicions having browsed the data record.

    There has also been work done in Colorado and in other places, I presume, linking increased dust (natural and industrial) to early snowpack depletion through decreased albedo. As the world population continues to grow, the demands on industry and land would support the dust component as small or big as that component may be.

  3. The dust situation is interesting. The Choi et al. (2010) article looks only at trends since the early 1970s. Lake cores suggest that dust deposition in alpine lakes has been quite high since western settlement (e.g., Neff et al. 2008). As such, it is unclear if dust deposition can explain the Choi et al. (2010) trends, at least over the western United States. On the otherhand, we don't have a good handle on how dust deposition varies on decadal and annual time scales, especially in specific regions.