Archive for May, 2013

end of snow drought and persistent sub-freezing for W Greenland

Sunday, May 26th, 2013

I’d reported on a highly abnormal snow drought that with more bare ground produced large negative albedo anomalies along west Greenland (Fig. 1).

Figure 1. Greenland reflectivity below 500 m elevation, including land areas. Notice the extreme low anomaly for 2013 that is by now erased.

Well, after about 4 months (1 Jun – 20 April) of that type of anomaly, the pendulum swung back late April, 2013, delivering a ~5 week return of snow showers that brought up to 300% of the normal snow for that period (Fig. 2) and relative cool weather (Fig. 3).  



Figure 2. End of snow drought. Blue and purple areas indicate abnormally high precipitation.

The snow drought is not actually ended everywhere. Along northeast Greenland, snow accumulation remains well below normal, 20% of normal for 1-Jan – 25 May. A @Promice_GL field workers had to transport from Zackenberg station to AP Oleson ice cap using a Argo track vehicle instead of snowmobiles.

Figure 3. Persistent cold for Greenland between 24 April and 19 May.

With the exception of melting 21-25 May, cold has been in place since 24 April. It’s clear now from the forecast for early June 2013 that temperatures will remain below freezing along much of west Greenland. It’s not extremely cold, just not yet melting much.

3 part study reconstructs Greenland ice sheet mass budget since 1840 and presents a theory connecting surface meltwater with ice deformational flow

Thursday, May 16th, 2013

It took 7 years to pull together a full ice sheet mass budget closure based on a fusion of observationally-based records from coastal and inland weather station temperature readings; ice cores; and regional climate modeling.

Below are links to pre-prints. The papers’ abstracts capture key results but are constrained by 250 word limits. I emphasize part III below. Parts I and II were foundational works with interesting aspects. Part III brings in necessary data from parts I and II.

Greenland ice sheet mass balance reconstruction. Part I: net snow accumulation (1600-2009)
Jason E. Box, Noel Cressie, David H. Bromwich, Ji-Hoon Jung, Michiel van den Broeke, J. H. van Angelen, Richard R. Forster, Clement Miège, Ellen Mosley-Thompson, Bo Vinther, Joseph R. McConnell
Abstract . PDF (3210 KB)

Greenland ice sheet mass balance reconstruction. Part II: surface mass balance (1840-2010)
Jason E. Box
Abstract . PDF (3322 KB)

Greenland ice sheet mass balance reconstruction. Part III: marine ice loss and total mass balance (1840-2010)
Jason E. Box, William Colgan
Abstract . PDF (2452 KB)

Paper III puts forth a theory* linking surface melting with ice flow dynamics. The two are by now too often examined in isolation. Our not so old science of glaciology, beginning in earnest in the late 1950s, can now begin unifying surface and ice dynamics processes at the ice sheet scale. In stark contrast to the messaging that the recent Nick et al modeling study produced, we may expect plenty more sea level contribution from Greenland than current models predict. The misreporting of otherwise good science refers to ice flow to the sea as “melt”. Ice deformational flow is a distinct process from melt. Yet, melt and ice deformational flow are in fact intertwined processes. Self-reinforcing amplifying feedbacks outnumbering damping feedbacks by a large margin (Cuffey and Patterson, 2010, chapter 14) ensure that given a climate warming perturbation, a.k.a. the Hockey Stick, we’ll see a stronger reponse of ice to climate than is currently encoded by models. More on that later.

Because the peak statistical sensitivity between meltwater runoff and ice flow discharge emerges at the decade scale (11-13 years), it seems that the ice softening due to more meltwater in-flow to the ice sheet, the Phillips Effect, if you will, is a central physical process behind a link between runoff and ice flow dynamics (Phillips et al. 2010; 2013).

Yet, on shorter time scales and resulting from a rising trend of surface melting, also to be considered is the effect of meltwater ejection at the underwater front of marine-terminating glaciers. The effect is to force a heat exchange between the glacier front and relatively warm sea water with the ice  (Motyka et al. 2003), melting it. This is, if you will, the Motyka Effect. Underwater melting undercuts the glacier front, promoting ice berg calving and thus providing a direct and immediate link between surface runoff and ice flow. Calving reduces flow resistance, causing ice flow acceleration.

January-February 2013, As I responded to 3 critical anonymous external reviewers and the sands of time were running low to make the 15 March, 2013 deadline for the IPCC AR5, in a bid to increase the likelihood of paper III’s acceptance, I brought on Liam Colgan. His fresh and sharp eyes would comb out any potential text and methodological snags from my major revision. While you may know Liam to be a frequent user of Monte Carlo methods, I already had that in this paper before thinking of his involvement. To his credit, Liam contributed the crevasse-widening aspect to the theory that builds coincidentally via Colgan et al. (2011)’s building on Pfeffer and Bretherton (1987). The Colgan Effect is thus the 3rd aspect of the unified theory this part III study puts forth.

As to the result of the mass budget reconstruction, it’s not surprising that Greenland ice sheet contribution to sea level has accelerated. After all, climate has emerged from the dim-sun Little Ice Age into the greenhouse gas-forced new post-industrial climate epoch, the Anthropocene. Greenland’s going. It’s a question of how fast. I’m happy to report that more to this story is in the works. So, stay tuned.

* A theory is a broad collection of knowledge based on hypotheses (emphasis plural) that have withstood skeptical inquiry and are accepted, unless otherwise proven, as Fact.

J. Climate Editor Anthony J Broccoli deserves thanks for, presumably, working extra in recognition of critical timeline.

Works Cited
  • Colgan, W., K. Steffen, W. McLamb, W. Abdalati, H. Rajaram, R. Motyka, T. Phillips, and R. Anderson, 2011a: An increase in crevasse extent, West Greenland: Hydrologic implications, Geophy. Res. Lett. 38, doi:10.1029/2011GL048491
  • Cuffey, K.M. and W.S.B. Paterson (2010). The Physics of Glaciers, Fourth Edition. Elsevier, 693 pp.
  • Motyka, R. J., L. Hunter, K. A. Echelmeyer, and C. Conner, 2003: Submarine melting at the terminus of a temperate tidewater glacier, LeConte Glacier, Alaska, U.S.A. Ann. Glaciol., 36, 57-65.
  • Nick, F.M., A. Vieli, M.L. Andersen, I. Joughin, A. Payne, T.L. Edwards, F. Pattyn & R.S.W. van de Wal, 2013, Future sea-level rise from Greenland’s main outlet glaciers in a warming climateNature 497, 235–238 (09 May 2013) doi:10.1038/nature12068
  • Pfeffer, W. and C. Bretherton, 1987: The effect of crevasses on the solar heating of a glacier surface, IAHS Publication, 170, 191-205. 
  • Phillips, T., H. Rajaram, and K. Steffen, 2010: Cryohydrologic warming: A potential mechanism for rapid thermal response of ice sheets, Geophys. Res. Lett., 37, L20503, doi:10.1029/2010GL044397.
  • Phillips, T., W. Colgan, H. Rajaram and K. Steffen. Evaluation of cryo-hydrologic warming as an explanation for increased ice velocities near the equilibrium line, Southwest Greenland. J. Geophys. Res. ,2012JF002584, submitted 7 July 2012, revised 31 December 2012.

Greenland “snow drought” makes big 2013 melt more likely

Friday, May 3rd, 2013

A friend in Greenland’s capital Nuuk reported (with a frown) that the backcountry skiing this year was poor due to a “snow drought”.

 

Figure 1. Western ice sheet snowfall totals are 30%-70% of normal. Brown areas have less than ‘normal’ precipitation. Blue/purple areas are anomalously ‘wet’. The precipitation anomalies are calculated from ‘re-analyses’ data after Kalnay et al. (1996).

Multiple melt factors combine to increase the odds of more melt water runoff from the ice sheet during the 2013 melt season:

  1. less ‘cold content’ of snow to melt away (ablate) for a given energy input before bare ice is exposed;
  2. a longer period of exposed darker bare ice, in this case weeks earlier bare ice exposure is likely unless a big snow dump before or during the coming warm season;
  3. Less snow leads to a smaller refreezing capacity in the lower accumulation area. Thanks Robert Fausto of GEUS for reminding me of this one.
  4. a possible higher concentration of light absorbing impurities per unit volume of snow, assuming that the impurities are deposited whether or not it snows.

This pattern results from a persistent atmospheric anomaly, blocking cold air transport southward along west Greenland, producing relatively warm temperatures there while northwestern Europe has had a cold winter (Figure 2).

Figure 2. The data after Kalnay et al. (1996) indicate tendencies toward offshore flow over western Greenland, opposite for what is needed to produce normal snowfall. 

The precipitation anomaly is manifesting in abnormally low land and ice sheet reflectivity (albedo) (Figure 3).

Figure 3. April 2013 surface albedo (a.k.a. reflectivity) anomaly. Substantially lower albedo anomalies on land are due to the dearth of snow revealing a much darker underlying tundra. The red areas across the northern 1/3 of Greenland are uncertain due to low solar illumination angles.

Low snowfall anomalies precondition Greenland ice for enhanced melt (Mote, 2003; Box et al. 2005; 2012), especially for the western ice sheet where the snowfall amounts are less than over the east.

From 20 March – 20 April, the snow drought drove ice sheet reflectivity well below values in 13 years of (NASA MODIS sensor) satellite observations since 2000 (Figure 4). Negative North Atlantic Oscillation (NAO) has promoted Greenland heating, melting and snow drought for now 6 summers in a row (Tedesco et al. 2013; Fettweis et al. 2013). Negative late winter NAO packs a similar punch. Negative NAO has prevailed much of the past decade and is largely to blame for Greenland’s astonishing melt increase. Whether negative NAO is promoted by an earlier loss of snow on land and declining Arctic sea ice area is something I’ve been wondering about.

Figure 4. Greenland ice sheet (land excluded) reflectivity or albedo updated after Box et al. (2012).

Then the weather flipped and ice sheet reflectivity rebounded toward normal values in the latest 10 days (Figure 4). Ice sheet reflectivity and accumulated precipitation remains lower than average for the year to date through 1 May (not shown), it therefore remains more likely than not that we’ll see a big melt in 2013.

References

  • Box, J.E., L. Yang, J. Rogers, D. Bromwich, L.-S. Bai, K. Steffen, J.C. Stroeve, and S.-H. Wang, 2005: Extreme precipitation events over Greenland: Consequences to ice sheet mass balance. Preprints, Eighth Conf. on Polar Meteorology and Oceanography, San Diego, CA, Amer. Meteor. Soc., CD-ROM, 5.2. PDF
  • Box, J. E., X. Fettweis, J.C. Stroeve, M. Tedesco, D.K. Hall, and K. Steffen: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821-839, doi:10.5194/tc-6-821-2012, 2012. open access
  • Fausto, Robert, provided point 3 above.
  • Fettweis, X., Hanna, E., Lang, C., Belleflamme, A., Erpicum, M., and Gallée, H.: Brief communication “Important role of the mid-tropospheric atmospheric circulation in the recent surface melt increase over the Greenland ice sheet”, The Cryosphere, 7, 241-248, doi:10.5194/tc-7-241-2013, 2013.
  • Mote, T., 2003: Estimation of runoff rates, mass balance and elevation changes on the Greenland ice sheet from passive microwave observations. Journal of Geophysical Research-Atmospheres, 108, 4056. DOI
  • Kalnay et al.,The NCEP/NCAR 40-year reanalysis project, Bull. Amer. Meteor. Soc., 77, 437-470, 1996.
    • The data are constrained by measurements from weather stations, weather balloons, ships, aircraft, and satellites. I total precipitation for 1 January – 30 April, 20130 and difference these with the average total for the same interval over each year in the most recent 30-year ‘climate normal’ spanning 1981-2010.
  • Tedesco, M., Fettweis, X., Mote, T., Wahr, J., Alexander, P., Box, J. E., and Wouters, B.: Evidence and analysis of 2012 Greenland records from spaceborne observations, a regional climate model and reanalysis data, The Cryosphere, 7, 615-630, doi:10.5194/tc-7-615-2013, 2013. open access