Archive for July, 2012

Recent NAO move to neutral: a signal of short term Greenland extreme melt pause?

Thursday, July 26th, 2012

The summer NAO index is useful, when negative, in explaining extreme Greenland melting. Box et al. (2012) includes a section to that effect (see section 4.4, figure 8). The recent late July 2012 NAO index data, illustrated below, have returned to a neutral (NAO index approx. = 0) suggest the south air flow that is overheating the ice sheet has paused. Does this mean the abnormal melt year is reverting to normal melt season? I think not, because, the ice sheet reflectivity (a.k.a. albedo) remains low and there is another month-plus of time with 24 h sunlight and air temperatures are at or near melting that will allow melting to proceed in an amplified mode. What can pause the abnormal melting would be summer snowfall which would brighten the surface and insulate the melted snow below from warm air. Summer snowfall over the ice sheet is more likely with a neutral or positive NAO index.

NAO index 1 April to ~20 July, 2012 from NOAA’s Climate Prediction Center

Work Cited

  • Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K.: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821-839, doi:10.5194/tc-6-821-2012, 2012. open access

My climate-cryosphere updates on Twitter

My Byrd Polar Research Center homepage

The North Atlantic Oscillation and the ice albedo feedback deliver a 1-2-3-4-5 punch, leading to extreme Greenland melting

Thursday, July 26th, 2012

The North Atlantic Oscillation (NAO) is measured as the atmospheric sea level pressure difference between the sub-tropical high and the low pressure that prevails over the North Atlantic. This difference ‘oscillates’ in time from positive to negative. From late May until ~20 July, 2012, the NAO was negative (the low pressure in the North Atlantic was relatively high and the sub-tropical high may have been relatively low). This summer pattern has persisted since 2007. year 2012 is the 6th year in a row with this anomalous pattern.

Negative summer NAO allowing more north-south heat exchange, northward (warm air) along west Greenland heated the ice at the same time the surface reflectivity (or albedo) was trending low.

NAO index 1 April to ~20 July, 2012 from NOAA's Climate Prediction Center

The negative NAO combined with the ice albedo feedback delivered a 1-2-3-4-5 punch… 1.) the high pressure suppressed cloud formation that could reduce (slightly, its a minor effect) the solar energy reaching the surface; 2.) less cloud development reduces snowfall which can brighten the surface reducing absorbed solar heating; 3.) the “cold content” of the snowpack and ice surface had been reduced from the previous years of warming and in summer the NAO had been negative just like in 2012; 4.) warm south air (and enhanced solar absorption) heated the snowpack  and ice surface to the melting point; and 5.) the heating rounded the jagged snow crystal edges, reducing the snow’s reflectivity, allowing more solar absorption, a process that amplifies melting.

My climate-cryosphere updates on Twitter

My Byrd Polar Research Center homepage

explanation and premonition of complete surface melting over Greenland

Wednesday, July 25th, 2012

Box et al. (2012) end the article’s abstract with an explanation of the atmospheric dynamics and thermodynamics that led to the extreme year 2012 Greenland melting …and a premonition:

“Abnormally strong anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme since 2007 enabled three amplifying mechanisms to maximize the albedo feedback: (1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; (2) increased surface downward shortwave flux, leading to more surface heating and further albedo reduction; and (3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net infrared and solar radiation for the high elevation accumulation area approached positive values during this period. Thus, it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.

Further, to the paper’s conclusions:

“Accumulation area radiation budget shift [2000-2011]

In the 12 years beginning in 2000, the reduced albedo combined with a significant increase in downward solar irradiance yielded an accumulation area net radiation increase from -0.9 to -0.2 Wm^2. Another similar decade may be sufficient to shift the average summer accumulation area radiation budget from negative to positive, resulting in an abrupt ice sheet melt area increase. The ice sheet mass budget deficit is therefore expected to become more sensitive to increasing temperatures via the ice albedo feedback, especially in negative summer NAO index conditions [which did persist in 2012]. Future work should therefore be concerned with understanding potential tipping points in ice sheet melt regime as the average radiation budget shifts from negative (cooling) to positive (heating), as it seems the threshold of this has just been reached. It will take some time, perhaps years for the cold content of the firn to be sufficiently eroded to allow continuous summer melting and an ice sheet surface characterized by 100% melt extent. Further warming would only hasten the amplification of melting that the albedo feedback permits.

Work Cited

  • Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K.: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821-839, doi:10.5194/tc-6-821-2012, 2012. open access

My climate-cryosphere updates on Twitter

My Byrd Polar Research Center homepage

Correspondence of Greenland ice sheet reflectivity decrease with US annual energy consumption

Wednesday, July 25th, 2012

Greenland ice sheet surface reflectively change for June is characterized by a 6% decline between years 2000 and 2012 (Fig. 1). Considering the average all-sky (clear and cloudy) solar receipt of 360+/-20 Watts per square meter of area (see Box et al. 2012), it is elementary to conclude that the ice sheet absorbed an additional 1020 Joules (1 Joule is a Watt per second) or 100 Exajoules more solar energy. According to the International Energy Agency, this monthly figure eclipses the annual energy consumption for the United States in year 2011 (91 Exajoules).

Fig. 1. June albedo time series for the Greenland ice sheet.

The reflectivity change for July is -7% for the 13 summers spanning 2000-2012. Similarly, the ice sheet absorbed another additional 120 EJ of energy in July 2012 as compared to July 2000. This energy is disposed by some combination of melting and perhaps more importantly in the raising of the temperature of the ice and snowpack on Greenland. Box et al. (2012) calculate that the 2000-2011 additional 148 EJ solar energy absorption from June-August is sufficient to completely erode the “cold content” of the 1.494106 km2 accumulation area to a depth of 14 cm, assuming its temperature, density, and specific heat to be -10 deg. C, 360 kg m/3, and 2110 J/kg/K, respectively. An important implication is that less energy will be required in future melt seasons to bring snowpack to the melting point. Earlier onset of melt is likely. As such, the albedo feedback can operate more strongly, amplifying the impact of warming on melt.

Fig. 2. July albedo time series for the Greenland ice sheet.

Notes

  • See Box et al. 2012, Fig. 2 and related discussion for the accuracy of downward solar irradiance.
  • The albedo data are according to the MODIS MOD10A1 product and are cross validated in Box et al. (2012).

Work Cited

  • Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K.: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821-839, doi:10.5194/tc-6-821-2012, 2012. open access
  • World Energy Statistics 2011, International Energy Agency, .pdf

Follow my climate-cryosphere updates on Twitter

My Byrd Polar Research Center homepage

updated map of ice sheet albedo decline

Wednesday, July 25th, 2012

It’s been 1 month since I updated this map. I had to re-adjust the color scale from the earlier post to accomodate greater values. The 1-20 July, 2012 anomaly is off the scale for albedo change I had established in earlier examinations of data beginning in year 2000.

Map showing where the albedo reduction is greatest; the southern ‘saddle’ region, the peripheral low elevation areas, and the northwest.

Averaged over the whole of the ice sheet, for nearly 2 months now, the ice sheet albedo has been ~2 standard deviations below the 2000-2012 average.

Averaged over the whole of the ice sheet, for nearly 2 months now, the ice sheet albedo has been ~2 standard deviations below the 2000-2012 average.

Earlier blog entries discuss these albedo visualizations; herehere and here.

Follow my climate-cryosphere updates on Twitter

My Byrd Polar Research Center homepage.

Greenland ice sheet record surface melting underway

Monday, July 23rd, 2012

While the potential impact of wildfires on darkening the Greenland ice sheet surface remain to be resolved, there is mounting evidence of an extreme year 2012 melt.

Melt signatures from active microwave remote sensing are stronger than in recent years over the upper areas of the ice sheet. Dark areas indicate absorption of the microwave signal emitted by the satellite. While, year 2010 and 2011 are recognized as being record melt years (Tedesco et al. 2011, van As et al. 2011), year 2012 melting appears to be more extensive. Reported here, a Japanese field team observed a slushy surface at 1500 m and rainfall while examining satellite observations similar to that below.

On July 12, 2012, the Watson River bridge in Kangerlussuaq, west Greenland, has been swamped by higher river levels than have been observed. Watch the stunning videos here and here.

An elevated occurrence of above melting temperatures are observed 11-14 July near the ice sheet topographic summit in an area typically considered to be melt-free, a.k.a. the “dry snow zone”. The dates of a 4 consecutive days with near-surface air temperature above melting at the Summit station coincide with the Watson river flooding.

In my recently accepted albedo paper (Box et al. 2012, below), see abstract, the statement: “it is reasonable to expect 100% melt area over the ice sheet within another similar decade of warming.” may be coming true already.

Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K.: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere, 6, 821-839, doi:10.5194/tc-6-821-2012, 2012. open access

Acknowledements

Thanks to Chris Biscan via Neven’s Arctic Sea Ice Blog for identifying an accessible source of Advanced Scatterometer (ASCAT) data.

Citations
  • Box, J. E., Fettweis, X., Stroeve, J. C., Tedesco, M., Hall, D. K., and Steffen, K.: Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers, The Cryosphere Discuss., 6,  593-634, doi:10.5194/tcd-6-593-2012, 2012.
  • Tedesco, M., Fettweis, X., van den Broeke, M. R., van de Wal, R. S. W., Smeets, C. J. P. P., van de Berg, W. J., Serreze, M. C., and Box, J. E.: The role of albedo and accumulation in the 2010 melting record in Greenland, Environ. Res. Lett., 6, 014005, http://dx.doi.org/10.1088/1748- 9326/6/1/014005doi:10.1088/1748-9326/6/1/014005, 2011.
  • van As, D., Hubbard, A., Hasholt, B., Mikkelsen, A. B., van den Broeke, M., and Fausto, R. S.: Surface mass budget and meltwater discharge from the Kangerlussuaq sector of the Greenland ice sheet during record-warm year 2010, The Cryosphere Discuss., 5, 2319–2347, http://dx.doi.org/10.5194/tcd-5- 2319-2011doi:10.5194/tcd-5-2319-2011, 2011.

Follow my climate-cryosphere tweets on Twitter

My Byrd Polar Research Center homepage.