Guest post by Dr. Walt Meier, National Snow and Ice Data Center
Winds, Temperatures, and Arctic Sea Ice Extent
As the summer sea ice melt season gets into high gear, I thought I’d do a post on sea ice processes and other tidbits about sea ice that may be useful as people watch the seasonal sea ice extent decline. My thanks to Anthony for the opportunity to share this information.
Often, much of the focus in the news is on the effect of warming air temperatures on observed decline in Arctic sea ice extent, such as in the The Economist article. Others have suggested, such as in last Saturday’s post, that winds are the key to understanding the extent decline. These are not competing viewpoints, but reflect complementary contributions to changes in sea ice extent. For a full description of how sea ice changes – day-by-day, month-by-month, and over the years and decades – both wind and air temperatures (along with other factors, e.g., the oceans) need to be considered.
Winds and daily variations in extent
Winds primarily affect sea ice extent by pushing ice around, either spreading the ice out over larger area (increasing extent) or compressing it into a smaller area (decreasing extent). Often, day-to-day changes in sea ice extent are primarily due to changes in winds and not freezing or melting. The winds can also open areas of water within the ice-pack, called leads, if they push floes of ice apart. Thus, even during winter, there are open water areas or areas of thin ice (as leads begin to re-freeze) throughout the ice-pack. It is this feature that has allowed submarines to surface at the North Pole since the 1950s, even though the overall sea ice thickness was much greater in the 1950s compared to today. (In other words, surfacing subs at the North Pole are not an indicator of Arctic sea ice conditions.)
Winds and interannual changes in extent
Winds are variable, blowing at different directions and speeds. Thus over time, the effect of the winds settles into an average pattern and their net effect on extent is smaller relative to temperatures. However, average wind patterns can themselves vary over longer periods of time due to large-scale climate oscillations, most notably for the Arctic Oscillation (AO). During the late 1980s and early 1990s, the AO was often in a positive mode that favors the motion of older, thicker sea ice out of the Arctic. The remaining younger, thinner ice cover was more easily melted completely in the subsequent summers. This contributed to some of the summer extent decline during that period, as was noted in papers by Rigor and Wallace (2004) and Rigor et al. (2002). However, in recent years, this relationship appears to have broken down. After very strongly negative AO winters in 2009-2010 and 2010-2011, the summer sea ice again reached low levels (Stroeve et al., 2011).
Winds and summer extent
Even over a season, variation in the winds can play an important role. They were a key factor in the record low extent of 2007, as noted for example by Ogi and Wallace (2012) and Zhang et al. (2008) , who found that ~30% of the record low extent could be attributed to unusual ice motion (driven by the winds). According to Ogi et al. (2010), 50% of the year-to-year variation in extent can be explained by the variation in winds. Ogi and Wallace (2012) noted that if the wind patterns were similar to 2007, the minimum extent during 2010 and 2011 would have likely been as low as or lower than 2007.
Effects of winds and temperature on long-term changes in sea ice
Winds can also influence the long-term trend in extent. Ogi et al. (2010) estimated that up to 33% of the trend for 1979-2009 could be explained by winds. One mechanism for this long-term influence is via long-term changes in the winds, which have been noted by Ogi et al. (2010) and Smedsrud et al. (2009). Another effect on extent due to winds is in how effective winds are pushing the ice around. Spreen et al. (2011) noted that while some increase in wind speed is observed (in agreement with the Ogi and Smedsrud papers), the speed of the ice increased much more. In other words, the winds are becoming more effective at pushing the ice around.
The motion of sea ice is affected not only by winds (and other smaller factors), but also by the ice itself. Thinner ice is more easily pushed around by the winds than thicker ice (Haas et al., 2008). And the sea ice cover has been getting substantially thinner through the loss of older, thicker ice (e.g., Maslanik et al., 2011; Kwok and Rothrock, 2009). Zhang et al. (2008) found that thinner ice cover was a crucial factor in the 2007 ice loss and if the ice pack were thicker, a record low would not have occurred under the same winds. As mentioned above, some of this loss can be ascribed to the positive AO of a couple decades ago. However, since then the AO has been in a mostly neutral or negative mode and yet older ice has continued to be lost. For those interested, a nice animation of changes in ice age can be seen at the NOAA Climate Watch website.
Thus, the long-term thinning trend is primarily a reflection of additional energy from globally warming temperatures. Thick ice still moves out of the Arctic (or melts within the Arctic), but the additional energy in the Arctic prevents the replenishment of thicker ice at the same pace. The system is out of equilibrium and older, thicker ice continues to decline (though with some year-to-year variability). The additional energy is not always indicated by warmer local air temperatures though, especially in ice-covered areas. The Danish Meteorological Institute (DMI) air temperature estimates for north of 80° N, shows summer temperatures just above freezing during summer and there is little or no trend. This is because the additional energy is used to melt the surface of the ice and not warm the atmosphere, which stays near the melting point through the summer.
So, overall, the long-term decline in sea ice is mostly due to increasing temperatures leading to thinner ice cover that is more easily melted completely during summer. Winds accelerate or slow the long-term decline through the motion of thick ice out of the Arctic for period of up to a few years. The effects of winds may have longer-term consequences because their effect on ice motion increases as ice thins. The interplay between the two factors – wind and temperature – is perhaps best exemplified by the estimates for September sea ice extent in the recently released Sea Ice Outlook. There is a wide spread between different outlooks – about 500,000 square kilometers; even the uncertainties of a single method are on the same order of magnitude. That 500,000 square kilometer uncertainty reflects uncertainty in how the winds will vary this summer. However, all of the outlook contributions are more than 1.5 million square kilometers below normal, which demonstrates the effect of the long-term warming trend.
Haas, C., A. Pfaffling, S. Hendriks, L. Rabenstein, J.-L. Etienne, and I. Rigor (2008), Reduced ice thickness in Arctic Transpolar Drift favors rapid ice retreat, Geophys. Res. Lett., 35, L17501, doi:10.1029/2008GL034457.
Kwok, R., and D. A. Rothrock (2009), Decline in Arctic sea ice thickness from submarine and ICESat records: 1958–2008, Geophys. Res. Lett., 36, L15501, doi:10.1029/2009GL039035.
Maslanik, J., J. Stroeve, C. Fowler, and W. Emery (2011), Distribution and trends in Arctic sea ice age through spring 2011, Geophys. Res. Lett., 38, L13502, doi:10.1029/2011GL047735.
Ogi, M., K. Yamazaki, and J. M. Wallace (2010), Influence of winter and summer surface wind anomalies on summer Arctic sea ice extent, Geophys. Res. Lett., 37, L07701, doi:10.1029/2009GL042356.
Ogi, M. and J. M. Wallace (2012), The role of summer surface wind anomalies in the summer Arctic sea ice extent in 2010 and 2011, Geophys. Res. Lett., 39, L09704, doi:10.1029/2012GL051330.
Rigor, I.G. and J.M. Wallace (2004), Variations in the Age of Sea Ice and Summer Sea Ice Extent, Geophys. Res. Lett., v. 31, doi:10.1029/2004GL019492.
Rigor, I.G., J.M. Wallace, and R.L. Colony (2002), Response of Sea Ice to the Arctic Oscillation, J. Climate, v. 15, no. 18, pp. 2648 – 2668.
Smedsrud, L. H., Sirevaag, A., Kloster, K., Sorteberg, A., and Sandven, S. (2011), Recent wind driven high sea ice area export in the Fram Strait contributes to Arctic sea ice decline, The Cryosphere, 5, 821-829, doi:10.5194/tc-5-821-2011.
Spreen, G., R. Kwok, and D. Menemenlis (2011), Trends in Arctic sea ice drift and role of wind forcing: 1992–2009, Geophys. Res. Lett., 38, L19501, doi:10.1029/2011GL048970.
Stroeve, J. C., J. Maslanik, M. C. Serreze, I. Rigor, W. Meier, and C. Fowler (2011), Sea ice response to an extreme negative phase of the Arctic Oscillation during winter 2009/2010, Geophys. Res. Lett., 38, L02502, doi:10.1029/2010GL045662.
Zhang, J., R. Lindsay, M. Steele, and A. Schweiger (2008), What drove the dramatic retreat of arctic sea ice during summer 2007?, Geophys. Res. Lett., 35, L11505, doi:10.1029/2008GL034005.
Latest graphs are on the WUWT Arctic Sea Ice page
- The Economist Provides Readers With Erroneous Information About Arctic Sea Ice (wattsupwiththat.com)
- Global warming: Arctic sea ice extent dips toward new lows (summitcountyvoice.com)
- Will Arctic Sea Ice Reach Record Low This Year? (news.discovery.com)