With the interest in the minimum areal coverage of Arctic sea ice this month, I am posting data provided by Bill Chapman of the University of Illinois (host of The Cryosphere Today) on the timing within the year of the maximum and minimum of Arctic and Antarctic sea ice coverage [thanks to Bill for doing providing this!].
The dates within each year of the maximum and minimum sea ice coverage are presented in the first set of two figures below for the Arctic and Antarctic regions, respectively. The Arctic is the subject of considerable scrutiny as the effect of human intervention into the climate system is hypothesized to be largest there [Holland and Bitz, 2003, Serreze and Francis, 2006; Graversen et al., 2008; Serreze et al., 2007]. For the Arctic, the time of year of both the maximum and minimum ice coverage is occurring slightly earlier (i.e., negative slopes) but neither trend is significant at the 2 sigma level although the slope for the time of occurrence of the ice maximum is almost significant at the 1 sigma level (p-value=0.66).
For the Antarctic, the time of occurrence of the sea ice maximum shows a slight trend towards later dates, while the minimum coverage shows a slight trend towards earlier dates, but neither trend is significant.
Figure: Date (fraction of year beginning in January) of maximum (red) and minimum (blue) sea ice areal coverage for (a) the Arctic region and (b) the Antarctic region. None of these regressions are significant at the 1-sigma level. Data to complete this figure courtesy of Bill Chapman.
Courtesy of Ben Herman and Mike Berlage, the two figures below show the average annual cycle of daily temperatures for the 60° to 82.5° latitude belts for both hemispheres in two 10-year periods: 1980 to 1989 in blue and 1998 to 2007 in red. The strange “humps” apparent in both hemispheres in early winter in the Northern Hemisphere and late fall in the Southern Hemisphere are likely due to the averaged effect of the sea surface emissivity. The MSU LT channel has about 10% of its signal coming from the surface, so the type of surface is important, particularly if it changes during the course of a year. Sea ice has a higher emissivity in the measured microwave band, so as water turns to ice in the fall (April in the Southern Hemisphere), the brightness temperature, which is what the MSU measures, increases until the area is totally frozen over, after which time the atmospheric temperature is the main driver. Since we are dealing with anomalies in MSU temperatures, this background freezing and thawing is fairly regular. The net impact of an actual loss in sea ice over time would be a small decrease in the measured brightness temperature which would result in an apparent small net cooling in the translational seasons.
Figure: The average of the daily temperature values for two 10-year periods over the latitudes from 60° poleward to 82.5° in (a) the Northern Hemisphere and (b) the Southern Hemisphere. The blue curve is the daily average for the 1980-1989 period, while the red curve is for the 1998-2007 period for each hemisphere. Figure courtesy of Ben Herman and Mike Berlage.
In the Northern Hemisphere (left figure above) it can be seen that the red curve is slightly above the blue curve for every month representing a warming of the later period compared to the earlier one. There appears to be a slightly slower warming rate from March to May of the later period and a slightly higher and later peak temperature also for the later period. For the Southern Hemisphere (right figure above) indicates smaller changes between the two time periods but with slightly colder winter and summer temperatures during the summer maximum and winter minimum periods. Any changes in warming and cooling rates during spring and fall are too small to be detected.
The time of occurrence of the maximum and minimum sea ice coverage in the Arctic showed slight trends towards occurring earlier in the year, although not significant. In the Southern Hemisphere, the trends were smaller and also not significant, but the time of ice maximum was becoming later, contrary to the other three trends.
References
Graversen, R.G., T. Mauritsen, M. Tjernstrom, E. Kallen, and G. Svensson (2008), Vertical structure of recent Arctic warming, Nature, 451, 53-56.
Holland, M.M, and C.M. Bitz (2003), Polar amplification of climate change in coupled models, Clim. Dyn., 21, 221-232..
Serreze, M.C., and J.A. Francis (2006), The Arctic amplification debate, Climatic Change, 76, 241-264.
Serreze, M.C., A.P. Barrett, A.J. Slater, M. Steele, J. Zhang, and K.E. Trenberth (2007), The large-scale energy budget of the Arctic, J. Geophys. Res., 112, D11122, doi:10.1029/2006JD008230.
Climate Science: Roger Pielke Sr. 