1. Interannual and Regional Variability of Southern Ocean Snow on Sea Ice Thorsten Markus and Donald J. Cavalieri Goal: To investigate the regional and interannual variability of Southern Ocean snow on sea ice Background Information: Snow on sea ice controls the heat exchange between ocean and atmosphere in winter because it is a good thermal insulator. For this same reason, the depth of snow also influences the growth and decay of sea ice. Procedure: In this study we make use of 12 years (1992–2003) of Special Sensor Microwave/Imager (SSM/I) radiances to investigate the interannual and regional variability of snow depth on sea ice. The minimum, maximum, average, and standard deviation of the September snow depth on the Antarctic sea-ice cover are shown in Fig. 1 based on the 12 years of SSM/I radiances. Antarctica Fig. 1. September SSM/I-derived Antarctic snow depth for the years 1992–2003. From left to right: minimum snow depth; maximum snow depth; average snow depth; and standard deviation multiplied by 5. Impact Statement: Twelve years of DMSP SSM/I radiances are used to assess the interannual and regional variability of Southern Ocean snow depth on sea ice.

2. Results:  The time series of average snow depth for each of the five Southern Ocean sectors (Fig. 2) show a slight positive trend (Fig. 3).  These trends range from 8 to 16mm per decade, but only the Indian Ocean sector and the entire Southern Ocean have significant trends at the 95% confidence level.  The difference in the significance is to a large extent caused by the lower interannual variability of the Indian Ocean sector and the entire Southern Ocean compared to the other sectors. These trends are well below the precision of the current snow-depth algorithm. Fig. 3. Average snow depth for each of the five Southern Ocean sectors for the month of September 1992–2003. The snow depth for the entire Southern Ocean is indicated by the thick solid line. Fig. 2. The five Southern Ocean sectors used in the study. Interannual and Regional Variability of Southern Ocean Snow on Sea Ice* Thorsten Markus and Donald J. Cavalieri Conclusion: There is a partial eastward propagation of maximum snow depths, which may be related to the Antarctic Circumpolar Wave. The overall trend and the variability of regional snow-depth distributions are also in agreement with regional cyclone densities.

3. The role of melt season length in the recent extreme Arctic summer ice extent minima Thorsten Markus, NASA/GSFC, Code 614.6 Problem: Melt Season length plays a role in the Arctic summer ice extent minima. Procedure: Melt-season duration, melt-onset and freeze-up dates are derived from satellite passive microwave data and analyzed from 1979 to 2005 over Arctic sea ice. Results (see images on next slide): Indication of a shift towards a longer melt season, particularly north of Alaska and Siberia, corresponding to large retreats of sea ice observed in these regions. Although there is large interannual and regional variability in the length of the melt season, the Arctic is experiencing an overall lengthening of the melt season at a rate of about 2 weeks per decade. All regions in the Arctic (except for the central Arctic) have statistically significant (at the 99% level or higher) longer melt seasons by >1 week per decade. The central Arctic shows a statistically significant trend (at the 98% level) of 5.4 days per decade. By comparison, the Arctic experienced its longest melt season in 2005, corresponding with the least amount of sea ice since 1979 and the warmest temperatures since the 1880s. Conclusion: Overall, the length of the melt season is inversely correlated with the lack of sea ice seen in September north of Alaska and Siberia, with a mean correlation of -0.8. Impact Statement: Quantification of the connection between the sea ice melt season length and the summer sea ice cover is a further step to a more complete understanding of the polar climate system.

4. Fig. 1. Average melt onset day (a), freeze-up day (b), and corresponding melt season length (c). Please note the different color scales. A day-of-year (DOY) of greater than 365 in (b) is needed for where the freeze-up occurs in the following year. The role of melt season length in the recent extreme Arctic summer ice extent minima Thorsten Markus, NASA/GSFC, Code 614.6 Impact Statement: Quantification of the connection between the sea ice melt season length and the summer sea ice cover is a further step to a more complete understanding of the polar climate system.