Discussion of Dec07-Jan08 anomalous Southern Hemisphere (Antarctic) sea ice

Sea Ice Forum - Discussion of Dec07-Jan08 anomalous Southern Hemisphere (Antarctic) sea ice

Please note that the opinions expressed here are not necessarily those of the ACE CRC or its partners. This is a research/discussion page only.

We saw in the Discussion of September 2007 Southern Hemisphere (Antarctic) sea ice that during September 2007 the Southern Hemisphere experienced the greatest sea ice area since comprehensive records began in 1979. By November 2007 the zonally averaged sea ice extent was close to the long term climatology, even below climatology later in the month. However, during December 2007 a large positive sea ice extent/area anomaly developed, and for the next few months the anomaly remained positive. This was by far the largest Southern Hemisphere sea ice extent anomaly since records began in 1979.

The facts

Figure 1: Plot showing the sea ice extent anomaly from June 2007 through June 2008. Anomalies are based on the climatology for the period 1979-2007. Click for a larger image.

Figure 2: Plot showing the sea ice extent and climatology from June 2007 through June 2008. Climatology is based on the period 1979-2007. Click for a larger image.

The total sea ice extent anomaly for December 2007 was about 1.5 million square kilometres and 1.7 million square kilometres for January 2008 (Figure 1). The previous record monthly mean extent anomaly of 1.2 million square kilometres was in December 1985. The sea ice extent in 1986 rapidly returned close to normal, as show in the historical sea ice extent anomaly plots. However, since January 2008 the total sea ice extent has not returned to near normal but has instead stayed distinctly greater than average all the way through to now (June 2008).

Figure 3: Plot showing the sea ice area anomaly from June 2007 through June 2008. Anomalies are based on the climatology for the period 1979-2007. Click for a larger image.

Figure 4: Plot showing the sea ice area and climatology from June 2007 through June 2008. Climatology is based on the period 1979-2007. Click for a larger image.

Just as dramatic are the sea ice area plots (Figures 3 and 4). In this plot, however, the large September sea ice area anomaly stands out - much more than it does in the sea ice extent plots above. The sea ice area anomaly in September 2007 is greater than the extent anomaly for that month. However, during Dec07-Jan08 the extent anomaly is greater than the area anomaly.
Several points are worth picking out in this event:

The steady increase in anomaly starting mid November 2007.
The decline starting mid December 2007.
A steady increase starting mid February 2008.
A steep decline starting mid April 2008.

Putting this in perspective

Figure 3: Plot showing the monthly mean sea ice extent anomaly from January 1979 through May 2008. Anomalies are based on the climatology for the period 1979-2007. Click for a larger image.

Figure 4: Plot showing the five-month running monthly mean sea ice extent anomaly from January 1979 through May 2008. Anomalies are based on the climatology for the period 1979-2007. Click for a larger image.

A longer term view of Southern Hemisphere sea ice extent puts the most recent anomaly into perspective. Figure 3 shows that there is considerable variability in the anomaly. It should also be noted that there has been a slight but statistically insignificant increase in the sea ice extent since 1979. But the latest event - Dec07-Jan08 - stands out quite significantly.

There have been other large positive sea ice extent anomalies. Up until this year all anomalies greater than one million square kilometres have occurred in either December or January (Dec 1981, Dec 1985, Dec 1988, Jan 1996). In the latest event four months have had anomalies over one million square kilometres: Dec 2007, Jan, Mar and Apr 2008). Viewing this as a five month running mean puts this in greater perspective (Figure 4). The latest event is shown to be considerably anomalous. So what has caused this to happen?

A closer look

Figure 5: Map showing the monthly mean sea ice concentration for December 2007 superimposed with the long term (1979-2000) median sea ice extent for December. Click for a larger image.

Figure 6: Map showing the monthly mean sea ice concentration for January 2008 superimposed with the long term (1979-2000) median sea ice extent for January. Click for a larger image.

A climatology of the daily sea ice extent during late November through to the end of December shows... It is shown in this movie loop (GIF animation ~2MB).

Maps of sea ice concentration for December 2007 and January 2008 with a superimposed long term median give us a regional perspective of where sea ice anomalies existed for those months (Figures 5 and 6). In December 2007 the usual summertime sea ice retreat off Queen Maud Land was delayed considerably. Positive sea ice extent anomalies also exist to the north of the Ross and Weddell Seas. The same patterns of anomaly continue through to January 2008, although here there was also a positive sea ice anomaly off the Amery Ice Shelf. For a day-by-day view of the sea ice transition see the movie loop (GIF animation ~2MB). Close inspection shows that the ice retreat near Maud Rise.

Matching in with large scale variability

Some thoughts:
SAM and ENSO
SAM: Dec 2.8, Jan 2.6, Feb 1.1, Mar 0.6, Apr -0.8
ENSO: La Nina for the whole period
Similar years
Positive SAM (3-month running mean > 1.5 in Dec): 1985, 1999, 2000, 2001
ENSO = La Nina: 1983, 1984, 1988, 1995, 1998, 1999, 2000, 2005, 2007
Similar ice years: 81, 83, 84, 85, 88, 91, 93, 97, 01, 02, 03, 07

Thanks and disclaimer

I would appreciate your comments or suggestions. So, please feel free to contact me on: p.reid[at]bom.gov.au. If you would like I can include your comments in the section below. Also, further maps and plots of sea ice, SST and atmospheric parameters are available on my web page. Or go to the excellent NSIDC sea ice web site. Climate change information for the Australian region is available from the Bureau of Meteorology's comprehensive Climate web page.

Also, thanks to a number of people who have helped me thrash out these ideas - although again, the opinions expressed above are not necessarily theirs. Thanks to Rob Massom, Tony Worby, Ian Allison...

Discussion

Data used to produce the NASA Goddard Space Flight Center (GSFC) sea ice concentration images are provided by the wonderful people at NSIDC. They are obtained from the Nimbus-7 SMMR and DMSP SSM/I Passive Microwave radiances at a resolution of 25 x 25 km. Anomalies are calculated from the 1979-2000 climatology. Citation for the data are:

Cavalieri, D., C. Parkinson, P. Gloersen, and H. J. Zwally. 1996, updated 2006. Sea ice concentrations from Nimbus-7 SMMR and DMSP SSM/I passive microwave data, [latest available]. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Meier, W., F. Fetterer, K. Knowles, M. Savoie, M. J. Brodzik. 2006, updated quarterly. Sea ice concentrations from Nimbus-7 SMMR and DMSP SSM/I passive microwave data, [all available]. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

SST data are the Reynolds SST analysis, otherwise known as the NOAA Optimum Interpolation Sea Surface Temperature Analysis, can be found at http://www.emc.ncep.noaa.gov/research/cmb/sst_analysis/.

Email me:: p.reid[at]bom.gov.au
Phone me:: (03) 62262983
Mail me:: Phil Reid; ACE CRC; Private Bag 80 Hobart; Tasmania 7001; Australia