Figure 1: Map showing the sea ice extent in Sep 2007. Click for a larger image.
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.
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Figure 1: Map showing the sea ice extent in Sep 2007. Click for a larger image. |
Figure 2: Map showing the sea ice extent anomaly in Sep 2007. Click for a larger image. |
Yes, in September 2007 the area of the sea ice around Antarctica was the greatest since satellite records began in 1979, and it was the second greatest sea ice extent, behind 2006.
Sep 2007 extent: 19.2 million square kilometres - second highest on record, behind 2006.
Sep 2007 area: 15.2 million square kilometres - highest on record.
Sep 2007 extent anomaly: 3% above the long term mean.
Sep 2007 area anomaly: 6% above the long term mean.
Figure 3: Map showing the sea ice extent anomaly in Sep 2007 superimposed on the MSLP. Blue arrows represent cold (southerly) air advection; red arrows represent warm northerly air advection. Click for a larger image. |
Figure 4: Map showing the sea surface temperature anomaly in Sep 2007. Blue arrows represent cold (southerly) air advection; red arrows represent warm northerly air advection. Click for a larger image. |
Similarly, the synoptic pattern influenced the sea surface temperatures (Figure 4), with warmer sea surface temperatures associated with warm northerly air flow and cooler sea surface temperatures associated with southerly air flow.
Figure 5: Map showing Shouthern Hemisphere precipitation anomalies for September 2007 (based on 1979-2008 climatology). Blue arrows show areas of positive sea ice extent anomalies and red arrows show negative sea ice extent anomalies. Click for a larger image. |
A number of people have asked about the role that precipitation may have played in regions of positive sea ice extent anomalies. A regional positive rainfall anomaly provides an increase of fresh water to the ocean surface. This may have two consequences - cause localised negative sea surface temperature anomalies and make it easier for sea ice formation (since fresh water freezes at a lower temperature than salty water). If freshwater flux played a part in September 2007's sea ice anomaly then we would expect positive precipitation anomalies around the area of sea ice extent anomalies.
In this case, September 2007, it is easy to see that ocean freshwater flux (precipitation) did not cause the positive sea ice extent anomalies (Figure 5). The blue arrows show where sea ice extent anomalies were positive - and they coincide with negative precipitation anomalies. Also, negative precipitation anomalies largely align with negative sea surface temperature (SST) anomalies (Figure 4). The two main regions of negative SST anomalies are to the north of the sea ice at around 60º-100ºE and 170º-220ºE and to a lesser degree at around 310º-320ºE (Figure 4). The former two regions (the main regions) coincide completely with broad scale negative rainfall anomalies while the latter region mostly coincides with negative rainfall anomaly.
What does this mean? It means that in September 2007 the Southern Hemipshere sea ice area anomaly was synoptic driven.
Further precipitation maps are available at the Southern Hemisphere precipitation web site.
Figure 6: Map showing the trend in Southern Hemisphere sea ice extent for Sep 1979-2007. This image taken from the NSIDC website. Click for a larger image. |
Figure 7: Map showing the trend in sea ice concentration for Sep 1979-2007. This image taken from the NSIDC website. Click for a larger image. |
Figure 7 shows the trend in Southern Hemisphere sea ice extent for September over the period 1979-2007. This month shows, as does every month, a small but non-significant positive trend in total sea ice extent. Such a zonal average hides significant spatial variability in sea ice trends. This is best revealed by a map showing trends in sea ice concentration, as shown in Figure 7. This shows quite distinct areas of sea ice increase (to the north of the Ross Sea and to the north of East Antarctic, between 10 and 80 degrees east) and other areas of sea ice decrease. What is causing these long term changes in sea ice concentration?
Figure 8: Map showing the trend in Southern Hemisphere September MSLP superimposed on the trend in sea ice concentration for September 1979-2007. Click for a larger image. |
Figure 9: Map showing the trend in Southern Hemisphere July MSLP superimposed on the trend in sea ice concentration for July 1979-2007. Click for a larger image. |
Figure 10: Map showing the trend in Southern Hemisphere August MSLP superimposed on the trend in sea ice concentration for August 1979-2007. Click for a larger image. |
The dramatic changes in sea ice concentration around 10 degrees east during July are not so evident in August (Figure 10). They have been washed out to some degree by a deepening of the circumpolar trough at around 0 degrees longitude. However, the deepening of the circumpolar trough to the north of the Ross Sea reinforces the changes in sea ice extent in that region.
So, we have seen that the record maximum sea ice area during September 2007 was not the anti-global warming event that some greenhouse sceptics might have hoped for. Hopefully, what I've presented here that shows that it is quite the reverse:
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...
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/.
