GCOS GOOS WCRP/OOPC IX/3
SEA ICE CONCENTRATION AND EXTENT FOR CLIMATE RESEARCH:
STATUS, ISSUES AND OPPORTUNITIES
Hadley Centre for Climate Prediction and Research, Met Office
The GCOS climate monitoring principles provide a useful check list of requirements
which should be satisfied by any data sets used to monitor climatic changes. The most
relevant of these requirements can be summarized in terms of: stability, homogeneity and
continuity. GCOS has recommended that high priority should be given to making
measurements in data sparse or climatically sensitive regions. In particular, the needs of
climate assessments like those of the IPCC should be integrated into the plans from the start.
In addition, users should be given easy access to data and uncertainties and any biases in the
data should be quantified and published.
There are a bewildering number of sea ice data sets apparently available for climate
studies. Many of these data sets are available from the National Snow and Ice Data Center
(NSIDC) in the U.S. They also provide helpful data summaries describing the strengths and
weaknesses of the most popular data sets. This archive contains different data sets based on
passive microwave retrievals from the ESMR, SSMR, SSM/I and AMSR E instruments on
various satellites, digitized chart collections and some field measurements. The passive
microwave data sets differ in the algorithms used to retrieve sea ice concentration by
combination of brightness temperatures sensed by different frequency channels of the
instrument. None are universally applicable and most have been validated over limited
regions or times. Fields of passive microwave retrieved sea ice concentration are not
presented with accompanying error estimates. Digitized chart collections include the Russian
charts from the Arctic and Antarctic Research Institute (AARI), Chapman and Walsh data,
plus charts for Alaska, Canadian Arctic and the Bering Sea and other Arctic regions
Other data sets are also referred to on, but not accessible from, the NSIDC web pages.
The Hadley Centre SST and sea ice analysis (HadISST1) is our attempt (see Rayner, et al.,
2003) at creating a more homogeneous sea ice concentration data set by blending chart
derived data from Walsh and Chapman and the National Ice Center with passive microwave
data. The Global Digital Sea Ice Data Bank (GDSIDB) was collected by JCOMM and
comprises operational ice charts from various countries. An analysis of these latter data
combined with data from Walsh and Chapman and climatology (where no actual data was
available) was recently created by Vasily Smolyanitsky during a visit to the Hadley Centre.
We hope to incorporate these into the next version of HadISST. Other operational charts have
not yet been collected into the GDSIDB, but there are plans for expansion.
Historical Ice Chart Archive contains historical sea ice observations in the Arctic region
between 30W and 70E in the form of digitized maps. It was created at the Norwegian Polar
Institute. The earliest chart dates from 1553, and the most recent is from December 2002. We
also hope to include these in the next version of HadISST.
To improve sea ice fields, it is necessary to identify the best passive microwave
algorithm (or combination of algorithms) for retrieval of sea ice concentration. Often
algorithms and brightness temperature fields are worked up into sea ice concentration data
sets for only a limited period, but algorithms are simple enough to apply to the whole