|Walsh and Chapman Northern Hemisphere Sea Ice Data Set|
|Variable(s)||Sea Ice Concentration|
|Land or Ocean||Ocean|
|Current Period of Record||1870-1998|
|Resolution||Monthly, Global (data present in Northern Hemisphere only), 1o x 1o|
|Description:||Mid-month values of sea ice concentration for the Arctic are digitized on a standard 1-degree grid (cylindrical projection) to provide a "relatively uniform set of sea ice extent for all longitudes , as a basis for hemispheric scale studies of observed sea ice fluctuations" (Walsh, 1978).|
|Reference:||Walsh J.E. (1978): A data set on Northern Hemisphere sea ice extent. World Data Center-A for Glaciology (Snow and Ice), "Glaciological Data, Report GD-2", part 1, pp.49-51.|
|Data Set Location:||Contact Bill Chapman at firstname.lastname@example.org|
|Technical Overview||Expert User Guidance||Relevant Arcticles|
Arctic Monthly Sea Ice Concentrations: 1870 - 1998
Mid-month values of sea ice concentration for the Arctic are digitized on a standard 1-degree grid (cylindrical projection) to provide a "relatively uniform set of sea ice extent for all longitudes, as a basis for hemispheric scale studies of observed sea ice fluctuations" (Walsh, 1978).
These data are a compilation of data from many sources integrated into a single gridded product by John Walsh and Bill Chapman, University of Illinois. The sources of data for each grid cell have changed over the years from infrequent land/sea observations, to observationally derived charts, to satellite data for the most recent decades. Temporal and spatial gaps within observed data are filled with a climatology or other statistically derived data.
Please note that large portions of the pre-1953, and almost all of the pre-1900 data is either climatology or interpolated data and the user is cautioned to use this data with care (see “Expert user guidance”, below).Data format:
The Arctic sea ice concentrations are a gridded ASCII dataset. The domain is global, though concentrations are provided only for the Northern Hemisphere. This was done to facilitate merging the sea ice data with other global climate and oceanographic datasets. The data can be read with the following FORTRAN code segment:
Data values are either -1.0 for a land grid cell or in the range 0.0 - 100.0 for ice concentration (%).Data Sources:
The data sources for the ice concentrations vary spatially and temporally. There are eight basic data sources for the ice concentrations:
1. Danish Meteorlogical Institute
(a) Temporal extension of Kelly ice extent analyses:
Sea ice extent data is provided by Kelly, et. al. 1988. The ice extent data is compiled for the months April-August for the majority of the period 1901-1956. In this dataset, we utilize the Kelly data to create an ice concentration data source for the early period of record. This data is given very low priority in the hierarchy of available data so that if there are data from any sources (except climatology), we replace the extended Kelly data with this new source data. The modification of the Kelly data is done in two parts: (1) conversion from ice extent to ice concentrations, and (2) temporal extension of the available data.
(1) We add a marginal sea ice zone to the Kelly ice extent data by computing average ice concentration drop-off rates for the period during which there are satellite observations. These drop-off rates indicate the rate at which ice concentrations decrease as a function of distance from open water and distance from 10/10 ice concentrations. The drop off rates vary with season; the summer melt season drop-off rate is about 0.5 that of the freeze-up season. We apply these drop-off rates to the Kelly ice extent data to create a marginal sea ice zone.
(2) Regional sea ice anomalies have been shown to persist for many months and even seasons (Chapman and Walsh, 1991). We attempt to capitalize on this persistence by extending the ice anomaly data from (1) forward and backward in time to fill in the months September-March for each year in the 1901-1956 period. We compute lagged autocorrelations for the period of satellite observations and use the autocorrelations as weighting functions in the temporally extended data. For example, an anomaly for November is made by summing the anomalies of the preceeding August and the subsequent April weighted by the -3 month lag autocorrelation and the +5 month lag autocorrelations, respectively.
We have attempted to stretch the useful information included in the Kelly ice extent data to extract as much information as possible from the data. We feel that the addition of these extensions make a more complete and detailed dataset useful for most applications.
(b) Satellite derived data updates to Walsh sea ice database
During October, 1996, updates were made to the Walsh sea ice database. The database previously contained data through December, 1990. Updates to this dataset are, and will continue to be made using ice concentrations obtained via the SSMI sources using the NASA Team algorithm.
In order to maintain a consistent data source for the last part of the period, all data from October, 1978 through December, 1998 are from the SMMR/ SSMI sources. Data from previous versions of this data set were replaced by SMMR and SSM/I data from Oct. 1978 - Dec. 1990.
|Expert User Guidance|
The temporal and spatial inhomogeneities in the data sources that went into the construction of this dataset require that any historical analysis of the data is done with caution and an understanding of the limitations of the data.
There are three periods for which the sources of the data change fundamentally:
1972-1998: Satellite period - hemispheric coverage, state-of-the-art data accuracy
Because most of the direct observations of sea ice (1870-1971 period) are from ships at sea, they are generally the most complete near the ice edge. The conditions north of the ice edge are often assumed to be 100% covered during this period. The satellite era has shown otherwise with concentrations between 70-90% frequently occurring well north of the ice edge in the post-1972 data. For this reason, we recommend using a measure of ice extent, when doing historical comparisons of hemispheric sea ice coverage for periods which include data prior to 1972. This is done by assuming that all grid points with ice concentrations greater than some threshold (15% is commonly used) is assumed completely covered by sea ice.
Regional or grid point analyses may benefit by using the concentration data as it is distributed but the completeness of the historical record will vary regionally. Please contact Bill Chapman (email@example.com) if you have a question regarding the inventory of data included in this dataset for a specific region.
Walsh, J.E. (1978): A data set on Northern Hemisphere sea ice extent. World Data Center-A for Glaciology (Snow and Ice), "Glaciological Data, Report GD-2", part 1, pp. 49 - 51.
Walsh, J.E. and C.M. Johnson (1979): Analysis of Arctic sea ice fluctuations 1953-77. "Journal of Physical Oceanography", 9(3), p. 580-591.
Kelly, P. M. 1979: An arctic sea ice data set, 1901-1956. Glaciological Data, Report GD-5: Workshop on Snow Cover and Sea Ice Data. World Data Center-A for Glaciology [Snow and Ice], 101-106.
Chapman, W.L. and J.E. Walsh (1991): Long-Range Prediction of Regional Sea Ice Anomalies in the Arctic. "Weather and Forecasting", 6(2), pp. 271-288.
Technical Report of the Japan Meteorological Agency (JMA) No. 109 (ISSN 0447-3868) titled "Report on 30 years of Observation of Sea Ice" by Marine Department, JMA. Published by JMA, Tokyo, February 1988.