James A. Maslanik

The Arctic Sea Ice‐Climate System: Observations and modeling

Significant advances are being made in our understanding of the Arctic sea ice-climate system. The mean circulation of the Arctic sea ice cover is now well defined through analysis of data from drifting stations and buoys. Analysis of nearly 20 years of daily satellite data from optical, infrared, and passive microwave sensors has documented the regional variability in monthly ice extent, concentration, and surface albedo. Advances in modeling include better treatments of sea ice dynamics and thermodynamics, improved atmosphere-ice-ocean coupling, and the development of high resolution regional models. Diagnostic studies of monthly and interannual sea ice variability have benefited from better sea ice data and geostrophic wind analyses that incorporate drifting buoy data. Some evidence exists for a small retreat of Arctic sea ice over the last 2 decades, but there are large decadal fluctuations in regional ice extent. Antiphase relationships between ice anomalies in different sectors can be related to changes in atmospheric circulation. Evidence suggests that episodes of significant salinity reduction in the North Atlantic, associated with extensive sea ice in the Greenland Sea, may be a manifestation of a decadal oscillation in the Arctic climate system. Aspects of the Arctic system in need of further attention include the surface energy budget and its variability, particularly with respect to the roles of cloud cover and surface types in summer. Sea ice thickness distribution data remain meager, and there are many unknowns regarding the circulation and hydrologic cycle of the Arctic Ocean and its links to the world ocean. Planned measurements from a new generation of satellites, supported by field programs, will provide much needed data to address these issues.

On the record reduction in 1998 western Arctic Sea‐ice cover

During summer 1998, record reductions in ice cover occurred in the Beaufort and Chukchi seas. Open water formed earlier than in prior years, and September ice extent in this region was 25 percent less than the previous minimum for 1953–1997. Seven percent of the Arctic Basin that had been perennially ice covered was ice-free in 1998. This reduction in western Arctic ice extent can be attributed in part to preconditioning by light-ice cover in autumn 1997 and to atmospheric circulation patterns during the following winter through autumn that favored southerly and easterly winds. Such decreases in northerly winds, and the associated weakening or displacement of the Beaufort Gyre, is found to be typical of winters that precede years with below-normal ice extent in the western Arctic.

Cloud classification from satellite data using a fuzzy sets algorithm - A polar example

Abstract Where spatial boundaries between phenomena are diffuse, classification methods which construct mutually exclusive clusters seem inappropriate. The fuzzy c-means (FCM) algorithm assigns each observation to all clusters, with membership values as a function of distance to the cluster centre. The FCM algorithm is applied to Advanced Very High Resolution Radiometer (AVHRR) data for the purpose of classifying polar clouds and surfaces. Careful analysis of the fuzzy sets can provide information on which spectral channels are best suited to the classification of particular features and can help determine likely areas of misclassification. General agreement in the resulting classes and cloud fraction was found between the FCM algorithm, a manual classification and an unsupervised maximum likelihood classifier.

Diagnosis of the record minimum in Arctic sea ice area during 1990 and associated snow cover extremes

The Arctic sea ice cover exhibited its record minimum area during 1990, characterized by extensive ice-free conditions during August along the Siberian coast. These reductions are consistent with warm, windy conditions in May and continued warmth in June promoting early melt and reductions in ice concentration, followed in August by strong coastal winds forcing a final breakup and retreat of the pack ice. The unusually warm Arctic conditions in 1990 are part of a larger-scale temperature anomaly pattern, linking the sea ice anomaly to accompanying record minima in Eurasian snow cover.

Airborne observations of summertime surface features and their effect on surface albedo during FIRE/SHEBA

Aircraft observations of the arctic surface were obtained during the recent Surface Heat Budget of the Arctic Ocean (SHEBA) and FIRE Arctic Clouds Experiment. A series of images were created from a downward looking video camera that was mounted on the underside of a research aircraft as it flew in the vicinity of the SHEBA camp, which drifted with the pack ice in the Beaufort and eastern Chukchi Seas. These data are processed to determine the distribution of melt ponds and open water during five flights in July 1998. Melt pond and open water coverage vary between 25–34% and 5–9%, respectively. Coincident observations of albedo derived from upward and downward looking broadband shortwave radiometers indicate that the albedo decreased through July until the last week, when the albedo began to rise. Pond fraction was observed to vary inversely with albedo through July. By relating the areal coverage of the surface features to the observed surface albedo, mean albedos of 0.26, 0.07, and 0.60 are found for melt ponds, open water, and pack ice, respectively.

Arctic sea ice concentrations from special sensor microwave imager and advanced very high resolution radiometer satellite data

Nearly coincident data from the special sensor microwave imager (SSM/I) and the advanced very high resolution radiometer (AVHRR) are used to compute and compare Arctic sea ice concentrations for different regions and times of the year. To help determine overall accuracies and to highlight sources of differences between passive microwave, optical wavelength, and thermal wavelength data, ice concentrations are estimated using two operational SSM/I ice concentration algorithms and with visible- and thermal-infrared wavelength AVHRR data. All algorithms capture the seasonal patterns of ice growth and melt. The ranges of differences fall within the general levels of uncertainty expected for each method and are similar to previous accuracy estimates. The estimated ice concentrations are all highly correlated, with uniform biases, although differences between individual pairs of observations can be large. On average, the NASA Team algorithm yielded 5% higher ice concentrations than the Bootstrap algorithm, while during nonmelt periods the two SSM/I algorithms agree to within 0.5%. These seasonal differences are consistent with the ways that the 19-GHz and 37-GHz microwave channels are used in the algorithms. When compared to the AVHRR-derived ice concentrations, the Team-algorithm results are more similar on average in terms of correlation and mean differences. However, the Team algorithm underestimates concentrations relative to the AVHRR output by 6% during cold months and overestimates by 3% during summer. Little seasonal difference exists between the Bootstrap and AVHRR results, with a mean difference of about 5%. Although the mean differences are less between the SSM/I-derived concentrations and concentrations estimated using AVHRR channel 1, the correlations appear substantially better between the SSM/I data and concentrations derived from AVHRR channel 4, particularly for the Team algorithm output.

Effects of weather on the retrieval of sea ice concentration and ice type from passive microwave data

Abstract Effects of wind, water vapour, and cloud liquid water on ice concentration and ice type calculated from passive microwave data are assessed through radiative transfer calculations and observations. These weather effects can cause overestimates in ice concentration and more substantial underestimates in multi-year ice percentage by decreasing polarization and by decreasing the gradient between frequencies. The effect of surface temperature and air temperature on the magnitudes of weather-related errors is small for ice concentration and substantial for multi-year ice percentage. The existing weather filter in the NASA Team Algorithm addresses only weather effects over open ocean; the additional use of local open-ocean tie points and an alternative weather correction for the marginal ice zone can further reduce errors due to weather. Ice concentrations calculated using 37 versus 18 GHz data show little difference in total ice covered area, but greater differences in intermediate concentration class...

Spatial and temporal variability of satellite-derived cloud and surface characteristics during FIRE-ACE

Advanced very high resolution radiometer (AVHRR) products calculated for the western Arctic for April-July 1998 are used to investigate spatial, temporal, and regional patterns and variability in energy budget parameters associated with ocean-ice-atmosphere interactions over the Arctic Ocean during the Surface Heat Budget of the Arctic Ocean (SHEBA) project and the First ISCCP (Internatonal Satellite Cloud Climatology Project) Regional Experiment - Arctic Cloud Experiment (FIRE-ACE). The AVHRR-derived parameters include cloud fraction, clear-sky and all-sky skin temperature and broadband albedo, upwelling and downwelling shortwave and longwave radiation, cloud top pressure and temperature, and cloud optical depth. The remotely sensed products generally agree well with field observations at the SHEBA site, which in turn is shown to be representative of a surrounding region comparable in size to a climate-model grid cell. Time series of products for other locations in the western Arctic illustrate the magnitude of spatial variability during the study period and provide spatial and temporal detail useful for studying regional processes. The data illustrate the progression of reduction in cloud cover, albedo decrease, and the considerable heating of the open ocean associated with the anomalous decrease in sea ice cover in the eastern Beaufort Sea that began in late spring. Above-freezing temperatures are also recorded within the ice pack, suggesting warming of the open water areas within the ice cover.

Winter atmospheric circulation in the Arctic Basin and possible relationships to the great salinity anomaly in the northern North Atlantic

Variations in synoptic activity are assessed in terms of potential influences on ice transport and indicators of shifts in the Arctic atmospheric circulation related to the “Great Salinity Anomaly” of the northern North Atlantic. Winter pressure anomalies suggest an increased contribution to the Fram Strait sea ice flux by multiyear ice from along the northern coasts of Greenland and Ellesmere Island. This may be related to a teleconnection involving changes in anticyclonicity over the central Arctic Ocean and Canadian Arctic Archipelago.

A Case Study of Regional Climate Anomalies in the Arctic: Performance Requirements for a Coupled Model

Abstract Simulations of Arctic climate require treatment of land, ocean, ice, and atmospheric processes, and are further complicated by the dynamic nature of the sea ice cover. Here, the ability of a climate system model to simulate conditions over the Arctic Ocean during April–September 1990, a period of anomalous atmospheric circulation and sea ice conditions, is investigated. Differences between observations and model results are used to gain insight into the mechanisms that contributed to the observed record reduction in ice extent in late summer. The coupled model reproduces the general patterns seen in comparison sea level pressure fields in most months, but the discrepancies significantly affect the model’s ability to simulate details of sea ice transport and warm air advection linked to the unusual ice conditions. The use of prescribed sea ice fraction in the climate model yields relatively small changes in the surface energy balance compared to the fully-coupled simulation with dynamic ice cover,...