ARCTIC SEA ICE ALBEDO VARIABILITY AND TRENDS,

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1 ARCTIC SEA ICE ALBEDO VARIABILITY AND TRENDS, Vesa Laine Finnish Meteorological Institute (FMI), Helsinki, Finland Abstract Whole-summer and monthly sea ice regional albedo averages, variations and trends for the months of e, y and ust from 1982 to 1998 have been processed from calibrated AVHRR data. In addition, the corresponding changes of sea ice concentration and sea ice extent have been extracted from passive microwave radiometer datasets. The potential relationships between the trends and anomalies of summer albedo and the mentioned parameters have been analyzed. In general, the sea ice albedo in the central Arctic is between 0.5 and 0.7. The highest albedo values are mainly found in the Arctic Ocean north of Greenland. The lowest albedo ( ) occurs in the fringe area of the Arctic Ocean, e.g., on the coasts of Alaska and Siberia. Low albedo values also exist on the east coast of Greenland and in Hudson Bay. Time series have been calculated for the sea ice cover for the Northern Hemisphere as a whole, and for six subregions: the Arctic Ocean, the Kara and Barents s, the Greenland, the Labrador, Hudson Bay, and the Canadian Archipelago. The results show a significant variability and slight negative trends for the sea ice albedo. The largest monthly slope for most of the regions is found for e and the lowest slope for ust. Among the sub-regions the Greenland has the steepest negative summer albedo trend during the period, while for the Arctic Ocean the albedo trend is near zero. 1. Introduction Realistic estimates of sea ice albedo at adequate spatial scales are a prerequisite for a realistic estimation of the global energy balance and for detecting changes in the radiation balance of the global climate system. At the present time the long-term trend in the sea ice albedo is insufficiently well known. In this study the sea ice regional albedo changes in the Arctic region are analyzed for a 17-year period using calibrated broad-band albedo data, that has been corrected both atmospherically and for bi-directional reflectance. The objective of this study is to present new information on the monthly and long-term annual sea ice albedo changes in the Arctic region. In the present study, summer and monthly sea ice regional albedo averages for the months of e, y and ust from 1982 to 1998 have been calculated for the entire Arctic sea ice area. In addition, regional analysis has been made of the albedo variations of the Arctic Ocean, the Kara and Barents s, the Greenland, the Labrador, Hudson Bay and the Canadian Archipelago. ice is thought to provide a positive sea-ice-albedo feedback in the polar regions. The melting surface induces ice retreat, darkens the bare ice and forms melt ponds and leads, resulting in a lower surface albedo and consequent increased solar radiation absorption at the surface, which in turn further produces melting and increases the area of open water and melt ponds. In the reverse case, a cold anomaly may produce a dryer snow surface or allow a deeper snow cover and more ice to form, increasing the surface albedo. This may in turn cause a further cooling of the ice surface. It is therefore of value to study the potential relationship between the trends and anomalies of summer sea ice regional albedo, sea ice concentration, sea ice extent and surface air temperature. 2. Data The Advanced Very High Resolution (AVHRR) Polar Pathfinder data provide long-time series of calibrated surface albedo and surface temperature data for the polar regions (Stroeve et al., 2001). In this research we have

2 used the AVHRR Polar Pathfinder 5-km EASE-Grid Composites from the National Snow and Ice Data Center (NSIDC), Boulder, Colorado. The scanning multichannel microwave radiometer (SMMR) and special sensor microwave imagers (SSMI) provide long time series ( ) of brightness temperature data for investigating the surface characteristics of the sea ice cover. For this study the sea ice concentrations have been extracted from the NSIDC data set: Bootstrap Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I. A detailed description of the data and its accuracy can be found in Comiso, The sea ice extents are calculated from the combined SMMR and SSMI data sets that have been developed by Cavalieri et al. (1997), Cavalieri et al. (1999), Parkinson et al. (1999), Gloersen et al. (1999), Parkinson and Cavalieri (2002), and Zwally et al. (2002). In this data set the SMMR/SSMI-derived sea ice concentrations have been used to determine extents for sea ice having a concentration of at least 15%. Thanks to the work of Polyakov et al (2003), several databases have been consolidated to produce available data sets of monthly surface air temperature and sea level pressure for the Arctic area. The datasets contain data from land stations, Russian NP stations, and drifting buoys operated by the International Arctic Buoy Programme (IABP). The summer surface air temperatures for this study have been extracted and averaged from these data sets, choosing the stations nearest the study regions Cloud masking To obtain the surface albedo, cloud-free pixels need to be separated from pixels contaminated by radiances due to reflection from clouds. The success of cloud detection is one of the critical points in obtaining the surface albedo. In polar regions, where the contrast between clouds and the ice surface in the visible and thermal bands is low, cloud screening is especially complicated. In this study a cloud detection method has been employed that uses multispectral threshold methods based on the AVHRR channels (Allen et al., 19; Derrien et al., 1993; Liljas, 1987 and Karlsson, 1989) Surface masking For albedo analysis as well as for analysis of sea ice extent it is necessary to define the ice area. The sea surface in the study region varies from open water to a % sea ice concentration. The lowest concentration was chosen to be 10 %. This means that all the pixels covered by first-year ice and multiyear ice having an ice concentration between 10% and % have been included in the sea ice albedo estimations. 3. Time series and trends The variability and trends in the sea ice albedo, the sea ice concentration, the sea ice extent and surface air temperature are presented in the form of time series of monthly or whole-summer averages for the years The time series are calculated for the sea ice cover for the Northern Hemisphere as a whole, and for six sub-regions: the Arctic Ocean, the Kara and Barents s, the Greenland, the Labrador, Hudson Bay, and the Canadian Archipelago (Figs. 2 and 3). The albedos for these regions have been calculated by time- and area-averaging all the albedo values from pixels identified as ice for each region. The albedo time series are presented separately for each month as well as for the whole summer season (averages from the beginning of e to the end of ust). The corresponding time series of sea ice concentration and sea ice extent for annual summer seasons have been calculated by averaging the monthly sea ice concentrations and extents for e, y and ust. The summer surface air temperature trends have been calculated by averaging the monthly surface air temperatures for e, y and ust from six stations located in the Arctic area as near as possible to the six sub-regions (see Polyakov et al., 2003). The temperature trend estimate for the Northern Hemisphere as a whole has been calculated by averaging the summer temperature trends from each of the six stations. The significance of each trend was tested using a standard F test at confidence levels of 95% and 98%. The letter S marks the statistical significance of those trends for which the F-observed statistics are greater than the F- critical values.

3 The Northern Hemisphere as a whole and all the sub-regions show slightly decreasing summer albedo trends. However, most of the trends are not statistically significant. In fact, the trends are statistically significant only in the Greenland and Hudson Bay. 4. Spatial distribution of sea ice albedo and concentration To illustrate the spatial variability of the albedo and concentration of the sea ice during the 17-year period, maps have been prepared of sea ice albedo and concentration from 1982 to The spatial distribution of the summer albedo over the entire Arctic sea ice region in each of the years in the period is shown in Fig. 4. The patterns of albedo for different years show large variability. In general the albedo is between 0.5 and 0.7 in the central Arctic (light green and brown areas). The highest albedo values are mostly found in the Arctic Ocean north of Greenland. The lowest albedo ( ) occurs in the fringe areas of the Arctic Ocean, such as along the coasts of Alaska and Siberia. Low albedo values are also to be found along the east coast of Greenland and in Hudson Bay. The average value for the summer sea ice albedo in the Northern Hemisphere is Among the subregions, the summer albedo is highest in the Arctic Ocean, having an average of In the other sub-regions the average summer albedo is near. The average of the summer sea ice concentration in the Northern Hemisphere is near 75%. The Arctic Ocean represents the largest continuous ice area with high sea ice concentrations (Fig. 5). High concentrations are also to be found in most years in the Canadian Archipelago. Lower concentrations can be seen in the fringe areas of the sea ice as well as in the coastal regions. 5. Spatial distribution of the albedo and concentration trends To gain a more illustrative and detailed view of the 17-year albedo and concentration trends, pixel-based spatial trend distribution maps were calculated for the entire Arctic ice area (Fig. 6a and b). The trends were calculated pixel by pixel using a multiple linear regression on the summer data. Areas of contiguous pixels having strongly negative trends can be found, notably in the Beaufort. Smaller regions of negative trends for albedo can be seen in the Greenland and in Hudson Bay. However, certain regions of positive trends for sea ice concentration are to be seen in these areas. Positive albedo trends can also be seen in the central and eastern part of the Arctic Ocean and in the Kara near Novaya Zemlya. 6. Discussion and Conclusions In Arctic regions, the onset of melting occurs in the late spring/summer period (Comiso and Kwok, 1996), during which the average surface air temperature from time to time exceeds the freezing temperature, the total surface energy balance changes, and the ice surface becomes wet. In autumn the surface cools again and freeze-up occurs. The air temperature and the duration of the melt season have a strong influence on the sea ice albedo, which decreases during the melting period. In spring, before the melting, the Arctic sea ice is usually covered by an optically-thick layer of snow with high albedo values. In early summer the snow cover begins to melt, and the ice surface turns into a mix of bare ice with a few shallow melt ponds having a much lower albedo. In late summer the snow is completely melted and the ponds on the bare ice are deeper and wider. Synoptic weather events, such as rain and snow can also cause changes in albedo. A rainfall event changes surface conditions from dry snow to wet snow, resulting in a decrease in albedo. On the other hand, snowfall can cause a dramatic temporary increase in albedo before melting again (Perovich et al., 2002). In the regions of thin ice (low sea ice concentration), ice thinning or growth (induced by temperature change) can be a significant cause of albedo change. ice melting and ice retreat are understandably connected with a lowering of the sea ice concentration. In most regions the trends of sea ice concentration and sea ice extent vary in a similar way from year to year. The close connection between the sea ice extent and the surface air temperature is also clearly to be seen in most regions. In cold summers the extents (and concentrations) are usually high and vice versa (Figs. 2 and 3). During the summer season, the month of lowest albedo varies from one sub-region to another. The abovementioned seasonal evolution, and the higher concentration and thickness of sea ice explains why the highest monthly albedo values in every sub-region occur in e. The occurrence of the lowest average albedo in the Arctic Ocean in y is probably due to the shorter duration of the above-freezing surface air temperature in the Arctic Ocean compared to sub-regions in lower latitudes. Even in the central Arctic the air temperature in the daytime can rise above the freezing point and the total surface energy balance can change, causing surface wetness and forming melt ponds.

4 The albedo variations and trends are consistent with the concentration variations in the fringe areas but not specifically in the central part of the Arctic Ocean (Figs. 4, 5, 6a and b). The conclusion can therefore be drawn that, in the central part of the Arctic Ocean, the ice concentration change does not play a very important role in albedo change. The albedo changes depend on the thickness of winter snow layer and the beginning of the melting period. Probable reasons for albedo changes include the evolution of sea ice (especially melt pond development) due to surface air temperature changes and synoptic weather events, such as snowfall. The complicated nature of Arctic sea ice properties makes analysis of possible causes for the changes and variability of sea ice albedo difficult. Hard evidence for the reasons behind the different Arctic phenomena are not always easily to be found; the results of the analysis are mostly more or less speculative. The results of this study show a significant variability and slight negative trends for the sea ice albedo from 1982 to Because of the continuing interest in the factors involved in world climate change, continuous monitoring of sea ice variability in the Arctic regions would seem to be called for. Along with observations of sea ice concentration, extent, and motion, and observations of the meteorological phenomena, the sea ice albedo is, and will remain, one of the most important variables for studying the global climatological state and its change. A more detailed study of the Arctic sea ice albedo and trends can be found in Laine, References Allen, R. C., Jr., P. Durkee and W. H. Carlyle, Snow/cloud discrimination with multispectral satellite measurements, J. Appl.. Meteor., 29, 994-4, 19. Cavalieri, D. J., P. Gloersen, C. L. Parkinson, J. C. Comiso, and H. J.Zwally, Observed Hemispheric Asymmetry in Global Ice Changes, SCIENCE, 272, , Cavalieri, D. J., C. L. Parkinson, P. Gloersen, J. C. Comiso, and H. J. Zwally, Deriving Long-Term Time Series of Ice Cover from Satellite Passive-Microwave Multisensor Data Sets, J. Geophys. Res., 104, 15,3-15,814, Comiso, J. C., Bootstrap Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I. Boulder, CO, USA: National Snow and Ice Data Center. Digital media, 1999, updated Comiso, J. C. and R. Kwok, Surface and radiative characteristics of the summer Arctic sea ice cover from multisensor satellite observations, J. Geophys. Res., 101(C2), 28,397-28,416, Derrien, M., B. Farki, L. Harang, H. LeGleau, A. Noyalet, D. Pochic and A. Sairouni, Automatic cloud detection applied to NOAA-11/AVHRR imagery, Remote Sens. Environ., 46, , Gloersen, P., C. L. Parkinson, D. J. Cavalieri, J. C. Comiso, and H. J. Zwally, Spatial Distribution of Trends and sonality in the Hemispheric Ice Covers: , J. Geophys. Res., 104, 20,827-20,836, Karlsson K. G., Developement of an cloud classification model, Int. J. Remote Sens., 10, , Laine V. (2004), Arctic sea ice regional albedo variability and trends, , J. Geophys. Res., 109, C027. Laine, V., A. Venäläinen, M. Heikinheimo and O. Hyvärinen, Estimation of surface solar global radiation from NOAA AVHRR data in high latitudes. J. Appl. Meteor., 38, , Liljas, E., Automated recognition of cloudtypes from satellites and its application to nowcasting, Int. Conf. on Agrometeorology, Cesena 1987, Fondzione Cesena Agricultura Publ., 27-45, 1987 Parkinson, C. L., and D. J. Cavalieri, A 21-year record of Arctic sea ice extents and their regional, seasonal, and monthly variability and trends, Annals of Glaciol., in press, 2002.

5 Parkinson, C. L., D. J. Cavalieri, P. Gloersen, H. J. Zwally, and J. C. Comiso, Variability of the Arctic Ice Cover , J. Geophys. Res., 104, 20,837-20,856, Perovich, D. K., T. C. Grenfell, B. Light and P. V. Hobbs, sonal evolution of the albedo of multiyear Arctic sea ice, J. Geophys. Res., 107, (C10), SHE , Polyakov, I., R. V. Bekryaev, G. V. Alekseev, U. Bhatt, R. Colony, M. A. Johnson, A. P. Makshtas and D. Walsh: Variability and trends of air temperature and pressure in the maritime Arctic, , Journal of Climate 16 (12), , Stroeve, J. C., J. E. Box, C. Fowler, T. Haran and J. Key, Intercomparison between in situ and AVHRR polar pathfinder-derived surface albedo over Greenland, Remote Sensing Environment 75, 3-374, Zwally, H. J., J. C. Comiso, C. L. Parkinson, D. J. Cavalieri, and P. Gloersen, Variability of Antarctic Ice , J. Geophys. Res., in press, 2002.

Northern Hemisphere Trend for e: -1.6±1.1 10-3 year -1 Trend for y: -0.9± 10-3 year -1 Trend for ust: -0.4±1.4 10-3 year -1 Trend for summer: -0.7±0.8 10-3 year -1 1 1 1 9.0 Trend: -9.3±6.

6 Northern Hemisphere Trend for e: -1.6± year -1 Trend for y: -0.9± 10-3 year -1 Trend for ust: -0.4± year -1 Trend for summer: -0.7± year Trend: -9.3± % year -1 Correlation between summer albedo and sea ice concentration: 0.56 Trend: -42.3± km 2 year -1, S = 95 Baker Lake Hudson Bay Labrador Canadian Archipelago Frobisher Bay Resolute Chukchi Barter Island Beaufort Central Arctic Arctic Ocean Greenland Jan Mayen East Siberian Laptev Barents Kara Ostrov Belyj Correlation between summer albedo and sea ice extent: Average of six stations: Central Arctic, Ostrov Belyj, Jan Meyen, Frobisher Bay, Baker Lake and Resolute Trend: 3.15± C year -1 Correlation between summer albedo and surface air temperature: Figure 2. Monthly and summer averaged Northern Hemisphere sea ice albedo from 1982 to averaged Northern Hemisphere sea ice concentration from 1982 to sea ice extent from 1982 to surface air temperatures from 1982 to S indicates statistical significance at the 95% and 98% confidence levels, using a standard F-test. Figure 1. Map of the Arctic sea ice region used for the analysis. In this context the Northern Hemisphere consists of the six sub-regions outlined in the map.

7 Arctic Ocean Trend for e: -1.5± year -1 Trend for y: -0.8± year -1 Trend for ust: 0.4± year -1 Trend for summer: -± year -1 Trend: -17.1± % year -1 Correlation between summer albedo and sea ice concentration: 0.34 Trend: -18.2± km 2 year - Correlation between summer albedo and sea ice extent: Central Arctic Trend: 3.5± C year -1 Correlation between summer albedo and surface air temperature: Kara and Barents Trend for e: -± year -1 Trend for y: -0.2± year -1 Trend for ust: 0.1± year -1 Trend for summer: -0.4± year -1 Trend: -8.1± km 2 year - Correlation between summer albedo and sea ice extent: 9 Ostrov Belyj Trend: 1.8± C year -1 Trend: -2± % year -1 Correlation between summer albedo and sea ice concentration: Correlation between summer albedo and surface air temperature: Greenland Trend for e: -2.7± year -1 Trend for y: -1.4± year -1 Trend for ust: -2.6± year -1, S = 95 Trend for summer: -3.8± year -1, S = Trend: 2.2± km 2 year Trend: -5.4± % year -1 Correlation between summer albedo and sea ice concentration: 0.37 Correlation between summer albedo and sea ice extent: Jan Mayen Trend: 4.3± C year -1 Correlation between summer albedo and surface air temperature: Labrador Trend: -15.6± % year -1 Correlation between summer albedo and sea ice concentration: 0.58 Trend for e: -2.8± year -1 Trend for y: -1.4± 10-3 year -1 Trend for ust: - 0.4± year -1 Trend for summer: -1.9± year -1 Trend: -5.1± km 2 year -1 Correlation between summer albedo and sea ice extent: Frobisher Bay Trend: ± C year -1 Correlation between summer albedo and surface air temperature: Hudson Bay Trend for e: -2.1± year -1 Trend for y: -1.1± year -1 Trend for ust: -1.7± year -1, S = 95 Trend for summer: -1.9± year -1, S = 98 Baker Lake Trend: -24.8± % year -1 Trend: -10.2± 10 3 km 2 year -1, S = 98 Trend: 9.5± C year -1 Correlation between summer albedo and sea ice concentration: 0.85 Correlation between summer albedo and sea ice extent: 0. Correlation between summer albedo and surface air temperature: -7 Canadian Archipelago Trend for e: -1.2± year -1 Trend for y: -1.2± year -1 Trend for ust: -0.3± year -1 Trend for summer: -0.4± year Trend: -67.6± % year -1, S = 98 Trend: -2.3± km 2 year -1 Resolute Correlation between summer albedo and sea ice concentration: 3 Correlation between summer albedo and sea ice extent: Trend: ± C year -1 Correlation between summer albedo and surface air temperature: Figure 3. Same as for Figure 2, except for the six subregions: the Arctic Ocean, the Kara and Barents s, the Greenland, the Labrador, Hudson Bay, and the Canadian Archipelago.

1982 1983 1986 1987 1991 19 1995 1994 1984 1982 1985

1991 1992 1993 1993 1996 1997 1996 1995 1994 1997 1998

8 Fig.4 sea ice albedo Fig. 5 sea ice concentration Fig.6 The 17-year trends in the sea ice albedo The 17-year trends in the sea ice concentration

Antarctic ice sheet and sea ice regional albedo and temperature change, , from AVHRR Polar Pathfinder data

Available online at www.sciencedirect.com Remote Sensing of Environment 112 (2008) 646 667 www.elsevier.com/locate/rse Antarctic ice sheet and sea ice regional albedo and temperature change, 1981 2000,