Ice extent in the Greenland Sea 1978–1995

Deep-Sea Research II 46 (1999) 1237}1254

Ice extent in the Greenland Sea 1978}1995 Leif Toudal* Danish Center for Remote Sensing, Technical University of Denmark, Building 348, DK-2800 Lyngby, Denmark Received 24 February 1998; received in revised form 11 November 1998; accepted 16 November 1998

Abstract During the European Subpolar Ocean Programme (ESOP) project from 1993 to 1995, very little ice was formed in the central Greenland Sea. The large-scale ice cover in the area during the period October 1978 to June 1995 has been mapped using passive microwave data from the scanning multichannel microwave radiometer (SMMR) and the special sensor microwave/imager (SSM/I). Special emphasis is put on the last three years of the period (the ESOP years), from the summer of 1992 to the summer of 1995. The results for the three winters compared with the average winter of the previous 12 yr show unusually little ice in the Greenland Sea during the winters of 1993}1995. In particular, the winters of 1994 and 1995 saw no ice (i.e. no Odden) in the central Greenland Sea at all. Another result is an observed upper limit of approximately 250,000 km to the seasonal ice cover in the study region (the Odden Area) within the central Greenland Sea over the period 1978}1995.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The large-scale ice cover in the Greenland Sea is believed to play a role in the process of deep-water formation, since the freezing of saline water into ice releases brine (salt) to the underlying water thus increasing the salinity of the water (Rudels, 1990). The freezing process may not directly cause water to sink to the deep layers, but is a necessary precondition since the fresher surface water needs increased density to penetrate the denser water below (Visbeck et al., 1995; Backhaus and KaK mpf, 1999). The large-scale interannual variability of the ice cover in the Greenland Sea will be investigated below.

* Fax: 0045-45-931635. E-mail address: [email protected] (L. Toudal) 0967-0645/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 0 2 1 - 1

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2. Instrumentation The present study was carried out using satellite passive microwave observations from the special sensor microwave/imager (SSM/I) on board the Defence Meteorological Satellite Program (DMSP) series of satellites as well as the scanning multichannel microwave radiometer (SMMR) on the NIMBUS-7 satellite. The use of microwaves to observe the ice conditions allows daily monitoring, even during the polar night and during periods with persistent cloud cover where traditional visible and infrared techniques have limited use. The microwave radiometers used covers the Greenland Sea area every day of operation, with swath widths of 800 km (SMMR)}1400 km (SSM/I). In October 1978, NASA launched the NIMBUS-7 satellite carrying a scanning multichannel microwave radiometer (SMMR). The instrument measured thermal microwave radiation at dual polarisation at 5 di!erent wavelengths. The NIMBUS-7 SMMR operated successfully until the summer of 1987 when it was succeeded by satellites of the US Defence Meteorological Satellite Program (DMSP) carrying another microwave radiometer system, the special sensor microwave/imager (SSM/I). This is also a multi-frequency, dual-polarised microwave radiometer system with a total of 7 channels at 4 di!erent wavelengths. Characteristics of the SMMR and SSM/I channels used here are shown in Table 1. The instruments measure the amount of thermal microwave radiation emitted by the surface of the Earth. The spectral composition of this emission di!ers substantially from ice-free to ice-covered ocean, and from multi-spectral measurements it is possible to derive the ice concentration in each observation cell. Data for the project were supplied by the National Snow and Ice Data Center in Boulder, Colorado, USA NSIDC (1992) on CD-ROM as gridded antenna temperatures. 2.1. Ice-concentration algorithm This section describes the algorithm used to convert satellite microwave observations into ice concentrations for the Greenland Sea area. The algorithm works directly on satellite-measured antenna temperatures, and it uses two channels. It is an adjusted version of an algorithm originally applied in

Table 1 Characteristics of the SMMR and SSM/I microwave radiometers. H/V corresponds to horizontal/vertical polarisation, respectively, and resolution is the size of the integrated "eld of view (antenna footprint) taking the integration time into account Frequency (GHz)

V

H

Resolution (SMMR) (km)

Resolution (SSM/I) (km)

18/19.35 37

x x

x x

55;55 27;27

69;43 37;28

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a one-channel version by Zwally et al. (1983) around Antarctica, and by Parkinson et al. (1987) in the Arctic. It was extended to two channels by Comiso (1986). Speci"c tie-points for the Greenland Sea area have been found for this study. The ice concentration is given by: ¹ !¹5 C" , ¹' !¹5

(1)

where C is the ice concentration, ¹ is the measured brightness temperature, ¹5 is the water tie-point, and ¹' is a calculated brightness temperature of ice. The algorithm can use either the (37V, 37H) or the (37V, 19V) channels, the latter being used here since it is less in#uenced by atmospheric disturbance (Pedersen, 1991). Since the retrieval requires use of a 18/19.35 GHz channel, the resulting ice concentrations have a spatial resolution of 69;43 km (SSM/I) and 55;55 km (SMMR). This means that only large-scale features can be seen in the images, and that sharp ice edges will be blurred by the averaging e!ect of the large antenna footprint. This e!fect does not limit the present study, which deals with long-term variability of large-scale features, 2.2. Tie-points In order to operate, the algorithm needs information about the signature of open water, "rst-year ice and multi-year ice. These signatures are not constant, but vary with season and the actual weather conditions since they in principle at any time must represent the surface as seen through the actual atmosphere (Pedersen, 1991).

Table 2 Tie-point antenna temperatures for the Greenland Sea area (in K) used to convert (a) SMMR and (b) SSM/I antenna temperatures into ice concentrations (a) SMMR antenna

W FY MY

37V (K)

37H (K)

18V (K)

195.5 243.3 195.6

147.9 234.8 183.4

162.7 246.9 215.4

37V (K)

37H (K)

19V (K)

208.9 246.9 185.4

151.2 234.8 170.3

185.9 251.2 219.6

(b) SSM/I antenna

W FY MY

Note: W indicates open water (ice-free), FY is "rst-year ice, and MY is multi-year ice.

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However, in this large-scale study, the tie-points are kept constant in order not to introduce biases from improper seasonal adjustment. This means that the derived ice concentrations will be slightly too high during the summer periods (0}5%) due to the stronger absorption during the warm part of the year when the atmosphere contains more water vapour and clouds liquid water. The tie-points used for the SMMR and SSM/I instruments can be seen in Table 2a and b. The tie points are found from analysing time series of the data from open-water areas and ice-covered areas in the Greenland Sea region (Pedersen, 1991). The tie-points thus di!er from the global tie points originally used by Comiso (1986). Since the tie-points are found locally, only slight adjustments were necessary to minimise the di!erence during the overlap period of the SMMR and the SSM/I instruments in July and August 1987.

Fig. 1. De"nition of area for the ice-cover investigation. Corner coordinates of the rectangular study area are (77.5N, 1.5E) (74.085, 12.364E), (67.657N, 8.412W) (69.885N, 18.390W). Greylevels corresponds to ice concentrations, black is no ice, and the brightest areas have close to a 100% ice cover.

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3. Historical (1978+1995) summary from passive microwave observations Daily (or bi-daily with the SMMR) microwave radiometer observations on a regular basis are available from October 1978 to the present time. In order to put the ESOP period (1993}1995) into perspective, this data set has been analysed with special emphasis on the Odden/Nordbukta area shown in Fig. 1. The peninsula of ice reaching out towards the east from the east coast of Greenland at approximately 723N is called the Odden area, the name is Norwegian for peninsula, and originates from Norwegian sealers in the 19th century. Nordbukta (northern Bay) is the Danish name of the bay of open water behind (to the west) of the Odden ice peninsula. The area is de"ned with a western boundary along the East Greenland continental shelf in order to exclude most of the ice carried southwards by the East Greenland Current over the shelf and only include the primarily locally grown ice in the central Greenland Sea. The eastern boundary is de"ned to include the largest ice cover in the Greenland Sea. The area is rectangular in the SSM/I grid that used for the data distribution by the National Snow and ice Data Center. Results are summarised in Fig. 2, and more details about the individual years can be seen in Figs. 3}7. The curves in Fig. 2 show mean and maximum ice conditions over this 17-yr period. Fig. 3a}n show the development during each of the winters since 1978}79. Some years had considerable ice during the early winter (December}January), whereas

Fig. 2. Weekly maximum and average ice cover in the Greenland Sea during the period 1978}1995 for the rectangular area outlined in Fig. 1. The maximum curve is drawn through points de"ned as the ice cover during the year between 1978 and 1995 where the ice cover was the largest on a weekly basis.

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Fig. 3. (a) Ice conditions in the Greenland Sea area during 1978}1979. The maximum and mean ice conditions from Fig. 2 are shown as reference; (b) 1979}1980; (c) 1980}1981; (d) 1981}1982; (e) 1982}1983; (f ) 1983}1984; (g) 1984}1985; (h) 1985}1986; (i) 1986}1987; (j) 1987}1988; (k) 1988}1989; (l) 1989}1990; (m) 1990}1991; (n) 1991}1992.

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Fig. 3. Continued.

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Fig. 3. Continued.

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Fig. 3. Continued.

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Fig. 3. Continued.

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Fig. 3. Continued.

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Fig. 3. Continued.

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Fig. 4. Ice conditions on February 21, 1987 as observed by the NIMBUS-7 SMMR overlaid with the outline of the de"nition of the Greenland Sea used here, and with bathymetry contours of 1000 m (dotted), 1500 m (dashed), 2000 m (thin-full) and 3000 m (thick-full). The graylevels of the image corresponds to ice concentration. The "gure shows the maximal ice cover in the Greenland Sea since October 1978.

others had more ice during the latter part of the winter period (March}April). Note in Fig. 2 how the maximum curve for sea ice in the Greenland Sea is fairly constant for the period from mid-December to late April. This indicates that there is an upper limit of approximately 250,000 km to the seasonal ice cover in the study region (the Odden Area) within the central Greenland Sea over the period 1978}1995. As can be seen from Fig. 3a}n, this maximum is reached during many of the years from 1978}1989, e.g. December}February 1978}79, several times during January}May 1982, February}May 1986, February}May 1987 and for the last time in December 1988}January 1989. It should be noted that the maximum ice cover corresponds to a situation where there is no open Nordbukta and thus no Odden as an ice peninsula, but where both the bay area (Nordbukta) and the Odden areas are ice-covered. Fig. 4 shows an example of this situation from February 1987 as observed with the NIMBUS-7 scanning multichannel microwave radiometer. The maximum ice-covered area is

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Fig. 5. Ice conditions in the Greenland Sea during the period 1992}1993 for the area shown in Fig. 1.

limited by the warm water of the Norwegian Atlantic Current on the warm side of the Arctic front, approximately at the Mohns Ridge (see Fig. 4). Years of exceptionally little ice cover during winter were 1982}83 and 1983}84 as well as 1990}1991. Figs. 5}7 show the daily ice cover during the 3 winters from 1992 to 1995. Only the 1992}93 winter produced an Odden feature, and it was quite a small one compared with the previous 12 winters, hardly even reaching the average ice cover for the area at any time. During the winters of 1993}94 and 1994}95, practically no ice formed in the Greenland Sea study area at all. In May}June 1995, some ice was present in the south western corner of the study area, maybe as a result of the enhanced ice transport through Fram Strait earlier that winter reported by Vinje et al. (1998) and Hilmer et al. (1998). Transport times from Fram Strait to the area are 2}4 months. The integrated amount of ice in the Greenland Sea during each of the winters from 1979 to 1996 de"ned as the time integral of the daily ice cover (Fig. 8) shows the anomalies of the integrated amounts with the average of 12.15 million km days subtracted. As can be seen from Fig. 8 the amount of ice varies dramatically from year to year. Notably heavy ice winters were 1978}79, 1981}82 and all winters in the period 1985}1989, whereas low integrated ice cover was encountered in 1982}83, 1983}84, and all the winters from 1990}1995, including all ESOP winters. Note that the annual variability is very large, being the same size as the mean value. In order to investigate further the variability of the ice conditions, surface air temperature observations from the manned Norwegian weather station at the island of Jan Mayen have been used.

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Fig. 6. Ice conditions in the Greenland Sea during the period 1993}1994 for the area shown in Fig. 1.

Fig. 7. Ice conditions in the Greenland Sea during the period 1994}1995 for the area shown in Fig. 1.

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Fig. 8. Integrated ice cover anomaly in million square kilometre days for the years of SMMR and SSM/I for the area shown in Fig. 1. The data set for the 1978}1979 and 1987}1988 winters is incomplete as seen from Fig. 3a and Fig. 3j. The average annual integrated ice cover is 12.15 million km days, so the maximum negative anomaly corresponds to practically no ice at all, and the maximum positive anomaly corresponds to almost twice the annual average.

Fig. 9 shows the 12 month running mean of the monthly air temperature anomaly at the island of Jan Mayen situated at 713N, 8330W just south of the centre of the area of investigation. The island is situated just at the Arctic front between the cold water to the northwest and the warmer Atlantic water to the southeast. Note the correspondence between the air temperatures in Fig. 9 and the annual integrated ice cover over the same years in Fig. 8. A lot of ice was formed every winter during the cold periods in the late 1970s and the 1980s with the exception of 1983}1985 when a brief warming occurred, and very little ice was present in the warmer period in the 1990s. The correspondence between the air temperatures and the ice cover may have one of two explanations. The presence of an ice cover in the proximity of the island leads to colder air masses in the area or vice versa, the cold air observed at the weather station cools the ocean to the freezing point and subsequently leads to ice formation. The correspondence is not perfect, in particular during the early 1980s where the small ice concentrations seems more associated with increasing temperatures than absolute positive anomalies, indicating that other mechanisms are involved as well. The large negative temperature anomaly in the late 1960s corresponds well with large positive sea-ice anomalies reported by the Freie UniversitaK t Berlin (Eckardt et al., 1992; M. Eckardt, personal communication).

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Fig. 9. 12-month running mean of the air temperature anomaly (3C) at the island of Jan Mayen situated just south of the centre of the area of investigation. The position of the island is almost outside the ice-covered region, but the temperature is still highly correlated with the ice cover on a monthly mean basis (Fig. 8). Anomalies are relative to the period 1922}1992.

4. Discussion From the historical analysis, the ice cover in the Greenland Sea over a 17-yr period showed large interannual variations, with strong ice years and weak ice years. The Greenland Sea during the ESOP years from 1992}1995 saw substantially less ice than normal compared to the previous 12 yr. This situation was even more pronounced during 1993}94 and 1994}95, when practically no ice was present within the Greenland Sea. Previous years have seen in the order of 240,000 km of ice in the area, but in the winter of 1993, less than 100,000 km were formed at the maximum, and in 1994 and 1995 we had less than 20,000 km most of the time. A correspondence between air temperature anomalies at the island of Jan Mayen and the observed ice cover is found back to the mid 1960s, indicating that the surface air temperatures may be used as an indicator of the ice cover earlier in the century.

Acknowledgements The work presented here was funded by the Commission of the European Communities under contract MAS2-CT93-0057. SMMR and SSM/I data was delivered by the

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National Snow and Ice Data Center, Boulder, Colorado NSIDC, (1992). Meteorological data from Jan Mayen were provided by the Norwegian Meteorological Institute.

References Backhaus, J., KaK mpf, J., 1999. Simulations of sub-mesoscale oceanic convection and ice}ocean interactions in the Greenland Sea. Deep-Sea Research II 46, 1427}1455. Comiso, J.C., 1986. Characteristics of Arctic winter sea ice from satellite multispectral microwave observations. Journal of Geophysical Research 91, 975}994. Eckardt, M., Gallas, J., Tonn, W., 1992. Sea ice distribution in the Greenland and Barents Seas based on satellite information for the period 1966}89. International Journal of Remote Sensing 13, 23}35. Hilmer, M., Harder, M., Lemke, P., 1998. Sea ice transport: a highly variable link between Arctic and North Atlantic. Geophysical Research Letters 25, 3359}3362. NSIDC - National Snow and Ice Data Center, 1992. DMSP F-11 SSM/I Brightness temperature and sea ice concentration grids for the polar regions on CD-ROM, user's guide. Co-operative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado. Parkinson, C.L., Comiso, J.C., Zwally, H.J., Cavalieri, D.J., Gloersen, P., Campbell, W.J., 1987. Arctic Sea Ice, 1973}1976; Satellite Passive-Microwave Observations, NASA SP-489, National Aeronautics and Space Administration, Washington, DC, 296 pp. Pedersen, L.T., 1991. Retrieval of sea ice concentration by means of microwave radiometry, LD 81. Electromagnetics Institute, Technical University of Denmark. Rudels, B., 1990. Haline convection in the Greenland Sea. Deep-Sea Research, Part A 37, 1491}1511. Vinje, T., Nordlund, N., Kvambekk, A., 1998. Monitoring ice thickness in Fram Strait. Journal of Geophysical Research 103, 10437}10449. Visbeck, M., Fischer, J., Schott, F., 1995. Preconditioning the Greenland Sea for Deep Convection: Ice formation and ice drift. Journal of Geophysical Research 100, 18489}18502. Zwally, H.J., Comiso, J.C., Parkinson, C.L., Campbell, W.J., Carsey F.D., Gloersen, P., 1983. Antarctic Sea Ice, 1973}1976; Satellite Passive-Microwave Observations, NASA SP-459. National Aeronautics and Space Administration, Washington, DC, 206 pp.