Variations of sea ice in the Antarctic and Arctic from 1997–2006

Abstract

Sea ice in polar areas is an important part of the global climate system. In order to obtain variations in sea ice extent for the Antarctic and Arctic, this paper analyzed the Special Sensor Microwave/Imager (SSM/I) sea ice data product dating from March 1, 1997 to December 31, 2006. During this period, the sea ice extent increased in the Antarctic with the trend of (0.5467±0.4933)×10⁴ km²·yr^–1, and decreased in the Arctic with the trend of (–7.6125±0.3503)×10⁴ km²·yr^–1. In different sectors of the Antarctic, variations of the sea ice extent are different. The sea ice extent increased in the Weddell Sea and Indian Ocean, but decreased in the Ross Sea, Western Pacific Ocean, and Bellingshausen/Amundsen Seas.

Keywords

sea ice concentration (SICN) / sea ice extent / linear trend

Introduction

Most areas of the Antarctic and Arctic are covered with snow and ice. Therefore, they play an important role as the main cold source of global atmosphere in global atmospheric circulation and the synoptic process (Bian et al., 2007; Jaiser et al., 2012). The sea ice extent affects the amount of solar radiation absorbed into its hemisphere, and alters the atmosphere-ocean exchanges of heat, moisture, and momentum (Gloersen and Campbell, 1991; Gloersen et al., 1993; Curry et al.,1995; Wu et al., 2004). Sea ice distribution is also a factor in climate change (McBean et al., 2005; Budikova, 2009; Overland and Wang, 2010). Climate change is amplified in the Northern Hemisphere due to the ice-albedo and snow-albedo feedbacks (Holland and Bitz, 2003). The Special Sensor Microwave/Imager (SSM/I), aboard the Defense Meteorological Satellite Program (DMSP) satellites, which served as the advanced sensor for monitoring atmospheric and land surface parameters, can provide long time series of satellite microwave measurements. These measurements can be used for studying changes in snow and sea ice in Polar Regions over a long period of time. Gloersen and Campbell (1991) analyzed the data from the scanning multichannel microwave radiometer (SMMR) dating from October 26, 1978 to August 20, 1987. They found that the sea ice extents in the Arctic had significantly decreased during those nine years; but there were no significant trends in the Antarctic. Parkinson et al. (1999) revealed the variations of sea ice extent in the Arctic from 1978 to 1996. They analyzed the data retrieved from SMMR and SSM/I measurements and found a significant decrease in sea ice extents in the Arctic in all seasons, especially in spring. Zwally et al. (2002) analyzed 20.2 years data set (from SMMR and SSM/I, 1978–1998) and pointed out that the total Antarctic sea ice extent had increased. They also found that the trends varied in different regions, even in the same region the trends differed from season to season. Comiso et al. (2008) analyzed the data from SMMR (late October 1978–1987) and SSM/I (1987–2007). They found that the decline of sea ice extent had accelerated. Recent researchers have reported that there was an overall positive trend in Antarctic sea ice extents of (17,100±2,300) km²·yr^-1, and an overall negative trend in Arctic sea ice extents of (-51.5±4.1)×10³ km²·yr^–1 from November 1978 to December 2010 (Cavalieri and Parkinson, 2012; Parkinson and Cavalieri, 2012). The above research on the variations of sea ice extent in the Antarctic and Arctic are summarized in Table 1.

In this paper, trends of sea ice extent in the Antarctic and Arctic will be analyzed using the sea ice data product from SSM/I aboard the DMSP F-13 satellite.

Descriptions of SSM/I and data product

The SSM/I, aboard the series satellites of DMSP, is a seven-channel, four-frequency, linear-polarized, and passive microwave radiometric system. As exhibited in Table 2 and Fig. 1, it measures atmospheric, ocean, and terrain microwave brightness temperatures (Hollinger et al., 1990). The SSM/I scans at a constant 45° angle from nadir and intersects the Earth’s surface at a constant incidence angle of 53.1° (Raytheon, 2000<FootNote>

Raytheon (2000). Special Sensor Microwave/Imager user’s interpretation guide.

</FootNote>). The sea ice data used in this paper is drawn from the SSM/I aboard the DMSP F13 satellite. The F13 spacecraft was launched on 24 March 1995. It travels in a near polar Sun-synchronous orbit at a nominal altitude of 850 km, with an inclination of 98.8° and an orbit period of 102 min (Julienne et al., 1998).

The data product analyzed in this paper was sea ice concentration (SICN), one of the Environmental Data Records (EDRs) parameters retrieved from brightness temperatures and mapped at a grid cell size of 0.5°×0.5° by NOAA/NESDIS from March 1, 1997 to December 31, 2006. SICN is a measurement of the fraction of ocean surface covered by sea ice (Cavalieri, 1992). There are several retrieval algorithms for SICN. The data used in this study were calculated using the Atmospheric Environment Service (AES)-York ice concentration algorithm. This algorithm calculated the SICN from the algebraic solution form of the radiation transfer equation by substituting the brightness temperatures of the four channels into the equation (19V, 19H, 37V, and 37H) (Ramseier and Cooper, 1991<FootNote>

Ramseier R O, Cooper S (1991). Sea ice validation. DMSP SSM/I calibration/validation.

</FootNote>; Steffen et al., 1992; Shokr and Markus, 2006).

Data processing

Due to the inclination degree (98.8°) of the satellite F13, there are some missing values from the 85°N to the 90°N latitude band in the Arctic. Considering that the sea surface would be covered with ice, it was assumed that the missing values of SICN in this area were 100%. Besides, the data on the coastline of Arctic and Antarctic were removed.

In this study, sea ice extent were calculated daily, by adding up the areas of all of the grid cells having at least 15% sea ice concentration (same method as Zawally et al., 2002) in the Antarctic (50°S–90°S) and Arctic (50°N– 90°N). Through calculating daily sea ice extent from March 1, 1997 to December 31, 2006, time series of the sea ice extent inter-annual variation could be created for both polar regions. Mean annual cycles were calculated by averaging inter-annual variations. The inter-annual anomaly variations were obtained by subtracting the mean annual cycles from inter-annual variations. Then, the outliers were removed by using the Biweight method (Zou and Zeng, 2006). The linear trends were calculated from the inter-annual anomaly variations using the Least Squares estimate (Zou, 2012).

Analysis

Arctic and Antarctic areas

Figure 2 shows areas with SICN greater than 15% on February 15 and September 15 of 1998 and 2006, in the Southern and Northern Hemispheres respectively. The figure shows that the coverage in 2006 was slightly larger than in 1998 in the Antarctic, while in the Arctic the coverage in 2006 was smaller than in 1998. The variations differed in different regions.

Figure 3 shows the annual variations of sea ice extent in the Antarctic and Arctic in 1998 and 2006. Antarctic sea ice extent in 2006 was smaller than in 1998 from January to May, and larger than in 1998 from June to October. Arctic sea ice extent in 2006 was smaller than in 1998 during the whole year. Inter-annual variations, mean annual cycles, and inter-annual anomaly variations of sea ice extent in the Antarctic and Arctic are presented in Figs. 4(a)–4(c). The minimum sea ice extent in the Antarctic was about 3×10⁶ km² in February and the maximum was about 18×10⁶ km² in September. The minimum sea ice extent in the Arctic was about 5×10⁶ km² in September, and the maximum was about 11×10⁶ km² in March. Green lines in Fig. 4(c) indicate linear trend lines. From March 1, 1997 to December 31, 2006, sea ice extent increased in the Antarctic and decreased in the Arctic, with linear trends of (0.5467±0.4933)×10⁴ km²·yr^–1 and (–7.6125±0.3503) ×10⁴ km²·yr^–1 respectively.

Five sectors in the Antarctic

Table 3 and Figure 5 present the whole Antarctic region in five sectors. Average SICN maps in February and September were obtained by averaging the SICN of each grid over the first 5 years (1998–2002) and the last 4 years (2003–2006). The top of Fig. 5 shows the difference between the first 5 years and the last 4 years (the last 4 years averages minus the first five years averages). In different sectors, the variations of SICN were different. In February, the variations mainly occurred in the Weddell Sea and the Ross Sea. In September, the SICN in the Ross Sea decreased, while in the Indian Ocean and the Weddell Sea it increased obviously (as shown in bottom of Fig. 5).

Table 4 and Figure 6 show the linear trends of the sea ice extent in each sector. Sea ice extent increased in the Weddell Sea and the Indian Ocean, especially in the Weddell Sea, with the trend of (3.0684±0.5827)×10⁴ km²·yr^–1, and decreased in West Pacific Ocean, Ross Sea, and Bellingshausen/Amundsen Seas, especially in Ross Sea, with the trend of (–3.2922±0.4187)×10⁴ km²·yr^–1.

Discussion and conclusions

Variations in sea ice extent in the Antarctic and Arctic are well-documented by many researchers. These phenomena are connected with other elements of the climate system. Simmonds and Jacka (1995) revealed a positive correlation between zonally averaged sea ice extent in the Antarctic from April to July with the Southern Oscillation Index (SOI) during most of the previous 12 months. But associations were different in the three ocean basins. Yuan (2004) pointed out that warm (cold) ENSO events usually generated positive (negative) temperature anomalies and negative (positive) sea ice anomalies in the Pacific centre of the Antarctic Dipole (ADP).

Regarding the decline of sea ice extent in the Arctic, Stroeve et al. (2012) pointed out that atmospheric patterns in favor of shrinking sea ice extent had more effect than before when compared with 20 years ago. Wang and Ikeda (2000) revealed sea ice area variations associated with the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). The AO operated in all seasons, producing a persistent pattern. The NAO prevailed in winter, spring, and autumn, but weakened in summer. Overland and Wang (2005) also believed summer sea ice extent in the Arctic was related to the AO.

The results presented here describe sea ice extend variations in the Antarctic and Arctic over nearly 10 years, ranging from 1997 to 2006. And the results indicate that sea ice extent increased slightly in the Antarctic with the trend of (0.5467±0.4933)×10⁴ km²·yr^–1, and decreased in the Arctic with the trend of (–7.6125±0.3503)×10⁴ km²·yr^–1 from March 1, 1997 to December 31, 2006. From the comparison of sea ice extent in 1998 and 2006, it was found that the sea ice extent of the Antarctic in 2006 was smaller than in 1998 from January to May, and larger than in 1998 from June to October. However, the sea ice extent of the Arctic in 2006 was smaller than in 1998, almost the whole year. The analysis of the variations in each sector of the Antarctic indicated that the variations varied in different sectors. Sea ice increased in the Weddell Sea and Indian Ocean, and decreased in the Western Pacific Ocean, Ross Sea, and Bellingshausen/Amundsen Seas.

Compared the results with other studies (Comiso et al., 2008; Cavalieri and Parkinson, 2012; Parkinson and Cavalieri, 2012) which calculating the trends from the data retrieved by NASA team algorithm (Shokr and Markus, 2006), the results are similar in the same periods. There also should be note that the trends were different in different periods. For example, the trend for the Arctic from 1997 to 2006 in this study was larger than the trend from 1979 to 2010 in Cavalieri’s study (Cavalieri and Parkinson, 2012) due to the acceleration of declining sea ice extend in the Arctic (Comiso et al., 2008). And for separate sectors, in Parkinson’s study, the trends in the Weddell Sea, Indian Ocean, Western Pacific Ocean, and Ross Ocean were positive, while in the Bellingshausen/Amundsen Seas were negative. In the Ross Sea, for instance, it could be found that the sea ice extent increased during the whole period of 1979 to 2010, but decreased from 1997 to 2006 (Parkinson and Cavalieri, 2012).

All in all, the studies of this paper confirm that there was a significant decrease in Arctic sea ice and a slightly increase in the Antarctic during 1997–2006. The trends of sea ice extent were different in different time periods and in different polar areas. Even in same polar area the trends were also different in different sectors. These may relate with other climate factors like atmospheric patterns, solar radiation, ocean current and so on.

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