Antarctic sea ice variability and trends, 1979–2010

C L Parkinson,D J Cavalieri

doi:10.5194/tc-6-871-2012

Abstract

Abstract. In sharp contrast to the decreasing sea ice coverage of the Arctic, in the Antarctic the sea ice cover has, on average, expanded since the late 1970s. More specifically, satellite passive-microwave data for the period November 1978–December 2010 reveal an overall positive trend in ice extents of 17 100 ± 2300 km2 yr−1. Much of the increase, at 13 700 ± 1500 km2 yr−1, has occurred in the region of the Ross Sea, with lesser contributions from the Weddell Sea and Indian Ocean. One region, that of the Bellingshausen/Amundsen Seas, has (like the Arctic) instead experienced significant sea ice decreases, with an overall ice extent trend of −8200 ± 1200 km2 yr−1. When examined through the annual cycle over the 32-yr period 1979–2010, the Southern Hemisphere sea ice cover as a whole experienced positive ice extent trends in every month, ranging in magnitude from a low of 9100 ± 6300 km2 yr−1 in February to a high of 24 700 ± 10 000 km2 yr−1 in May. The Ross Sea and Indian Ocean also had positive trends in each month, while the Bellingshausen/Amundsen Seas had negative trends in each month, and the Weddell Sea and western Pacific Ocean had a mixture of positive and negative trends. Comparing ice-area results to ice-extent results, in each case the ice-area trend has the same sign as the ice-extent trend, but the magnitudes of the two trends differ, and in some cases these differences allow inferences about the corresponding changes in sea ice concentrations. The strong pattern of decreasing ice coverage in the Bellingshausen/Amundsen Seas region and increasing ice coverage in the Ross Sea region is suggestive of changes in atmospheric circulation. This is a key topic for future research.

Highlights

Sea ice spreads over millions of square kilometers of the Southern Ocean at all times of the year and over an area larger than the Antarctic continent in the midst of the austral winter, with February typically the month of minimum ice coverage and September typically the month of maximum ice coverage (Fig. 1)
The Southern Hemisphere slope is 17 500 ± 4100 km2 yr−1 (1.5 ± 0.4 % decade−1) (Table 1), increased from a slope of 11 500 ± 4600 km2 yr−1 for the shorter period ending in December 2006 (Cavalieri and Parkinson, 2008) and within 3 % of the slope for the monthly deviations, which have a somewhat smaller slope in part because of including at the start of the record the initial months of November and December 1978, both of which have ice extent values above the line of least squares fit (Fig. 3b)
The upward trends in sea ice extents and areas reported here for the Southern Hemisphere are in sharp contrast to the situation in the Arctic, where downward trends have been reported since the late 1980s (Parkinson and Cavalieri, 1989) and have become stronger over time (Johannessen et al, 1995; Parkinson et al, 1999; Meier et al, 2007; Comiso et al, 2008; Cavalieri and Parkinson, 2012)

Summary

Introduction

Sea ice spreads over millions of square kilometers of the Southern Ocean at all times of the year and over an area larger than the Antarctic continent in the midst of the austral winter, with February typically the month of minimum ice coverage and September typically the month of maximum ice coverage (Fig. 1). Through satellite passive-microwave observations, there exists a solid record of the distribution and extent of Arctic and Antarctic sea ice coverage and their changes since the late 1970s. To obtain longterm trends for the monthly results, the seasonal cycle was removed by creating monthly deviations, which were calculated by subtracting from each individual monthly average the 32-yr average for that month (or, in the case of November and December, the 33-yr average). This follows the procedure in Parkinson et al (1999) and subsequent works. As in Cavalieri and Parkinson (2008) and earlier studies, results are presented for the following five regions of the Southern Ocean: Weddell Sea (60° W–20° E, plus the small ocean area between the east coast of the Antarctic Peninsula and 60° W), Indian Ocean (20–90° E), western Pacific Ocean (90–160° E), Ross Sea (160° E–130° W), and the combined Bellingshausen and Amundsen Seas (130–60° W) (Fig. 2)

Southern Hemisphere total

Regional results

Discussion