a. Annual cycle

Adding Arctic and Antarctic sea ice extents month by month for the period November 1978–December 2013 yields a global time series that shows a strong seasonal cycle with minimum global ice extent occurring in February of each year, maximum ice extent occurring in October or November of each year except 1979, and a minor secondary maximum often occurring in the June–July time frame (Fig. 1). In the anomalous year 1979, the customary June/July secondary maximum is instead the primary maximum (Fig. 1).

The fairly systematic annual cycle of global ice extents depicted in Fig. 1 is most readily understood through comparison with the two component annual cycles. With that in mind, Fig. 2 presents the global average annual cycle for the full 1979–2013 period, averaging the 35 full annual cycles in Fig. 1, along with the corresponding curves for the Arctic and Antarctic individually. Both polar regions have large-amplitude annual cycles, with maximum ice extent coming in late winter and minimum ice extent coming in mid-to-late summer for the respective hemispheres. In the Antarctic, on average, monthly ice extents range from a minimum of 3.1 × 106 km2 in February to a maximum of 18.5 × 106 km2 in September, for an amplitude of 15.4 × 106 km2. The month of minimum was February for every year of the 35-yr record, and the month of maximum was September in every year except 1988, when it was October. In the Arctic, the range is from a minimum of 6.4 × 106 km2 in September to a maximum of 15.2 × 106 km2 in March, for an amplitude of 8.8 × 106 km2 (Fig. 2). Here too the timing is quite systematic, with September being the month of minimum Arctic ice extent in each of the 35 years and March the month of maximum ice extent in every year except 1987, 1989, and 1998, when it was February.

Not only is the amplitude of the average annual cycle greater in the Antarctic than in the Arctic (Fig. 2), but the record daily minimum in the Arctic, 3.4 × 106 km2 on 17 September 2012, is not as low as the average February ice extent in the Antarctic, and the record daily maximum in the Arctic, 16.3 × 106 km2 on 1 March 1979, is not even close to reaching the level of the average September ice extent in the Antarctic (Fig. 2).

The much greater amplitude of the annual cycle of Antarctic versus Arctic ice extents results in the annual cycle of global sea ice extents being more in line, overall, with the Antarctic cycle than the Arctic cycle (Fig. 2). Globally, on average the monthly ice extents range from a minimum of 18.2 × 106 km2 in February to a maximum of 26.6 × 106 km2 in November. Hence the global and Antarctic cycles share the February timing of ice extent minimum, while the global maximum comes two months after the Antarctic maximum. Another prominent feature of the global plot is the dip to a secondary minimum in August/September, resulting from the July–September rapid summer retreat of the Arctic ice outweighing the more gradual July–September winter expansion of the Antarctic ice. Both hemispheres have winter maxima that are relatively broad and summer minima that are sharper, resulting in both minima being visible in the global plot (Fig. 2).