Daily sea ice extent data is processed to extract an estimation of the melting/freezing periods for each hemisphere.
Since the seasons are at opposite times of the year, comparing NH melting to SH melting would involved a 6 month shift in one direction or the other. In order to compare the same March-September interval in both hemispheres, the complement of Antarctic freezing period is compared to the Arctic melting season.
The processing of the daily ice extent data to derive the turning points, minimising the influence if localised weather events, is explained in detail here:
Arctic melting/freezing is close to an even 6 month split. Antarctic melting is consistently less than half a year and reflects the continued increase in ice extent throughout the record.
Cross-correlation of the two series shows a significant peak ( 95% confidence against random change ) with variations in the Arctic leading Antarctic by three years ( one datum per year resolution ).
Here the data are lagged to allow visual comparison.