Period-specific thresholds reduced four-sector Antarctic low-ice days from 16.2% to 1.95% in 2016-2025

We test joint low sea ice extent in five Antarctic regions. Sea Ice Index Version 4 provides 65,700 daily regional values for 1990-2025. Values are five-day trailing means. At least four regions were below their calendar-day 1990-2015 20th percentiles on 0.35% of earlier days and 16.16% of 2016-2025 days. When each period has its own regional thresholds, the recent rate is 1.95%. Independent alignment of the recent regional years gives an exact expected rate of 0.672%. A whole-year permutation test gives upper-tail p = 0.0518. Of the 71 recent days under these local thresholds, 56 occur in 2023. Removing 2023 and fitting new thresholds gives a rate of 1.19% and p = 0.3198. These tests separate changes in regional distributions from their joint low tail. They do not establish a lasting change in physical coupling. The short recent record, fitted thresholds, trends, and sensor changes limit inference. This is an extension of earlier campaign studies with the same observations, not independent confirmation.

Antarctic sea ice · compound extremes · regional dependence · quantile thresholds · year permutation