Something strange has been swirling in the waters around Antarctica. From the 1970s until a decade ago, the floating sea ice that radiates from the continent had been expanding, even with climate change already in full swing. Then, in 2016, it suddenly and dramatically contracted — and has yet to recover — as rising global temperatures seemed to catch up with the Southern Ocean. Far from being just a local issue, the loss of sea ice has huge implications for Antarctica’s vast ice sheet, which would drive sea levels up 190 feet if it disappeared. Now scientists say they’ve identified what’s behind this rise and sudden fall, thanks to an assist from deep-diving robots. It all comes down to salinity, winds, and churn.…