摘要：Sea ice surviving the summer melt season to become multi-year ice in the Arctic Ocean is of interest because multi-year ice significantly affects the ice-thickness distribution and the dynamics and thermodynamics of the ice pack in subsequent seasons. However, the amount of ice surviving summer melting has not been well determined because the time of the minimum ice area varies from region to region. A concept of local temporal minimum (LTM) accounts for non-simultaneity of the melt–freeze transition by determining the minima ice concentrations ( C LTM ) on local spatial scales. C LTM are calculated for 25 km gridcells using 24 years (1979–2002) of satellite passive-microwave data. The total area of ice surviving the summer melt ( A LTM ) is given by spatial integration of C LTM . Over 24 years, the average A LTM is 2.6 × 10 6 km 2 (excluding ∼0.7 × 10 5 km 2 above 84° N). In contrast, the average area (3.8 × 10 6 km 2 ) of all ice types ( A SM ), measured when the total (simultaneous) ice cover is a minimum in daily maps in mid-September, is an often-used estimate of ice surviving the summer melting that is ∼45% too large. Over 24 years, the A LTM decreased by 9.5 ± 2.2% (10 a) −1 (0.27 ± 0.06 × 10 6 km 2 (10 a) −1 ), which is similar to the rate of decline of A SM and about three times the rate of the annual average. The time-of-occurrence of the LTM averaged over the perennial ice pack increased by 8 days from around 11 to 19 August, indicating a later ending of the melt season by about 3 days (10 a) −1 as the summer pack declines. Estimates of multi-year ice in midwinter from passive microwave observations are ∼17% smaller than A LTM , suggesting that the microwave algorithm does not measure all the multi-year ice.
