Abstract: This study found that the ratio of vertically polarized brightness temperature of AMSR-E passive microwave data at 18.7 and 36.5 is the ratio of sea ice microwave emissivity for first-year ice. However, for multi-year ice, this ratio is also affected by sea ice temperature. Furthermore, the ratio for older ice is larger than for younger ice. The contrast ratio is a suitable parameter with which to ascertain the threshold between first-year and multi-year ice because the maximum gradient of the contrast ratio appears at the threshold. This threshold varies seasonally; it is relatively steady during winter but changes considerably during summer because of the influence of meltwater. Little difference was found in the multi-year ice coverage of the Arctic section of the Atlantic Ocean when ascertained by the contrast ratio and NASA Team2 (NT2) algorithm; however, large differences were found in the Arctic section of the Pacific Ocean. In comparison to the NT2, the coverage of multi-year ice is commonly found to be larger when ascertained using the contrast ratio.

Key words: Arctic, brightness temperature, first-year ice, multi-year ice, contrast ratio