Review article: the false–bottom ice

Abstract. Nansen from his observations in the Beaufort Sea published in 1897 noted that heat transfer from the fresh water (with a~temperature of 0 °C) to the arctic salt water (with a temperature of −1.6 °C) is the only source of ice accretion during the polar summer. This transfer mechanism, unusual at first sight, is responsible for the initiation and evolution of a false bottom ice, changing ice properties to a great extent and affecting various processes while interacting with the ocean and the atmosphere. The processes of false bottom ice growth from below (i.e. from the ocean to the atmosphere) become of prime importance in the era of global warming and climate change. In this review, we summarize the theoretical approaches, field and laboratory observations, conducted during more than 100 yr, in order to address the problem of false bottoms to a broad community of readers. We also discuss the recent modeling advances to which we have contributed. A "false bottom" is a thin layer of ice which forms in summer underneath the floe, where fresh water lies between the salt water and the ice. Such false bottoms represent the only significant source of ice growth in the Arctic during the spring-summer period. Their evolution influences the mass balance of the Arctic sea-ice cover, which is recognized as an indicator of climate change. However, the quantity, aerial extent and other properties of false bottoms are difficult to measure because coring under the surface melt ponds leads to direct mixing of surface and under-ice water. This explains why their aerial extent and overall volume is still not known despite the fact that the upper limit of the present-day estimate of the false bottom ice coverage is approximately half of the sea ice surface. The growth of false bottoms also leads to other important consequences for various physical, chemical and biological processes associated with their dynamics.