The Melting of Ice in the Arctic Ocean: The Influence of Double-Diffusive Transport of Heat from Below

Abstract

This investigation was originally prompted by two oceanographic observations: an increased rate of melting of sea ice in the Arctic Ocean, and the advance of an anomalously warm tongue of Atlantic water intruding across the Arctic below the halocline over the past few decades. A series of laboratory model experiments has previously been carried out to explore the possibility that the extra heating at depth could be responsible for the enhanced melting rate. These experiments have demonstrated that a one-dimensional heat flux from below through a series of double-diffusive layers can in principle lead to faster melting of floating ice. However, it is now essential to test these ideas quantitatively under ocean conditions and to compare the results with other possible mechanisms of melting. A simple calculation shows that there is enough heat in the intruding Atlantic water to melt all the ice in the Arctic in a few years if all the heat could be brought to the surface in this time. The vertical double-diffusive transport of heat is slower than this, but it is large enough to make a substantial contribution to the increased rate of melting over the last three or four decades. Another proposed mechanism for melting is the solar input to the surface mixed layer from the atmosphere. In particular years when detailed measurements and calculations have been made, this atmospheric input can explain both the seasonal cyclic behavior of ice and the increased melting rate. Given the large heat content in the intruding Atlantic layer, however, it seems worth exploring further other advective two-dimensional mechanisms that could transport this heat upward more rapidly than the purely vertical double-diffusive convection. For example, dense salty water produced by freezing on the shelves around the Arctic Basin could flow down the slope and penetrate through the halocline, thus mixing with the warm water and bringing it to the surface.