Double-Diffusive Convection and Interleaving in the Arctic Ocean – Distribution and Importance

Beneath its ice cover the Arctic Ocean is a low energy environment. The weak turbulent activity allows other, more esoteric mixing mechanisms to become important in transforming the water masses. One such process is double-diffusive convection, which is triggered by the different molecular diffusion rates of heat and salt and utilises the potential energy stored in the unstably stratified component, heat or salt, to increase the vertical transports. Cold, fresh water above warm, saline water leads to the formation of diffusive interfaces, while warm and saline water above cooler and fresher water results in saltfingers. The former situation is more representative of the Arctic Ocean and the vertical heat transport through diffusive interfaces could contribute significantly to the upward flux of heat from the subsurface warm Atlantic water to the upper layers. The lateral property contrasts between the different inflow branches to the Arctic Ocean and between the boundary current and the water columns of the different basins allow finite lateral disturbances to create intrusions and inversions in the temperature and salinity profiles. These in turn cause double-diffusive transports, which generate convergences and divergences in the vertical buoyancy fluxes, reinforcing the lateral interleaving between the different water masses. Although the interleaving structures are most prominent at frontal zones, they appear to extend over entire basins and are encountered in almost any depth range and in all types of background stratification, saltfinger unstable, diffusively unstable, and when both components stably stratified. The interleaving could be a mechanism that redistributes heat and salt from boundary current at the rim to the interior of the deep basins. However, the formation process(es) for the interleaving layers has (have) not yet been adequately described, and their importance for lateral mixing have not been determined. Here the main features and the distribution of the intrusions in the Arctic Ocean are presented, different explanations for the large extent of the interleaving layers are discussed, and their possible significance for the water mass transformations is evaluated.