Simulating Freshwater Flux Induced Abrupt Climate Change in the Arctic/Atlantic Ocean
OCCI Funded Project: 2006
Proposed ResearchAbrupt climate change at high latitudes, such as the Arctic, is intimately related to a rapid melting of sea ice or iceberg drafting. During ice formation and melting, there are freshwater and salt exchanges across the water-ice interface that drive ocean circulation. Since it is difficult to directly measure such exchanges over large regions, we use numerical models to simulate these physical processes in the ocean. In this project, we carried out a numerical study on ice-water interaction, whereby a large influx of freshwater induces abrupt changes in ocean circulation and, thus, climate change.
Two major theoretical problems related to ice formation exist for modelers. First, how do we treat the exchange of freshwater and salt across the water-ice interface? Second, how do we deal with the change in pressure in the upper surface of the water column? For a long time, a suitable way of mathematically handling these issues remained unclear.
In this project, we started from the fundamental physics related to the exchange of freshwater and salt and postulated suitable calculations for handling the ice-water interface during ice freezing and melting. Ice formation is similar to evaporation in that a certain amount of freshwater is removed from the water column during both processes. The freshwater associated with evaporation is diffused into the atmosphere where air currents continually redistribute air masses and render the local atmospheric pressure over the sea nearly unchanged. After ice formation (or evaporation) water below the sea surface is slightly heavier - salt water is denser than fresh water. This process changes the pressure in the surface water, inducing currents, as water masses shift to accommodate new pressure gradients.
In the numerical experiments, we tested boundary conditions suitable for simulating freshwater flow through the air-sea interface with simple geometry. We devised a new formulation and demonstrated it through a simple, analytical model. Numerical simulation through experiments based on an oceanic general circulation model also confirmed our theory.
With a more accurate formulation of boundary conditions, motions related to ice formation and melting can be handled more accurately in our models. In particular, we developed a model based on pressure coordinates, which can handle problems associated with sea ice melting and drafting of icebergs with thicknesses on the order of a few hundred meters.
Results from this study have been described in the manuscript, “On the boundary conditions applied to the sea-ice coupled model” (R. X. Huang and X.-Z. Jin), which was submitted to the Journal of Geophysical Research and accepted for publication.