AN INTERMEDIATE ONE-DIMENSIONAL THERMODYNAMIC SEA-ICE MODEL FOR INVESTIGATING ICE-ATMOSPHERE INTERACTIONS

A one-dimensional thermodynamic model of sea ice is presented that foc uses on those features that are most relevant to interactions with the atmosphere, namely the surface albedo and leads. It includes a surfac e albedo parameterization that interacts strongly with the state of th e surface, and explicitly includes meltwater ponds. The lead parameter ization contains a minimum lead fraction, absorption of solar radiatio n in and below the leads, lateral accretion and ablation of the sea ic e, and a prescribed sea ice divergence rate. The model performed well in predicting the current climatic sea ice conditions in the central A rctic when compared with observations and other theoretical calculatio ns. Results of parameter sensitivity tests produced large equilibrium ice thicknesses for small values of ice divergence or large values of minimum lead fraction as a result of positive feedback mechanisms invo lving cooling of water in the leads. The ice thickness was also quite sensitive to the meltwater runoff fraction and moderately sensitive to the other parameters in the melt pond parameterization, a result of t he strong dependence of the surface albedo, and hence the net flux, on the surface conditions. To further investigate the physical interacti ons and internal feedback processes governing the sea ice-lead system, sensitivity tests were also performed for each of the external forcin g variables. The model's equilibrium sea ice thickness was extremely s ensitive to changes in the downward longwave and shortwave fluxes and atmospheric temperature and humidity, moderately sensitive to the valu e of the ocean heat flux, and insensitive to values of wind speed, sno wfall, and rainfall in the immediate vicinity of the baseline forcing, although significant changes in thickness occurred for larger variati ons in wind speed and snowfall. Four important positive feedback loops were identified and described: (1) the surface albedo feedback, (2) t he conduction feedback, (3) the lead solar flux feedback, and (4) the lead fraction feedback. The destabilizing effects of these positive fe edbacks were mitigated by two strong negative feedbacks: (1) the outgo ing longwave flux feedback, and (2) the turbulent flux feedback. Consi dering the strong influence which sea ice has on global atmospheric an d oceanic circulation patterns, it is essential that climate models be able to treat these feedback processes appropriately.