Assessing ocean mixing under sea ice and lead in climate models

The polar ocean is covered by inhomogenous sea ice, broken by leads. Climate models typically use multi-category sea ice model to resolve the ice thickness distribution in one model grid, while communicate with one ocean grid under the ice. In summer, the modeled lead water is saltier and colder than observed, and a 1-D multi-column ocean model (each ocean column corresponding to one ice thickness category) coupled with ice model showed much improved results. In wintertime, freezing in leads accounts for half of the total amount of new ice formation in the wintertime Arctic Ocean, even though the total area of leads only accounts for a few percent of the whole ice-covered area. We expect that this localized brine rejection can modify the upper ocean in ways that is not adquatly represented in models that do not account for variability in ocean mixing under a hetrogenous ice pack. This study uses a high-resolution 3-D ocean model forced by sea surface brine rejection in lead to quantify the discrepancies of ocean mixing between one-column and multi-column ocean model settings. We assume a climate model grid range of 30km by 30km and the two model settings are different by having brine rejection in lead area only or in the entire grid. The vertical profiles of the modeled salinity and temperature averaged over the 30km by 30km are compared between the two model schemes. It is found that multi-column ocean model get lower salinity in the surface but higher salinity at the base of the halocline because the dense water produced in lead mainly sink to the base of halocline and then spread horizontally rather than add to the surface salinity as did if those brine rejection in lead was added evenly over the 30km by 30km area. The vertical heat flux is also different because warmer water under the halocline can be brought up in multi-column ocean model, and with local circulation generated by the dense water sinking. We are exploring ways to parameterize these ocean mixing mechanisms in climate models such as CCSM and GFDL.