Improve ocean mixing caused by subgrid-scale brine rejection using multi-column ocean grid in a climate model

Heterogeneous ice pack with sporadic narrow but long leads in the polar oceans was unresolved in typical climate model grid. Although multi-category sea ice thickness distribution was used in one sea ice model grid to calculate separate heat, salt and tracer fluxes through each category, the ocean models use only single-column grid to communicate with the averaged fluxes from all categories. When the lead is resolved by the grid, the added salt at the sea surface will sink to the base of the mixed layer and then spread horizontally. When averaged at a climate-model grid size, this vertical distribution of added salt is lead-fraction dependent. When the lead is unresolved, the model errors were systematic leading to greater surface salinity and deeper mixed-layer depth (MLD). An empirical function was developed to revise the added-salt-related parameter n from being fixed to lead-fraction dependent. Application of this new scheme in climate model showed significant improvement in modeled wintertime salinity and MLD as compared to series of CTD data sets in 1997/1998 and 2006/2007. The results showed the most evident improvement in modeled MLD in the Arctic Basin, similar to that using a fixed n = 5, as recommended by the previous Arctic regional model study, in which the parameter n obtained is close to 5 due to the small lead fraction in the Arctic Basin in winter.

极地海洋中存在零星狭长冰间水道（leads）的非均匀海冰群，难以在典型气候模式的网格中被显式解析。尽管部分海冰模式采用多分类海冰厚度分布方案，以分别计算各分类下的热、盐与示踪剂通量，但海洋模式仅通过单柱网格与所有分类的平均通量进行耦合。当冰间水道被网格显式解析时，海表新增的盐分会下沉至混合层底部，随后沿水平方向扩散。若以气候模式的网格尺度进行平均，新增盐分的垂直分布与冰间水道占比呈显著相关关系。若冰间水道未被网格显式解析，模式将出现系统性误差，表现为表层盐度偏高、混合层深度（mixed layer depth, MLD）更深。研究人员开发了一款经验函数，将原本固定的新增盐分相关参数n修正为与冰间水道占比相关的变量。将该新方案应用于气候模式后，相较于1997/1998年与2006/2007年的一系列温盐深仪（Conductivity, Temperature, Depth, CTD）数据集，模式模拟的冬季盐度与混合层深度均得到显著改善。结果显示，北极海盆区域的模式模拟混合层深度改善最为显著，这与此前北极区域模式研究推荐的固定参数n=5的效果相近——此前研究中，由于冬季北极海盆的冰间水道占比较小，所得参数n接近5。