不可逆混合扩散是引起海洋水团变性、维持大洋经向翻转环流、影响海洋气候数值模拟的关键物理过程。尽管当前有许多涡致扩散定义和诊断方法，但不同方法所得结果不仅差异较大，也无法反映局地瞬时的不可逆混合过程，因而为混合参数化方案带来巨大不确定性。对此本项目在“等值线”坐标空间框架下提出一种新的拉格朗日扩散定义，尝试将拉格朗日扩散和有效扩散这两种常用的扩散定义从理论上统一起来，以澄清目前物理海洋界对这两种定义理解和认识上的误区，减少不同定义之间的理论和计算差异。由于新定义直接将涡致扩散与分子扩散联系起来，能清楚地反映涡旋引起的不可逆混合过程，并给出局地的、瞬时的混合扩散分布，因而克服了传统扩散诊断量的不足。基于新的扩散定义，本项目拟进一步利用真实海洋观测资料对全球大洋不可逆混合的时空分布特征进行诊断分析，建立一套准确合理的不可逆混合参数化新方案，为提高我国海洋数值模拟和预测水平提供理论支持。

Irreversible mixing is a key process for the transformation of water mass and the maintenance of the global meridional overturning circulation. It also affects the numerical simulation of the ocean climate. Although there are many mixing diagnostics, estimates from different diagnostics generally result in different magnitudes and patterns. Besides, there is no clear way to demonstrate local instantaneous irreversible mixing. The present project proposes a new definition of Lagrangian diffusivity in the framework of ‘contour-based’ coordinates, and tries to unify the Lagrangian diffusivity and effective diffusivity, which are two commonly used diagnostics, on a theoretical foundation. The unification of the two diffusivities could help to clarify the misunderstandings of the two diagnostics in the community. Using the new definition, Lagrangian diffusivity can be directly linked to small-scale molecular diffusivity and thus is regarded as a robust diagnostic for local instantaneous irreversible mixing, which is generally impossible for other diagnostics. This new definition is then applied to diagnose mixing and diffusion over the global ocean, with the aim at revealing the full temporal and spatial variations of irreversible mixing. A new parameterization scheme will be developed based on the new diagnostic, which eventually provides theoretical supports for improving oceanic simulation and prediction.