浪致混合对亚热带冬季海洋混合强度的影响

The reduced winter vertical mixing in the subtropical oceans by the surface wave-induced mixing

上层海洋在全球气候系统中起着至关重要的作用。对上层海洋层结及混合的模拟偏差一直是海洋和气候数值模式发展中悬而未决的问题。本文首先评估了CMIP5中45个模式对上层海洋层结模拟的偏差,确认了冬季亚热带地区海洋模式垂向混合偏强。随后,基于自然资源部第一海洋研究所地球系统模式(FIO-ESM v1.0),分别开展了1986-2005年期间包含和关闭海浪垂向混合情况下的数值实验,分析浪致混合对亚热带冬季海洋混合强度模拟的影响及机制。发现浪致混合使得气候模式中亚热带海域冬季的海洋层结增强,增强的层结使上层海洋更加稳定。首次揭示了增加浪致混合反而降低了海洋总体的垂向混合率:浪致混合使北半球冬季亚热带海域混合率从无浪实验的227 cm^2/s降低到有浪实验的178 cm^2/s,降低了21.6%;南半球冬季亚热带海域混合率从无浪实验的189 cm^2/s降低到有浪实验的165 cm^2/s,降低了12.7%。进一步分析发现,浪致混合主要是通过增加冬季亚热带海域上层海洋的热含量从而强化了海洋的层结,最终改善了气候模式对上层海洋混合的模拟。

The ocean mixing is one of the most important parameters in the global climate system. The simulation biases of the stratification and the ocean mixing are still open questions. Compared to observations, the simulated multi-model mean stratification during winter in the subtropical regions of both hemispheres shows week bias from45 CMIP5 climate models. Our results from two numerical experiments using one of CMIP5 models, FIO-ESM v1.0, show that the non-breaking surface wave-induced vertical mixing can serve as a remedy. It increases the temperature of the upper ocean in winter which then stabilize the upper ocean and increase the stratification in subtropical regions. As a result, the stronger stratification restrains the ocean vertical mixing. The simulation of ocean mixing reduce from 227 cm/m^2 to 178 cm/m^2 in the north subtropical in winter, and from 189 cm/m^2 to 165 cm/m^2 in the south subtropical. They reduced by 21.6% and 12.7%, respectively. Further analysis indicated that the the surface wave-induced mixing strengthen the stratification by the increasing the upper ocean heat content and then improve the simulation of the ocean mixing.