Surface flux–induced salinity change and its effects on ocean stratification in response to global warming

Global warming induced by increased CO 2 has caused marked changes in the ocean. Previous estimates of ocean salinity change in response to global warming have considerable ambiguity, largely attributable to the diverse sensitivities of surface fluxes. This study utilizes data from the Flux-Anomaly-Forced Model Intercomparison Project to investigate how ocean salinity responds to perturbations of surface fluxes. The findings indicate the emergence of a sea surface salinity (SSS) dipole pattern predominantly in the North Atlantic and Pacific fresh pools, driven by surface flux perturbations. This results in an intensification of the “salty gets saltier and fresh gets fresher” SSS pattern across the global ocean. The spatial pattern amplification (PA) of SSS under global warming is estimated to be approximately 11.5 %, with surface water flux perturbations being the most significant contributor to salinity PA, accounting for 8.1 % of the change after 70 years in experiments since pre-industrial control (piControl). Notably, the zonal-depth distribution of salinity in the upper ocean exhibits lighter seawater above the denser water, with bowed isopycnals in the upper 400 m. This stable stratification inhibits vertical mixing of salinity and temperature. In response to the flux perturbations, there is a strong positive feedback due to consequent freshening. It is hypothesized that under global warming, an SSS amplification of 7.2 %/°C and a mixed-layer depth amplification of 12.5 %/°C will occur in the global ocean. It suggests that the salinity effect can exert a more stable ocean to hinder the downward transfer of heat, which provides positive feedback to future global warming. 摘要 由于二氧化碳 (CO 2 ) 增加导致的全球变暖已经对海洋产生了显著变化. 以往对海洋盐度变化响应于全球变暖的估计存在相当大的不确定性, 这主要归因于模式对表面通量的敏感性差异. 研究利用“通量异常强迫模式比较计划” (Flux-Anomaly-Forced Model Intercomparison Project) 的数据, 探讨全球变暖背景下海洋盐度变化如何响应表面通量的扰动. 结果表明, 在北大西洋和太平洋, 表面通量扰动导致了海表面盐度 (SSS) 偶极模态的出现. 使得全球海洋中“咸的变得更咸, 淡的变得更淡”的SSS分布模态的进一步增强. 其中, SSS的空间模式放大 (PA) 估计约为11.5 %. 其中表面水通量扰动是盐度PA 增强的最显著贡献者, 对比对照实验 (piControl) 开始后70年内总变化的8.1 %. 值得注意的是, 通量强迫导致上层海洋的盐度层结化加强, 显示出大部分海洋上层盐度降低, 上层400米处的等密度线呈现“碗”型状态. 稳定的层结抑制了盐度和温度的垂直混合. 作为对通量扰动的响应, 上层海洋淡化作用, 对全球变暖产生了强烈的正反馈. 研究表明, 在全球变暖背景下, 全球海洋的SSS将放大7.2 %/°C, 混合层深度的强度变化将放大12.5 %/°C. 这表明盐度效应可以使海洋更加稳定, 从而阻碍热量向下传输, 对未来的全球变暖提供正反馈.