Mixed layer heat and salinity budgets during the onset of the 2011 Atlantic cold tongue

The mixed layer (ML) temperature and salinity changes in the central tropical Atlantic have been studied by a dedicated experiment (Cold Tongue Experiment (CTE)) carried out from May to July 2011. The CTE was based on two successive research cruises, a glider swarm, and moored observations. The acquired in situ data sets together with satellite, reanalysis, and assimilation model data were used to evaluate box-averaged ML heat and salinity budgets for two subregions: (1) the western equatorial Atlantic cold tongue (ACT) (23°-10°W) and (2) the region north of the ACT. The strong ML heat loss in the ACT region during the CTE was found to be the result of the balance of warming due to net surface heat flux and cooling due to zonal advection and diapycnal mixing. The northern region was characterized by weak cooling and the dominant balance of net surface heat flux and zonal advection. A strong salinity increase occurred at the equator, 10°W, just before the CTE. During the CTE, ML salinity in the ACT region slightly increased. Largest contributions to the ML salinity budget were zonal advection and the net surface freshwater flux. While essential for the ML heat budget in the ACT region, diapycnal mixing played only a minor role for the ML salinity budget. In the region north of the ACT, the ML freshened at the beginning of the CTE due to precipitation, followed by a weak salinity increase. Zonal advection changed sign contributing to ML freshening at the beginning of the CTE and salinity increase afterward.

2011年5月至7月开展的专项实验——冷舌实验（Cold Tongue Experiment, CTE），针对热带大西洋中部海域的混合层（mixed layer, ML）温度与盐度变化展开了系统性研究。该实验依托两次连续的研究航次、滑翔机群观测以及系泊观测完成，研究团队将获取的原位观测数据集与卫星数据、再分析资料及同化模式数据相结合，用于评估两个子区域的箱平均混合层热量与盐度收支：(1) 西赤道大西洋冷舌区（ACT）（23°W至10°W）；(2) 冷舌区以北海域。研究发现，冷舌区在实验期间存在显著的混合层热损失，其源于海面净热通量带来的增温与纬向平流、跨等密面混合引发的降温之间的平衡；冷舌区以北海域则表现为微弱降温，其热量收支主要由海面净热通量与纬向平流共同主导。在实验启动前不久，赤道海域（10°W附近）出现了显著的盐度升高现象，实验期间冷舌区的混合层盐度仅小幅上升，其盐度收支的主要贡献项为纬向平流与海面净淡水通量。跨等密面混合虽对冷舌区的混合层热量收支至关重要，但对盐度收支仅起到次要作用。冷舌区以北海域在实验初期因降水发生海水淡化，随后盐度出现微弱回升，纬向平流的作用符号发生改变：实验初期其贡献为海水淡化，后续则推动盐度升高。