As a manifestation of mixing dynamics in the upper ocean, interannual and decadal variability of subtropical mode water (STMW) properties in the North Atlantic Ocean provides a valuable insight into ocean–atmosphere interaction in a changing climate. Here, we use hydrographic data from the Bermuda Atlantic Time-Series Study and Hydrostation S sites near Bermuda, as well as various ocean reanalysis products, to evaluate the modern variability of STMW properties. Our study finds an 86–93% loss of STMW thickness at these sites between 2010 and 2018 and a comparable loss throughout the western subtropical gyre, culminating in the weakest STMW pentad on record. We correlate this decline with a reduction in the annual outcropping volume and northward excursions of the formation region, suggesting a gyre-wide signal of weakening STMW generation. The outcropping volume of STMW is anti-correlated with surface ocean heat content, foreshadowing future STMW loss in the face of continued warming.

作为上层海洋混合动力的一种表现，北大西洋亚热带模式水（STMW）的年际和年代际变化为气候变化中的海洋与大气相互作用提供了宝贵的见解。在这里，我们使用来自百慕大大西洋时间序列研究和百慕大附近的Hydrostation S站点以及各种海洋再分析产品的水文数据来评估STMW属性的现代变异性。我们的研究发现，在2010年至2018年期间，这些地点的STMW厚度损失了86-93％，整个西亚热带回旋区的损失也相当，最终导致有记录以来最弱的STMW五单元组。我们将此下降与形成区域的年露头量和北移次数的减少相关联，这表明在全回转范围内减弱了STMW产生的信号。