DataSheet_1_A seasonal climatology of the upper ocean pycnocline.pdf

Climatologies of the mixed layer depth (MLD) have been provided using several definitions based on temperature/density thresholds or hybrid approaches. The upper ocean pycnocline (UOP) that sits below the mixed layer base remains poorly characterized, though this transition layer is an ubiquitous feature of the ocean surface layer. Available hydrographic profiles provide near-global coverage of the world’s ocean and are used to build a seasonal climatology of UOP properties – intensity, depth, thickness – to characterize the spatial and seasonal variations of upper ocean stratification. The largest stratification values O(10−3s−2) are found in the intertropical band, where seasonal variations of the UOP are also very small. The deepest (> 200 m) and least stratified O(10−6s−2) UOPs are found in winter along the Antarctic Circumpolar Current (ACC) and at high latitudes of the North Atlantic. The UOP thickness has a median value of 23 m with limited seasonal and spatial variations; only a few regions have UOP thicknesses exceeding 35 m. The UOP properties allow the characterization of the upper ocean restratification that generally occurs in early spring and is generally associated with large variability. Depending on the region, this restratification may happen gradually as around the Rockall plateau or abruptly as in the Kuroshio Extension. The UOP is also likely to merge intermittently with the permanent pycnocline in winter. The upper edge of the UOP is eventually close to MLD estimates, except in a few notable regions such as in the Pacific Warm Pool where barrier layers are important, and during wintertime at high latitudes of the North Pacific.

混合层深度（MLD）的气候学数据已通过基于温度/密度阈值或混合方法的多种定义提供。位于混合层底部之下的上层海洋跃层（UOP）仍缺乏充分表征，尽管这一过渡层是海洋表层普遍存在的特征。可用的水文剖面数据为全球海洋提供了近全面的覆盖，并用于构建UOP特性的季节性气候学——包括强度、深度、厚度——以描绘上层海洋层结的空间和季节性变化。在赤道带，发现了最大的层结值（O(10−3s−2)），而UOP的季节性变化也极小。在南极环流（ACC）沿线的高纬度和北大西洋的高纬度地区，发现了最深（> 200米）且层结最弱的O(10−6s−2) UOP。UOP的厚度中位数为23米，季节和空间变化有限；只有少数地区的UOP厚度超过35米。UOP特性使得能够描述上层海洋在春季早期普遍发生的再分层，这通常与大的变率相关。根据地区不同，这种再分层可能如洛克雷尔高原附近那样逐渐发生，或者在黑潮延伸区那样突然发生。UOP也可能会在冬季间歇性地与永久性跃层合并。UOP的上界最终接近MLD的估计值，但在太平洋暖池等少数著名地区，以及北太平洋高纬度地区的冬季，存在屏障层，因此情况例外。