Quantifying spread in spatio-temporal changes of upper-ocean heat 1 content estimates: 2 an internationally coordinated comparison

The Earth system is accumulating energy due to human-induced activities. More than 90% of this energy has been stored in the ocean as heat since 1970, with ~64% of that in the upper 700 m. Differences in upper ocean heat content anomaly (OHCA) estimates, however, exist. Here, we evaluate spread in upper OHCA estimates arising from choices in instrumental bias corrections and mapping methods, in addition to theeffect of using a common ocean mask. The same dataset was mapped by six research groups for 1970–2008, with six instrumental bias corrections applied to expendable bathythermograph (XBT) data. We find that use of a common ocean mask may impact estimation of global OHCA by 2–13%. Uncertainty due to mapping method dominatesover XBT bias correction at a global scale and is largest in the Indian Ocean and in the eddy-rich and frontal regions of all basins. Uncertainty due to XBT bias correction is largest in the Pacific Ocean within 30°N–30°S. In both mapping and XBT cases,spread is higher since the 1990s. Important differences in spatial trends among mapping methods are found in the well-observed Northwest Atlantic and the poorly observed Southern Ocean. Although our results cannot identify the best mapping or bias correction schemes, they identify where and when greater uncertainties exist, and so where further refinements may yield the largest improvements. Our results highlightthe need for a future international coordination to evaluate performance of existing mapping methods.

受人类活动影响，地球系统正持续积累能量。自1970年以来，该积累能量中超过90%以热能形式储存在海洋中，其中约64%储存在700米以浅的上层海洋。不过，上层海洋热含量距平（upper ocean heat content anomaly, OHCA）的估算结果存在差异。为此，我们评估了由仪器偏差校正方案选择、制图方法选取，以及使用通用海洋掩膜所带来的上层OHCA估算结果的离散程度。本研究针对1970–2008年的同一数据集，由6个研究团队分别开展制图，并对投弃式温深计（expendable bathythermograph, XBT）数据采用6种不同的仪器偏差校正方案。研究发现，使用通用海洋掩膜可能会使全球OHCA的估算结果产生2%–13%的偏差。在全球尺度上，制图方法带来的不确定性大于XBT数据偏差校正带来的不确定性；该不确定性在印度洋以及所有海盆的多涡区与锋面区域最为显著。而XBT偏差校正带来的不确定性，则在南北纬30°之间的太平洋海域最为突出。无论是制图方法还是XBT数据校正方案所带来的离散程度，自20世纪90年代以来均有所升高。在观测资料充足的西北大西洋以及观测资料匮乏的南大洋，不同制图方法得到的空间趋势存在显著差异。尽管本研究无法确定最优的制图方案或偏差校正方案，但我们明确了不确定性较高的时空分布区域与时段，从而指明了进一步优化可带来最大提升效果的方向。本研究结果凸显了未来开展国际协作以评估现有制图方法性能的必要性。