The Pacific Walker circulation (PWC) is one of the most important components of large-scale tropical atmospheric circulations. The PWC and its influences have been studied extensively by numerical models and reanalysis. The newly released ERA5 and NCEP2 are the most widely used reanalysis datasets and serve as benchmarks for evaluation of model simulations. If the results of these datasets differ significantly, this could lead to a bias in projected long-term climate knowledge. For better understanding of future climate change, it is necessary to evaluate PWC reanalysis productions. As a result, we compared the PWC structures between the ERA5 and NCEP2 datasets from month to seasonal time scales. We used the zonal mass streamfunction (ZMS) over the equatorial Pacific to indicate the strength of the PWC. The PWC’s average monthly or seasonal cycle peaks around July. From February to June, the NCEP2 shows a higher PWC intensity, whereas the ERA5 shows greater intensity from July to December. The circulation center in the NCEP2 is generally stronger and wider than in the ERA5. The ERA5, however, revealed that the PWC’s west edge (zero line of ZMS over the western Pacific) had moved 10 degrees westward in comparison to the NCEP2. In addition, we compared the PWC mean state in the reanalysis and CMIP6 models; the mean state vertical structures of the tropical PWC in the CMIP6 multi-model ensemble (MME) are similar to those of the reanalyses in structure but weaker and wider than in the two reanalysis datasets. The PWC is broader in CMIP6, and the western boundary is 7 and 17 degrees farther west than in the ERA5 and NCEP2, respectively. This study suggests that, when using reanalysis datasets to evaluate PWC structural changes in intensity and western edge, extreme caution should be exercised.

中文翻译：

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再分析和 CMIP6 描绘的太平洋沃克环流结构

太平洋沃克环流（PWC）是大规模热带大气环流的最重要组成部分之一。PWC 及其影响已通过数值模型和再分析得到广泛研究。新发布的 ERA5 和 NCEP2 是使用最广泛的再分析数据集，可作为评估模型模拟的基准。如果这些数据集的结果显着不同，则可能导致预测的长期气候知识存在偏差。为了更好地了解未来的气候变化，有必要对 PWC 再分析产品进行评估。因此，我们比较了 ERA5 和 NCEP2 数据集之间从月到季节时间尺度的 PWC 结构。我们使用赤道太平洋上空的纬向质量流函数 (ZMS) 来指示 PWC 的强度。PWC 的平均月度或季节性周期在 7 月左右达到峰值。从2月到6月，NCEP2显示出更高的PWC强度，而ERA5从7月到12月显示出更高的强度。NCEP2 中的流通中心通常比 ERA5 中的更强大和更广泛。然而，ERA5 显示，与 NCEP2 相比，PWC 的西边缘（西太平洋上的 ZMS 零线）向西移动了 10 度。此外，我们比较了再分析和 CMIP6 模型中的 PWC 平均状态；CMIP6多模式集合（MME）中热带PWC的平均状态垂直结构与再分析的结构相似，但比两个再分析数据集中的更弱和更宽。CMIP6 中的 PWC 更宽，西部边界分别比 ERA5 和 NCEP2 向西偏西 7 度和 17 度。