The annular mode at the surface, referred to as the Arctic Oscillation (AO) in the Northern Hemisphere and the Antarctic Oscillation (AAO) in the Southern Hemisphere, is statistically defined by the leading mode of variability of sea level pressure (SLP) or geopotential height. Its principal component time series is set as the AO or AAO index. Although this metric is widely used, it needs an empirical orthogonal function analysis which introduces the complexity in the multi-model analysis. As an alternative measure, the zonal index (ZI), which is traditionally defined as the zonal-mean SLP difference between the two reference latitudes, has also been used. Here we re-evaluate the interannual and trend relationships between the ZI and the AO/AAO index using the two reanalysis datasets and 35 climate model simulations for both the present and future climate. For all datasets, the spatio-temporal variability of the Southern-Hemisphere ZI is almost identical to that of the AAO index. The ZI is still useful to examine the AO-related circulation variability and change in the Northern Hemisphere but mostly in the cold season. The data-dependent ZI, optimized for each data, exhibits a rather weak relationship with the AO index. This result suggests that the interannual variability and long-term trend of the midlatitude circulation can be concisely quantified by computing the fixed-latitude ZI in all seasons in the Southern Hemisphere and in the cold season in the Northern Hemisphere, not only for reanalysis data but also for climate model datasets which have significant biases varying from model to model.

从表面上看，环形模式在北半球称为北极涛动（AO），在南半球称为南极涛动（AAO）。高度。其主成分时间序列设置为AO或AAO索引。尽管此度量标准已被广泛使用，但它需要经验正交函数分析，这会在多模型分析中引入复杂性。作为一种替代措施，还使用了纬向指数（ZI），传统上将其定义为两个参考纬度之间的纬向平均SLP差异。在这里，我们使用两个重新分析数据集和针对当前和未来气候的35个气候模型模拟，重新评估ZI和AO / AAO指数之间的年际关系和趋势关系。对于所有数据集，南半球ZI的时空变异性几乎与AAO指数的时空变异性相同。ZI仍可用于检查北半球（主要是在寒冷季节）与AO相关的循环变化和变化。为每个数据优化的数据相关ZI与AO索引之间的关系很弱。这一结果表明，可以通过计算南半球所有季节和北半球寒冷季节的固定纬度ZI来精确量化中纬度环流的年际变化和长期趋势，