东亚夏季风指数（1851-2021）

东亚夏季风是北半球夏季地球气候系统最活跃的成员之一，指数是研究东亚夏季风的重要工具。东亚夏季风及其变率既包含了赤道和中纬度的环流系统，又涉及对流层低层和高层的系统。基于此，用200hPa纬向风场定义了一个新的东亚夏季风指数。
NEWI = Nor[u(2.5°–10°N, 105°– 140°E) - u(17.5°–22.5°N, 105°– 140E) + u(30°– 37.5°N, 105°– 140°E)]
其中，Nor表示标准化，u表示200hPa纬向风场。
该指数可以很好地抓住东亚夏季风的年际和年代际变率。与之前的指数相比，新指数在描述东亚降水和温度方面有更好的表现，还可以表征前夏和后夏的不同气候异常特征。另外，新指数与东亚-太平洋遥相关型（太平洋-日本遥相关型）有很好的联系。同时，新指数与ENSO以及赤道印度洋海表温度联系密切，在观测和模式中都能得到很好的预测。利用NCEP，ERA，20CR高空200hPa纬向风场数据计算了1851-2021年东亚夏季风指数，指数强代表长江流域降水偏多。新指数对东亚夏季风的研究、监测和预测有重要意义；同时，该指数可用于古气候研究、模式评估以及季风的未来预估方面（Zhao et al，2015，J Clim）。

The East Asian Summer Monsoon (EASM) is one of the most active components of the Earth’s climate system during boreal summer, and monsoon indices serve as critical tools for EASM research. The EASM and its variability encompass both equatorial and mid-latitude circulation systems, as well as systems in the lower and upper troposphere. Against this backdrop, a new EASM index is defined using the 200-hPa zonal wind field:
NEWI = Nor[u(2.5°–10°N, 105°–140°E) - u(17.5°–22.5°N, 105°–140°E) + u(30°–37.5°N, 105°–140°E)]
where Nor denotes normalization, and u represents the 200-hPa zonal wind field. This index well captures the interannual and interdecadal variability of the EASM. Compared with existing indices, the new index performs better in describing East Asian precipitation and temperature, and can also characterize distinct climate anomaly patterns between early and late summer. Additionally, the new index exhibits a strong correlation with the East Asia-Pacific (EAP) teleconnection pattern (also known as the Pacific-Japan (PJ) teleconnection pattern). Meanwhile, the new index is closely linked to ENSO and the sea surface temperature (SST) in the equatorial Indian Ocean, and can be well predicted in both observational datasets and numerical models. The 1851–2021 EASM index was calculated using 200-hPa zonal wind field data from NCEP, ERA, and 20CR reanalysis datasets. A strong index value corresponds to above-normal precipitation over the Yangtze River Basin. The new index holds significant implications for EASM research, monitoring and prediction; meanwhile, it can be applied to paleoclimate research, model evaluation, and future projection of monsoons (Zhao et al., 2015, *Journal of Climate*).