The East Asian summer monsoon index (1851-2018)

The East Asian summer monsoon (EASM) and its variability involve circulation systems in both the tropics and midlatitudes as well as in both the lower and upper troposphere. Considering this fact, a new EASM index (NEWI) is proposed based on 200-hPa zonal wind, which takes into account wind anomalies in the southern (about 5⁰N), middle (about 20⁰N), and northern areas (about 35⁰N) of East Asia.
NEWI = Nor[u(2.5⁰–10⁰N, 105⁰– 140⁰E) - u(17.5⁰–22.5⁰N, 105⁰– 140E) + u(30⁰– 37.5⁰N, 105⁰– 140⁰E)]
where Nor represents standardization and u is JJA-mean 200-hPa zonal wind. When easterly anomalies appear around 20⁰N and westerly anomalies appear around 5⁰ and 35⁰N, the index is positive, and the EASM is stronger.
The NEWI can capture the interannual EASM-related climate anomalies and the interdecadal variability well. Compared to previous indices, the NEWI shows a better performance in describing precipitation and air temperature variations over East Asia. It can also show distinct climate anomalous features in early and late summer. The NEWI is tightly associated with the East Asian–Pacific or the Pacific–Japan teleconnection, suggesting a possible role of internal dynamics in the EASM variability. Meanwhile, the NEWI is significantly linked to El Niño–Southern Oscillation and tropical Indian Ocean sea surface temperature anomalies. Furthermore, the NEWI is highly predictable in the ENSEMBLES models, indicating its advantage for operational prediction of the EASM. The physical mechanism of the EASM variability as represented by the NEWI is also explicit. Both warm advection anomalies of temperature by anomalous westerly winds and the advection of anomalous positive relative vorticity by northerly basic winds cause anomalous ascending motion over the mei-yu–changma–baiu rainfall area, and vice versa over the South China Sea area. Hence, this NEWI would be a good choice to study, monitor, and predict the EASM (Zhao et al，2015，J Clim).