Two novel indices of the East Asian Summer Monsoon (EASM): MVPI and DAI 1951-2025

Two novel indices for quantifying the East Asian Summer Monsoon (EASM) were proposed by Yang and Li (2025). The first is the monsoon vector projection index (MVPI), which is defined as the summer (JJA) difference in regional mean Monsoon Vector Projection (MVP) between a southern region composed of the four vertices: (105°E, 25.5°N), (105°E, 19.5°N), (121.5°E, 33°N) and (121.5°E, 27°N), and a northern region defined by (112.5°E, 36°N), (112.5°E, 30°N), (124.5°E, 42°N) and (124.5°E, 36°N). Fig. 1 shows the normalized time series of the MVPI of EASM. A positive MVPI indicates a stronger-than-average EASM with enhanced Meiyu-Changma-Baiu precipitation, while a negative value reflects a weaker EASM with reduced precipitation (Fig. 4).

Fig. 1 The normalized time series of the MVPI of EASM (1951–2025). The summer here is JJA.

The second index is the Directed Angle Index (DAI), calculated as the JJA difference in regional mean directed angle (DA) between a northern domain (31.5°–37.5°N, 100.5°–129°E) and a southern domain (25.5°–31.5°N, 109.5°–129°E). To filter out small-scale noise, a five-point-smoothing is applied to wind vector during the calculation of DA. Fig. 2 shows the normalized time series of the DAI of EASM.A positive DAI indicates anomalous cyclonic shear over the Meiyu-Changma-Baiu region, enhancing low-level convergence and contributing to above-average precipitation (Fig. 5). Conversely, a negative DAI reflects anticyclonic shear associated with suppressed convergence and below-average precipitation.

Notably, the MVPI and DAI exhibit an opposite sign to traditional EASM indices in representing monsoon strength; however, they offer a physically grounded interpretation by directly capturing the convergence intensity and directional shear over the Meiyu-Changma-Baiu region.

　Fig. 2 The normalized time series of the DAI of EASM (1951–2025).

There is an apparent positively correlation between the MVPI and DAI summer (JJA) rainfall in the Meiyu-Changma-Baiu band, indicating wet years over the band are associated with the strong EASM and drought years with the weak EASM (Fig. 3).

 Fig. 3 (a1)–(a2) Correlation coefficient maps (shading) between JJA GPCM (mainland China) precipitation anomalies and the MVPI and DAI of EASM for 1951-2023, respectively. The regions with oblique lines denote the significant values at the 95% confidence level, based on the Student’s t-test. (b1)–(b2) and (c1)–(c2) are the same as (a1)–(a2), but for JJA GPCC and CMAP (East Asia) precipitation anomalies, respectively.

Fig. 4 Composite GPCC precipitation (mm) in (a) strong and (b) weak EASM years defined by the MVPI.

Fig. 5 as in Fig. 4, but for the DAI.

References

• Yang, Y. N., and J. P. Li*, 2025: Novel monsoon indices based on vector projection and directed angle for measuring the East Asian summer monsoon. Clim. Dyn., 63, 210. https://doi.org/10.1007/s00382-025-07696-7. PDF