Relationship between spring Arctic Oscillation and East Asian summer monsoon

The spring Arctic Oscillation (AO) is considered one of the most important precursory signals for the East Asian summer monsoon (EASM), although the relationship appears to be unstable. Gao et al. (2014) found a sharp transition in the correlation coefficient between these two variables from +0.77 during 1979–1997 to −0.62 during 1998–2007. This sift was accompanied by a change in the propagation of the spring AO-associated wave activity from the high-latitude path (via the Eurasian continent to the North Pacific) to the subtropical path (via the Middle East toward South Asia).

Here, the AO-EASM relationship is reexamined using STARS. As in Gao et al. (2014), the EASM index was defined by the difference in mean May, June and July (MJJ) zonal wind anomalies at 850hPa between two areas (5N–15N, 90E–130E) and (22.5N–32.5N, 110E–140E). The spring (MAM) AO index is the time coefficients of the first empirical orthogonal function (EOF) of the monthly SLP north of 20N. Unlike Gao et al. (2014), no high-pass filtering was applied. The data range for both indices was 1950-2014. Since no regime shifts in the mean and variance were detected, the correlation module in STARS was applied directly to the raw data.

As shown in Fig. 1 below, the correlation coefficient between AO and EASM was very low (r = 0.14) during 1948-1979, making the AO index practically useless in predicting EASM. During the 1980-1997 period, the correlation jumped to 0.65. This is slightly lower than the correlation coefficient found in Gao et al. (2014). Since the only difference with their work was the Butterworth filter they applied to the data, the difference in the correlation coefficients can probably be attributed to that. During 1980-2008, the correlation coefficient became negative (r = -0.27), although not as strong as in Gao et al. (2014), and not statistically significant even at the 70% confidence level. In more recent years (2009-2014), the correlation coefficient jumped again to strongly positive (r = 0.73). The p-values for these three shifts in the correlation coefficient (1980, 1998, and 2009) are all less than 0.05.

References

Gao, M.-N., J. Yang, D.-Y. Gong, and S.-J. Kim, 2014: Unstable relationship between spring Arctic Oscillation and East Asian summer monsoon. Int. J. Climatol., 34, 2522–2528, doi:10.1002/joc.3849.