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Intraseasonal Variation of East Asian Summer Monsoon in La Niña Years

Feb 21, 2017

ENSO (the El Niño–Southern Oscillation) consists of two basic modes, i.e., El Niño and La Niña. There is a higher sea surface temperature (SST) in the tropical Pacific in El Niño years and a lower SST in La Niña years. A large body of evidence has shown that the western Pacific subtropical high (WPSH), during El Niño decaying summers, tends to extend westward with stronger intensity, resulting in more rainfall over the Yangtze River basin. By comparison, the influence of La Niña on the East Asian summer monsoon (EASM) is weak, which is simply ascribed to an anti-symmetric aspect of El Niño.
Based on multi-year reanalysis data, Prof. XUE Feng from Institute of Atmospheric Physics and his student ZHAO Junjie studied the intraseasonal variation of the EASM and summer rainfall over East Asia in La Niña years (Xue and Zhao, 2017). They find that owing to a higher SST over the western Pacific warm pool in the preceding winter and spring, warm pool convection in summer is enhanced.

As a result, the WPSH extends more northeastward. This anomalous pattern tends to change with the march of the seasons, with a maximum anomaly in July (Figure 1). The rainfall distribution in East Asia in La Niña summers is characterized by a zonal pattern of less rainfall over eastern China and more rainfall over the oceanic region of the western Pacific. By comparison, a meridional pattern is found in El Niño summers, with less rainfall over the tropics and more rainfall over the subtropics and mid-latitudes. 

 

Figure 1. Rainfall anomaly in La Niña years (units: mm d−1) and the 500 hPa western Pacific subtropical high (units: gpm). Regions with statistically significant results at the greater than 95% confidence level are shaded. The black and red contours represent the WPSH in La Niña years and the climatological mean, respectively. (Image by Xue and Zhao, 2017)

They also find that owing to an earlier eastward retreat of the WPSH in La Niña summers, the seasonal march in East Asia is accelerated, with an earlier onset and termination of the mei-yu season and less rainfall over the Yangtze River basin. Therefore, a La Niña signal is useful for mei-yu prediction. By contrast, the maximum anomaly in El Niño summers is found in August.  

"Except in some strong El Niño years, an El Niño signal is of little use to mei-yu prediction, although El Niño signals are statistically stronger than La Niña signals." Prof. XUE observed, "The influence of La Niña is not anti-symmetric to that of El Niño." 

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