Impacts of pure ENSO events on Summer climate of China

Many scientists have examined the ENSO impacts on summer monsoon variations in China in recent decades, as mentioned in section 4.1-4.3 in this Chapter. However, they had never linked the IOD with summer climate conditions of China before the IOD was discovered. To understand more clearly the ENSO impacts on China climate, it must be helpful to have the IOD signals removed from the data before analyzing the ENSO influences. In some studies (e.g., Ashok et al., 2001; Saji and Yamagata, 2003b; Guan et al., 2003), the partial correlations are employed to assess the individual influences of IOD and ENSO. Here, we do the composites instead to analyze the individual influences for showing magnitudes of the anomalies of the climate variables.

The differences of composite variables in 'pure' El Niño years from those in 'pure' La Niña years are plotted in Fig.4.7. In 'pure' El Niño years, more rainfall is received in reaches of Yangtze River (Fig.4.7a) whereas less rainfall in reaches of Yellow River. Vertically integrated water vapor or precipitable water (Fig.4.7b) is significantly less than normal in the northern and northeastern parts of China, India, and western part of north Pacific, but it is relatively more in reaches of Yangtze River and the southern part of China. This distribution of water vapor anomalies facilitates more rainfall in this region. Vertically integrated water vapor transports (Fig.4.7c) show clearly the convergence of water vapor in Reaches of Yangtze River whereas divergence in the northern part of China, South China Sea, the western North Pacific, and Indian continent, being consistent with the anomalous rainfall pattern as shown in Fig. 4.7a. Usually, more clouds are expected where more rainfall is received, which hence induces less shortwave radiation at the earth surface, and possibly more evaporation at a wetter surface in this area. These lead to significant air temperature changes as seen in Fig.4.7d and lower temperatures appear in Reaches of Yangtze River and the northeastern part of China.

Fig. 4.7 JJA mean differences of composite anomalies of physical quantities between "pure" El Niño years and "pure" La Niña years. Values above 90% level of confidence are shaded using the Student's t test. (a) The rainfall (in mm-month-1), (b) the moisture content (in kg) which is vertically integrated from the earth surface up to 300hPa, (c) the velocity potential of the moisture flux (in 107kg.s-1), and (d) the air temperature (in °C). From Yang et al. (2007).

Fig. 4.7 JJA mean differences of composite anomalies of physical quantities between "pure" El Niño years and "pure" La Niña years. Values above 90% level of confidence are shaded using the Student's t test. (a) The rainfall (in mm-month-1), (b) the moisture content (in kg) which is vertically integrated from the earth surface up to 300hPa, (c) the velocity potential of the moisture flux (in 107kg.s-1), and (d) the air temperature (in °C). From Yang et al. (2007).

These results above have reconfirmed that the ENSO events indeed have significantly simultaneous influences on the Summer climate of China. The anomalous rainfall pattern looks similar to the pattern occurred in a developing stage of El Niño events as discussed in section 4.2.1. This reconfirmation is very convincing and encouraging. Those results are helpful for predicting and understanding variabilities of the summer climate in China on inter-annual time scales as long as the cases of IOD-ENSO cooccurrence are kicked out.

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