Comparison Of Characteristics Of 10-20d Low-frequency Oscillation Of Summer Precipitation In Eastern China Of Two Types Of ENSO

The low-frequency circulation characteristics of summer rainfall in eastern China in decaying year of two types of ENSO events were compared and analyzed,using the monthly mean precipitation data of 160 meteorological observation stations and the daily mean precipitation data of 756 meteorological observation stations in China,HadISST monthly mean Sea Surface Temperature reanalysis data,and daily mean reanalysis data from NCEP/NCAR.Wavelet analysis and power spectrum were used to determine the low-frequency period of precipitation while Lanczos filter was used for bandpass filtering.The correlation between the 10-20 d low-frequency oscillation of atmosphere and the low-frequency precipitation in eastern China was discussed through statistical methods such as synthetic analysis,and the main conclusions were as follows:(1)In the following summer of EP type of El Ni?o(La Ni?a)events,precipitation in eastern China presents a three-pole-type of “+-+”(“-+-”)inverse distribution from north to south while precipitation during the following summer of CP type of El Ni?o(La Ni?a)events in eastern China presents a dipole-type of “+-”(“+-”)inverse distribution;The Western Pacific Subtropical High(WPSH)and the South Asia High(SAH)in the following summer of El Ni?o events are much stronger than climatological normal while during the following summer of La Ni?a events,the intensity of the WPSH and the SAH was weaker than climatological normal.10-20 d was the prominent oscillation period of summer precipitation in eastern China during decaying year of both types of ENSO events.In the extremely active phase of low-frequency precipitation of El Ni?o(La Ni?a)events,zonal positions of the SAH and the SWPH were near(far),while low-level southwest airflow was significant and vertical rise motion was strong(weak).(2)Low-frequency oscillations in different levels of troposphere had different effects on 10-20 d low-frequency precipitation of CP ENSO.1)In the upper troposphere,low-frequency circulation system and low-frequency divergence field matched well,low-frequency cyclone(anticyclone)was always accompanied by low-frequency convergence(divergence).Low frequency wave train of CP El Ni?o(La Ni?a)maintained for a long(short)time.In the extremely active phase of low-frequency precipitation,eastern China were dominated by low-frequency divergence and positive(negative)anomaly of low-frequency height,and there was a strong(weak)northwest-southeast wave-active flux(WAF)transport.2)In the middle troposphere,the range and intensity of the SWPH during decaying year of CP El Ni?o was larger and stronger than that of CP La Ni?a,and the low-frequency height field anomaly affected the location of the SWPH and the East Asia coastal ridge and the East Asian trough.3)In the lower troposphere,the main circulation system affecting low-frequency summer precipitation of CP El Ni?o was the low-frequency cyclone over eastern China,while the low-frequency cyclone over northern China and the low-frequency anticyclone over the South China Sea were the crucial systems of CP La Ni?a,which affecting low-frequency convergence and divergence in eastern China.4)Low-frequency water vapor transport in the lower troposphere and low-frequency vertical movement also played an important role in low-frequency precipitation.The low-frequency water vapor circulation system and the warm-wet moisture transport channels that affected low-frequency precipitation of CP El Ni?o and La Ni?a were significantly different,and low-frequency vertical movement corresponding to the low-frequency precipitation process of CP El Ni?o was stronger and broader than that of CP La Ni?a,causing the low-frequency precipitation also stronger.(3)Atmospheric low-frequency circulation characteristics of low-frequency precipitation during the next summer of EP ENSO were as follows: 1)In the upper troposphere,the main circulation systems affecting low-frequency summer precipitation of EP El Ni?o were the low-frequency cyclone in the downstream of Heilongjiang and the low-frequency anticyclone in southern China,making the middle reach of Yangtze river in strong low-frequency divergence control,with low-frequency highly negative anomalies and strong southwestward(westward)WAF transport controlling in the same region;while the low-frequency cyclone over northern China and the low-frequency anticyclone over the South China Sea were the crucial systems of CP La Ni?a,causing low-frequency convergence in eastern China.Low-frequency highly negative anomalies in the middle reach of Yangtze river,has weaker southwest WAF energy delivery.2)In the middle troposphere: the low-frequencyheight field anomaly affected the evolution of the WPSH and the baikal ridge.The deepening of EP type El Ni?o in east Asia was mainly due to the enhancement of low-frequency height negative anomaly along the east Asian coast,while the deepening of EP La Ni?a in east Asia was mainly due to the development of low-frequency height negative anomaly in central and western China.3)In the lower troposphere: the circulation system affecting the low-frequency precipitation of EP El Ni?o in summer were the low-frequency anticyclone near Heilongjiang and the low-frequency cyclone in eastern China,leading to the low-frequency convergence airflow in the middle reaches of the Yangtze river;The summer low-frequency precipitation of EP La Ni?a were due to the combination of low-frequency cyclone in south China,low-frequency anticyclone near Japan island and low-frequency anticyclone in south China sea.4)low-frequency water vapor and low-frequency vertical motion: during the next summer of EP ENSO,low frequency water systems and water vapor have significant differences,the strength of low-frequency vertical motion EP El Ni?o was stronger compared with EP La Ni?a,so the low-frequency rainfall of the same phase was also stronger.