Study Of Impact Process Of Global And Local Atmosphere Anomaly Caused By ENSO On Precipitation During The Meiyu Period

The relations between the Meiyu rainfall (MYR) and pre-winter sea surface temperature (SST) and relations between MYR and circulation during the Meiyu period are studied by using singular value decomposition (SVD), extend singular value decomposition (ESVD), correlation, composition and linear regression methods with 730 China stations ten-days mean rainfall, NCEP/NCAR daily and monthly mean reanalysis data, extended reconstruct sea surface temperature (ERSST). The formation processes of EAP during the Meiyu period caused by ENSO are discussed. And the actions of global and regional circulation anomalies aroused by ENSO are studied too. In the end we carried some numerical experiments to model the formation of EAP teleconnection by using IAP 9L AGCM. Main conclusions are as follows:1) The SVD results of the MYR and pre-winter SST show: the MYR over the Yangtze River reaches not only has tight relation to the middle and east equatorial Pacific SST, but to other regions SST locked with ENSO such as equatorial Indian Ocean, warm pool, Kuroshio extent, oyashio extent et al.. The MYR anomalies are results long corresponding to the abnormal SST patterns during the ENSO mature phase.2) The SVD results of the MYR and homochronous circulation show: the MYR has tight relation to the EAP teleconnection formed by west pacific subtropical high, the Meiyu trough and the Okhotsk high. It is the three circulation systems work wholly that leads to the MYR anomalies.3) The ESVD results of the prewinter SST and December to June Geopotential height at 500 hPa level show: from December to June the height is above normal over the tropical Indian Ocean and Pacific, which resulting in the west subtropical high stronger than normal. And the atmosphere circulation over middle and high latitude also has considerable continuities, and transforms in some manners. During April and May the correlation pattern: positive in low latitude, negative in middle latitude and positive in high latitude correlation has come into being by and large over the eastern Asia and west pacific. These results furtherly confirm that the EAP teleconnection which has tight relation to the MYR over the Yangtze River is the results corresponding to the abnormal SST patterns during the ENSO mature phase too. 4) Via the ESVD anaylisis, we put forward that there are three key processes linking prewinter ENSO to the EAP teleconnection during the Meiyu Period. These key processed are as follows:(a) The anti cyclone's forming and maintaining leads to the west Pacific subtropical high more southward and stronger than normal.(b)The East Asia trough keeps continually weaker than normal, which favors the cold air moving path more eastward and northward. Such circulation benefits Oyashio SST lower and Kuroshio SST higher. And such SST anomalies distribution over the west Pacific is very important to the formation of EAP teleconnection during the Meiyu period. (c)The continual development of PNA teleconnection causes the height to increase over Arctic regions, which offers a favored background for the formation of Okhotsk high.5) The evolvement of time series in SVD and ESVD results show that the MYR, EAP teleconnection and SST anomalies Pattern in prewinter are not only consistent in interannual scale, but in interdecadal scales. They all experience a change from negative period to positive period in the end of 1970's.6) The analysis of the anomalies of global winter 1000-500hPa thickness for El Nino and La Nina year respectively shows that during the El Nino mature phase, the mean temperature increases in low troposphere over the global tropical regions, not just located over Indian Ocean and Pacific, increasing amplitude about 1.0~1.5 standardized deviations. At the same time the temperature decreases in the middle and high latitude regions of both hemispheres, decreasing amplitude about -0.5~1.0 standardized deviations, the 1000-500 hPa thickness anomalies over the tropical regions are opposite to the anomalies over middle and high latitude in sign. Results for La Nina are similar to El Nino, but the sign is in reverse to El Nino. This further confirms that the global influence of ENSO. For El Nino phase the longitudinal gradient of Temperature's is enlarged, which is favorable for the zonal circulation development. On the contrary, the situation is favorable for longitudinal circulation development in La Nina phase.7) The evolvement of 500 hPa circulation indexes for global, north hemisphere and south hemisphere from 1961-2000 shows that they are all consistent with SST over Nino3 area. The correlation coefficients between these indexes and Nino3 SST are 0.82(Global), 0.90 (South hemisphere) and 0.82 (North hemisphere), all above the 99.9% confident level. It shows that all 500 hPa circulation indexes (Global, South hemisphere and North hemisphere) vary in a cycle about 3~5 years.8) The 3~5 years circulation index cycle forced by ENSO is different with the high and low index cycle in period about some weeks caused by the internal dynamics of the atmosphere. Because the 3~5 years circulation index cycle represents certain circulation styles at the same time. As for the Asia-Pacific region, the East Asia is weaker, an anticyclone hovers over the West tropical Pacific, and the PNA teleconnection develops over East Pacific-North America in El Nino phase; and opposite in La Nina phase. So the circulation forced by El Nino offers a very important background for the formation of EAP teleconnection during the Meiyu period.9) The analysis of continuities of circulation and SST during ENSO decaying year shows that the continuities of circulation and SST are greater in ENSO decaying years than in normal years. Further studies show that the greater continuities of circulation and SST are due to the stronger coupling relations between circulation and SST in ENSO decaying years, which slows down the evolvement speed of circulation and SST , leading to stronger continuities of circulation and SST in ENSO decaying years than in normal year.10) The study about regional air-sea action over north west Pacific shows that the coupling and influencing on each other between air and sea, leads to SST lower over warmpool, higher over Kuroshio and lower over Oyashio, presenting a anomalies feature that negative over warmpool, positive over Kuroshio and negative over Oyashio from winter to early summer in El Nino decaying year. This SST anomalies distribution forming and continuing over West Pacific leads to the formation of EAP teleconnection in early summer.11) The numerical experiment succeed in modeling the formation of EAP teleconnection and PNA teleconnection in early summer, and the numerical experiment also succeed in modeling the precipitation anomlies over east China in early summer of ENSO decaying year. These results furtherly prove that the EAP teleconnection and precipitation anomlies ove the Yangtze River valleys are response of atmosphere circulation over East Asia to ENSO SST anomlies remote forcing in long time period.12) A possible mechanism about the influence of ENSO on the MYR is brought forward in the end of this paper.