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Study Potential Impact of EI Ni(?)o Events of Two Types on the Following Summer Precipitation in the Eastern China

Author: QinJianZhao
Tutor: WangYaFei
School: Chinese Academy of Meteorological Sciences
Course: Meteorology
Keywords: EP-El Ni(?)o CP-El Ni(?)o OKJ pattern circulation WNPSH
CLC: P732
Type: Master's thesis
Year: 2014
Downloads: 7
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As we all known, there are two types of El Ni o, that is Eastern Pacific type of El Ni o(EP-El Ni o) and Central Pacific type of El Ni o (CP-El Ni o), whose warming centerslocated in equatorial Eastern and Centre Pacific, respectively. It is very important significanceto study distribution of precipitation over China in two types of ENSO decaying summer.Therefore, firstly, this study defines two indices for two types of ENSO, according to theirdifferent equatorial Pacific sea surface temperature (SST) patterns. Secondly, from the view ofa large-scale wave energy propagation and western Pacific subtropical high (WPSH), usingthe new indices to discuss circulation and rainfall anomalies over China in following summerfor two types of El Ni o. Finally, using model simulation, investigating the following summeratmosphere response to tropical Pacific SST anomalies for two types of preceding El Ni o, inorder to supplement and validate diagnostic analysis above. The present study consists of thefollowing three parts:1、Taking into account the different equatorial Pacific sea surface temperature (SST)patterns obtained from a combined EOF-regression analysis for EP-ENSO and CP-ENSO,this study constructs IEP(EP-ENSO Index) and ICP(CP-ENSO Index) for two types of ENSO.The analysis is performed for the period of1950—2011using the monthly mean Met OfficeHadley Centre SST analyses data. Some conclusions are as follows. Firstly, the two indicesare nearly orthogonal and the skewness coefficients for IEPand ICPare1.56and-0.43,respectively. In addition, EP-ENSO and CP-ENSO show different periods. The former hastwo dominate periods, one is near2a band and the other is near4—8a band. The latter hastwo signifcant powers around2—4a and12a band from1965to1995. Therefore, lowcorrelation, different leading periodicity and skewness are all desirable properties for theabove defined indices to separate the EP and CP type of ENSO. Secondly, IEPand ICPindicescan describe location of SST anomalies in the mature period of two types of El Ni o and LaNi a events. Thirdly, from the perspective of probability and statistics of random events, twotypes of El Ni o and La Ni a events are more strictly defined by using this pair of newindices, which could be applied to monitoring two types of ENSO effectively and in real-time.Finally, the two new indices can also depict characteristics of the equatorial Pacific SST zonal propagation and evolution during ENSO events, their running lagged correlations capturedifferent ENSO phase propagations and ENSO regime changes associated with the climateshift in1976/1977. It is also noticed that most strong El Ni o events belong to EP type, butmost strong La Ni a events are CP type, and CP type of La Ni a events tend to occur after EPtype of El Ni o events.2、Using multiple observed data, this part focuses its analysis on the evolutioncharacteristics of SST anomaly (SSTA) for EP-El Ni o or CP-El Ni o, and tropicalatmospheric response to them, respectively, and the relationship between two types of El Ni oand the following summer precipitation over China is further investigated. The results asfollows: firstly, two types of El Ni o show different development and evolution features ofSSTA. Associated with the SSTA, distinct response of the anomalous Walker Cell overtropical Pacific-Indian Ocean. Secondly, for the case of EP-El Ni o, Okhotsk-Japan (OKJ)teleconnection pattern exists in mid-latitude in June and July during warm events decayingyear. This pattern linked with the development of Okhotsk High could keep a steadycirculation that suppresses northward progress of the Western North Pacific Subtropical High(WNPSH) and Meiyu front. WNPSH brings plentiful moisture to the south of the YangtzeRiver region, resulting in enhanced rainfall in that domain. On the contrary, for CP-El Ni o,anomalous cyclonic will appear in Okhotsk Sea and WNPSH’s location is easterly to normal,which induces water vapor transport to South China, accompanied by no obvious rainfallanomalies in south of the Yangtze River in June during warm events decaying year, thecirculation over East Asian in July is similar to the last month, but WNPSH’s location isnortherly and water vapor transports to North China. Thirdly, the influence of two types ofwarm and cold events on rainfall anomalies is asymmetric during Meiyu season in south ofthe Yangtze River. In terms of EP-ENSO, the relationships are only statistically significant forpositive events, while there is no statistically significant for CP-ENSO.3、Using the NCAR CAM3.0model, we designed several sensitive experiments tosimulate their response of the atmospheric circulation and precipitation in summer to SSTanomalies (SSTA) over the equatorial Eastern or Central Pacific for EP El Ni o or CP El Ni o,respectively. The result as follows: Firstly, when forced with realistic, time-varying SSTanomalies from event developing September to decaying August, model successfullyreproduced circulation and precipitation over East Asian. This confirms previous diagnosticanalysis. Secondly, when SST anomaly in May can continued to August, the simulated resultsare similar to observation, which proved that the two types of El Ni o events have laggedeffect on circulation and precipitation over China. Lastly, model with strong or weak SSTanomaly, it found that for EP-El Ni o, forced with the stronger SST anomaly, the morerainfall following summer in south of the Yangtze River, and when the SST anomaly is strong enough, there is significant positive rainfall anomalies in the region, which is close to theactual; while for CP-El Ni o, accompanied with different SST anomaly, precipitation appearsno certain evolution trend in the above region.

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