Principal Modes Of Zonal-Mean Flow And Their Connections With The AO,AAO And ENSO

The NCEP/NCAR reanalysis data have been employed to diagnose variations of the zonal mean flow in summer. The results are summarized as follows:(1)Two leading EOF modes are found to dominate the spatial and temporal changes of the boreal summertime (6-8 month) zonal-mean zonal winds in the troposphere. These two EOF modes are AO and ENSO related, respectively. EOF1 shows the distribution of zonal-mean flow related to AO, whereas the EOF2 shows the distribution of zonal-mean flow in association with ENSO. The EOF1 and EOF2 show respectively the periodicities similar to those of AO and ENSO. The quasi-geostrophic non-acceleration theorem can be used to partly explain the possible mechanisms of the maintenance of the two principal modes. The composite differences of E-P flux divergence between positive and negative years as indicated by the time series of EOF1 and EOF2 display the similar patterns as the EOF1 and EOF2 do, respectively. The planetary other than the synoptic waves dominate the behaviors of the E-P fluxes.(2)Only is one leading EOF mode (EOF1) found to dominate the spatial and temporal changes of the austral summertime (12-2 month) zonal-mean zonal winds in the troposphere. EOF1 relates to AAO and ENSO. In order to see the influence of AAO and ENSO on the Southern zonal-mean flow clearly, the Nino3 signal has been removed from the EOF1 using the regression method. Therefore, AEOF1 yields, which is independent of ENSO. Correlation analysis shows that AEOF1 displays a structure of the zonal-mean flow related to AAO. To examine the ENSO related component, the EOF analysis has been performed for the Southern zonal-mean flow from which the AEOF1 has been removed. SEOF1 has hence been obtained, which is highly correlated with Nino3 index (correlation coefficient: 0.9), SEOF1 suggests that the eastly in the tropics and the westly in middle latitudes be weakened (strengthened) in the Southern Hemisphere during the E1 Nino (La Nina) years.(3)The simultaneous correlations of EOF1 of boreal summer with both the summer rainfall and the temperature in China are not significant. The EOF2 that relates to ENSO does not well correlate with rainfall anomolies in China. However, it is significantly anti-correlated with the anomalous temperature in the Northern part of China; summertime temperature in North China is lower in E1 Nino years whereas it is higher in La Nina years.(4)AEOF1 showing the vacillations of the zonal winds in middle and high latitudes in the Southern Hemisphere, relating to the Antactic Oscillation, is strongly and negatively correlated with the boreal winter temperature in the northeastern and northern parts of China. No signigicant simultaneous correlation of AEOF1 with winter rainfall in China is observed. Checking in a similar way, the correlated with the winter rainfall in South China. No significant relation between SEOF1 and temperature changes in China is suggested.