Principal Modes of Summertime Zonal-Mean Flow and Their Connections with the AO and ENSO

The NCEP/NCAR reanalysis data have been employed to diagnose variations of the zonal mean flow in boreal summer.Two leading EOF modes are found to dominate the spatial and temporal changes of the summertime zonal mean winds in the troposphere.EOF1 shows the distribution of zonal-mean flow anomalies with higher variance in the North Polar Region,whereas the EOF2 shows the distribution of zonal-mean flow anomalies with higher variance in tropical and extra-tropical regions.The EOF1 and EOF2 have respectively the periodicities similar to those of AO and ENSO.Significant lag correlations have been found between EOF1 and ENSO,and between EOF2 and AO,in the seasons including spring,autumn, and winter.However,no significant correlations have been found between EOF1 in summer and ENSO in any other seasons,and between EOF2 in summer and AO in other seasons,no matter how big the lag that represents number of seasons has been set.These results suggest that the principal modes of summertime zonal mean flow could be statistically separated from each other.Hence,EOF1 and EOF2 are physically related to the AO and ENSO respectively.A theory called quasi-geostrophic non-acceleration theorem has been used to partly explain the possible mechanisms of the maintenance of the two principal modes.The composite differences of the divergence of Eliassen-Palm flux(E-P flux) between positive and negative years as obtained from the time series of EOF1 and EOF2 display the distributions that contribute to the zonal mean wind anomalies represented by EOF1 and EOF2,respectively.The planetary other than the synoptic waves dominate the behaviors of the E-P fluxes,suggesting the crucial role of the planetary waves in the maintenance of the zonal mean flow anomalies.The residual circulation as well as the friction,which cancel the divergence of the E-P flux,also play an important role in some places.These results are very helpful for our better understanding how the anomalous zonal mean flows maintain and how the ENSO and AO influence the global climate variations.