Vertical Structure of Zonal‐Mean Circulation Anomalies in the Southern Hemisphere in Austral Summer and Its Relationship with AAO and ENSO

Abstract

The anomalous vertical structure of the austral summer (December∼February) zonal mean circulation [u] in association with Antarctic Oscillation (AAO) and ENSO has been investigated by means of 1979∼2004 NCEP/NCAR monthly reanalysis data. The Empirical Orthogonal Function (EOF) analysis has been performed for the zonal mean zonal wind anomalies in the present article. It is found that the EOF1 dominates the anomalous distribution of [u] in vertical with a positive-negative-positive sign pattern in the polar region, 70 • S∼50 • S and regions from 50 • S to the equator, respectively. The EOF1 exhibits the AAO signature and a close linkage with ENSO. The AAO is found to be statistically correlated with Nino3 index. To further clarify the effects of AAO and ENSO upon [u] variability in conjunction with the resulting structures, a one-variable regression scheme is utilized through Nino3 index to filter out the ENSO impact, and the remaining portion is decomposed with EOF analysis, obtaining AEOF1 for the [u] without the ENSO effect involved. It is displayed that AEOF1 represents the anomalous pattern of AAO-associated [u]. The residual of [u] after removing AAO-associated AEOF1 is analyzed by means of EOF, leading to a mode SEOF1, which is correlated with Nino3 index with a correlation coefficient of 0.9, thereby resulting in modes of [u] unrelated and related to ENSO. Besides the interdecadal and 3∼7-year interannual variations, the zonal mean zonal flow [u] shows weakening westerlies in the polar region, enhanced westerlies in the sub-polar zone, enfeebling subtropical westerlies and intensifying tropical easterlies as the long-term trends during the past 30 years.