Interannual variability of the upper tropospheric circulation

Seventeen years of sea level pressure (SLP), 200-hPa zonal wind and 500-hPa geopotential height data were used to investigate the boreal winter and su mmer interannual (LA) circulation patterns. The IA patterns for these varia bles and for their zonally asymmetric (ZA) part were determined by performi ng empirical orthogonal function (EOF) analyses on the SLP and on ZA SLP. T he corresponding patterns for the other variables were obtained by correlat ing their time series with the amplitude time series of these EOF analyses. For both seasons, the SLP and ZA SLP show a zonal wavenumber one pattern e xtending from the tropics into the winter hemisphere extratropics, which is consistent with the circulation anomalies related to the El Nino/Southern Oscillation (ENSO) cycles. The zonal wavenumber one pattern observed for th e boreal winter describes the SLP and ZA SLP variations related to the matu re state of the El Nino and La Nina episodes, and that for the summer, the SLP and ZA SLP variations associated with the initial or decay stages of th ese phenomena. The 200-hPa zonal wind and 500-hPa geopotential height patte rns exhibit strong seasonal dependence, and the ZA parts of these two varia bles show even more pronounced seasonal differences. These results indicate that the seasonal cycle of the atmospheric circulation, in particular at t he upper tropospheric levels, might play an important role in extending the IA wavetrain-like structure into the subtropics as noted for the 200-hPa z onal wind and its ZA part in the Pacific/Americas sector. This wavetrain-li ke structure shows its Southern Hemisphere (SH) and Northern Hemisphere (NH ) branches for the boreal winter, and only its SH branch, for the boreal su mmer. Thus, the effects of the seasonal cycle of the atmospheric circulatio n on the IA patterns seem to be stronger for the NH.