EOF representations of the Madden-Julian oscillation and its connection with ENSO

Although recent El Nino events have seen the occurrence of strong intraseasonal winds apparently associated with the Madden-Julian oscillation (MJO), the usual indices of interannual variability of the MJO are uncorrelated with measures of the ENSO cycle. An EOF decomposition of intraseasonal outgoing longwave radiation and zonal wind identifies two modes of interannual variability of the MJO: a zonally stationary variation of amplitude that is unrelated to ENSO and a roughly 20 degrees -longitude eastward extension of the MJO envelope during El Nino events. The stationary mode is represented by the first two EOFs, which form the familiar lag-correlated quadrature pair, and the eastward-extending mode is represented by the third EOF, which is usually ignored although it is statistically significant. However, the third EOF also has a systematic phase relation with the first pair, and all three should be considered as a triplet; rotating the EOFs makes the phase relation clear. The zonal shift represents about 20% of total MJO variance (which itself is about 55% of intraseasonal variance over the tropical strip). Although the eastward shift is small when compared with the global scale of the MJO, it produces a large proportional shift of MJO activity over the open Pacific, where physical interactions with ENSO processes can occur.