Interannual Variations of the Boreal Summer Intraseasonal Oscillation in the Asian–Pacific Region

Abstract

A finite-domain wavenumber?frequency analysis was proposed to objectively measure the interannual variability of the boreal summer intraseasonal oscillation (ISO) in the Asian?Pacific region. The strongest interannual variations of the ISO are found in the off-equatorial western North Pacific (WNP). In summers when El Niño is developing, both the westward- and northward-propagating waves with periods of 15?40 and 8?10 days are enhanced in July?October. The northward-propagating ISO in the Indian summer monsoon region, however, has little linkage with El Niño?Southern Oscillation (ENSO). ENSO affects the northwestward-propagating ISO mode in the WNP through changing the mean circulation. During July?October in the El Niño developing year, the easterly vertical shears over the tropical western Pacific are considerably increased, which in turn promote development and northwestward emanation of Rossby waves away from the equatorial western-central Pacific, reinforcing the WNP ISO. In the Indian summer monsoon region, the ENSO-induced circulation changes are too weak to significantly modify the strong easterly sheared monsoon mean circulation. Therefore, the northward-propagating ISO is insensitive to ENSO. Unlike the wintertime Madden?Julian oscillation (MJO), which is uncorrelated with ENSO, the May?July MJO is strengthened during El Niño developing years. The questions of why there is a seasonal dependence of the MJO?ENSO relationship and how ENSO directly affects the May?July MJO require further investigations.