Energy Spectrum Characteristics of Boreal Summer Intraseasonal Oscillations: Climatology and Variations during the ENSO Developing and Decaying Phases*

Abstract

Abstract The geographic-dependence characteristics of the energy spectrum of the boreal summer intraseasonal oscillation (BSISO; May–October) over the Indo–western Pacific region were analyzed using 25-yr (1979–2003) observational data. The BSISO energy spectrum distribution exhibits a distinctive regional characteristic. The stationary and eastward-propagating modes are most pronounced at the equator (5°S–5°N), while the westward-propagating modes are dominant in the off-equatorial region (10°–20°N). While the eastward intraseasonal oscillation (ISO) spectrum agglomerates on the 30–60-day period and zonal wavenumber 1, the westward mode covers wider spatial (wavenumber) and temporal (period) range. Along the Arabian Sea, Bay of Bengal, and South China Sea (SCS) latitudes, the dominant wavenumber 1 mode is the eastward (westward) propagation at the 30–60-day (10–20 day) period; for zonal wavenumber 2, the dominant mode is the westward propagation at both the 30–60-day and 10–20-day periods. Compared to the absolute amplitude of both zonal and meridional mode energy spectrum, northward propagation is the most predominant mode in boreal summer over the Indo–western Pacific regions. The strongest northward-propagating BSISO signal appears in the eastern tropical Indian Ocean. The variation of BSISO differs significantly in the El Niño and La Niña developing and decaying phases. During the El Niño (La Niña) developing summer, the eastward propagation is enhanced (weakened) at the equator, while the northward propagation is also strengthened (weakened) over the western Pacific (east of 140°E). During the El Niño (La Niña) decaying summer, the eastward propagation weakens (strengthens) at the equator, opposite to that in the developing summer; the westward propagation off the equator and the northward propagation over SCS and the western Pacific are suppressed (enhanced). The amplitude of the BSISO variation is stronger in the decaying summer than that in the developing summer. This asymmetry in BSISO variations is primarily attributed to the asymmetry of the background mean flow change associated with the developing and decaying phases of ENSO.