Origins of Quasi-Biweekly and Intraseasonal Oscillations over the South China Sea and Bay of Bengal and Scale Selection of Unstable Equatorial and Off-Equatorial Modes

The daily outgoing longwave radiation (OLR) field in boreal summer shows significant power spectrum peaks on quasi-biweekly (10–20-day) and intraseasonal (20–80-day) timescales over the Indo–western Pacific warm pool, especially over the South China Sea and Bay of Bengal. The quasi-biweekly oscillation (QBWO) originates from off-equatorial western North Pacific, and is characterized by a northwest–southeast oriented wave train pattern, propagating northwestward. The intraseasonal oscillation (ISO), on the other hand, originates from the equatorial Indian Ocean and propagates eastward and northward.

Why the equatorial mode possesses a 20–80-day periodicity while the off-equatorial mode favors a 10–20-day periodicity is investigated through idealized numerical experiments with a 2.5-layer atmospheric model. In the off-equatorial region, the model simulates, under a realistic three-dimensional summer mean flow, the most unstable mode that has a wave train pattern with a typical zonal wavelength of 6000 km and a period of 10–20 days, propagating northwestward. This is in contrast to the equatorial region, where a Madden–Julian oscillation (MJO) like mode with a planetary (wavenumber-1) zonal scale and a period ranging from 20 to 80 days is simulated. Sensitivity experiments with different initial conditions indicate that the QBWO is an intrinsic mode of the atmosphere in boreal summer in the off-equatorial Indo–western Pacific region under the summer mean state, while the MJO is the most unstable mode in the equatorial region.