Structure, genesis and scale selection of the tropical quasi-biweekly mode

Abstract

The quasi-biweekly mode (QBM) and the 30-60 day mode are two major intraseasonal oscillations (ISOs) in the tropics. The QBM is known to have a major influence in determining the active and break conditions of the Indian monsoon during the northern summer. A westward-propagating equatorial Rossby wave with quasi-biweekly period influences the Australian monsoon during the northern winter. Universality between the summer and winter QBM is established through analysis of daily circulation and convection data for 10 years. It is shown that the mean spatial structure of the QBM in circulation and convection resembles that of a gravest meridional mode equatorial Rossby wave with wavelength of about 6000 km and westward phase speed of approximately 4.5 m s^-1. However, the maximum zonal wind occurs at around 5°N (5°S) during the northern summer (winter). The wave structure appears to be translated northward (southward) by about 5° during the northern summer (winter). The relationship between outgoing long-wave radiation and circulation data indicates that the mode is driven unstable by coupling with moist convection. Similarity in temporal and spatial characteristics of the mode during the two seasons leads us to propose that the same mechanism governs the genesis and scale selection of the mode in both the seasons. An acceptable mechanism for genesis and scale selection of the QBM has been lacking. In the present study, a mechanism for genesis and scale selection of the observed QBM is proposed. A simple 2½-layer model that includes a steady Ekman boundary layer (BL) formulation incorporating effect of entrainment mixing is constructed for the convectively coupled equatorial waves. Without influence of the background mean flow, moist feedback in the presence of frictional BL convergence drives the gravest meridional mode equatorial Rossby wave unstable with observed wavelength and period but with zonal winds symmetric about the equator. Potential temperature perturbation associated with the Rossby wave is in phase with relative vorticity perturbation at low level. The BL drives moisture convergence in phase with the relative vorticity at the top of the BL. Release of latent heat associated with the BL convergence enhances the potential temperature leading to a positive feedback. The mean flow over the Indian Ocean and western Pacific at low levels is such that the zero ambient absolute vorticity or the 'dynamic equator' shifts to around 5°N (5°S) during summer (winter) and results in a shift of the unstable Rossby waves towards the north (south) by about 5°. The resulting structure of the unstable Rossby mode resembles the observed structure of the biweekly mode. It is shown that neither evaporation-wind feedback nor vertical shear of the mean flow is crucial for the existence of the mode. However these processes marginally modify the growth rate and make the structure of the unstable wave more realistic.