Southwestward propagating quasi-biweekly oscillations over the South-West Indian Ocean during boreal winter: Characteristics and propagation mechanism

Abstract. An analysis of outgoing longwave radiation (OLR) and winds over the South-West Indian Ocean (SWIO) yields regular, poleward propagating, large-scale, convectively coupled systems of alternating cyclonic and anticyclonic circulation with a quasi-biweekly period during boreal winter. Composites from 30 years (1980/81–2009/10) of OLR and reanalysis data show well-formed rotational gyres in the lower troposphere (700 hPa) that can be tracked from near the equator to almost 30° S appearing west of Sumatra and propagating towards Madagascar, i.e., with mean southwest propagation. The gyres show a marked northwest-southeast tilt, giving rise to a northeast-southwest oriented wavetrain. The scale of the gyres is about 30°–35°, their period is 14–18 days and they have a westward phase speed of approximately 3.5 ms^-1. The group velocity of these wave packets is near-zero. In early stages, the gyres are associated with weak convection, but when they move poleward and cross 10° S, convective coupling is enhanced. Velocity fields and OLR indicate that the maxima of moist convective activity lies in the eastern sector of the gyres and a comparison between column-integrated moisture and OLR anomalies shows they are highly collocated, indicating the applicability of the moisture mode framework. A moisture budget reveals that once the gyres reach 10°–20° S, moistening is mainly due to northerlies in the eastern sector of the cyclonic gyre acting on the meridional gradient of background moisture, which eventually gives rise to anomalous convection in this region. This moistening process continues up to 30° S while the gyres traverse southwestward. Subsequently, background easterlies advect anomalous moisture and along with moistening via so-called 'column-processes', convection is observed to extend inside the gyre from the eastern side. A vorticity budget reveals that the β effect plays a leading role in the south-westward propagation, horizontal advection assists the westward movement of vorticity anomalies due to prevailing easterlies and moist coupling (via stretching) is important in reducing the speed of propagation of this mode. In fact, the relatively slow southwestward movement of the system is because moist coupling reduces the effect of β and horizontal advection terms. Moreover, as convection primarily takes place on the eastern side of the vortex, and somewhat inside the vortex too, it also follows a southwestward path along with the QBWO vortices.