South-westward Propagating Quasi-biweekly Oscillation over South-western Indian Ocean during Boreal Winter

The Quasi-Biweekly Oscillation (QBWO), with a period between the synoptic scale and the Madden Julian Oscillation (MJO), is an important component of tropical intraseasonal variability (ISV). While this mode has received a lot of attention over India (during the summer monsoon) and South China sea region, less attention has been paid on South-western Indian Ocean (SWIO). Apart from improving our understanding of ISV, this mode is important in the SWIO during boreal winter as this is an active basin for tropical cyclones (TC), and the QBWO significantly influences TCs. Here we study details of the genesis of the QBWO, its propagation, vertical structure and evolution. The data used comprises of NCEP-NCAR and ERA-interim reanalysis and NOAA outgoing longwave radiation (OLR) from 2000-2010. A composite analysis based on 10-30 day filtered data during the boreal winter reveals a well-organized convectively-coupled wave-train pattern, namely the QBWO, over the SWIO. It emerges from south of the equator (5S), between 50-80E, and then propagates south-westward. The horizontal structure exhibits a slight southwest-northeast tilt, but mainly longitudinal elongation. After arriving Madagascar, the system shows more pronounced southward migration. Further, negative (positive) OLR anomalies are tightly coupled with a cyclonic  (anticyclonic) circulation at 850 hPa. In the end, the QBWO with associated anomalous convection dies down near 40S, between 40-70E. Overall, over this oceanic basin, the QBWO has a period of approximately 18 days, wavelength of about 5000-6000 km, a southward (westward) phase speed of 1.9 (2.7)°/day and a near zero group velocity. Near equator, the system emerges with an equivalent barotropic structure. When mature with strongest convection around 10-20S, the system becomes weakly baroclinic, with relative vorticity anomaly changing sign near 400 hPa, and in the dying phase after 30S, the QBWO becomes equivalent barotropic again. Finally, the centres of relative vorticity and vertical motion near the equator are consistent with the characteristics of equatorial Rossby waves, whereas the cyclonic circulation is tightly coupled with anomalous convection as the wave moves away from the equator.