Quasi-biweekly oscillation over South-West Indian Ocean during boreal winter: features, propagation dynamics and its impact on Tropical Cyclone genesis and tracks

Abstract

<p>An analysis of NOAA outgoing longwave radiation (OLR) data and NCEP-NCAR reanalysis winds spanning 10 years (2000/2001-2009/2010) over the South-West Indian Ocean (SWIO) yields regular, poleward propagating, large-scale, convectively coupled systems with a quasi-biweekly period during the boreal winter (DJFM). A composite of 14−30 day filtered data shows wavetrains with well-formed alternating rotational gyres that can be tracked from approximately 5° S to almost 40° S appearing west of Sumatra and going past Madagascar, i.e., with mean southwest propagation. This Quasi-biweekly oscillation (QBWO) has a period of 18−20 days, a wavelength of about 5000−6000 km, a phase speed of approximately 4−5 m/s, and a near-zero group velocity. Further, maxima of moist convective activity lie in the northeast sector of the gyres and is likely a result of both convergence and poleward rotational advection of moist air. A vorticity budget analysis in the lower levels of the atmosphere reveals that the β-effect term plays a leading role in the propagation of the QBWO, though the stretching term reduces its speed of propagation. The influence of QBWO on Tropical Cyclones (TCs) is well-established in other ocean basins, but hasn't gotten much attention in the SWIO, which is one of the most active basins in terms of TCs. From a composite study of large scale fields known to influence cyclone formation, we find that active QBWO phases create an environment that is particularly conducive for cyclogenesis. Namely, anomalous 850 hPa relative vorticity , anomalous 500 hPa vertical velocity and  600 hPa relative humidity fields show that the favorable conditions are provided by the QBWO, and with the southwestward movement of the QBWO the most favourable region for cyclogenesis also moves southwest. Further, the convective gyre of QBWO, while passing over monsoon trough-like background cyclonic circulation around 10° S makes the region highly favorable for cyclogenesis. In fact, our composite analysis shows that after making the open ocean environment favourable for cyclogenesis, the QBWO reaches the Mozambique channel, and in turn, this region becomes conducive for TC genesis, whereas, during this phase, conditions over the open ocean north of Madagascar do not support the birth of cyclones. In terms of the track of TCs, the wind composite shows that after reaching Madagascar, the QBWO influences the lower level large-scale background flow of subtropical anti-cyclone and the prevailing easterlies by pushing them further eastward. By modulating the steering flow, this potentially has implications for the track of TC by helping it to divert south/southeastward before hitting Madagascar. Finally, a case study of a long-lived QBWO of 2008-09 season and few other case studies from 2000/2001–2009/2010 elucidate the connection between QBWO and TC genesis and track in this basin.  </p>