Preprints
https://doi.org/10.5194/wcd-2021-66
https://doi.org/10.5194/wcd-2021-66

  25 Oct 2021

25 Oct 2021

Review status: this preprint is currently under review for the journal WCD.

Southwestward propagating quasi-biweekly oscillations over the South-West Indian Ocean during boreal winter

Sambrita Ghatak1, and Jai Sukhatme1,2, Sambrita Ghatak and Jai Sukhatme
  • 1Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India
  • 2Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India
  • These authors contributed equally to this work.

Abstract. An analysis of outgoing longwave radiation (OLR) 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 10 years (2000/01 – 2009/10) of OLR and reanalysis data show well-formed rotational gyres that can be tracked from near the equator to almost 35° S appearing slightly west of Sumatra and going 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 18–20 days and they have a westward phase speed of approximately 4 ms−1. The group velocity of these wave packets is near-zero. Velocity fields with OLR indicate that maxima of moist convective activity lie in the northeast sector of the gyres (in the Southern Hemisphere), likely a result of both convergence and the poleward rotational advection of moist air. Wavetrains comprising the quasi-biweekly oscillation (QBWO) are born near the equator with a barotropic profile; a first baroclinic form emerges as they move southward and couple with moisture. In their decaying stage, convective activity decreases and the systems regain an equivalent barotropic structure. A vorticity budget reveals that the β effect plays a leading role in the propagation of the QBWO, though moist coupling (via stretching) is important in reducing the speed of propagation of this mode. Further, these two terms with horizontal advection account for much of the observed vorticity tendency. Finally, apart from their contribution to intraseasonal variability, moist convection and strong circulation anomalies in the QBWO lows (cyclonic gyres with negative OLR anomalies) — especially in combination with the vorticity of the background flow during the boreal winter season — are shown to provide favourable conditions for the genesis of tropical cyclones (TCs). In particular, depressions are spawned within QBWO lows, some of which mature into TCs that affect Madagascar, other SWIO islands and the coast of southeast Africa.

Sambrita Ghatak and Jai Sukhatme

Status: open (until 12 Dec 2021)

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  • RC1: 'Comment on wcd-2021-66', Anonymous Referee #1, 23 Nov 2021 reply

Sambrita Ghatak and Jai Sukhatme

Sambrita Ghatak and Jai Sukhatme

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Short summary
A south-westward propagating moist wavetrain with quasi-biweekly period is shown to exist in the South-West Indian Ocean during winter. This oscillation is interpreted in terms of an equatorial Rossby wave. Differential planetary rotation is responsible for its propagation, and moist convection (via stretching) reduces its speed. Its circulation provides favourable conditions for cyclogenesis, with potential implications for the forecast of cyclones that affect Madagascar and southeast Africa.