the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Oct 2021
Southwestward propagating quasi-biweekly oscillations over the South-West Indian Ocean during boreal winter
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.
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RC1: 'Comment on wcd-2021-66', Anonymous Referee #1, 23 Nov 2021
The authors present evidence for the existence of a quasi-biweekly oscillation in the southern Indian Ocean. They show some composites, a vorticity budget analysis, and show how the mode may modulate TC activity. If confirmed, this study could reveal our understanding of these quasi-biweekly modes. However, I have multiple concerns about the contents of the manuscript, which I outline below. Because of this I recommend major revisions.
Major comments:
- The manuscript feels long and disjointed. There is a lack of organization of the manuscript which makes reading it difficult and exhausting. Mean state plots should be shown together, as well as the plots about the vorticity budget. Some plots could be coalesced to save space or even gain new insights. There is also some parts of the manuscript that feel unnecessary or are not well-justified. For example, why discuss equatorial Rossby waves in the Introduction? The mode shown here is not an equatorial Rossby wave. If there’s a point to this, the authors should be more clear about it. Overall, I think the authors can trim a lot of the content that is currently shown and focus on the essentials, as well as focus more on the two other major concerns below.
- Statistical significance of the mode: The analysis shown here is based on a composite analysis on a box over the southwest Indian Ocean. It is unclear why this box was chosen, and no attempt is made to show that the quasi-weekly mode is statistically significant. This could be done by showing that the power spectrum of vorticity or OLR is above the red spectrum at the 99% confidence interval. An EOF analysis showing that the eigenvalues corresponding to this mode are statistically distinct could also be shown. However, the analysis as shown in its current form is not sufficiently convincing. This is important, as it is otherwise unclear why the authors chose the filtering process outlined in the paper – it seem ad hoc.
- The vorticity budget is not enough to justify the main points of the paper. There is discussion about moisture advection throughout the paper yet not discussion about a moisture budget. This should be included. Even better would be an MSE budget or a weak-temperature gradient balance-based moisture budget (see Chikira 2014, Wolding et al. 2016, Adames and Ming 2018a). The authors should also check whether the water vapor explains most of the precipitation variance. On when examining the evolution of moisture can we better understand how convection is modifying the evolution of the vortex.
Minor comments:
Figures: The contents of the figure should be shown in the title. The color bars should say what fields its showing, and the abscissa and ordinate should be labeled. They are not labeled in most figures.
Figure 1: Grid lines are obstructive. Consider removing.
Fig. 3: Same comment as Fig. 1. Lags should be shown in the title to make it easier and more intuitive for the reader.
176-177: outlined in the Data and Methods section. This part of the sentence is unnecessary. Please remove.
Citation: https://doi.org/10.5194/wcd-2021-66-RC1 - AC1: 'Reply on RC1', Sambrita Ghatak, 25 Mar 2022
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RC2: 'Comment on wcd-2021-66', Anonymous Referee #2, 17 Dec 2021
This paper documents the convectively coupled quasi-biweekly oscillation (QBWO) in the South west Indian ocean. The paper is easy to read and is rather descriptive in nature. The introduction is expansive and provides relevant background on the topic and ends with a clear statement of the goals of the paper. The discussion section recaps some of the physical mechanisms of the genesis of this oscillation.
Despite some promising initial discussion of background moisture distribution (the authors appeared to hint at some moisture mode type behavior) a vorticity budget was the route taken here. This does not address the organization and modulation of convection (perhaps a moisture or moist static energy budget would be useful for that). The key result here is that planetary vorticity advection accounts for the propagation of the wave and stretching to its amplification. The former is consistent with the notion that the wave is an ER type mode and the latter points to the vorticity generation by convergence/divergence associated with convection.
In section 5, the paper presents some material on tropical cyclone formation during the QBWO of 2008--2009. This is also easy to read and is again descriptive in its treatment with no calculations or diagnostics (beyond maps of filtered fields)
Over all, the paper provides documentation of the QBWO in a basin that has not received as much attention as compared to other basins. The results are not necessarily novel but will be useful reference points for future work( such as evaluation of theoretical and conceptual models of this phenomenon).Other comments:
- The authors might wish to consider calculating statistical significance for their composite anomalies and only show values that are deemed significantly different from zero.
- The data and methods seem reasonable
- Line 160: Just to be sure, can you add a few lines (connecting constant phase) on Fig. 2 to illustrate the wave (phase) propagation. Can you also estimate the southward phase speed and check if they are realistic and the patterns in the Hovmoller represent propagation.
- Line 165: Same as above, but for the group speed.
- Line 207: How does an oscillation die? Is it being damped or absorbed by the background flow? Or is the "weakening" of the composite anomalies simply because one is averaging a band-passed field many days away from the reference time (lag 0).
Figure 8: Any idea why the structure changes from 1st baroclinic to a tilt? Is it really tilting or is that simply an artifact of the contouring/shading? - Line 484: OLR anomalies are the visible outcome of moist convection. Please rephrase this sentence to make it less redundant.
- Section 5 on the impact of the QBWO on troical cyclone formation is again very qualitative in the way it is presented. No real issues here but a more comprehensive study would need simulations with a full physics model and sensitivity experiements.
Citation: https://doi.org/10.5194/wcd-2021-66-RC2 - AC2: 'Reply on RC2', Sambrita Ghatak, 25 Mar 2022
Status: closed
-
RC1: 'Comment on wcd-2021-66', Anonymous Referee #1, 23 Nov 2021
The authors present evidence for the existence of a quasi-biweekly oscillation in the southern Indian Ocean. They show some composites, a vorticity budget analysis, and show how the mode may modulate TC activity. If confirmed, this study could reveal our understanding of these quasi-biweekly modes. However, I have multiple concerns about the contents of the manuscript, which I outline below. Because of this I recommend major revisions.
Major comments:
- The manuscript feels long and disjointed. There is a lack of organization of the manuscript which makes reading it difficult and exhausting. Mean state plots should be shown together, as well as the plots about the vorticity budget. Some plots could be coalesced to save space or even gain new insights. There is also some parts of the manuscript that feel unnecessary or are not well-justified. For example, why discuss equatorial Rossby waves in the Introduction? The mode shown here is not an equatorial Rossby wave. If there’s a point to this, the authors should be more clear about it. Overall, I think the authors can trim a lot of the content that is currently shown and focus on the essentials, as well as focus more on the two other major concerns below.
- Statistical significance of the mode: The analysis shown here is based on a composite analysis on a box over the southwest Indian Ocean. It is unclear why this box was chosen, and no attempt is made to show that the quasi-weekly mode is statistically significant. This could be done by showing that the power spectrum of vorticity or OLR is above the red spectrum at the 99% confidence interval. An EOF analysis showing that the eigenvalues corresponding to this mode are statistically distinct could also be shown. However, the analysis as shown in its current form is not sufficiently convincing. This is important, as it is otherwise unclear why the authors chose the filtering process outlined in the paper – it seem ad hoc.
- The vorticity budget is not enough to justify the main points of the paper. There is discussion about moisture advection throughout the paper yet not discussion about a moisture budget. This should be included. Even better would be an MSE budget or a weak-temperature gradient balance-based moisture budget (see Chikira 2014, Wolding et al. 2016, Adames and Ming 2018a). The authors should also check whether the water vapor explains most of the precipitation variance. On when examining the evolution of moisture can we better understand how convection is modifying the evolution of the vortex.
Minor comments:
Figures: The contents of the figure should be shown in the title. The color bars should say what fields its showing, and the abscissa and ordinate should be labeled. They are not labeled in most figures.
Figure 1: Grid lines are obstructive. Consider removing.
Fig. 3: Same comment as Fig. 1. Lags should be shown in the title to make it easier and more intuitive for the reader.
176-177: outlined in the Data and Methods section. This part of the sentence is unnecessary. Please remove.
Citation: https://doi.org/10.5194/wcd-2021-66-RC1 - AC1: 'Reply on RC1', Sambrita Ghatak, 25 Mar 2022
-
RC2: 'Comment on wcd-2021-66', Anonymous Referee #2, 17 Dec 2021
This paper documents the convectively coupled quasi-biweekly oscillation (QBWO) in the South west Indian ocean. The paper is easy to read and is rather descriptive in nature. The introduction is expansive and provides relevant background on the topic and ends with a clear statement of the goals of the paper. The discussion section recaps some of the physical mechanisms of the genesis of this oscillation.
Despite some promising initial discussion of background moisture distribution (the authors appeared to hint at some moisture mode type behavior) a vorticity budget was the route taken here. This does not address the organization and modulation of convection (perhaps a moisture or moist static energy budget would be useful for that). The key result here is that planetary vorticity advection accounts for the propagation of the wave and stretching to its amplification. The former is consistent with the notion that the wave is an ER type mode and the latter points to the vorticity generation by convergence/divergence associated with convection.
In section 5, the paper presents some material on tropical cyclone formation during the QBWO of 2008--2009. This is also easy to read and is again descriptive in its treatment with no calculations or diagnostics (beyond maps of filtered fields)
Over all, the paper provides documentation of the QBWO in a basin that has not received as much attention as compared to other basins. The results are not necessarily novel but will be useful reference points for future work( such as evaluation of theoretical and conceptual models of this phenomenon).Other comments:
- The authors might wish to consider calculating statistical significance for their composite anomalies and only show values that are deemed significantly different from zero.
- The data and methods seem reasonable
- Line 160: Just to be sure, can you add a few lines (connecting constant phase) on Fig. 2 to illustrate the wave (phase) propagation. Can you also estimate the southward phase speed and check if they are realistic and the patterns in the Hovmoller represent propagation.
- Line 165: Same as above, but for the group speed.
- Line 207: How does an oscillation die? Is it being damped or absorbed by the background flow? Or is the "weakening" of the composite anomalies simply because one is averaging a band-passed field many days away from the reference time (lag 0).
Figure 8: Any idea why the structure changes from 1st baroclinic to a tilt? Is it really tilting or is that simply an artifact of the contouring/shading? - Line 484: OLR anomalies are the visible outcome of moist convection. Please rephrase this sentence to make it less redundant.
- Section 5 on the impact of the QBWO on troical cyclone formation is again very qualitative in the way it is presented. No real issues here but a more comprehensive study would need simulations with a full physics model and sensitivity experiements.
Citation: https://doi.org/10.5194/wcd-2021-66-RC2 - AC2: 'Reply on RC2', Sambrita Ghatak, 25 Mar 2022
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