Review of « Drivers and impacts of westerly moisture transport events in East Africa » by Peal and Collier

This article analyzes events of westerly moisture transport across Africa (and nearby regions), and links those events to the phase of the Madden-Julian Oscillation (MJO), and the presence (or lack) of tropical cyclones (TCs) over the Indian Ocean. It finally provides an estimation of the precipitation that can be attributed to those westerly moisture transport events (WMTEs). 

The authors developed a detection algorithm, quite similar to those used to monitor atmospheric rivers, to identify the WMTEs. The algorithm is unique in that ARs have a strong poleward component, while inter-tropical WMTEs are mostly zonal. The approach is sound and there was a clear need to have more in-depth understanding of those WMTEs. 

The following analyses are less convincing. They remain very descriptive. The authors build samples consisting of different phases of the MJO, combined with the presence or absence of TCs, but the main object proposed in this work (the WMTEs) are not analyzed with more detail than just their presence or absence. Another possible issue comes from the attribution of rainfall to WMTEs: the justifications are not very clear but I understand (Fig. 1) that precipitation extending far beyond the contours of the detected WMTEs can be considered as linked to it. This is clearly not what is usually done when working with those detected moisture transports.

For those reasons, I believe that there is a very good study to be done with those WMTEs, but I think that the current version of the manuscript required major modifications and improvements before it can be accepted for publication. Even though I recommend major corrections, I would like to encourage the authors — there are really great ideas here, we’d just need more physical characterizations (and understanding) of the WMTEs, their properties, and their mechanisms (see detailed comments below).

Major comments

1. What are the WMTEs, physically speaking? ARs have been described as “A long, narrow, and transient corridor of strong horizontal water vapor transport that is typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone” according to the AMS Glossary. This is because ARs are a concept mostly used for extra-tropical climate and weather, where transient disturbances (atmospheric highs and lows) have fronts that separate air masses, where baroclinic instability develops. This definition (and even the schematics that are often provided in some studies) are useful to conceptualize the AR objects, from a physical point of view. The concept of WMTEs strongly differs, even if it is detected by algorithm that are not so different. This is not only because WMTEs are more zonal, but also, because they develop in a tropical climate, where vorticity is very low because of the proximity to the equator. In those regions, large-scale cellular circulations (that are, partly, conceptual objects) can develop (including zonal ones), as part of the MJO at the intraseasonal timescale, or ENSO at the interannual timescale. Are WMTEs linked, or even part, of those large-scale cells? Do they correspond to “bursts” or transient increases in their lower branch, when moisture convergence is located further east over the Indian Ocean sector (since you use a quite high threshold to define them, you only depict the most intense phases of those westerly circulations)? Do the WMTEs help refine (regionally) the conceptual schemes of the MJO as proposed by Madden and Julian (e.g., Fig. 3 of their 1994 paper, cited in this work), or propose more detailed vertical cross-sections? So, overall: what are WMTEs, physically speaking? 

The same could be said about the interannual timescale, especially in OND when the EEA region experiences the Short Rains whose interannual variability is strongly tied to the Walker circulation (= are WMTEs partly driven by changes in the Walker-type circulation ?). See e.g. an (already old) paper by Hastenrath about the detection of those cellular circulations: Hastenrath, S (2000) Zonal circulations over the equatorial Indian Ocean : Journal of Climate 13, 2746-2756. 

When reading the article I had a similar question, about their interpretation, when the dominant winds are easterly (usually the case where/when trade winds are well developed) vs. the regions where westerlies prevail (like the northwestern Indian Ocean in boreal summer, due to the monsoonal circulation). In other words, how can the threshold used differentiate transient from seasonal circulations. But the questions raised above are more general and encompass that particular case. All these questions are, in my opinion, super important, to know what are the objects we’re talking about, and that we consider so extensively in this work.

2. There are no metrics here to characterize the WMTEs, like their length, width, tilt / direction, duration, integrated moisture transport (and location of the maximum), … The AR community produced tens of articles showing that those descriptors are important to better analyze their impacts on rainfall, and help better understand the mechanisms responsible for rainfall, and its space-time variability (including daily amounts or even extremes). In addition, relating rainfall to the location of the outflow boundary of the WMTEs might give potentially interesting results; similarly, the inflow location might give insight into the moisture sources.

3. Attribution of precipitation to WMTEs. I understand you’ve considered the Boolean union of the WMTE and Precipitation > 1mm.day-1 contours and attributed all rainfall falling within that new contour to WMTEs. This would mean that precipitation occurring outside the WMTE contour is yet attributed to it. This would be clearly an issue, especially since WMTE can be linked to the MJO that promoted the development of large-scale convective clusters (some of which can reach 10,000km diameters). This would imply you could attribute MJO-caused precipitation to WMTEs. Considering the Boolean intersection instead of the union would certainly decrease the contribution of WMTE to rainfall totals, but the approach would be more conservative and more robust. This is the one traditionally used by the AR community. Yet, double (or triple!) counts are still possible (i.e., in the worst-case scenario, attributing rainfall to WMTEs, MJO and TCs). See e.g. Dacre’s papers about the interactions between atmospheric rivers, warm conveyor belts and cyclones: 10.1038/s41612-025-00942-z; 10.1029/2023jd040557; 10.1175/JHM-D-18-0175.1.

4. Relationship between WMTEs and TCs: is it a “chicken and egg” problem? Do the WMTEs feed TCs with moisture, or do WMTEs respond to the development of TCs (or convection, more generally) through ageostrophic circulations? While the causality itself might deserve dedicated studies, it would be quite straightforward to see whether the WMTEs develop before, or after the TC. This could be useful as a first clue to understand how both circulations behave and interact.

Minor points. There are not many of them because I mostly focused this first review on the main points listed above.

l. 92. How precisely is the 70th percentile of moisture transport calculated? By considering both signs (i.e. easterly and westerly), or just westerly transport occurrences?

Figure 2. If the main interest in this work is to assess westerly moisture transport across Africa and reaching the EEA region, then the choice of the domain is a bit strange — shifted eastwards, and giving more importance to the Indian Ocean region. Previous work (e.g. on the regional influence of the MJO) discussed zonal moisture convergence between the Congo basin and the Indian sector, so the WMTEs of interest, advecting moisture towards EEA, should be placed over inter-tropical or equatorial Africa. Moisture sources might be continental (Congo basin) or oceanic (Atlantic sector). Why such an eastward-shifted domain? WMTEs have lesser importance for EEA if they occur east of it?

Figure 3. Why show the curl of the moisture fluxes, rather than their convergence? I’m not saying this is wrong, but this needs to be explained. Moisture convergence may make more sense for rainfall analysis. Vorticity is certainly more meaningful when assessing the links with cyclogenesis.

l. 156. TCs themselves are not independent of the MJO. The authors did not discuss this point.

Bessafi, M. & Wheeler, M. C. (2006) Modulation of south Indian Ocean tropical cyclones by the Madden-Julian Oscillation and convectively coupled equatorial waves. Mon. Weather Rev. 134, 638–656

Klotzbach, P. J. (2014) The Madden-Julian Oscillation’s Impacts on Worldwide Tropical Cyclone Activity. J. Clim. 27, 2317–2330

Diamond, H. J. & Renwick, J. A. (2015) The climatological relationship between tropical cyclones in the southwest pacific and the Madden-Julian Oscillation. Int. J. Climatol. 35, 676–686

Figure 6. What is the use of defining an EEA region like in Finney et al. if it’s not used in this work, e.g. to compute regional indices?