the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Jul 2026
Dynamical Controls on Interannual ITCZ Variability over the Indian Summer Monsoon Domain
Abstract. The Intertropical Convergence Zone (ITCZ) is a narrow band of intense convection formed by the convergence of tropical trade winds, leading to intense convection and precipitation. Its seasonal and interannual variations have significant socio-economic impacts, particularly over the Indian subcontinent, where the shifts in ITCZ position influence monsoon rainfall. Using long-term precipitation and atmospheric reanalysis data, this study investigates the interannual variability of boreal summer ITCZ latitudes, quantified using multivariate probabilistic approach, over the Indian region and its associated circulation changes. Extreme northward ITCZ shift years are associated with enhanced convection, strengthened monsoon westerlies, and intensified ascent leading to increased precipitation over the monsoon core zone along with poleward-displaced regional Hadley circulation, while extreme southward shift exhibits opposite features. Large-scale climate modes such as the El Niño–Southern Oscillation modulate tropical heating and circulation, influencing ITCZ position. Additionally, a significant long-term northward shift of the ITCZ during 1940–2022 is identified, which is closely linked to changes in moist static energy transport. A corresponding poleward shift of the meridional energy transport indicates that this migration is driven by changes in atmospheric energy balance.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2861', Anonymous Referee #2, 28 Jul 2026
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RC2: 'Comment on egusphere-2026-2861', Anonymous Referee #1, 06 Aug 2026
In this study, Kumari et al. propose a mechanism to explain the long-term variations of the ITCZ over the Indian subcontinent. The authors suggest that the ITCZ is shifting northward over the Indian sector due to changes in MSE export, while attributing its interannual variations to ENSO-modulated shifts in the Walker circulation.
While the manuscript addresses an important aspect of Indian summer monsoon dynamics, the overarching novelty of the study remains unclear. Furthermore, the proposed mechanisms appear fragmented and are not fully developed.
My primary concern centres on how meridional shifts in the ITCZ within the Indian sector are framed. The authors attempt to explain interannual variability entirely through meridional shifts; however, as shown in their Fig. 7, East-West (zonal) shifts are actually more prevalent—a finding consistent with Walker cell dynamics. Additionally, the manuscript invokes different physical mechanisms for long-term trends (meridional MSE transport) versus interannual variations (zonal Walker cell shifts) without explaining why these two timescales operate under distinct regimes or what fundamentally drives the long-term trends in MSE transport.
In my opinion, a major overhaul of the manuscript is required to address these conceptual and diagnostic gaps. I recommend Major Revision. The study could be substantially strengthened by refocusing on a clear, core question—for instance: Why have southern excursions of the ITCZ reduced post-2000 AD?
Major Comments
- Conceptualization of Long-Term Shifts
The authors claim there is a long-term northward shift of the ITCZ over the Indian region. However, as shown in Figure 1, the pattern is primarily characterized by a reduction of the southern excursions of the ITCZ latitude, rather than a northward shift of its northern boundary. A reduction in southern excursion does not strictly equate to a northward shift of the whole convergence zone. Does this truly qualify as a "northward shift"? Clarification and refined terminology are needed.
- Physical Mechanism of Long-Term Shifts
The authors attribute long-term ITCZ shifts to a northward movement of MSE export—specifically, the latitude where vertically integrated MSE flux vanishes (the Energy Flux Equator, or EFE). However, this is not conclusively demonstrated:
- Missing Diagnostic: A plot showing the latitude of the EFE over the study period is essential to support this claim directly.
- Contradiction at 18N: In Fig. 3c, the trend in vertically integrated MSE flux is zero near 18N, which is the mean latitude of the ITCZ in the Indian sector. A zero trend at the mean ITCZ position implies no local change in MSE flux, contradicting the argument of a forced shift.
- Underlying Drivers: The fundamental physical causes driving these long-term trends in MSE and MSE flux are left unexplained.
- Zonal (East-West) vs. Meridional Shifts
The manuscript focuses almost exclusively on meridional shifts in the ITCZ. However, zonal shifts are equally critical in this region (e.g., Boos & Korty, 2016), as corroborated by the rainfall anomalies presented in Fig. 7. A comprehensive assessment must account for both zonal and meridional components of ITCZ variability.
- Marine vs. Terrestrial ITCZ
During the boreal summer, two distinct convergence zones coexist in the Indian sector: the equatorial Marine ITCZ and the Terrestrial Convergence Zone (15-20N). The manuscript does not explain how its objective ITCZ definition accounts for this bimodal structure. For example, in Fig. 5a, the presence of the equatorial Marine ITCZ is evident during years marked by "northward shifts," yet this dual-zone interaction is left unaddressed.
- Mechanism of Interannual Variability
- Novelty: The attribution of interannual variability to ENSO-modulated Walker circulation shifts is already well established in monsoon literature. The unique contribution of this manuscript needs to be clearly articulated.
- Physical Linkage: The authors do not adequately explain how a zonal displacement of the Walker cell physically induces a meridional shift in the ITCZ over India.
- Inconsistent Frameworks across Timescales: The manuscript uses thermodynamic diagnostics (MSE fluxes) for long-term trends, but shifts to pure dynamics for interannual variability. How do MSE fluxes and the EFE behave on interannual timescales? Long-term SST trends and interannual variations in vertically integrated MSE flux (and the EFE) can be evaluated within a single, unified framework.
- Role of ENSO: It is well known that ENSO accounts for only a fraction of Indian summer monsoon variability. Stating that ENSO governs interannual variations in ITCZ latitude oversimplifies the influence of other factors E.g. Indian Ocean Dipole.
- Causal Relationship Between Diabatic Heating and ITCZ Shifts
The authors argue that ENSO-driven Walker cell shifts alter the location of monsoon diabatic heating, which subsequently drives shifts in the ITCZ. This framing presents a logical inconsistency: diabatic heating and ITCZ convergence are co-located, coupled manifestations of the same convective system. Stating that one leads or causes the shift of the other should be supported with enough evidence.
Minor Comments
- Lines 33–34: Please add appropriate citations to support this statement.
- Lines 35–36: Please include relevant citations here.
- Line 40: Change "discussed about trends" to "discuss the trends".
- Line 48: Change "Summer" to lowercase "summer".
- Line 90: Change "they are having" to "they have".
- Line 91: Correct "south Aisan region" to "South Asian region".
- Line 93: Remove word "data".
- Line 93: Change "and has" to "who have".
- Line 122: Change "Kg" to lowercase "kg" (standard SI notation).
- Line 152: Clarify precisely which SST asymmetry metric/region is being referred to here.
- Lines 157–159: Clarify the statement that large-scale circulation is driven directly by MSE. Large-scale atmospheric circulations are primarily governed by the sources and sinks of MSE (i.e., net atmospheric energy input; see Neelin & Held, 1987). Please revise and add appropriate citations.
- Line 162: Please add a line plot showing vertically integrated MSE flux as a function of latitude.
- Lines 197 & 201: The description of Indian Ocean SST spatial patterns in the text does not match the corresponding figures. Please re-check the figure references.
- Lines 218–219: The assertion that regional circulation patterns dominate interannual ITCZ latitude variations conflicts with the authors' prior claim that interannual variations are driven by ENSO (a large-scale, remote phenomenon). Please reconcile this statement.
Citation: https://doi.org/10.5194/egusphere-2026-2861-RC2
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The manuscript is well written, approach and methods are sound, contains significant contribution to scientific progress, Recommended for publication