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arxiv: 2509.17526 · v2 · submitted 2025-09-22 · ⚛️ physics.ao-ph

Key role of the Madden-Julian Oscillation on tropical and subtropical humid heat and heatwaves

Claire Rocuet (UPF , SECOPOL) , Takeshi Izumo (IRD [Polyn\'esie] , Bastien Pagli (UPF , IRD [Polyn\'esie] , Neil J Holbrook (IMAS) , Sophie Cravatte (IRD [Nouvelle-Cal\'edonie] , LEGOS)
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Marania Hopuare (UPF GePaSUD) Maxime Colin (ZMT)
This is my paper

Pith reviewed 2026-05-18 15:10 UTC · model grok-4.3

classification ⚛️ physics.ao-ph
keywords Madden-Julian Oscillationhumid heatheatwavestropicssubtropicsspecific humiditywet-bulb temperatureintraseasonal variability
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The pith

The Madden-Julian Oscillation can double or halve humid heatwave likelihood across large tropical and subtropical areas year-round.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that the Madden-Julian Oscillation and its boreal summer counterpart drive much of the week-to-month variability in humid heat and heatwaves. This modulation reaches over land and ocean in the tropics and subtropics in every season. The main pathway is through changes in specific humidity that alter wet-bulb temperature, supplemented by wind-driven advection of moisture and temperature in subtropical boundary layers. A sympathetic reader would care because the 30-60 day cycle supplies a source of subseasonal predictability that could improve risk forecasts for health, ecosystems, and economies.

Core claim

The MJO and the associated BSISO have a significant influence on humid heat and heatwaves over much of the tropics and subtropics across all seasons, both over terrestrial and marine regions. Humid heatwave likelihood can double or halve, depending on the MJO phase, in large areas of the Earth. The MJO/BSISO's influence on wet-bulb temperature is primarily via specific humidity rather than dry-bulb temperature anomalies. In the subtropics and other regions where we typically do not find a strong signal of the convection, we find that intraseasonal anomalies of specific humidity and dry-bulb temperature are influenced by horizontal advection in the planetary boundary layer.

What carries the argument

MJO phase composites that isolate intraseasonal anomalies in specific humidity and planetary-boundary-layer advection to modulate wet-bulb temperature.

If this is right

  • Subseasonal forecasts of humid heat risk can incorporate MJO phase information for lead times of several weeks.
  • Risk-management planning in tropical and subtropical regions can adjust for expected doubling or halving of heatwave likelihood by MJO phase.
  • Humid heat variability is tied to intraseasonal oscillations rather than solely to longer-term climate trends.
  • Marine and terrestrial humid heat responses are both organized by the same MJO-driven moisture and wind anomalies.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar composite analyses could test whether other intraseasonal modes produce comparable humid-heat modulations.
  • Future changes in MJO frequency or strength under warming would be expected to alter baseline humid heatwave statistics.
  • The advection pathway identified in the subtropics suggests testable links to regional wind-climatology interactions in climate models.

Load-bearing premise

MJO phase-based composites cleanly isolate causal effects on wet-bulb temperature without substantial confounding from other modes, trends, or data selection choices.

What would settle it

Finding no consistent change in humid heatwave frequency or wet-bulb temperature across MJO phases in an independent global reanalysis or observational record would falsify the central claim.

Figures

Figures reproduced from arXiv: 2509.17526 by Bastien Pagli (UPF, Claire Rocuet (UPF, GePaSUD), IRD [Polyn\'esie], LEGOS), Marania Hopuare (UPF, Maxime Colin (ZMT), Neil J Holbrook (IMAS), SECOPOL), Sophie Cravatte (IRD [Nouvelle-Cal\'edonie], Takeshi Izumo (IRD [Polyn\'esie].

Figure 1
Figure 1. Figure 1: Composite maps of intraseasonal anomalies of precipitation rate (colour shading) during the [PITH_FULL_IMAGE:figures/full_fig_p012_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Composite maps of intraseasonal anomalies of wet [PITH_FULL_IMAGE:figures/full_fig_p014_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Hovmöller diagram of latitudinally averaged surface anomalies between 10°N and 10°S for composites of MJO phases 1-8 (y-axis) for a) wet-bulb temperature Tw with black contours showing precipitation anomalies (intervals of 1 mm day-1, with zero omitted), and b) surface pressure (hPa) with black contours showing zonal surface (10m) wind anomalies (intervals of 0.4 ms-1, with zero omitted). c)- d) same as a)… view at source ↗
Figure 4
Figure 4. Figure 4: Map of the ratio (i.e. the increase, if ratio >1, or decrease, if ratio <1, of humid heatwave days [PITH_FULL_IMAGE:figures/full_fig_p017_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Contribution map of specific humidity (via dewpoint temperature) and dry [PITH_FULL_IMAGE:figures/full_fig_p019_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Coefficient of regression (colour shading) and correlation (black hatching lines represents coefficient above 0.25) between horizontal advection anomalies at 950hPa and ∂q′/∂t at 950hPa for a) NDJFMA and b) MJJASO. Overall, at the large scale, for both seasons, horizontal moisture advection in the planetary boundary layer influences specific humidity anomalies. Regions where the coefficient of regression v… view at source ↗
Figure 7
Figure 7. Figure 7: Coefficient of regression (colour shading) and correlation (black stippling represents coefficient above 0.5) between horizontal advection anomalies at 950hPa and a) !−V′ %%%⃗. ∇q* b) !−V %⃗ . ∇q′* and c) +−% V%%⃗! . ∇q′, and for NDJFMA. d)-e) for MJJASO. The dominant contributor to horizontal advection across the subtropics is the anomalous wind acting on the climatological humidity field ,−𝑉123′ 11111111… view at source ↗
read the original abstract

Humid heat stress and heatwaves pose significant risks for living organisms, from humans and wildlife to insects. These threats have wide-ranging health, ecological, and socio-economic impacts that are expected to worsen with climate change. How large-scale climate modes drive the week-to-month variability of humid heat remains poorly understood at the global scale. This limitation hinders the development of accurate forecasts necessary for risk-management measures, notably in the heavily populated and ecologically fragile regions of the tropics and subtropics. With forecast lead times up to several weeks, the Madden-Julian Oscillation (MJO), a global-scale intraseasonal tropical atmospheric disturbance circumnavigating earth in around 30-60 days, provides considerable predictability for weather conditions, and meteorological and oceanic extremes. Here we show that the MJO, and the associated boreal summer intraseasonal oscillation (BSISO), have a significant influence on humid heat and heatwaves over much of the tropics and subtropics across all seasons, both over terrestrial and marine regions. Humid heatwave likelihood can double or halve, depending on the MJO phase, in large areas of the Earth. The MJO/BSISO's influence on wet-bulb temperature is primarily via specific humidity rather than dry-bulb temperature anomalies. In the subtropics and other regions where we typically do not find a strong signal of the convection, we find that intraseasonal anomalies of specific humidity and dry-bulb temperature are influenced by horizontal advection in the planetary boundary layer. Particularly in the subtropics where advection of the climatological moisture and temperature gradient by MJO-related anomalous winds is an important term.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper claims that the Madden-Julian Oscillation (MJO) and associated Boreal Summer Intraseasonal Oscillation (BSISO) exert a significant influence on humid heat and heatwaves over much of the tropics and subtropics in all seasons, both over land and ocean. Using MJO phase composites, it reports that humid heatwave likelihood can double or halve depending on phase, with the effect on wet-bulb temperature driven primarily by specific humidity anomalies and, in subtropics, by horizontal advection of climatological gradients in the planetary boundary layer.

Significance. If the results hold after addressing potential confounders, the work would establish intraseasonal tropical modes as important drivers of subseasonal humid heat variability at global scale. This could improve extended-range forecasts of heat stress and support risk management in vulnerable regions, while the land-ocean and all-season coverage broadens its relevance for both human health and ecological impacts.

major comments (2)
  1. [Data and Methods] Data and Methods (composite construction): The MJO/BSISO phase composites for wet-bulb temperature and heatwave probability do not describe any removal or regression of ENSO, IOD, or long-term trends. Because MJO amplitude and phase are known to be modulated by ENSO, and ENSO directly alters tropical specific humidity and temperature, unremoved interannual signals could be aliased into the reported phase-dependent likelihood changes, undermining the isolation of the intraseasonal effect.
  2. [Results] Results (quantitative claims): The assertion that humid heatwave likelihood doubles or halves in large areas lacks accompanying details on the precise heatwave definition, baseline frequencies, number of events per phase, or statistical significance testing. Without these, the magnitude of the reported effect cannot be evaluated for robustness.
minor comments (2)
  1. [Abstract] Abstract: The final sentence on advection in the subtropics is grammatically incomplete ('is an important term.') and should be revised for clarity.
  2. [Abstract] Abstract: Clarify how the BSISO is defined and applied outside boreal summer, given that the MJO is active year-round.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and insightful comments, which have helped us improve the clarity and robustness of our analysis. We agree that potential confounding by interannual modes and additional quantitative details on the heatwave results warrant attention. Our responses to the major comments are provided below, along with the revisions we will implement.

read point-by-point responses

  1. Referee: [Data and Methods] Data and Methods (composite construction): The MJO/BSISO phase composites for wet-bulb temperature and heatwave probability do not describe any removal or regression of ENSO, IOD, or long-term trends. Because MJO amplitude and phase are known to be modulated by ENSO, and ENSO directly alters tropical specific humidity and temperature, unremoved interannual signals could be aliased into the reported phase-dependent likelihood changes, undermining the isolation of the intraseasonal effect.

    Authors: We agree this is a valid concern that could affect the isolation of the intraseasonal signal. To address it, we will add a new subsection in the Methods describing the regression of ENSO (Niño 3.4 index) and IOD indices, as well as removal of linear trends, from the specific humidity and temperature fields prior to phase compositing. We will then recompute the key composites and likelihood changes; if the primary findings remain robust, we will report both the original and revised results for transparency. revision: yes

  2. Referee: [Results] Results (quantitative claims): The assertion that humid heatwave likelihood doubles or halves in large areas lacks accompanying details on the precise heatwave definition, baseline frequencies, number of events per phase, or statistical significance testing. Without these, the magnitude of the reported effect cannot be evaluated for robustness.

    Authors: We acknowledge that these supporting details are needed for a full assessment of robustness. In the revised manuscript we will expand the Methods to state the exact heatwave definition (wet-bulb temperature exceeding the local 95th percentile for at least three consecutive days), provide baseline climatological frequencies, report the number of events per MJO/BSISO phase, and include statistical significance testing via a 1000-member bootstrap resampling of the phase composites. These elements will be added to the text and indicated on the relevant figures. revision: yes

Circularity Check

0 steps flagged

No circularity: observational composites on external MJO index

full rationale

The paper's central claim rests on standard phase-based composites of an externally defined MJO/BSISO index applied to observed wet-bulb temperature and heatwave metrics. No equations, fitted parameters, or self-citations are described that reduce the reported influence (doubling/halving of likelihood) to the input data by construction. The analysis is self-contained against external benchmarks such as the Wheeler-Hendon MJO index and reanalysis datasets, with no self-definitional steps or load-bearing self-citations.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard definitions of the MJO index and wet-bulb temperature from prior literature, with no new free parameters or invented entities introduced in the abstract.

axioms (2)
  • standard math MJO phases are defined using established indices such as RMM or similar
    Common practice in intraseasonal variability studies
  • domain assumption Humid heat is quantified via wet-bulb temperature
    Standard metric for heat stress in the field

pith-pipeline@v0.9.0 · 5909 in / 1346 out tokens · 51647 ms · 2026-05-18T15:10:48.162788+00:00 · methodology

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Reference graph

Works this paper leans on

4 extracted references · 4 canonical work pages

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    #(%!)'(()(

    Introduction Global air temperature has risen since the pre-industrial period (IPCC, 2023) with the past ten years (2015-2024) all being in the top ten of the warmest years in global temperature data records going back to 1850 (NOAA, 2025). Global warming trends and associated increased extreme events such as heatwaves have, for a long time, focused on me...

    work page 2023

  2. [2]

    Results a. Key characteristics of the MJO Figure 1 shows composite maps of the precipitation anomalies in the tropics and subtropics, with surface wind overlaid, for different MJO phase pairs for the two ‘extended seasons’, November to April (NDJFMA) and May to October (MJJASO), thus characterising the MJO cycle. Fig. 1. Composite maps of intraseasonal an...

    work page 1966

  3. [3]

    El Niño–Madden-Julian Oscillation

    Summary and discussion We have quantified the influence of the Madden-Julian Oscillation on intraseasonal variations of humid heat and heatwaves (based on the wet-bulb temperature, Tw) across the tropics and subtropics. Our analysis has revealed statistically significant (at the 95% confidence level) and robust anomalies of Tw (and other heat stress metri...

    work page doi:10.5065/zgjd-9b02 2008

  4. [4]

    https://doi.org/10.1175/JCLI-D-19-0337.1 Huffman, G

    Journal of Climate, 33(8), 3333–3349. https://doi.org/10.1175/JCLI-D-19-0337.1 Huffman, G. J., Adler, R. F., Morrissey, M. M., Bolvin, D. T., Curtis, S., Joyce, R., McGavock, B., & Susskind, J. (2001). Global Precipitation at One-Degree Daily Resolution from Multisatellite Observations. Journal of Hydrometeorology, 2(1), 36–50. https://doi.org/10.1175/152...

    work page doi:10.1175/jcli-d-19-0337.1 2001