The choice of the Madden-Julian Oscillation (MJO) as the centerpiece of the study is no accident. The MJO is a vast, eastward-propagating envelope of tropical convection that circles the planet at roughly 5 meters per second, with a characteristic cycle of 40 to 50 days—a fact first documented by Roland Madden and Paul Julian in 1972. It is the dominant source of weather variability on weekly-to-monthly timescales in the tropics and, through atmospheric teleconnections, exerts a powerful influence on precipitation over regions as distant as the western United States. For many forecast applications, accurately simulating the MJO is a prerequisite for skillful S2S precipitation prediction. Yet despite five decades of research, the MJO remains notoriously difficult for even the most advanced Earth system models to capture, and disagreements about its fundamental generation mechanisms persist.

The team targeted three well-observed MJO events that occurred during the 2011 Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign, spanning mid-October through December 2011. These events offer an exceptionally well-documented framework for evaluating how models represent MJO-related convection and precipitation. The experimental design was elegantly layered. A control simulation, launched on 1 September 2011, used ERA5 atmospheric fields—generated through the HICCUP tool—for its atmospheric initial conditions and land states derived from an offline E3SM Land Model run forced by ERA5 data stretching back to 1979. Ten additional experiments, initialized at approximately five-day intervals from 5 September to 20 October, formed an ensemble whose mean the authors call E_ERA5. Against these, the team ran two counterfactuals: one in which only the land initial conditions came from an equilibrium state, and one—the fully equilibrium experiment—in which both the atmosphere and land began from a balanced state obtained after roughly twenty years of coupled model simulation.

The results are striking in their consistency. When precipitation from the experiments was compared against the CMORPH satellite-based precipitation dataset over the period from 20 October to 31 December 2011, the equilibrium-initialized simulation showed a time-averaged precipitation difference of 0.89 mm/day in the MJO region, while the ERA5-initialized control showed only −0.32 mm/day. Globally, the gap was smaller but still clear: 0.55 mm/day for the equilibrium run versus just 0.02 mm/day for the ERA5-initialized experiment. Root-mean-square errors told the same story—14.56 mm/day in the MJO region for the ERA5-initialized control versus 15.01 mm/day for the equilibrium case, and 9.45 versus 9.73 mm/day globally. In other words, the traditional equilibrium approach, prized because it minimizes the model’s adjustment shock, produced systematically worse precipitation simulations once the model had settled down.

Perhaps the most consequential finding concerns how long these initialization effects endure. By tracking the evolution of precipitation errors as a function of lead time across the eleven ERA5-initialized experiments, the researchers found that the simulations generally reached their optimal performance around 40 days of integration in the MJO region and around 50 days in the global domain. The control run, initialized from realistic conditions, actually showed larger early errors than the equilibrium experiment during the first weeks—evidence of the so-called initialization shock as the model adjusts toERA5-imposed atmospheric fields—but its errors declined below those of the equilibrium run after approximately 40 days. The authors note that the roughly ten-day lag between the global and MJO-region persistence timescales mirrors the 5-to-9-day lag with which MJO influences propagate to the midlatitudes, an intriguing hint that the memory of initial conditions is physically coupled to the oscillation itself.

To place these results on firm statistical footing, the team conducted additional ten-member ensemble experiments, perturbing initial atmospheric temperatures with small-amplitude white noise. They then evaluated MJO prediction skill using the bivariate anomaly correlation coefficient (ACC) applied to the real-time multivariate (RMM) index, a standard metric constructed from observed outgoing longwave radiation (OLR) and zonal winds at 850 and 200 hPa following Wheeler and Hendon’s widely used methodology. An ACC above 0.5 is conventionally regarded as useful forecast skill. The ERA5-initialized control maintained ACC values above 0.5 for roughly the first 65 lead days during the first MJO event—longer than reported in many previous studies, a result the authors attribute partly to the relatively weak MJO activity in that period. More tellingly, Student’s t-tests across the ensembles showed that differences between equilibrium and ERA5 land initializations were statistically significant at the 95 percent confidence level across lead days 44–54, and that differences attributable to atmospheric initial conditions remained significant across lead days 44–58. ERA5-based land initial conditions also reduced the ACC spread by an average of 0.15 over that window.

The mechanism work is where the study makes its most novel contribution. In the MJO region, the improved precipitation skill turned out to be tightly linked to a better representation of outgoing longwave radiation, the satellite-observed signature of deep tropical convection. The correlation between precipitation and OLR errors in the E_ERA5 ensemble reached 0.5 over lead days 11 to 73, and the OLR error itself reached its minimum around day 40—mirroring the precipitation error evolution almost exactly. Zonal wind errors, after an initial spike from the initialization shock, stabilized near day 40 at both 200 and 850 hPa. At the global scale, by contrast, the controlling variable was surface latent heat flux—the evaporation-driven energy exchange between surface and atmosphere—whose errors correlated with precipitation errors at 0.72. The authors interpret this as evidence that regional tropical precipitation is fundamentally a convection problem, while global precipitation is more strongly constrained by the atmospheric energy budget.

The land component of the story centers on the Maritime Continent, the archipelagic region of Indonesia and surrounding islands where MJO convection often stalls or reorganizes. When the team compared their ERA5-land and equilibrium-land ensembles during 15–25 October 2011—the window of maximum divergence—they found that the equilibrium-land simulation systematically underestimated surface latent heat flux over equatorial land areas relative to ERA5. That deficit limits the transport of moisture from the land surface into the lower troposphere, drying the planetary boundary layer and creating hostile conditions for deep convection: rising convective parcels suffer enhanced entrainment dilution and evaporative cooling, suppressing convective development. The ERA5-land experiment, with more realistic moisture fluxes, sustained a moister lower troposphere and more robust deep convection. The equilibrium-land run also displayed a widespread warm bias in surface temperature over equatorial land, which the ERA5-land initialization substantially reduced, restoring more realistic land-surface energy partitioning and boundary-layer thermodynamics.

These surface improvements propagated upward into the convection and circulation fields. The equilibrium-land run overestimated OLR near 10°N, 90°E and underestimated it near 150°E—hallmarks of misplaced convective activity—while the ERA5-land experiment sharply reduced those biases, particularly near 150°E. At 850 hPa, the equilibrium-land run’s overestimate of zonal winds near 130°E, reflecting distorted low-level convergence, was likewise corrected. Together, the chain of evidence—surface fluxes to boundary-layer moisture, moisture to convection, convection to OLR and circulation—demonstrates that land initial conditions act as a genuine, statistically significant, though secondary, source of MJO predictability through their modulation of land–atmosphere coupling.

The findings carry practical weight for the forecasting community. S2S prediction occupies a critical gap in operational meteorology: weeks three through six, beyond the reach of deterministic weather prediction but before the slow ocean drivers of seasonal climate dominate. Current forecast systems, from ECMWF’s SEAS5 to NOAA’s unified forecast system, depend heavily on initialization quality, and the results here suggest that investing in realistic land-surface initialization—alongside atmospheric data assimilation—can yield measurable gains at timescales where skill is scarce. The study also highlights an unresolved tension: a 2024 analysis of the Community Earth System Model version 2 by Richter and colleagues found no significant impact of land initial state on S2S skill, a discrepancy the authors suggest may reflect how differently models represent atmosphere-land interactions, and how those differences become magnified during active MJO periods.

The authors are careful to frame their work as a case study, albeit one whose robustness is supported by eleven experiments initiated across September and October, and they acknowledge limitations: ten-member ensembles cannot establish strong statistical significance on their own, and ocean initial conditions—the most obvious missing ingredient, given that MJO events predominantly occur over the ocean—were not considered. Ocean coupling, they note, warrants investigation as a next step, ideally through coupled data assimilation approaches that would initialize atmosphere, land, and ocean coherently. In an era when exascale computing is making such ambitious initialization strategies feasible, this study offers both a technical roadmap and a clear demonstration of the payoff: better initial conditions, even imperfect ones, remember themselves far longer than the field has generally assumed—long enough, in fact, to matter for the forecasts that people most need.

Recent decades have witnessed a notable divergence in sea surface temperature (SST) trends within tropical oceans. While many regions, including the Indian Ocean and parts of the Maritime Continent, experienced significant warming, large swathes of the central and eastern equatorial Pacific have exhibited comparatively cooler temperatures. This anomalous oceanic cooling bears a resemblance to persistent La Niña conditions, thereby establishing a La Niña–like background state distinctively different from previous decades. Such an asymmetric SST distribution raises compelling questions about its implications for atmospheric dynamics, specifically how these spatial disparities in ocean warming influence the propagation characteristics of the MJO.

Addressing this challenge, researchers from Pusan National University conducted a comprehensive analysis of MJO behaviors over two distinct periods: 1979–1998, representing pre-1999 oceanic conditions, and 2003–2022, encapsulating the era marked by La Niña–like asymmetric warming patterns. Utilizing an integrated approach combining satellite-derived observations, such as outgoing longwave radiation (OLR) as a proxy for convection, along with sophisticated atmospheric reanalysis datasets, the team was able to track and characterize intraseasonal convective variability while correlating these dynamics with changes in sea surface temperatures and atmospheric circulation patterns. Their findings elucidate how uneven ocean warming reshapes the fundamental propagation and intensity features of the MJO.

Professor Kyung-Ja Ha, who led the study, articulated a transformative insight: “The recent asymmetric tropical ocean warming has driven contrasting changes in the regional propagation of the Madden–Julian Oscillation, with faster eastward progression over the Indian Ocean and Maritime Continent, contrasted by a pronounced slowdown over the western Pacific Ocean.” This statement encapsulates the emergence of a complex, regionally heterogeneous response of the MJO to evolving thermal and atmospheric conditions across the tropical belt, an evolution that carries significant ramifications for climate models and forecasting systems.

Mechanistically, the research highlights the critical interplay between atmospheric moisture gradients and stability in modulating MJO dynamics. Over the Indian Ocean, the intensification of horizontal moisture gradients ahead of the propagating convective envelope fosters enhanced pre-moistening—a key process facilitating deeper convection and faster eastward movement. Concomitantly, an increase in upper-tropospheric atmospheric stability acts to sharpen the vertical structure of the MJO, further assisting rapid propagation. The Maritime Continent presents additional complexity due to its intricate mosaic of landmasses and seas; however, even here, the MJO’s eastward movement accelerated, albeit to a lesser extent compared to the Indian Ocean.

Conversely, the western Pacific Ocean showcases a starkly different picture. This region experienced a deceleration of the MJO’s eastward propagation, attributed primarily to weakened moisture gradients and a suppression of vertical motion critical for convective development. Furthermore, a destabilization of the upper atmosphere in this zone reduces the efficacy of moist convective processes, thereby limiting the MJO’s ability to sustain its movement at prior speeds. The combined effect of these atmospheric and oceanic changes essentially reshapes the MJO’s lifecycle regionally, underscoring the sensitivity of tropical convection to shifting ocean surface temperatures.

A pivotal contribution of this study lies in emphasizing atmospheric stability as a vital diagnostic parameter for the MJO’s evolution. Traditionally, moisture supply and large-scale circulation dominated understanding of MJO dynamics; however, this research demonstrates that vertical thermodynamic structure—specifically atmospheric stability—modulates how intraseasonal convection evolves and propagates. By quantifying changes in stability alongside moisture variations, the study provides a more comprehensive framework to represent and predict MJO behavior in coupled ocean-atmosphere models.

The implications for climate science and meteorological applications are profound. Accurate simulation of the MJO’s propagation speed and amplitude is essential for improving forecasts of extreme rainfall events, tropical cyclones, and monsoonal variability. Prof. Ha emphasizes, “Improving how climate models incorporate the effects of asymmetric ocean warming on MJO behavior will enhance the reliability of seasonal-to-decadal predictions concerning rainfall distribution and drought potential.” Achieving this progress would empower governments, planners, and communities to devise more resilient strategies in agriculture, water resource management, and infrastructure development, particularly in regions vulnerable to the severe impacts of erratic tropical weather.

Beyond immediate forecasting benefits, these findings enrich the broader understanding of climate variability in a warming world. By revealing how ocean warming patterns modulate weather-driving oscillations like the MJO, this study contributes to deciphering feedback mechanisms within the Earth system. Such insights are integral to projecting future climate scenarios under continued anthropogenic forcing, where shifts in tropical convection and circulation could trigger unanticipated atmospheric responses globally.

Methodologically, the research harnessed state-of-the-art satellite measurements and atmospheric reanalysis frameworks, capturing nuanced variations in intraseasonal convective activity via outgoing longwave radiation (OLR) anomalies. Simultaneously analyzing sea surface temperature distributions allowed the team to disentangle ocean-atmospheric coupling processes that dictate MJO evolution. Vertical profiles of temperature and humidity further informed assessments of atmospheric stability, enabling a multi-dimensional portrayal of the physical environment conducive or restrictive to MJO progression.

In conclusion, the revelation that uneven tropical ocean warming substantially alters the Madden–Julian Oscillation’s regional propagation heralds a paradigm shift in tropical meteorology. This oscillation, long recognized as a cornerstone of tropical and global weather, now emerges as a sensitive barometer of oceanic thermal asymmetries and their cascading atmospheric consequences. As climate change continues to rewrite Earth’s thermal landscape, understanding and integrating these dynamic feedbacks into predictive models will prove critical for safeguarding communities worldwide from the intensifying vagaries of weather and climate.

This important study was carried out under the auspices of the PNU Global—Learning & Academic Research Institution for Master’s, PhD students, and Postdocs (G-LAMP) Program, embodying cutting-edge interdisciplinary climate science innovation emerging from Pusan National University.

Subject of Research: Not applicable

Article Title: Recent asymmetric tropical ocean warming has altered regional propagation of Madden-Julian Oscillation

News Publication Date: 14-Aug-2025

Web References: https://doi.org/10.1038/s43247-025-02652-z

References: DOI: 10.1038/s43247-025-02652-z

Image Credits: Pusan National University

Keywords: Climatology, Climate change, Climate variability, Ocean surface temperature, Monsoons, Weather forecasting, Ocean warming, Precipitation

The Madden-Julian Oscillation is a crucial element of tropical meteorology, characterized by large-scale atmospheric circulation patterns that transit eastward around the equator. Normally, this oscillation manifests as a series of moisture waves and convection, which can significantly affect weather variations, including monsoonal rains in South Asia and hurricane activity in the Atlantic. This new study highlights how recent changes in the warming patterns of tropical oceans are already causing shifts in the strength and patterns associated with the MJO, with potential ramifications for millions worldwide.

One of the striking revelations of this research is the asymmetrical nature of the warming occurring in the tropical oceans. Unlike uniform warming, the researchers identified that certain regions of the ocean surface are heating at rates that defy previous models and expectations. This asymmetry raises questions about the traditional understanding of ocean-atmosphere interactions and their roles in global weather patterns. As a result, the propagation speed and strength of the MJO fluctuations are affected, suggesting that these observed deviations from the norm are aligned with wider climatic shifts.

The researchers utilized extensive datasets, covering various geographical regions and employing sophisticated climate models, that allow them to simulate ocean-atmosphere interactions with unprecedented accuracy. The findings indicate distinct differences in temperature indices between the eastern and western regions of the tropical oceans. This disparity not only alters the dynamics of the ocean as a whole but also affects the atmospheric responses, with cascading effects on circulation patterns that may influence regional climates across vast distances.

An intriguing aspect of this research is its potential to explain why climate models have struggled to predict weather patterns accurately over the recent decades. The traditional models often assume uniform ocean temperatures, failing to capture the intricate dynamics of asymmetric warming. This misunderstanding may have contributed to gaps in forecasting abilities, particularly concerning events like the onset of tropical storms, droughts, and flooding—an alarming concern as climate variability becomes more pronounced.

The study’s authors emphasize the urgency of fine-tuning existing climate models to incorporate these new empirical findings on ocean warming. By doing so, future predictions can become more reliable, which is critical for disaster preparedness and resource management, particularly in regions vulnerable to extreme weather events. As global temperatures continue to rise, understanding how tropical ocean dynamics interact with atmospheric systems will be essential for mitigating risks associated with climate change.

Moreover, the implications of these findings extend beyond immediate weather-related concerns. The impact of altered MJO patterns can influence agricultural yields, water supply stability, and even marine biodiversity. In regions where monsoon rains are crucial for food production, a shift in rainfall patterns could lead to significant socio-economic challenges. Climate resilience and adaptive strategies will need to be developed based on these new insights to ensure that communities can withstand potential disruptions.

The global community needs to take heed of these findings, considering the interconnected nature of climate systems. As we continue to grapple with the repercussions of climate change, investing in research that enhances our understanding of tropical ocean dynamics can provide important guidance for policymakers. The study by Kim and colleagues is a clarion call to recognize the urgency of addressing altered climate patterns, providing a roadmap to navigate an increasingly complex reality.

Next, the research team outlined their further exploration into how these findings may also reshape our understanding of global weather patterns, specifically in relation to El Niño and La Niña events. These oscillations interact with the MJO and are critical determinants of climate variability across the globe. The layered relationships among ocean temperatures, atmospheric feedbacks, and historical weather data create a rich area for further study, which could yield invaluable insights.

Ultimately, while the research sheds light on the new landscape of tropical ocean warming, it also serves as a reminder of the complexities within our planet’s climate system. As interrelated dynamics continue to evolve, our understanding must adapt accordingly. It is hoped that further studies will build upon this groundwork, refining predictive models that account for new phenomena like this asymmetric warming and how it impacts global systems.

In an era marked by climate uncertainties, the crucial connections illustrated in this study could ignite further exploration and lead to ambitious global collaborative efforts aimed at combatting the deleterious effects of climate change. The outcomes of this research may mark a pivotal moment in both climate science and policy, serving as a springboard for future inquiries that can create an adaptive global community prepared to tackle the urgent challenge of climate change.

As the world continues to observe these changes unfold, the essence of resilience against climate impacts lies in collaboration, education, and a deeper understanding of the profound connections between the ocean and atmosphere. Such efforts can ensure that societies are equipped not just to endure, but to thrive despite the challenges posed by a warming world.

Subject of Research: Recent asymmetric tropical ocean warming and its effects on the Madden-Julian Oscillation.

Article Title: Recent asymmetric tropical ocean warming has altered regional propagation of Madden-Julian Oscillation.

Article References:

Kim, HR., Ha, KJ., Roxy, M.K. et al. Recent asymmetric tropical ocean warming has altered regional propagation of Madden-Julian Oscillation.
Commun Earth Environ 6, 663 (2025). https://doi.org/10.1038/s43247-025-02652-z

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s43247-025-02652-z

Keywords: Tropical Ocean Warming, Madden-Julian Oscillation, Climate Change, Weather Patterns, Climate Models, Environmental Impact, Global Climate Dynamics.