The study used a Warm-Wet Index, or WWI, designed to represent the combined influence of temperature, precipitation and surface energy exchanges rather than examining heat or rainfall in isolation. The index incorporates annual precipitation, near-surface air temperature, sensible heat flux and latent heat flux through a moisture-and-energy balance based on the Budyko framework. Because most of China receives the majority of its annual precipitation in summer, the investigators consistently applied the summer-dominant formulation. A higher WWI indicates a climate state in which a greater share of available turbulent surface energy supports evaporation, which the authors associate with relatively warmer and wetter conditions. This approach captures the fact that precipitation, evaporation and surface heating interact differently across China’s humid southeast, arid northwest and high-elevation Tibetan Plateau.

ERA5 reanalysis supplied the principal meteorological fields at their native resolution, including 2-m air temperature, precipitation, wind speed, vertical velocity, specific humidity, sea-surface temperature, and sensible and latent heat fluxes. The authors also used the gridded CN05.1 observational dataset, constructed from records at 2,416 meteorological stations, to evaluate ERA5 precipitation over China, especially in areas with complex terrain and pronounced spatial heterogeneity. The PDO index came from the National Center for Atmospheric Research. This combination of records and reanalysis allowed the authors to examine both the surface hydroclimatic signal and the large-scale ocean and atmospheric patterns associated with it, although the study was observational in design and did not establish the PDO as the sole cause of China’s warm-wet variability.

The background climate over the 63-year period showed substantial warming and uneven precipitation change. China’s national mean temperature rose at approximately 0.30 °C per decade, with trends locally exceeding 0.4–0.5 °C per decade over the Tibetan Plateau. Annual precipitation increased by 2.64 mm per decade nationally, but the spatial pattern differed markedly: northwestern China experienced a robust increase, whereas North China retained a drying tendency. The authors also identified changes in surface energy partitioning, including stronger upward sensible heat transfer in northern China and enhanced upward latent heat flux in association with greater evaporative transfer of energy from the land surface to the atmosphere.

The resulting WWI displayed a northwest-to-southeast gradient, with relatively low values in the arid interior and higher values in the humid southeast. A local maximum appeared over the Tianshan Mountains in northern Xinjiang, where orographic uplift promotes moisture convergence and precipitation despite the surrounding aridity. Temporal variation was more complex than a single warming-and-wetting trend: the national index declined during much of the 1960s and 1970s, recovered in the early 1980s, remained relatively high through much of the 1981–2000 period, and then fluctuated after 2000 with a gradual rebound. Its linear trend was weak, at about −0.01 per decade, whereas its interdecadal component changed on approximate 15–25-year timescales. Empirical orthogonal function analysis separated two leading spatial modes, explaining 20.2% and 13.8% of the total variance and revealing contrasting large-scale patterns in China’s warm-wet conditions.

To isolate the slow-changing component, the authors applied Ensemble Empirical Mode Decomposition to the national WWI and PDO series, then combined the third and fourth intrinsic mode functions as their interdecadal signal. The resulting national WWI and PDO series had a correlation of 0.58. That coefficient indicates a moderately strong association, but its conventional statistical significance did not persist after the effective degrees of freedom were reduced to account for the strong temporal autocorrelation of the low-frequency series. The limited effective sample size therefore requires caution. Regional spatial analyses, which also accounted for effective degrees of freedom, nevertheless identified statistically significant PDO–WWI relationships in several areas. Major PDO phase transitions in the mid-1970s also slightly preceded corresponding changes in the WWI, a timing pattern consistent with the ocean acting as a background modulator rather than merely coinciding with short-term atmospheric fluctuations.

Singular value decomposition provided an ocean–land diagnostic by identifying coupled patterns that maximize the squared covariance between North Pacific sea-surface temperature anomalies and China’s WWI. The leading coupled mode accounted for 58% of the total squared covariance. Its sea-surface temperature pattern resembled the positive PDO “horseshoe,” with negative anomalies across the central and western North Pacific and positive anomalies along the basin’s eastern boundary and near western North America. The associated Chinese pattern formed a meridional tripole: enhanced warm-wet conditions in Northeast China, northern Xinjiang and southeastern coastal regions, and reduced conditions across North China and the Tibetan Plateau. Coherent changes in the paired time coefficients, together with agreement between the coupled mode, the leading empirical orthogonal function modes and the EEMD results, support a physically consistent association between PDO-related ocean variability and China’s warm-wet conditions.

Composite analyses showed that precipitation was the main pathway connecting PDO phase changes to the integrated index. During positive PDO phases, detrended precipitation generally increased over central-eastern and southeastern China, extending northward toward Northeast China, while declining over North China and the Tibetan Plateau. Negative PDO phases produced a broadly reversed pattern, with precipitation deficits across southeastern China, Xinjiang and Northeast China. Multiple linear regression supported this interpretation: precipitation contributed more than 0.6–0.8 to WWI variation across much of eastern, central and southern China and large parts of the Northeast, whereas temperature and the two surface heat-flux terms had smaller, more geographically confined contributions. The authors described precipitation as the dominant statistical control, not a complete explanation for every regional signal.

The circulation diagnostics supplied the dynamic mechanism. Positive PDO phases were associated with sea-surface temperature anomalies in the tropical central-eastern Pacific and Indian Ocean that favored an East Asia–Pacific-type teleconnection response. A pronounced cyclonic circulation developed over much of China, with an anomalous wave train extending from the Philippine Sea toward the Sea of Japan. Enhanced upward motion and moisture convergence appeared over southeastern China, Northeast China and northern Xinjiang, supporting increased rainfall and higher WWI, while subsidence over North China suppressed precipitation. The westerly jet shifted southward, a configuration favorable for more rainfall in southern China, and the Pacific–North American teleconnection was also strengthened. During negative PDO phases, the pressure and circulation anomalies generally reversed, with anticyclonic flow and widespread subsidence replacing ascent over the same regions. These findings describe a consistent ocean–atmosphere–land pathway, but they do not prove that the PDO alone drives the observed changes; the Atlantic Multidecadal Oscillation, internal atmospheric variability and other factors may also contribute. The authors also identify separation of externally forced trends from internal decadal variability, seasonal mechanisms and future climate-model projections as unresolved questions.

Subject of Research: Interdecadal variability of China’s integrated warm-wet conditions and its association with the Pacific Decadal Oscillation

Article Title: Decadal variability of the integrated warm-wet conditions over China and its modulation by the pacific decadal oscillation

Article References: Yang, X., Ji, F., Zhang, S., Chang, J., Hu, S., Luo, M., Chen, Y., & Guan, X. (2026). Decadal variability of the integrated warm-wet conditions over China and its modulation by the pacific decadal oscillation. Frontiers in Environmental Science, 14, 1992362. https://doi.org/10.3389/fenvs.2026.1992362

Image Credits: AI Generated

DOI: 10.3389/fenvs.2026.1992362

Keywords: China, Pacific Decadal Oscillation, warm-wet index, interdecadal variability, precipitation, atmospheric circulation, ERA5 reanalysis, moisture transport, climate teleconnections, regional climate, Decadal, variability

The study rests on a simple but powerful epidemiological premise: antibodies of the immunoglobulin G class persist for years after infection or vaccination, so their prevalence in a population acts as a cumulative record of exposure. Because hepatitis A and hepatitis E are both transmitted primarily through the faecal-oral route, their seroprevalence patterns are tightly coupled to sanitation, water quality, hygiene practices and socio-economic conditions. When a community improves its water and sanitation infrastructure, the age at which children first encounter these viruses tends to rise, shifting the burden of susceptibility toward older individuals in whom infection is more likely to cause symptomatic disease. This phenomenon, known as epidemiological transition, is one of the central concerns for vaccination policy in low- and middle-income countries undergoing rapid urban development.

To test whether such a transition was underway in Vellore, the team drew on biobanked serum samples from 600 participants in the 2013 survey and 558 participants in the 2022 survey, all aged between one and forty years. Each sample was tested for IgG antibodies specific to hepatitis A virus and hepatitis E virus. The investigators then stratified seroprevalence estimates by age, gender and residential cluster, allowing them to detect not only overall changes between the two survey years but also geographic heterogeneity within the city. Statistical comparisons of seropositivity between years were carried out using proportion tests or Fisher’s exact tests, depending on the structure of the data.

The hepatitis A results were striking in their consistency. Seroprevalence exceeded 80 percent among children aged one to five years in both surveys and climbed to 100 percent among individuals aged sixteen and above, in 2013 as well as 2022. No statistically significant differences in hepatitis A seroprevalence were observed between the two survey years in any age stratum. In practical terms, nearly every resident of urban Vellore had been infected with hepatitis A virus by early adulthood in both eras, and infection continued to occur early in childhood. The virus, in this setting, remains firmly endemic, and the population’s collective immunity remains high.

Hepatitis E told a different story. IgG seroprevalence against this virus was low among children and adolescents in both surveys and increased progressively with age, a pattern consistent with sporadic rather than sustained childhood transmission. Among adults aged twenty-six to forty years, seroprevalence rose from 18 percent in 2013, with a 95 percent confidence interval of 12 to 27 percent, to 24 percent in 2022, with a 95 percent confidence interval of 16 to 32 percent. Although this increase suggests a possible gradual accumulation of exposure in adulthood, the change did not reach statistical significance, and the authors are careful not to overinterpret it. Across both viruses, no significant gender-based differences in seropositivity were detected.

Cluster-wise analysis added a spatial dimension to these findings. Hepatitis A seroprevalence was uniformly high across all residential clusters sampled in Vellore, reflecting the pervasive nature of early-life exposure to the virus throughout the urban environment. Hepatitis E seroprevalence, by contrast, was both low and heterogeneous across clusters, indicating that exposure to this virus is patchy and likely driven by localised factors such as intermittent contamination of water supplies, sanitation gaps in specific neighbourhoods, or differences in food handling practices. This heterogeneity matters for surveillance design, because a citywide average can easily mask pockets of elevated risk where outbreaks of hepatitis E, which can be particularly dangerous for pregnant women, may originate.

Beyond simple prevalence counts, the study employed more sophisticated quantitative tools to probe hepatitis A transmission dynamics. The researchers applied contact matrix and mixture modelling to the serological data to estimate the effective reproduction number, Re, for hepatitis A in each survey year. Mixture modelling, which treats the antibody distribution in a population as a blend of distributions from susceptible and immune individuals, allows researchers to infer the force of infection even from cross-sectional snapshots. The analysis yielded an Re below 1 in both serosurveys, indicating that secondary transmission of hepatitis A was not sustaining epidemic growth in either era, despite the accumulation of susceptible individuals in older age groups over time. This finding suggests that while the age profile of susceptibility may be shifting slowly, the underlying transmission intensity has not yet crossed the threshold that would fuel outbreaks among older, more vulnerable populations.

The implications for vaccination policy are significant. In many middle-income countries that have improved sanitation, hepatitis A has transitioned from a disease of early childhood, where infection is usually asymptomatic, to one affecting older children and adults, in whom clinical illness and occasional severe outcomes are more common. This shift is the classic argument for universal childhood hepatitis A vaccination: immunising children early both protects them and reduces circulation, indirectly shielding older susceptible individuals. The Vellore data show that this transition, while anticipated, has not yet materialised in measurable form; immunity remains high and acquired early, and the effective reproduction number remains below one. The authors note that their findings should inform age-specific considerations for hepatitis A vaccination policy in urban India as it undergoes transition, implying that the window for deciding on vaccination strategy remains open but should be monitored with continued serosurveillance.

For hepatitis E, the picture is one of limited but persistent and age-dependent exposure. The low seroprevalence among the young means that a large fraction of the population reaches adulthood without prior immunity, and the gradual rise in seropositivity with age reflects cumulative adult exposure. The authors highlight the need for targeted hepatitis E surveillance, particularly because the virus causes substantial morbidity in pregnant women and can trigger large outbreaks when water supplies are compromised. The modest, non-significant rise in adult seroprevalence between 2013 and 2022 is consistent with continued low-level transmission that could accelerate under adverse conditions, making sustained monitoring essential.

Perhaps the clearest message of the study is a reaffirmation of the value of water, sanitation and hygiene interventions. The stability of hepatitis A immunity over the decade, and the continued containment of hepatitis E transmission below epidemic thresholds, support continued investment in WASH infrastructure as the backbone of enteric virus control in urban India. At the same time, the study demonstrates the power of repeated, geographically stratified serosurveys using biobanked samples to detect epidemiological change before it becomes clinically visible. As Indian cities continue to grow and modernise, the balance between endemic early-childhood infection and emerging adult susceptibility will determine whether hepatitis A vaccination becomes a public health priority, and whether hepatitis E remains a smouldering, localised threat or flares into wider outbreaks. The Vellore data provide a rigorous baseline against which that future can be measured.

Subject of Research: Decadal changes in IgG seroprevalence of hepatitis A and E virus in urban Vellore, India

Article Title: Decadal changes in IgG seroprevalence of hepatitis A and E virus in Urban Vellore, India: Persistent Endemicity or Epidemiological Shift?

Article References: Madhavan, R., Eswaran, A., Varghese, T., Gandhi, V., Raj, L., Raju, R., Joseph, J. S., Xavier, J. V. L., Rose, W., & John, J. (2026). Decadal changes in IgG seroprevalence of hepatitis A and E virus in Urban Vellore, India: Persistent Endemicity or Epidemiological Shift?. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14413-0

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14413-0

Keywords: Hepatitis A, Hepatitis E, IgG seroprevalence, serosurvey, enteric infections, WASH, epidemiological transition, urban India, effective reproduction number, vaccination policy, Decadal, changes