The Intertropical Convergence Zone (ITCZ), a crucial regulator of global weather and climate, significantly influences atmospheric circulation and the distribution of air pollutants. Its location, typically defined by the latitude of peak ascent averaged over a specific longitude range and season, is dynamic, migrating seasonally and exhibiting interannual variability. While a global poleward ITCZ shift is anticipated due to Northern Hemisphere warming, regional variations exist. The Maritime Continent, a major source of aerosols from biomass burning and wildfires, experiences frequent haze events, impacting public health. Black carbon (BC), a component of these emissions, is particularly harmful, contributing significantly to premature mortality. The connection between ITCZ location, BC distribution, and resultant health impacts in the Maritime Continent remains understudied. This research utilizes multiple reanalysis datasets and CMIP6 simulations to examine the long-term shift of the ITCZ in the Maritime Continent, analyzing its relationship with anthropogenic greenhouse gas emissions, its effects on BC concentration via changes in atmospheric circulation, and ultimately, its impact on public health in the region. The goal is to understand climate change's role in regional air pollution and inform environmental mitigation strategies.

Existing literature on ITCZ shifts reveals conflicting findings regarding spatiotemporal scales. While some studies suggest an annual-scale tropical expansion, long-term variations in seasonal ITCZ location remain uncertain. Zonal variations are more apparent, with a general poleward shift predicted for a warming Northern Hemisphere due to anthropogenic factors. However, regional discrepancies exist, for example, contrasting northward shifts in Eastern Africa and the Indian Ocean versus southward shifts in the Eastern Pacific and Atlantic Oceans. These inconsistencies stem from varying datasets and methodologies used to define ITCZ location. Recent research suggests that divergent atmospheric energy transport plays a crucial role in determining zonal ITCZ latitude. Overall, the need for regional-scale investigation of ITCZ shifts under climate change is highlighted, particularly in regions like the Maritime Continent, which serves as a significant heat source for global circulation and a major aerosol source.

This study employed four reanalysis datasets (ERA5, MERRA-2, NCEP-DOE, and ERA-Interim) to analyze ITCZ location, intensity, and width in boreal summer (June-September) from 1980 to 2014. ITCZ location was defined as the latitude of the peak 500 hPa ascent, intensity as the peak ascent value, and width was calculated as area/length of the region where 500 hPa ascent was below -15 hPa/day. To attribute equatorial warming to anthropogenic activities, 10 CMIP6 models were used with both All-hist (all forcings) and Nat-hist (natural forcings only) simulations of near-surface air temperature. Probability density functions (PDFs) were compared using the Kolmogorov-Smirnov test. Composite analysis was used to examine the impact of equatorward ITCZ shifts (defined as anomalies relative to 1980-2014 mean) on BC concentration and 850 hPa circulation. BC concentration data were obtained from MERRA-2. The impact on public health was assessed using a concentration-response function (CRF) derived from epidemiological studies, linking BC concentration to all-cause premature mortality rates using population data from WorldPop and mortality rates from the Global Burden of Disease (GBD) study. The uncertainty in health impact estimates was evaluated using a Monte-Carlo approach based on a triangular probability distribution for the CRF coefficient.

The study revealed a significant equatorward shift of the boreal summer ITCZ in the Maritime Continent by approximately 2.1° in the recent decade (2005-2014) compared to the historical decade (1980-1989), supported by all four datasets. This shift was significantly associated with increased regional equatorial sea surface temperature (SST) anomalies. CMIP6 simulations further indicated that anthropogenic activities contributed substantially to this equatorial warming. The equatorward ITCZ shift led to spatially heterogeneous impacts on BC concentration. In maritime areas (Central Sumatra and Borneo), BC concentration decreased significantly due to enhanced updrafts, increased rainfall, and wet deposition. However, in continental areas, BC concentration increased due to weakened updrafts. The shift also weakened summer circulation, suppressing BC transport from maritime to the Philippines. Analysis of BC changes in relation to ITCZ shift and rainfall showed a significant decrease in BC concentration in the maritime area, less pronounced changes in the Philippines, and a significant increase in continental areas. The health impact assessment revealed that in maritime countries, the equatorward ITCZ shift decreased BC-associated premature mortality by ~13% (2010 baseline), while in continental countries, it increased mortality by ~6%. These effects were amplified under extreme ITCZ shifts (<-2.1°).

The findings demonstrate that anthropogenic-induced equatorial warming significantly contributes to the equatorward shift of the ITCZ in the Maritime Continent, leading to spatially heterogeneous impacts on BC concentration and public health. The mechanisms involve altered atmospheric circulation (updrafts, low-level winds) and wet deposition. While the maritime area benefits from reduced BC pollution and related mortality, continental areas face increased health risks. This spatial heterogeneity highlights the importance of considering regional specificities in developing climate change adaptation strategies. The weakening of the East Asia Summer Monsoon, possibly linked to changes in the Western North Pacific Subtropical High position rather than strength, further contributes to the observed patterns. Future research needs to consider other pollutants besides BC and improve the spatial resolution of precipitation datasets for a more complete analysis.

This study reveals a significant equatorward shift of the boreal summer ITCZ in the Maritime Continent, driven by anthropogenic-induced equatorial warming. This shift differentially impacts BC concentration and public health, decreasing mortality in maritime areas but increasing it in continental areas. The study emphasizes the importance of regional-scale considerations in developing climate change adaptation strategies for air pollution mitigation and public health protection. Future research should focus on higher-resolution data analysis, incorporating other air pollutants, and employing numerical simulations to further elucidate the intricate mechanisms involved.

The study has some limitations. Firstly, the coarse spatial resolution of long-term precipitation datasets may limit the precise detection of ITCZ shifts. Secondly, model biases in long-term precipitation simulations need to be addressed. Thirdly, the analysis focused on monthly-scale variations, potentially overlooking the impact of daily-scale processes such as BC aging and local wet deposition. Fourthly, while the study linked BC changes to ITCZ shifts using statistical methods, quantitative assessments based on numerical simulations are needed for a more complete understanding of the mechanism. Finally, the health impact assessment relies on a concentration-response function that involves uncertainties.