This study relies entirely on publicly available data, covering the period from 2003 to 2023. We used data products from various satellite measurements, ground-based observations, and reanalysis products.

The anomalies in the various parameters for April 2023 were estimated by subtracting the April background long-term mea (2003–2022) from April 2023 value.

The magnitude of the AOD / CO enhancement in April 2023 above the long-term background was determined by comparing the average of April 2003–2022. We obtained the percentage change in AOD / CO relative to the respective background using Eq. (1): 1Relative change in percentage=xi-x‾x‾×100 where xi represents the monthly mean of April in 2023, and x‾ is the long-term mean of April calculated using the data from 2003 to 2022.

Aerosol optical depth (AOD) is a common metric for measuring atmospheric aerosol loading and is crucial for radiative forcing assessments (Hirsch and Koren, 2021). In this study, we use AOD data from the MODIS instruments aboard the Aqua and Terra satellites for the period. Before analyzing the unusual AOD conditions observed in April 2023, we first characterize the long-term AOD behavior over the study area using two decades of MODIS data. The spatial patterns of the long-term annual mean AOD and its associated standard deviation across the Asian region are shown in Fig. 2a and b, respectively. Additionally, the long-term monthly-mean variability of AOD over the two main BB regions surrounding the SCS, namely PSEA and the MC, is shown in Fig. 2c and d. The relatively low AOD levels and small standard deviations over the SCS suggest a predominantly clean marine environment. The seasonal cycle of AOD shows clear peaks linked to regional BB activities (Fig. 2c and d). Over PSEA, AOD peaks during March–April, while over the MC, the peak occurs in September–October. These seasonal maxima align with well-known BB activity periods in these areas (Ravindra Babu and Lin, 2023; Chang et al., 2024). The fire season over the MC usually runs from August to October, whereas PSEA experiences intense BB activity from January to April, with a notable peak in March.

To emphasize the anomalous conditions in April 2023, we present the spatial distribution of April AOD over the study region for both the long-term average and April 2023. The long-term average April AOD for 2003–2022 and the corresponding April 2023 AOD distribution are shown in Fig. 3a and b. Moreover, the time series of monthly mean AOD from 2003 to 2023 over northern PSEA (17–23° N, 99–106° E) and the SCS (11–18° N, 109–119° E) are displayed in Fig. 3c and d. The AOD distribution in April over two decades indicates high aerosol loading from northern Laos to coastal South China (15–25° N, 100–120° E). In April 2023, extreme AOD values extended from PSEA to South China and SCS, with the highest values centered between northern Laos and the SCS. Record-breaking AOD levels were observed for the area averaged over the SCS in April 2023, showing a nominal increase in northern PSEA (Fig. 3c). However, the highest AOD value for northern PSEA in April 2023 correlates with record AOD over the SCS. Long-term monthly mean AOD from Aqua and Terra (2003–2023) exhibited a strong correlation of 0.97, confirming the consistency and reliability of these observations (Fig. S1a in the Supplement). To assess the magnitude of the increase, we estimated the percentage change in AOD by comparing April 2023 with the long-term April average from 2003 to 2022.

Figures 4a and 4b illustrate the spatial extent of AOD anomalies, shown as percentage changes, based on data from MODIS Aqua and Terra satellites. In April 2023, there was a widespread and unexpected increase of about 150 % across most of the SCS and southern Bay of Bengal (BoB), with anomalies surpassing roughly 4 standard deviations. The area-averaged AOD anomalies (%) over the SCS, indicated by the black line for Aqua and the red line for Terra, reveal that the April 2023 peak was the highest on record compared to data from 2003 to 2023, emphasizing the extreme AOD intensity that month. These satellite observations were also supported by ground-based measurements from AERONET. The only operational AERONET remote station downwind of PSEA BB, with over a decade of continuous AOD measurements (Fig. S2a in the Supplement), within the SCS region, is located on Dongsha Island (also known as Pratas Island, 20.70° N, 116.73° E; 5 m a.s.l.). Analysis of monthly mean AOD data from Dongsha Island indicates that April 2023 had the highest AOD value during the observational period from January 2009 to December 2023 (Fig. S2c in the Supplement). AERONET comparisons show strong correlations with MODIS AOD: 0.86 for Aqua and 0.87 for Terra (Fig. S3 in the Supplement), supporting the reliability of the satellite observations. Because AOD is a column-integrated measure, it does not provide information on the vertical distribution of aerosols. To address this limitation, we further analyzed vertical distribution images of aerosols from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), which reveal pronounced enhancements in smoke aerosol over the SCS (Fig. 5). Elevated smoke layers were also observed over the southern BoB in April 2023, predominantly within the mid-troposphere at altitudes of approximately 2–5 km. Consistent with these CALIPSO lidar observations, MERRA-2 reanalysis data indicate substantial increases in aerosol mass concentrations in 2023, with black carbon (BC) increasing by ∼ 250 % and organic carbon (OC) by ∼ 350 % (Fig. S4 in the Supplement). The most pronounced enhancements occur between 700 and 600 hPa, closely matching the altitude range identified by CALIPSO. The concurrence of satellite (MODIS and CALIPSO) and reanalysis data points to a severe pollution episode in April 2023 over and surrounding regions of the SCS, characterized by an elevated aerosol layer indicative of long-range-transported BB smoke. Notably, the SCS is a relatively clean remote marine region with limited local aerosol sources. In such environments, enhanced aerosol loading is typically associated with long-range transport of pollutants from surrounding continental regions (Pani et al., 2023). Given the potential influence of long-range transport of pollutants, we further analyzed variations in carbon monoxide (CO), a widely used tracer of BB emissions due to its relatively long atmospheric lifetime (∼ 1–2 months) and strong association with incomplete combustion (Ravindra Babu et al., 2023). We investigated CO changes across the study region using measurements from the MOPITT and AIRS satellites, which together provide more than two decades of continuous CO observations. CO data at 700 and 500 hPa from both satellites were analyzed for the period 2003–2023, with the 500 hPa level representing the altitude of maximum sensitivity for CO retrievals (Buchholz et al., 2021).

The observed CO anomalies from the two satellites are shown in Fig. 6, revealing significantly elevated CO concentrations over the SCS in April 2023, reaching up to 3σ above the 2003–2022 climatology. Although MOPITT displays more spatially concentrated anomalies than AIRS, both datasets consistently show positive CO anomalies at both pressure levels, indicating a substantial increase in mid-tropospheric CO during this period. For MOPITT CO retrievals, primary sources of uncertainty include limitations in vertical sensitivity and potential retrieval biases (Sayer et al., 2014). However, the observed enhancements (> 3σ) are supported by independent AIRS CO measurements, reinforcing the robustness of the detected anomalies. The comparison between MOPITT and AIRS CO at 500 hPa over the SCS further shows a strong positive correlation (R≈0.89; Fig. S1b in the Supplement). Furthermore, the spatial distribution of CO anomalies closely resembles that of AOD anomalies (Fig. 4). The area-averaged anomalies of AOD and 500 hPa CO over the SCS in April during 2003–2023 exhibit a strong positive correlation (R=0.81; Fig. 7), suggesting that long-range transport of pollution plays an important role in modulating aerosol variability in this region. The bubble chart further highlights the exceptional severity of the April 2023 event compared with previous years (Fig. 7). Because CO is primarily produced by incomplete combustion, elevated concentrations far from major traffic or industrial sources strongly suggest BB and wildfire emissions. The strong similarity between the spatial distributions of CO and AOD anomalies suggests that the increased aerosol loading was driven by BB in April 2023. This indicates that smoke was likely transported from surrounding regions toward the SCS and the BoB, which are located near major BB hotspots, including the MC and PSEA. The MC fire season typically occurs from August to October, whereas PSEA experiences a BB season from January to April, peaking in March (Fig. 2). These seasonal characteristics strongly suggest that the elevated AOD levels observed over the SCS in April 2023 were likely linked to BB activity in PSEA PSEA (Lin et al., 2009, 2017). Overall, the April 2023 event is notable for its exceptional intensity and extensive spatial coverage. The MODIS AOD anomalies were approximately four times higher than the long-term mean across much of the SCS and the southern BoB during April 2023. Such anomalously high aerosol loading over the SCS may have important implications for the regional climate and the hydrological cycle, highlighting the need for further investigation into the underlying drivers and physical mechanisms responsible for this event.

From the previous section, it is clear that most aerosols are free-troposphere-dominated and located primarily in the ∼ 2–5 km region over the SCS. To determine the source regions responsible for elevated aerosol concentrations over the SCS in April 2023, backward air-mass trajectories were computed using the NOAA HYSPLIT model. Trajectories were initialized at 15° N, 115° E, at 3 km a.s.l., corresponding to the altitude range (2–5 km) of the elevated aerosol layer observed in CALIPSO and MERRA-2 profiles. Daily 72 h trajectories (Fig. 8a) indicate that air masses arriving over the SCS predominantly originated from northern PSEA. When overlaid on the monthly mean MODIS AOD (Fig. 8b), these trajectories reveal transport pathways that coincide with regions of high aerosol loading, strongly suggesting that long-range transport of PSEA BB smoke likely transported to the SCS. To examine the BB activity during April 2023, MODIS fire counts and burned area (BA) data were used. The spatial distributions of MODIS fire counts and BA for April 2023 (Fig. 9a–b) show that BB activity was overwhelmingly concentrated in northern PSEA, particularly in northern Laos and adjacent regions of Myanmar and Thailand. Minimal fire activity and BA were observed over the MC and southern China; therefore, these regions were excluded from source-region calculations. Analysis of inter-annual variability in April BB activity over PSEA from 2003 to 2023 (Fig. 9c–d), considering only fire detections above the 80 % confidence level, indicates substantial year-to-year fluctuations. Notably, April 2023 recorded the highest burned area (∼ 2.27 Mha) over the 21-year period, highlighting the exceptional intensity of BB activity that month.

Country-level statistics (Table 2) further highlight the dominant contribution from Laos, which accounted for 11 877 fires (56.0 %), 1.53×106 MW of fire radiative power (FRP; 63.5 %), and 1.08×106 ha of burned area (47.7 %). Notably, the BA observed in April 2023 represents the highest monthly value in the available dataset (2002–2023; Fig. S5 in the Supplement). Approximately 60 % of Laos is forested (Fig. S6 in the Supplement), with much of this forest located in northern Laos where the majority of fires occurred during April 2023 (Fig. 9a). Myanmar contributed the second-largest share, with 7 054 fires (33.3 %), 7.78×105 MW of FRP (32.3 %), and 9.15×105 ha of burned area (40.3 %). Thailand, Vietnam, and Cambodia contributed comparatively smaller burned areas of 1.21×105 ha, 1.30×105 ha, and 2.20×104 ha, respectively. In total, 21 198 fires across PSEA produced approximately 2.41×106 MW of FRP in April 2023. These results strongly suggest that intense BB activity over northern PSEA, likely contributed substantially to the exceptional aerosol loading observed over the SCS in April 2023. However, two key questions remain: (i) what factors triggered the anomalously strong fire activity in Laos during April 2023, and (ii) why were BB aerosols transported unusually far southward across the SCS and into the southern BoB, rather than following the more typical transport pathways toward Taiwan and the north western Pacific? To address these questions, we next examine the large-scale meteorological and dynamical circulation conditions prevailing during April 2023.

In this section, we examine the meteorological and large-scale dynamical circulation anomalies associated with the April 2023 event. Figure 10 presents the spatial distribution of anomalies in key meteorological parameters during April 2023 relative to the 2003–2022 climatological mean. As shown in Fig. 10, Outgoing Longwave Radiation (OLR) anomalies indicate suppressed convective activity over PSEA during April 2023. Reduced convection was accompanied by negative precipitation anomalies, elevated surface temperatures, and pronounced soil moisture (SM) deficits. These conditions are spatially consistent with the regions of enhanced fire counts and BA observed by MODIS over northern Laos (Fig. 9). Long-term SM anomalies over northern Laos reached exceptionally low values in April 2023, representing the lowest levels in the past two decades (Fig. S7a–b in the Supplement). To further examine the persistence of these conditions, we analyzed the temporal evolution of SM anomalies during 2021–2023. The results show maximum positive anomalies in March 2022 and a transition to strong negative anomalies by April 2023, indicating a prolonged drought from winter 2022 to April 2023 (Fig. S7c in the Supplement). We further assessed the relationship between SM anomalies and fire activity over Laos. Standardized fire anomalies and SM anomalies (Fig. S8 in the Supplement) reveal a significant inverse relationship: reduced SM is associated with enhanced fire activity. In April 2023, extreme negative SM anomalies coincided with strong positive fire anomalies, highlighting the role of severe surface drying in promoting intense BB activity. A similar co-occurrence of anomalously low SM and elevated fire activity is also evident in 2016, a year previously identified as having intensified regional burning. These findings provide additional observational evidence that SM deficits are linked to enhanced fire activity over PSEA. Previous studies have shown that negative SM anomalies can promote positive geopotential height anomalies in the upper troposphere, which tend to reinforce local high-pressure conditions and enhance surface warming (e.g., Fischer et al., 2007; Dong et al., 2023). Motivated by this mechanism, we next examine the large-scale dynamical circulation patterns during April 2023 to understand how atmospheric dynamics may have contributed to the observed meteorological anomalies and the anomalous transport of BB aerosols.

Our analysis focuses on geopotential height and horizontal wind fields at the 700 and 500 hPa levels. The geopotential height fields (Z700 and Z500) in April 2023 show clear departures from the April climatology (1991–2020) (Figs. S9–10 in the Supplement). At 700 hPa, the climatological high-pressure system over the Indian region extended eastward toward PSEA, while at 500 hPa, the western Pacific subtropical anticyclone shifted westward to lie directly over PSEA. Geopotential height anomalies relative to the 1991–2020 mean (Fig. 11a–b) reveal a pronounced anti-cyclonic circulation (positive anomalies) centered over northern PSEA (∼ 20° N, 100° E) at 500 hPa. Similar anti-cyclonic anomalies are also evident in the Z700 field over the BoB. At the same time, significant cyclonic anomalies (negative geopotential height anomalies) are observed over the Arabian Sea and the WNP at both levels. These geopotential height anomaly patterns are broadly consistent with the observed sea surface temperature (SST) anomaly distribution. Positive SST anomalies are evident over the Arabian Sea, whereas negative SST anomalies occur over the BoB (Fig. 11). Positive SST anomalies are also observed over the WNP warm pool region, where negative geopotential height anomalies are present. Collectively, these patterns suggest a low–high–low (L–H–L)-like wave train extending from the Arabian Sea to the WNP warm pool region. These findings are consistent with those of Liu et al. (2026), who proposed that enhanced convection over the Horn of Africa and Arabian Peninsula (HAAP) generated downstream Rossby wave propagation across South Asia and mainland Southeast Asia, contributing to anomalous upper-level cyclonic circulation over South Asia and anti-cyclonic circulation over the Indochina Peninsula (ICP) and southern China. However, their analysis was primarily based on upper-tropospheric (∼ 300 hPa) circulation fields. Although our analysis focuses on mid-tropospheric circulation anomalies at 700 and 500 hPa, the observed patterns are broadly consistent with a vertically coherent large-scale circulation response during spring 2023. These circulation anomalies, particularly mid-tropospheric anti-cyclone that suppressed convection over PSEA, are consistent with positive OLR anomalies and reduced precipitation (Fig. 10). Subsidence associated with the anti-cyclonic circulation promoted clear-sky conditions (Fig. 10) and surface warming, which, together with severe soil moisture deficits, created favorable conditions for intense BB across northern Laos. Liu et al. (2026) further showed that upper-tropospheric convergence and dynamical subsidence associated with this wave pattern contributed to persistent heat and drying over the ICP during spring 2023, potentially supporting the lower- and mid-tropospheric anti-cyclonic anomalies identified here. The circulation pattern also altered regional transport pathways. The combined influence of the WNP cyclone, the BoB anticyclone at 700 hPa, and the PSEA anticyclone at 500 hPa modified the background flow over Southeast Asia. Meridional wind anomalies (Fig. S11b–c in the Supplement) show persistent northerly flow over the SCS, enabling southward transport of smoke from northern PSEA toward the SCS and into the southern BoB. Zonal wind anomalies further indicate weakened mid-latitude westerlies and locally reversed easterlies near Japan and Taiwan associated with a strong anti-cyclonic anomaly (Fig. S11a–b in the Supplement), suppressing the typical eastward transport of smoke. Consistent with this, AOD observations from the high-altitude mountain background station, namely “Lulin Atmospheric Background Station (LABS)” obtained through AERONET show no notable enhancement in April 2023 (Fig. S12 in the Supplement), indicating reduced smoke transport toward the northwestern Pacific. AERONET comparisons show strong correlations with MODIS AOD: 0.83 for Aqua and 0.84 for Terra (Fig. S13 in the Supplement), further supporting the reliability of the satellite observations as shown in the Dongsha data. Overall, the anomalous circulation pattern, characterized by a BoB anticyclone and a WNP cyclone, redirected BB plumes from northern PSEA toward the SCS and southern BoB, producing the exceptional aerosol loading observed in April 2023.

A key question is whether April 2023 was an exceptional BB year relative to PSEA, or whether similar conditions have occurred in other high-BB years. High-BB years were identified using standardized fire anomalies derived from April MODIS fire counts following the method of Vadrevu et al. (2019). For this analysis, April fire counts from 2003–2023 were standardized relative to the 2003–2022 climatology over northern PSEA (17–23° N). Years with anomaly values exceeding 0.5 were classified as high-BB years (Fig. S14), including 2003, 2006, 2008, 2010, 2014, 2015, 2016, and 2023. Composite analyses of MODIS AOD, 500 hPa geopotential height (Z500), and wind vectors were conducted to characterize typical aerosol and circulation patterns during high-BB conditions. Figure 12a–c presents the spatial distributions of composite AOD for high-BB years, AOD during April 2023, and their anomalies, respectively, while Fig. 12d–f shows the corresponding Z500 and wind vector fields. Relative to the high-BB composite, April 2023 exhibited substantially higher AOD across PSEA, the SCS, and the southern BoB, indicating enhanced regional aerosol loading. The Z500 field further showed pronounced positive anomalies over northern PSEA, reflecting a stronger, more spatially coherent anti-cyclonic circulation than under typical high-BB conditions. Correspondingly, wind anomalies revealed enhanced northerly flow over the SCS during 2023, suggesting strengthened meridional aerosol transport. An additional comparison between April 2016 and April 2023 (Fig. S15 in the supplement) showed similar spatial patterns, including enhanced AOD over southern PSEA. However, the magnitude and spatial extent of the AOD enhancement were greater in 2023. The AOD anomaly between 2023 and 2016 further revealed pronounced increases over the SCS and southern BoB, coinciding with enhanced northerly anomalies. Overall, these results suggest that anomalous circulation conditions in 2023 likely amplified aerosol transport and accumulation beyond those typically associated with high-BB years.

Another question is whether inter-annual variability associated with climate oscillations, such as the El Niño–Southern Oscillation (ENSO), influenced BB activity and circulation in 2023. SST anomalies associated with ENSO strongly modulate Indo-Pacific circulation and hydro-climate, with El Niño events typically enhancing springtime fires through drought, reduced precipitation, and suppressed moisture transport (Yin, 2020; Zhu et al., 2021; Zheng et al., 2023). We constructed composites of MODIS AOD and 500 hPa wind vectors for El Niño–ensuing and La Niña–ensuing Aprils during 2003–2022 (Fig. S16 in the Supplement). The results reveal higher AOD over northern PSEA and coastal South China during El Niño-ensuing Aprils, accompanied by a stronger anti-cyclonic system over PSEA extending from the BoB, consistent with enhanced aerosol accumulation. In contrast, Aprils following La Niña show weaker circulation and lower AOD. April 2023 occurred during the transition from a triple-dip La Niña to a developing El Niño, deviating from typical ENSO-fire patterns. This unusually large-scale background is consistent with Liu et al. (2026), who noted that the extreme spring 2023 heat event over the ICP differed fundamentally from canonical ENSO-linked events, as severe regional warming and subsidence developed despite antecedent La Niña conditions.

Overall, systematic analysis of MODIS fire counts, burned area (BA), HYSPLIT back trajectories, and MERRA-2 large-scale circulation fields, we conclude that the extreme aerosol event in April 2023 was shaped not only by BB activity but also by anomalous large-scale circulation patterns. These findings highlight the combined influence of BB emissions and background meteorological conditions in producing the exceptional smoke event.