Critical role of biomass burning aerosols in enhanced historical Indian Ocean warming

The tropical oceans have shown substantial warming over the past century, exhibiting distinct regional patterns. Relative to the tropical mean, sea surface temperature (SST) warming is amplified in the TIO, western Pacific, and eastern Atlantic, but suppressed in the central-eastern Pacific. The enhanced warming in the TIO, a climatically warm region, is particularly noteworthy because it's linked to various climate impacts through teleconnections, including a strengthened positive phase of the North Atlantic Oscillation (NAO), decreased Sahel rainfall, an enhanced Pacific Walker Circulation, the occurrence of a North Atlantic "warming hole", and an intensified Atlantic Meridional Overturning Circulation. The observed TIO relative warming (TIO warming exceeding tropical-mean warming) is a robust feature across different datasets, especially prominent after the 1950s. While the absolute TIO warming rate shows large data uncertainties and is positively correlated with the tropical mean warming rate, the relative warming trend remains a consistent characteristic. This faster warming in the TIO compared to the tropical average suggests an increase in TIO rainfall, potentially driving the mentioned global teleconnections, but this remains debated due to limited direct observations. The physical mechanisms behind the observed TIO relative warming are unclear, with proposed explanations for absolute TIO warming (e.g., increased greenhouse gas concentrations or changes in El Niño-Southern Oscillation properties) potentially not applicable to the relative warming. Given the crucial role of relative SST changes in shaping tropical rainfall patterns, the study aims to determine if the observed TIO relative warming is a response to external radiative forcing, the role of internal climate variability, and the responsible forcing agent(s). Addressing these questions requires analyzing single-forcing, large-ensemble coupled model simulations alongside observations, which allows the identification of the previously overlooked role of BMB aerosol forcing in the historical TIO relative warming.

Previous research has explored various mechanisms to explain the absolute warming of the Tropical Indian Ocean (TIO), including the increase in greenhouse gas concentration and changes in El Niño-Southern Oscillation properties. However, these mechanisms may not fully account for the relative warming observed in the TIO compared to the global average. Studies highlighting the importance of relative SST changes in shaping the tropical rainfall patterns further underscore the need to understand the specific drivers behind the relative warming trend in the TIO. Existing literature also notes discrepancies between the observed TIO relative warming and the simulations produced by Coupled Model Intercomparison Project Phase 5 (CMIP5) coupled climate models, which tend to simulate a more uniform surface warming over the tropical oceans. This study aims to address these gaps by employing a different modeling approach and focusing on the role of biomass burning aerosols, a factor largely overlooked in previous studies of TIO relative warming.

To investigate the mechanisms driving TIO warming, the study utilizes initial-condition Large Ensembles (LEs) from the Community Earth System Model version 1 (CESM1), a widely used, fully coupled general circulation model participating in CMIP5. The LE approach helps separate externally forced climate responses from internal climate variability. Besides the standard all-forcing (ALL) historical LE simulations, "all-but-one-forcing" LE simulations are used to isolate the impacts of greenhouse gases (GHG), anthropogenic aerosols (AAER), and BMB aerosols. The simulations cover the period 1920–2005, with analysis focusing on 1925–2005 to minimize the initial ocean condition influence. The study uses four monthly observational SST datasets (ERSSTv5, HadISST, COBE-SST, Kaplan SST) for comparison. The CESM1 ALL LE, despite underestimating the observed tropical-mean ocean warming rate, accurately captures the TIO relative warming, suggesting that external forcing may partly explain the observed pattern. The model's realistic depiction of natural variability and forced response in TIO trends is evident because the LE's ensemble spread in simulated TIO SST variations (both absolute and relative) includes the observed variations. To identify the primary driver of TIO relative warming, the impacts of individual forcing agents (GHG, AAER, and BMB aerosols) are examined using the "all-but-one-forcing" LEs. BMB aerosols, while playing a minor role in tropical mean surface temperature trends, significantly contribute to the spatial variability of tropical SST warming. The study then employs an ocean mixed layer heat budget analysis to decompose SST changes induced by external radiative forcing (BMB aerosols) into seven components: changes in shortwave (SW) and longwave (LW) radiative fluxes, sensible heat flux (SH), ocean dynamics (OCN), and latent heat flux changes due to surface wind speed (Hu), relative humidity (HRH), and air-sea temperature gradient (HAT). The analysis assesses the contribution of each component to the BMB-induced TIO relative warming. Finally, the study investigates the potential climate impacts of the BMB aerosol-induced TIO relative warming, including changes in rainfall, salinity, zonal winds, and North Atlantic jet stream, analyzing their connection to the NAO. The NAO pattern and index are computed as the first EOF and PC of annual mean sea level pressure anomalies within a specified region.

The study's key findings demonstrate the critical role of BMB aerosol changes in shaping the pattern of tropical SST warming. Although BMB aerosols contribute minimally to the absolute TIO SST trend, they are the primary driver of TIO relative warming in the historical period. Greenhouse gases (GHGs) and anthropogenic aerosols (AAERs) are major contributors to absolute TIO SST changes but not relative changes. Analysis of the ocean mixed layer heat budget reveals that the most significant contributors to BMB-induced TIO relative warming are changes in shortwave radiative flux (T_SW) and surface wind speed (T_Hu), with ocean dynamics (T_OCN) primarily affecting absolute, not relative, warming. Specifically, the BMB reduction over the TIO increases clear-sky shortwave radiation, inducing local warming. Simultaneously, increased BMB aerosols over the eastern Pacific and Atlantic cool these areas. Changes in surface wind speed, primarily in the northern and southern TIO, lead to decreased latent heat flux and warming. The BMB aerosol-induced TIO relative warming results in increased rainfall over most of the TIO and decreased rainfall in the western Pacific. This creates a zonal dipole in rainfall anomalies, suggesting a westward displacement of the Indo-western Pacific warm pool convective center. The TIO becomes fresher, and the western Pacific saltier, consistent with the precipitation changes. These changes are also observed in the all-forcing experiments but with larger amplitudes, highlighting BMB's critical role in shaping the Indo-western Pacific warm pool rainfall pattern. The BMB aerosol-induced changes affect the North Atlantic jet stream, which is significantly enhanced and shifted northward, leading to increased rainfall in northwestern Europe and reduced rainfall in southern Europe and the Mediterranean. These changes are consistent with a shift toward a positive phase of the NAO. The study estimates that BMB aerosol-induced changes explain about half of the observed positive NAO trend, with the other half primarily due to GHG-induced changes. The results align with previous findings suggesting a link between TIO warming and a positive NAO.

The study's findings highlight the significant, yet often overlooked, role of BMB aerosols in historical climate change. While BMB aerosols' impact on global mean temperature change might be secondary compared to GHGs or AAERs, their influence on the pattern of warming is substantial. Over 80% of global BMB aerosol emissions originate from tropical regions, emphasizing the importance of these aerosols in shaping the tropical SST warming pattern during the past century. The study's use of CESM1 large ensembles indicates that BMB aerosol changes are the dominant contributor to the forced component of TIO relative warming compared to GHGs and AAERs. Observational data show relative TIO warming, primarily after 1960, with considerable uncertainties in magnitude, suggesting internal variability's contribution. The BMB aerosol-induced TIO relative warming leads to enhanced rainfall and a westward shift of the Indo-western Pacific warm pool convective center. The resulting changes in the North Atlantic jet stream likely influence North Atlantic/European climate via teleconnections. Future work should validate these findings using other climate models to determine the model-dependency of the results. The study's importance lies in its revealing of a previously underestimated climatic factor and suggesting necessary improvements in GCM representation of BMB aerosols for better climate predictions.

This research reveals a critical role for biomass burning aerosols in driving historical Tropical Indian Ocean warming, exceeding the impact of greenhouse gases and anthropogenic aerosols in shaping the relative warming pattern. The study highlights the significant influence of BMB aerosols on global climate, notably contributing to changes in rainfall, salinity, and the North Atlantic jet stream. Future research should focus on refining the representation of BMB aerosols in General Circulation Models (GCMs) to better understand and predict climate change. This includes improving the modeling of BMB aerosol emissions, their chemical and physical properties, atmospheric transport, cloud interactions, and impact on the carbon cycle.

The study primarily relies on CESM1 model simulations. While CESM1 is a widely used and comprehensive model, the results may be model-dependent. Further research using other climate models is necessary to validate these findings and assess their generalizability. The observed TIO relative warming shows considerable uncertainty, particularly in the magnitude of warming before the 1960s, potentially influencing the precise quantification of BMB aerosols' effect. Additionally, the complexity of atmospheric and oceanic interactions means that isolating the sole impact of BMB aerosol changes on observed climate trends remains challenging. The decomposition of the ocean heat budget relies on approximations, particularly the linearized bulk formula for latent heat flux, which could introduce uncertainties in the component analysis.