The southeast Atlantic (SEA) region, characterized by persistent stratocumulus clouds, has one of the highest uncertainties in aerosol radiative forcing and significant variability across climate models. During the austral spring (August to October), biomass burning emissions from neighboring southern African fires introduce considerable variability into aerosol forcing assessments in the SEA, prompting several international field campaigns. Despite this, the region's year-round prolific production of marine aerosols and their potential impact on regional climate dynamics through interactions with low-level marine clouds have been largely overlooked. To address this, our study employs the GEOS-Chem atmospheric chemical transport model (version 13.3.3) to analyze the seasonally varying role of marine aerosol sources and to identify key uncertainties in aerosol composition at cloud-relevant altitudes across the SEA (40°W-20°E, 0-40°S). We simulate aerosol optical depth (AOD) and speciated aerosol concentrations, evaluating them against ground-based Aerosol Robotic Network (AERONET) AOD data and measurements from aircraft campaigns such as ORACLES and CLARIFY conducted in 2017. Additionally, we conduct multiple nested grid simulations at a horizontal resolution of 0.5° by 0.625° to explore the sensitivity of marine aerosols to various emission sources. To quantify the impact of marine sources on sulfate aerosols within the stratocumulus cloud layer, we perform a simulation excluding SO2 and SO4 emissions from anthropogenic sources, biomass burning, volcanic activity, ships, and aviation. We also investigate the sensitivity of dimethylsulfide (DMS) emission fluxes to variations in ocean surface DMS concentrations through two distinct simulations: one using a climatological dataset from the Global Surface Seawater DMS Database, and another utilizing a remote-sensing algorithm that integrates satellite-derived estimates of chlorophyll and light penetration. Our findings indicate that the model consistently underestimates AOD compared to AERONET data, particularly at remote oceanic sites. However, when compared with aerosol mass concentrations from aircraft campaigns during the peak biomass burning months of August-September, the model performs adequately at cloud-relevant altitudes, with a normalized mean bias (NMB) between -3.5% (CLARIFY) and -7.5% (ORACLES). At these altitudes, organic aerosols (63%) dominate during the biomass burning period, while sulfate (41%) prevails during austral summer (November to February), when DMS emissions peak in the model. Our results show that marine sulfate can account for up to 69% of total sulfate during high DMS period. Sensitivity analyses focusing on DMS emission rates and oxidation mechanism indicate that refining these processes in the model may increase sulfate aerosol production from marine sources, highlighting their overall importance. In conclusion, this study enhances our understanding of the seasonal dynamics of aerosols in the SEA and underscores the imperative need to refine marine emissions and their chemical transformations to better predict aerosol-cloud interactions and reduce uncertainties in aerosol radiative forcing over the southeast Atlantic.