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<p>The western Andaman Sea (AndS) is a marginal basin of the Bay of Bengal that plays a crucial role in regional air-sea interaction, yet its upper-ocean variability remains poorly characterized due to sparse observations. This study utilizes a theoretical framework to estimate mixed layer depth (MLD) using high-resolution (~ 6&#xa0;km) HF Radar surface currents during 2016–2020, based on an inertial slab dynamics approach. MLD is derived using both partial and total derivative formulations, with the latter demonstrating superior skill at capturing wind-driven variability, showing higher correlation with ensemble-based estimates (<i>r</i> = 0.85) than the former (<i>r</i> = 0.72), as it incorporates lateral advection. The results reveal a strong seasonal cycle, with the deepest MLDs (~ 60–80&#xa0;m) during June–October, intermediate depths (~ 20–40&#xa0;m) during November–January, and the shallowest layers (~ 10–20&#xa0;m) during February–April. These variations exhibit an inverse relationship with the potential energy anomaly, which peaks above 100&#xa0;J m⁻³ during pre-monsoon stratification. Turner Angle analysis indicates distinct stability regimes, ranging from salinity-controlled stability in June–October to diffusive convection in November–January, and a doubly stable regime during February–April. Further, the buoyancy flux diagnostics reveal that haline forcing dominates during the monsoon, constraining wind-driven mixing despite strong winds, whereas thermal forcing governs convective instability during the winter months. Overall, this study demonstrates the potential of HF Radar surface currents to resolve fine-scale MLD variability in data-sparse AndS, with implications for upper ocean stratification and mixing.</p>

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Estimating Mixed Layer Depth and Variability along the Western Andaman Sea: Insights from HF Radar Surface Currents

  • 期刊论文

  • 发表日期：

    2026年3月25日

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  • Arun Kumar,
  • Samiran Mandal,
  • Basanta Kumar Jena

摘要

The western Andaman Sea (AndS) is a marginal basin of the Bay of Bengal that plays a crucial role in regional air-sea interaction, yet its upper-ocean variability remains poorly characterized due to sparse observations. This study utilizes a theoretical framework to estimate mixed layer depth (MLD) using high-resolution (~ 6 km) HF Radar surface currents during 2016–2020, based on an inertial slab dynamics approach. MLD is derived using both partial and total derivative formulations, with the latter demonstrating superior skill at capturing wind-driven variability, showing higher correlation with ensemble-based estimates (r = 0.85) than the former (r = 0.72), as it incorporates lateral advection. The results reveal a strong seasonal cycle, with the deepest MLDs (~ 60–80 m) during June–October, intermediate depths (~ 20–40 m) during November–January, and the shallowest layers (~ 10–20 m) during February–April. These variations exhibit an inverse relationship with the potential energy anomaly, which peaks above 100 J m⁻³ during pre-monsoon stratification. Turner Angle analysis indicates distinct stability regimes, ranging from salinity-controlled stability in June–October to diffusive convection in November–January, and a doubly stable regime during February–April. Further, the buoyancy flux diagnostics reveal that haline forcing dominates during the monsoon, constraining wind-driven mixing despite strong winds, whereas thermal forcing governs convective instability during the winter months. Overall, this study demonstrates the potential of HF Radar surface currents to resolve fine-scale MLD variability in data-sparse AndS, with implications for upper ocean stratification and mixing.