Formation of an Intrathermocline Eddy Triggered by the Coastal‐Trapped Wave in the Northern Bay of Bengal

The present study investigates the characteristics and mechanism of intrathermocline eddy (ITE) formation in the northern Bay of Bengal (BoB) using observations from Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) buoys moored at 15°N, 12°N, and 8°N along 90°E and simulations from a high‐resolution numerical model. Among the RAMA locations, a relatively large number of anticyclonic ITEs are observed in the northern parts (15°N) compared to the other two (12°N and 8°N). The observed ITEs are characterized by a double‐convex shape of isotherms in the thermocline with a vertical extent of about 100–200 m and a positive sea level anomaly in the surface. Water mass properties show that Andaman Sea (AS) water is trapped inside the core of the ITEs. Trajectories of these ITEs (especially at 15°N) suggest that most of them originated in the vicinity of Preparis Channel, the northernmost passage between BoB and AS. Our study shows that vorticity developed downstream of Preparis Channel due to the strong subsurface flow (∼100 cm/s), which carries AS water into the BoB, results in the formation of observed anticyclonic ITE in the northern BoB. Further analysis revealed that this strong subsurface flow through the Preparis Channel is caused by the propagation of downwelling coastal‐trapped waves along the boundaries of BoB, which originates from the equatorial Indian Ocean. Our study suggests that ITEs could be an important pathway of water mass distribution in the intermediate layers in this region. Plain Language Summary Intrathermocline eddies (ITEs) are subsurface‐intensified eddies that are often seen as double‐convex shapes in the thermocline. This study investigates the characteristics, variability, and formation of ITEs in the Bay of Bengal (BoB) using observations and numerical simulations. A relatively large number of ITEs in the BoB are observed in the northern part compared to the south. These eddies are characterized by the Andaman Sea (AS) water inside its core and tracks of these eddies suggest that they originate in the vicinity of the passage between BoB and AS, called Preparis Channel. It is found that most of these eddies are generated downstream of strong westward flow through the Preparis Channel caused by the propagation of coastal‐trapped waves from the equatorial Indian Ocean. This study suggests that the propagation of downwelling coastal‐trapped waves plays an important role in the formation of ITEs i