<p>The seasonal thermocline in the southern Bay of Bengal (BOB) deepens markedly during boreal summer (July to September), yet its phase evolution and underlying causes remain unclear. Using the Array for Real-time Geostrophic Oceanography and a 1.5-layer reduced gravity model, we quantify the dominant spatiotemporal evolutions of the deepened thermocline. Results indicate that the combined effects of distinct phase-locked local and equatorial remote forcing in the BOB jointly shape the anomalously thermocline deepening, forming two unique stages during boreal summer. In the formation stage (June), semiannual Rossby waves generated remotely in the equatorial Indian Ocean reflect from the eastern boundary and constructively interfere with annual Rossby waves forced by negative wind stress curl, producing basin-wide thermocline deepening. During the maintenance stage (July-September), local forcing remains in a positive phase, while remote forcing shifts negative. This phase difference yields an east-west asymmetry: in the west, strong local forcing sustains the deepened thermocline despite the opposing remote signal, whereas in the east, the negative remote forcing dominates, leading to thermocline shoaling. This study highlights the complex interferences between local-annual and remote-semiannual Rossby waves in modulating the seasonal thermocline evolution and provides insights to enhance the forecasting of subsurface seasonality in the southern BOB.</p>