Near‐Inertial Kinetic Energy and Diapycnal Heat Flux to the Mixed Layer During Tropical Cyclone Bulbul in the Northern Bay of Bengal Using High‐Resolution Moored Buoy Observation

Abstract High‐resolution mooring observations at 17.8°N, 89.5°E in the northern Bay of Bengal (BoB) are used to investigate the fraction of near‐inertial (NI) wind power input () at the ocean surface that propagates below the mixed layer (ML) and its role in modulating diapycnal heat flux‐induced ML temperature (MLT) cooling during Tropical Cyclone (TC) Bulbul (5–9 November 2019). The induces a sharp rise in ML NI Kinetic Energy (NIKE) (∼80 Jm −3 ), followed by its downward propagation, with subsurface NIKE (∼40 Jm −3 ) about half of the ML value and lagging by approximately 2 days. Though the ML NIKE budget indicates that approximately 10% of the is radiated downward from the ML, the enhancement of NIKE below the ML is plausible due to stalling of NIWs at the base of the ML. Radiation of NIKE at the ML base increases vertical shear of currents, leading to a fourfold enhancement of diapycnal diffusivity to 1.25 × 10 −3 m 2 s −1 compared to the pre‐cyclonic value (3.3 ± 0.04 × 10 −4 m 2 s −1 ). As a result, diapycnal heat flux at the ML base increased to −126.4 ± 11.4 Wm −2 from the pre‐cyclonic value of ∼2.7 ± 1.9 Wm −2 . During the peak cooling phase of ML, diapycnal heat flux (−200 Wm −2 ) shows a comparable contribution with respect to the net heat flux term. During the post‐TC phase, the ML heat storage term has exhibited a pronounced high‐frequency variability ranging between −4,000 and −2,000 Wm −2 due to the NI current in the presence of cold wake.