The Bay of Bengal (BoB) receives disproportionately large freshwater input yet its subsurface thermal structure has warmed persistently, raising the question of whether surface halocline and deep thermal tendencies evolve as a coupled or progressively decoupled vertical system. Using ORAS5 reanalysis, ERA5 atmospheric forcing, and NOAA OISST spanning 1990–2024, we demonstrate that these tendencies are not vertically coherent but reflect a systematic thermal decoupling organised around a three-phase cascade of regime transitions propagating upward from the deep layer over two decades.

Basin-mean SST increased at +0.24°C decade⁻¹ while SSS freshened at −0.037 psu decade⁻¹. Subsurface warming was detected at 100m (+0.17°C decade⁻¹), 300m (+0.08°C decade⁻¹), and 700m (+0.04°C decade⁻¹), with the 300–700m layer accumulating OHC fastest (+5.26 × 10⁶ J m⁻² decade⁻¹), exceeding the surface layer (+4.60 × 10⁶ J m⁻² decade⁻¹). After detrending, surface and deep temperatures were statistically independent (r = −0.27, n.s.), confirming vertical thermal decoupling. Lead-lag analysis revealed a bottom-up thermal cascade with 700m anomalies propagating upward to 100m with an approximate one-year lag.

Barrier layer thickness increased significantly (+1.18 m decade⁻¹), driven by isothermal layer deepening (+0.96 m decade⁻¹) rather than mixed layer shoaling (−0.21 m decade⁻¹). Halocline strength weakened (−0.005 psu m⁻¹ decade⁻¹) and near-surface stratification (N² at 50m) declined (−1.28 × 10⁻⁵ s⁻² decade⁻¹), concurrent with thermocline-level strengthening (N² at 100m: +1.27 × 10⁻⁵ s⁻² decade⁻¹), indicating a downward migration of the stratification maximum. Pettitt change-point detection identified a three-phase temporal cascade: deep density reorganisation (~2002), basin-wide thermal and haline restructuring (~2007–2008), and surface stratification response (~2014). Neither WSC nor MHT exhibited significant trends, ruling out local atmospheric and advective forcing as primary drivers.

These findings reveal a progressively decoupling vertical thermal structure in the BoB, with implications for barrier layer dynamics, deep heat sequestration, and regional climate feedbacks.

Keywords: Bay of Bengal, upper ocean heat content, barrier layer, halocline, stratification regime shift