Lagrangian Thermodynamic Transformation of North Atlantic Air Masses Reaching the Azores: The Roles of Ocean–Atmosphere Turbulent Heat Fluxes and Boundary-Layer Coupling

Air masses reaching remote oceanic regions integrate the effects of surface forcing encountered during transport. This study investigates the Lagrangian thermodynamic transformation of North Atlantic air masses reaching the Azores during December 2019 using 93 five-day HYSPLIT backward trajectories at 100, 500, and 1000 m AGL combined with ERA5 fields. Potential temperature (θ), equivalent potential temperature (θe), specific humidity (q), moist static energy (MSE), turbulent heat fluxes, and boundary-layer exposure were reconstructed along each trajectory. Mean θe increased by 25.97, 18.43, and 7.23 K at 100, 500, and 1000 m, respectively; q increased by 6.31, 5.99, and 3.72 g kg−1, and MSE by 25.56, 17.76, and 7.21 kJ kg−1. In contrast, θ increased at 100 and 500 m but decreased at 1000 m. Latent-dominated turbulent heat exchange peaked 48–24 h before arrival. Integrated turbulent heat exposure was positively associated with Δθe (r = 0.53) and ΔMSE (r = 0.54), while boundary-layer exposure was also positively associated with ΔMSE (r = 0.55; Spearman ρ = 0.60; p < 0.001). These results indicate that the magnitude and vertical structure of air-mass transformation during the sampled period reflect the combined influence of upstream ocean–atmosphere exchange, transport history, and boundary-layer coupling.