Topographic control of gyre dynamics under thermal and wind forcing in rotating stratified systems

• Topographic modulation of gyres was investigated using rotating tank experiments under controlled thermal and wind forcing. • Seasonally reversing surface flows were induced by thermal forcing: counterclockwise in summer and clockwise in winter, intensified near slopes and depressions. • Wind forcing alone generated persistent counterclockwise gyres with offshore-displaced centers and pressure-driven inward flows in depressions. • Combined forcing produced nonlinear amplification, with enhanced surface gyres, thermocline-intensified vorticity, and suppressed deep flow due to stratification. Topographic features exert fundamental control on large-scale oceanic circulation, yet their role under the combined influence of thermal and wind forcings remains insufficiently understood. Here, rotating tank experiments in stratified fluids are employed to investigate how slopes and depressions modulate gyre dynamics. Thermal forcing alone generated seasonally reversing surface circulations: summer heating induced robust counterclockwise gyres, whereas winter cooling produced clockwise circulation. These seasonal patterns were amplified near slopes and depressions owing to localized heat retention. In tanks with depressions, surface velocities weakened, giving rise to seasonally reversing, localized vortices. Under wind forcing, persistent counterclockwise gyres developed, with their centers displaced offshore by spatial heterogeneity in wind stress. Depressions generated pressure minima that drew fluid inward, producing centrally confined counterclockwise eddies shaped by the Coriolis force. When thermal and wind forcings act simultaneously, gyres markedly intensify, resulting in enhanced vorticity near the thermocline and flow suppression at depth due to stratification. Strikingly, the observed velocities exceeded the linear superposition of the individual forcings, demonstrating a nonlinear interaction. These results underscore the decisive role of small-scale topography in modulating rotating, stratified flows and provide mechanistic insights into the dynamics of basin-scale circulation in natural water bodies.