Subtropical western boundary currents (WBCs) are major conduits of ocean heat and momentum, playing a fundamental role in Earth’s climate. Satellite observations reveal complex zonal movements and a pronounced meridional asymmetry in WBC shifts, yet lack a unifying explanation. Here, using quasi-geostrophic theory and numerical experiments, we demonstrate that wind stress curl and stratification variations compete to control WBC position: enhanced wind stress curl drives an eastward shift, opposing the westward shift driven by stratification. This competition explains the observed divergence in zonal trends among WBCs, while meridional variations in the background potential vorticity gradient account for the asymmetric response. Our findings provide a predictive framework for assessing WBC responses to climate change.