Temperature and salinity can reinforce or compensate each other in setting ocean stratification, with important consequences for winter mixing and dense-water formation. Here, we use the Stratification Control Index (SCI), which distinguishes temperature-stabilised alpha regimes (SCI > 1), salinity-stabilised beta regimes (SCI < −1), and transition regimes (−1 < SCI < 1), to diagnose the thermohaline structure below the mixed layer. We construct SCI and mixed-layer depth (MLD) climatologies for 2004–2024 based on the EN.4.2.2 observation dataset and compare them with 26 CMIP6 historical simulations processed with the same definitions. In EN.4.2.2, winter SCI separates subtropical alpha oceans from polar beta oceans, with deep mixed layers occurring mainly for SCI > 1. Summer SCI is smoother and shifted toward more alpha-like values because of the seasonal thermocline. CMIP6 shows much larger discrepancies in winter than in summer, with a systematic shift toward stronger haline control at high latitudes. In the North Atlantic north of 40°N, the winter CMIP6 mean product has SCI = −1.11 and a beta-regime area of 8.6 × 106 km², compared with SCI = 0.40 and 4.3 × 106 km² in EN.4.2.2. Across the models with available Atlantic Meridional Overturning Circulation (AMOC) estimates, North Atlantic SCI and beta-regime area covary with overturning strength. These systematic winter biases indicate that many CMIP6 models misrepresent the thermohaline preconditioning of regions central to dense-water formation. Since this preconditioning shapes the response of Atlantic and Southern Ocean overturning to warming and freshwater forcing, such mean-state errors limit confidence in projections of future overturning changes.