Sea surface temperatures (SSTs) vary not only due to heat exchange across the air-sea interface but also due to changes in effective heat capacity as primarily determined by mixed layer depth (MLD). Here, we investigate seasonal and regional characteristics of the contribution of MLD anomalies to SST variability using observational datasets. We propose a metric called Flux Divergence Angle (FDA), which can quantify the relative contributions of surface heat fluxes and MLD anomalies to SST variability. Using this metric, we find that MLD anomalies tend to amplify SST anomalies in the extra-tropics, especially in the eastern ocean basins, during spring and summer. This amplification is explained by a positive feedback loop between SST and MLD via upper ocean stratification. In contrast, MLD anomalies tend to suppress SST anomalies in the eastern tropical Pacific. The MLD contribution in the summer hemispheres is more pronounced on seasonal timescales than on sub-monthly timescales.