Observational data and historical model simulations nearly ubiquitously show strong seasonality in rates of sea surface temperature (SST) warming over the mid-latitude oceans between 1970 and present, with late summer warming about 40% faster than late winter. The seasonality in SST warming since 1970 also coincides with strong seasonality in anthropogenic sulfate aerosol concentration and forcing. In this project we investigate (1) the chemical processes responsible for seasonality in sulfate aerosol over the mid-latitude oceans, (2) the resulting impact on direct and indirect (Twomey effect) radiative forcing by sulfate aerosols, and (3) the thermodynamic processes responsible for the strong seasonality in SST warming. The project uses direct climate model simulation, analysis of historical model simulations, and simulation using an entraining mixed-layer model that represents mid-latitude ocean-atmosphere interactions to investigate seasonality of historical warming. The project highlights the cause of seasonal SST trends over the mid-latitude oceans, and shows that from 1970 to present summer sulfate aerosol forcing over the mid-latitudes has likely exceeded CO₂ forcing.