A critical role for seasonality in forcing, feedback, and climate system response over the mid-latitude oceans

Observational data, historical model simulations, and future model projections nearly ubiquitously show strong seasonality in rates of sea surface temperature (SST) warming over the mid-latitude oceans, with late summer warming about 1.5 – 2 times faster than late winter. This project investigates seasonality in warming by examining seasonality in (i) forcing, (ii) climate feedbacks, and (iii) physical processes that determine the climatic response. Climate model simulations and analyses show that (i) sulfate aerosol forcing over the North Atlantic and Pacific is strongly seasonal due to seasonality in sulfate chemistry, depositional processes, cloud / aerosol interactions, and incident radiation; the net result is that sulfate aerosol forcing is only active during the warm season. Theoretical arguments and radiative transfer model calculations show that (ii) air-sea and water vapor feedbacks tend to amplify the seasonal response of the climate system to external forcing, including aerosols and greenhouse gasses. Finally, a one-dimensional entraining oceanic mixed layer model demonstrates that (iii) seasonal mixed-layer dynamics alter the timing of the climate system response to external forcing, with seasonal warming resulting from forcing that occurred several months earlier. Results highlight a critical role for seasonality in understanding the climate system’s spatial and temporal structure to imposed forcing.