Seasonality of Sulfate Aerosol, and Forced SST Variations over the Mid-latitude Oceans
Over the mid-latitude oceans, late-summer sea surface temperatures (SST) have been increasing about 80% faster than winter SST over the last five decades. This result is found in observations, and ubiquitously in climate models. At the same time, sulfate aerosol amount and subsequent forcing over the North Atlantic and Pacific exhibit strong seasonal variations with maximum forcing during the boreal summer. We examine the cause of this seasonal trends using a variety of model simulations and analyses, and using a simple seasonally entraining mixed layer model of the mid-latitude ocean. Specifically, we investigate the following questions:
- Why do anthropogenic sulfate aerosol amount and forcing exhibit such strong seasonality over the mid-latitude oceans when there is no seasonality in precursor SO2 emissions? (It’s chemistry!)
- Is the seasonality in forcing responsible for the seasonality in the response? (Spoiler alert – no.)
- What processes ARE responsible for the seasonality in the mid-latitude response to forcing? (No spoilers here, you’ll have to show up to find out!)
Results highlight the importance of considering seasonality in physical processes – especially feedbacks in the climate system – when considering the climatic response to forcing.