[Department of Ocean Science] OCES Seminar: Seasonality and its effects on ocean stratification, circulation, and the carbon cycle
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Abstract:
The high latitude oceans are subject to a seasonal cycle in surface forcing that swamps the annual mean. The effects of this seasonality are profound, for example with the shallowing and deepening of the upper boundary layer and with the growth and retreat of sea ice, but an understanding of how it imprints on broader features of the ocean circulation is lacking. Here, I will present three separate studies that explore the impacts of upper ocean seasonality and its rectified effects on the ocean interior. In the first, we consider the impact of Southern Ocean seasonality on the residual overturning circulation. In an idealised ocean model, we find that the overturning strength and structure is significantly altered by the introduction of seasonality, mediated through both buoyancy effects and eddy activity. In the second study, we explore the seasonal cycle of the upper ocean carbon cycle in the Southern Ocean sea ice zone. Again in an idealised ocean-sea ice-biogeochemistry model, we elucidate the competing effects of ocean stratification and biological processes in dictating carbon increases or decreases during ice growth and retreat. Finally, we consider the global ocean surface and the seasonality of the global warming signal. We show that the warming of sea surface temperature is amplified in fall relative to spring — an effect arising from the seasonality of the mixed layer depth. We note, however, an inconsistency between inferred and observed mixed layer depth changes, and consider its implications for our understanding of upper ocean dynamics in a warming world.
Biography:
Graeme MacGilchrist is an oceanographer and climate scientist, and currently a UKRI Future Leaders Fellow in the School of Earth and Environmental Sciences at the University of St. Andrews. Prior to St. Andrews he was a post-doc at GFDL in Princeton/NOAA with Jorge Sarmiento and Stephen Griffies, and he complete his DPhil at the University of Oxford supervised by Helen Johnson and David Marshall. His research is concerned with understanding how climate-relevant tracers, such as heat and carbon dioxide, are taken up by, stored within, and transported around the ocean. He uses a host of tools in his research, including large-scale numerical simulations, process models, theoretical ideas, trajectory analysis, and ocean observations.