In this study we introduce a pair of idealized tracers to quantify how
changes in physical advection and mixing under climate change affect the
nutrient supply, new production, and particulate export rates. The low
cost and simplicity of these tracers allows us to explore the
sensitivity of the model biogeochemistry, and in particular its response
to a changing physical environment, to the choice of model parameters.
Using CESM2.1 with active ocean and ice only, at nominal one-degree
resolution, under initial conditions and forcing representative of 2000
and 2100, our idealized nutrient and particulate are within the spread
of nitrate and export from CMIP5 models. The simple form of the tracers
allows us to identify the physical controls on the changing rates of
supply, production, and export throughout the year, which together form
the different seasonal cycles. We find that the ocean basins with the
largest changes in the seasonal cycle over the 21st century are the
North Atlantic, the Arctic, and the eastern tropical Pacific. We present
results comparing the controls across basins, focusing on shifts in the
timing of deepening mixed layers and maximum production rate in the
northern North Atlantic through the Arctic, and changes in the spatial
and temporal patterns of vertical advective exchange in the tropics and
subtropics of the Pacific and Indian Oceans. In both cases we discuss
how much these changes depend on the biogeochemical model parameter
values.