5:15 PM - 7:15 PM
[AOS13-P07] Biogeochemical Response Thresholds and Drivers in the North Pacific Subtropical Gyre Under Idealized Global Warming
Keywords:North Pacific Subtropic Region, Global Warming, Nutrient Budget, Ocean General Circulation Model
Future projections suggest that surface nutrient concentrations will decline as ocean warming progresses. The primary cause of this decline is increased ocean stratification, which reduces vertical nutrient supply. However, the extent and nature of these changes are not uniform across ocean regions, and the potential existence of biogeochemical response thresholds remains unclear. The North Pacific Subtropical Gyre (NPSG), a major region for global fish catches and oceanic carbon sequestration, provides an ideal setting to investigate these dynamics. In this study, we use numerical simulations to examine how nutrient budgets and their controlling processes respond to warming, with a particular focus on identifying critical thresholds in nutrient availability and their biogeochemical drivers.
To achieve this, we conducted numerical experiments using an ocean general circulation model COCO ver.4.0 coupled with an ocean biogeochemical model. A standard ocean model experiment was performed under pre-industrial climate conditions (CTRL experiment), and additional experiments were conducted under idealized surface warming scenarios, in which the sea surface temperature boundary condition was uniformly increased by 1°C to 4°C (1°C to 4°C experiment).
We first analyzed the nutrient budget in the surface layer of the NPSG. The budget analysis in the control experiment revealed that nutrients in this region are primarily supplied through the convergence of horizontal advection from both the north and south. While vertical advection generally removes nutrients from the surface layer, it acts as a source of nutrients in the western boundary region. This imbalance plays a crucial role in sustaining biological production.
In the warming experiments, surface nutrient concentrations generally declined as temperature increased. Notably, the largest decrease was observed between the +1°C and +2°C experiments. A comparison of these cases revealed that in the Southern Ocean, where deep water forms, enhanced stratification led to a shallower mixed layer, weakening the deep ocean circulation in the Pacific originating from Antarctic Bottom Water. This weakening of deep ocean circulation reduces nutrient transport from the deep Southern Ocean to the entire North Pacific, ultimately leading to a decline in surface nutrient concentrations. The reduced nutrient supply, in turn, led to decreased biological production in the subtropical North Pacific. The rate of decline in biological production exceeded the global average, indicating that this region is particularly sensitive to global warming.
Thus, as warming progresses, marine ecosystems in this region may experience greater stress compared to other ocean regions. These findings highlight the role of large-scale ocean circulation changes in shaping biogeochemical thresholds and regional nutrient dynamics. Identifying critical tipping points in nutrient availability and biological productivity will be key to understanding long-term ocean biogeochemical feedback to climate change. We plan to conduct a more detailed budget analysis by further breaking down individual processes to investigate the origins of nutrient changes.
To achieve this, we conducted numerical experiments using an ocean general circulation model COCO ver.4.0 coupled with an ocean biogeochemical model. A standard ocean model experiment was performed under pre-industrial climate conditions (CTRL experiment), and additional experiments were conducted under idealized surface warming scenarios, in which the sea surface temperature boundary condition was uniformly increased by 1°C to 4°C (1°C to 4°C experiment).
We first analyzed the nutrient budget in the surface layer of the NPSG. The budget analysis in the control experiment revealed that nutrients in this region are primarily supplied through the convergence of horizontal advection from both the north and south. While vertical advection generally removes nutrients from the surface layer, it acts as a source of nutrients in the western boundary region. This imbalance plays a crucial role in sustaining biological production.
In the warming experiments, surface nutrient concentrations generally declined as temperature increased. Notably, the largest decrease was observed between the +1°C and +2°C experiments. A comparison of these cases revealed that in the Southern Ocean, where deep water forms, enhanced stratification led to a shallower mixed layer, weakening the deep ocean circulation in the Pacific originating from Antarctic Bottom Water. This weakening of deep ocean circulation reduces nutrient transport from the deep Southern Ocean to the entire North Pacific, ultimately leading to a decline in surface nutrient concentrations. The reduced nutrient supply, in turn, led to decreased biological production in the subtropical North Pacific. The rate of decline in biological production exceeded the global average, indicating that this region is particularly sensitive to global warming.
Thus, as warming progresses, marine ecosystems in this region may experience greater stress compared to other ocean regions. These findings highlight the role of large-scale ocean circulation changes in shaping biogeochemical thresholds and regional nutrient dynamics. Identifying critical tipping points in nutrient availability and biological productivity will be key to understanding long-term ocean biogeochemical feedback to climate change. We plan to conduct a more detailed budget analysis by further breaking down individual processes to investigate the origins of nutrient changes.