5:15 PM - 7:15 PM
[ACG36-P17] Pathways of the North Pacific intermediate circulation estimated by Lagrangian particle tracking based on high-resolution modeling
Keywords:North Pacific Intermediate Circulation, Lagrangian particle tracking, eddy-resolving ocean model
The North Pacific Ocean is known as the endpoint of the global thermohaline circulation and it contains deep water rich in carbon and nutrients. Due to its high biological productivity, this region has the highest carbon dioxide absorption from the atmosphere globally. The amount of carbon dioxide absorbed at the sea surface and transported to the deep ocean and the time it takes to return to the surface and be released back into the atmosphere significantly influence climate change. Therefore, understanding the overall structure and variability of the North Pacific deep circulation is essential for assessing its future impacts on ecosystems and the climate system. However, the physical mechanisms and specific pathways of the North Pacific deep circulation have not been fully understood. It has been suggested that apart from the deep circulation, there is the North Pacific intermediate circulation which transports North Pacific Intermediate Water formed by sea surface cooling. The intermediate circulation is thought to transport heat to the deep ocean through tide-induced vertical mixing near Kuril Islands and thus drives the deep circulation (Kawasaki and Hasumi, 2010). However, there are still many uncertainties regarding the structure, transport volume, and timescales of the intermediate circulation and the interaction between the intermediate and deep circulation. Previous studies have shown that eddies play an important role in the transport of North Pacific Intermediate Water, so it is necessary to employ an eddy-resolving resolution to properly capture the North Pacific intermediate circulation in ocean models. In this study, we investigate the three-dimensional structure of the North Pacific intermediate circulation using an eddy-resolving ocean model. Based on the high spatiotemporal resolution flow field obtained by the ocean model, we estimate the pathways and timescales of the North Pacific intermediate circulation quantitatively by using Lagrangian particle tracking.
We employed a nested global ocean model, where a model with 1/12° horizontal resolution for the Pacific Ocean is nested into a global model with 1/4° horizontal resolution. It was initialized by the temperature and salinity of World Ocean Atlas 2018 and forced repeatedly by 1990 JRA55-do atmospheric reanalysis data. The intensity of vertical mixing was set to the value that has shown the highest consistency with observational data (Kawasaki et al., 2021). After integrating the model until it reached a quasi-steady state, we verified the consistency of the temperature and salinity distributions with observational data and utilized the velocity and diffusivity fields for particle tracking. A large number of virtual particles were evenly distributed in the North Pacific intermediate layer, defined by density and latitude. Based on 20 years’ tracking, we analyzed the residence time of intermediate water and its pathways after leaving the intermediate layer.
Among the particles initially placed in the intermediate layer, 80% left the layer, and about 60% of those eventually returned to the intermediate layer within 20 years. Further analysis is planned to investigate differences in the pathways of particles that return within a short time (1–2 years) and those that do not return for a long period. The analysis of the density when particles left the intermediate layer revealed that about 40% of the particles moved into a lighter density layer, while another 40% sank into a denser layer. In the presentation, we will discuss the detailed pathways of the North Pacific intermediate circulation, the governing processes for the pathways, and the ventilation timescale of the intermediate layer.
We employed a nested global ocean model, where a model with 1/12° horizontal resolution for the Pacific Ocean is nested into a global model with 1/4° horizontal resolution. It was initialized by the temperature and salinity of World Ocean Atlas 2018 and forced repeatedly by 1990 JRA55-do atmospheric reanalysis data. The intensity of vertical mixing was set to the value that has shown the highest consistency with observational data (Kawasaki et al., 2021). After integrating the model until it reached a quasi-steady state, we verified the consistency of the temperature and salinity distributions with observational data and utilized the velocity and diffusivity fields for particle tracking. A large number of virtual particles were evenly distributed in the North Pacific intermediate layer, defined by density and latitude. Based on 20 years’ tracking, we analyzed the residence time of intermediate water and its pathways after leaving the intermediate layer.
Among the particles initially placed in the intermediate layer, 80% left the layer, and about 60% of those eventually returned to the intermediate layer within 20 years. Further analysis is planned to investigate differences in the pathways of particles that return within a short time (1–2 years) and those that do not return for a long period. The analysis of the density when particles left the intermediate layer revealed that about 40% of the particles moved into a lighter density layer, while another 40% sank into a denser layer. In the presentation, we will discuss the detailed pathways of the North Pacific intermediate circulation, the governing processes for the pathways, and the ventilation timescale of the intermediate layer.