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Modeling ²³⁰Th (and ²³¹Pa) : as an approach to study the intermediate and deep water circulation in the Arctic Ocean Yu, Xiaoxin

Abstract

Recently observed ²³⁰Th concentrations in 2007 and 2009 documented very high ²³⁰Th values within the Atlantic layer in the Canada Basin of the Arctic Ocean. Similar levels of high ²³⁰Th had only been previously observed in the Alpha Ridge region, implying that the Alpha Ridge is the potential source of the high ²³⁰Th waters. As the Alpha Ridge is downstream in the classic cyclonic circulation, that circulation is believed to have changed. Motivated by this, a three-dimensional Arctic ²³⁰Th model is configured for the first time to study such change. To simulate the tracer, I coupled a scavenging model, which describes the exchange of ²³⁰Th (and ²³¹Pa) between the dissolved and particulate phases, to an offline NEMO model (the Nucleus for European Modelling of the Ocean) that provides the advection and mixing processes that redistribute the tracers within the ocean. As the scavenging rates of such tracer elements are strongly affected by oceanic particle concentrations, the scavenging rates are parameterized as a function of ice concentration, which, to a great extent, influences the biological processes in the water. Model output produced an increase of ²³⁰Th concentration in the south Canada Basin. Sensitivity experiments confirm such change is not caused by a change in the particle field but a change in the intermediate circulation from cyclonic to anticyclonic throughout the Amerasian Basin. This shift in circulation is the reason for a subsequent transport of high ²³⁰Th concentration from the Alpha Ridge to the south Canada Basin. The model circulation and density fields suggest that the change in the flow is caused by increased dense water flux into the Arctic Ocean, primarily through the Barents Sea route. This increase of dense water inflow alters the density distribution in the Arctic and results in a quick adjustment in the Atlantic layer (~1 year) through propagation of boundary trapped internal Kelvin waves.

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