Bachelor defense by Naya Sophie Rye Jørgensen – Niels Bohr Institute - University of Copenhagen

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Bachelor defense by Naya Sophie Rye Jørgensen

Title: Investigating Bimodal Variability of the Kuroshio with High Resolution Climate Model

The Kuroshio current, the western boundary current of the North Pacic subtropical gyre, exhibits bimodal variability on interannual to decadal timescales characterized by two states; a stable, non-large meander, and an unstable, large meander. This low frequency variability is a signature feature of the Kuroshio, not yet observed in other western boundary currents. The Kuroshio is one of the strongest air-sea heat loss regions in the ocean and so understanding the interannual variability of the Kuroshio is key to understanding and predicting climate in the region. There is currently no consensus about the mechanisms driving the low frequency transitions of the Kuroshio.
The main question addressed in this study is whether the bimodal behaviour of the Kuroshio is a result of external time varying wind forcing, acting through a linear time-dependent Sverdrup balance, or through nonlinear processes internal to the ocean system. [Usui et al. 2013] [Knauss 2005] [Pierini et al. 2009]. We base our analysis on output from the high resolution Community Earth System Model. With a theoretical basis in the geostrophic and hydrostatic balance we compute the horizontal gradient of the mean annual sea surface height anomalies provided by the CESM and use it to determine the barotropic surface velocity in the Kuroshio region. We develope a method to track the position of the Kuroshio along the longitude where it displays the most signicant path difference between its stable and unstable state. We test the hypothesis that the variability of the Kuroshio is a result of external time varying wind forcing by examining the transitions of the Kuroshio with a prescribed annual atmospheric evolution in a control integration in the CESM. Our findings suggests that the bimodal variability of the Kuroshio is not a result of external time varying
wind forcing. We conduct a pertubation experiment and introduce an atmospheric forcing through a change in the zonal winds in the Southern Ocean, the effect of which can only be transmitted to the Kuroshio region through internal ocean mechanics. Our results support the hypothesis that mechanisms internal to the ocean system are linked to the low frequency bimodal variability of the Kuroshio.