Date of Award
Doctor of Philosophy in Oceanography
D. Randolph Watts
The Kuroshio Extension is the continuation of the Kuroshio western boundary current in the North Pacific Ocean. In 2004 an array of current and pressure recording inverted echo sounders was deployed within the Kuroshio Extension as part the Kuroshio Extension System Study (KESS). One of the goals of KESS was to investigate the relationship between upper-ocean circulation patterns and the deep-barotropic currents. In order to understand the processes which couple the upper and deep ocean in the Kuroshio Extension it is helpful to first provide a comprehensive investigation of dynamical balances in the deep Kuroshio Extension.
In the deep Kuroshio Extension, variability is generated locally and remotely. Like the Gulf Stream, meandering of the upper-baroclinic jet locally generates deep cyclones and anticyclones by stretching and squashing the lower water column. While this baroclinic stretching process dominates some events, there are other cases in which a seemingly spontaneous spin-up of an abyssal cyclone occurs without meander-induced baroclinic stretching. These strong deep eddies had cyclonic vorticities greater than 0.2 ƒ0, where ƒ0 is the Coriolis parameter, and are caused by topographic stretching when water columns are advected off isolated seamounts in the region. Strong incident currents drive water columns off seamounts to form cyclones with relative vorticity consistent with a layered potential vorticity conservation calculation.
Besides locally-generated strong eddies, larger-scale and weaker deep cyclones and anticyclones were observed to propagate into the KESS region from the northeast. Daily snapshots of deep and upper ocean streamfunction demonstrated that as the deep eddies encountered the Kuroshio Extension, upper-ocean meanders steepened and deep eddies intensified. The joint intensification of the upper and deep circulations was also seen by strong energy and vertical coherence in the 30–60 day band near the Kuroshio jet axis. Using complex empirical orthogonal function analysis, the phase speeds and wavelengths were calculated for the deep-pressure signals, identifying them as short barotropic topographic Rossby waves. Joint complex empirical orthogonal function analysis of upper and deep streamfunctions revealed that near the Kuroshio Extension axis, upper and deep signals were phase shifted laterally downstream, with the deep signal leading the upper. This arrangement is consistent with the joint baroclinic development patterns. Observations were then compared against an two-layer intermediate geostrophic model and the Ocean General Circulation Model for the Earth Simulator (OFES), both of which agreed with the KESS observations.
Since OFES reproduced upper and deep pairs of cyclonic and anticyclonic features that propagated southwestward across the Kuroshio Extension, with phase speeds and intensities matching those observed in KESS, it was further utilized to help provide regional context to the KESS observations. Outside the KESS region, 30–60 day bandpass filtered bottom pressures were also consistent with Rossby-wave propagation on a combined planetary and topographic beta plane. This result also suggested that the likely origin of the waves was the Shatsky Rise. Furthermore, the meandering Kuroshio Extension was shown to be the likely generation source of the waves insomuch as the dominant Kuroshio Extension meander in the 30–60 day band has a zonal wavelength that can couple to the topographic Rossby wave field on the Shatsky Rise.
Greene, Andrew Dale, "DEEP VARIABILITY IN THE KUROSHIO EXTENSION" (2010). Open Access Dissertations. Paper 1602.