Stream-coordinate structure and variability of the Kuroshio Extension

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Fine horizontal-scale surveys performed during the Kuroshio Extension System Study in May 2004 provide near-synoptic ADCP and CTD data along cross-jet transects just up-stream of the first meander trough of the Kuroshio Extension. An array of current- and pressure-recording inverted echo sounders (CPIES) deployed over a ∼ 600 × 600 km region centered on the first meander trough provide time series of bottom pressure and currents as well as acoustic travel time (τ), which is converted via the gravest empirical mode method to profiles of temperature, salinity, and specific volume anomaly. These datasets are used here to determine the mean velocity, hydrographic, and potential vorticity structure of the Kuroshio Extension in its "weakly meandering" state in a stream-coordinate system, which avoids the lateral smearing of the jet structure that results from an Eulerian approach. Maximum surface down-stream velocities range from 1 to 2 m/s, averaging around 1.4 m/s. Down-stream velocities reach the bottom south of the core with average magnitudes of 1-5 cm/s but vary in magnitude and direction depending upon the presence of deep barotropic eddies. In the first meander crest, mean cross-stream flux (O(0.5-2 cm/s)) is towards the cyclonic side, while entering the trough it is towards the anticyclonic side. However, cross-stream flows appear to be event-driven, with fluctuations in steepness of the meander pattern driven by jet instability. Transports above 5300 dbar decrease west to east from 138 Sv at 143 . 75{ring operator} E in the first crest, to 124 Sv at the trough and 75 Sv entering the second crest at 148 . 5{ring operator} E. The mean from the surveys shows relative vorticity (ζ) making contributions as high as 72% of f on the cyclonic side and - 41 % of f on the anticyclonic side, while the "twisting" term due to vertical shear and horizontal density gradients reaches a maximum in the mean of 45% of f north of the core. Comparison of the structure in crests and troughs from the CPIES dataset reveals differences in the distribution of relative vorticity across the core. Both datasets suggest the presence of four isopycnal potential vorticity gradient layers, where strong cross-stream gradients in shallower layers represent a barrier to cross-stream flow. Comparison of the Kuroshio Extension to the Gulf Stream suggests that mean velocities and gradients are generally about 30% weaker in the Kuroshio Extension, and the strong jet structure penetrates to about 25% greater depths in the Gulf Stream. © 2009 Elsevier Ltd. All rights reserved.

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Deep-Sea Research Part I: Oceanographic Research Papers