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R. L. Bernstein
and
W. B. White

Abstract

Several hundred XBT observations in the western North Pacific were collected each month over a 2½ year period 1976–78. They have been assembled into a monthly map sequence of thermocline temperature in the Kuroshio Extension Current, having 100 km spatial resolution. The overall time, men of the maps exhibits mesoscale (200–600 km) perturbations which correlate with several major bathymetric features, especially the Shatsky Rise. Time variability about the mean decreases significantly cast of the Shatsky Rise. In addition, time-variable mesoscale disturbances propagate zonally westward at ∼3.8 cm s−1. Attempts to explain the observed propagation phase velocity through simple analytic baroclinic Rossby wave theory lead to the implication that there exists in the region an eastward deep mean flow of 3 or 4 cm s−1. Direct current measurements of long duration are required to help resolve and explain these observations.

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R. L. Bernstein
and
W. B. White

Abstract

A set of 19 zonal vertical sections of temperature were collected in 1975 with XBT observations at 80 km spacing, made from ships-of-opportunity transiting the mid-latitude North Pacific. In the region of the Kuroshio Extension Current, around 35°N, 165°E to 170°W, cross-spectral analysis reveals a significant 30° phase shift, upwards to the west, for the dominant mesoscale features of ∼500 km wavelength. This phase shift implies that a poleward eddy flux of heat exists, associated with these mesoscale eddies. A simple calculation of the meridional eddy heat flux based on these results indicates an eddy flux of 3 × 1014 W over the length of the Kuroshio Extension. This value is 17% of that estimated by Oort and Vonder Haar (1976) to be the total ocean heat flux at 40°N over the entire Northern Hemisphere.

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W. B. White
and
R. L. Bernstein

Abstract

Reported here are the statistical results leading to the design of an optimum oceanographic network in the interior midlatitude North Pacific from 30 to 50°N, the primary function of which is to detect the generation and evolution of large-scale temperature anomalies in both the surface and subsurface layers of the upper 50 m of ocean. The method used in this optimum network design is based on linear least-squares estimation developed by Gandin (1963), wherein it is necessary to determine the first and second statistical moments (i.e., mean and covariance distribution, respectively) of the variable field, leading to the estimation of the dominant space and time scales of variability, as well as the signal-to-noise ratio. Having determined this information on the statistical structure of the thermal field in the interior midlatitude North Pacific (i.e., Lx = 1500 km, Ly = 1000 km, T = 10 months, S/N = 0.5), the minimum sampling density (i.e., 1 station per 200 km square per month) and maximum instrument error (a lσ accuracy of 0.2°C) are defined, necessary to detect the large-scale thermal variability. This latter information is then used in the actual construction of an oceanographic network, where since January 1976 XBT systems (having a lσ accuracy of 0.07°C) have been placed aboard 22 ships of opportunity that ply the trade routes between the west coast of North America and Japan. Examples of temperature anomaly maps, constructed monthly from 300 XBT's taken randomly over the region 30–50°N, 140–150°E, are presented. As a check on map reliability, the surface and subsurface temperature maps produced by this XBT network are compared with surface maps constructed by ship-injection temperature and subsurface maps constructed from research vessel XBT data.

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W. C. Patzert
and
R. L. Bernstein

Abstract

Two west-to-cast temperature sections (∼2500 km in length) have been constructed from closely spaced (∼37 km) XBT measurements in the central South Pacific collected during Legs J and K of the Pacific GEOSECS Expedition. For these portions of track passing through the islands of the Tuamotu Archipelago, the main thermocline exhibited 50 m station-to-station variability on a scale too short to be resolved. In the open ocean away from these islands, the variability was smaller, 10–20 m, with only one 200 km scale eddy spatially resolved.

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R. L. Bernstein
and
W. B. White

Abstract

The time and length scales of baroclinic eddies in the eastern half of the North Pacific Subtropical Gyre are examined using four data sets: 1) single vertical sections of temperature, based on closely spaced bathy-thermograph (BT) casts (cast spacing <50 km); 2) a set of similarly spaced BT observations distributed in both horizontal dimensions; 3) sub-surface temperature times series of four months to ten years duration from fixed moorings; and 4) a data set identical to the first above, only repeated along the same section at one month intervals for 16 months.

The first data set allows calculation of wavenumber power spectra which reveal that the major contribution to the temperature variance comes from wavelengths >100 km. The spectra attenuate toward higher wavenumbers as the square of the wavenumber over the range 200 to 600 km. Spatial autocorrelation in both horizontal dimensions of the second data set yields a correlation matrix with a first zero crossing at 150 km in both dimensions, implying significant energy at 600 km wavelength, isotropically distributed. Oceanic subsurface perturbations of this isotropic scale are referred to as mesoscale eddies.

The third data set allows calculation of frequency power spectra which reveal that the major temperature variance contribution comes from periods >2 days. The frequency spectra roughly follow an inverse square law bearing a similarity to the wavenumber spectrum over the range of 2 days to 6 months. This similarity indicates a linear relationship between time and length scales exceeding 2 days and 200 km, respectively. This suggests that the eddies are “frozen” features, carried along by the mean flow. To further examine this possibility, we consider the fourth data set, wherein baroclinic eddies of 480 km wavelength were dominant, propagating westward from one month to the next at a speed of 4.5±2 cm sec−1. This is comparable to, but definitely larger than, the observed maximum mean baroclinic flow observed in the fourth data set (1.4±0.8 cm sec−1 to the west).

These results indicate that the eddies are not composed of a uniform water mass carried along by the mean flow. Rather the view is adopted that the eddies possess a wave-like nature. The type of waves that have the same time and length scales as the eddy motions, in addition to sharing the property of baroclinicity, are non-dispersive baroclinic planetary waves. These waves have a phase speed (−4.2 cm sec−1) that is in excellent agreement with the observed eddy propagation speed.

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