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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 N. E. Clark

Abstract

No abstract available.

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G. J. McNally and W. B. White

Abstract

Wind-driven flow in the upper 90 meters during autumn–winter in the midlatitude North Pacific is investigated using satellite-traced drifting buoys (i.e., drifters) deployed nearly simultaneously but drogued at different depths. The difference in the relationship between drifter velocity and wind stress as a function of drogue depth is observed to change when its drogue entered the deepening mixed layer. This change is characterized by a sudden increase in the amplitude of the near-inertial motions observed by the drifters and by the onset of a persistent net displacement whose downwind component is approximately 3 times as large as its crosswind component. Attempts to model this downwind flow as a windage result in a large unexplained residual downwind velocity component. On the other hand, 80–90% of the observed crosswind displacement is explained by an Ekman slab model (i.e., with flow uniform over the mixed layer). This large residual downwind velocity component combined with the Ekman driven crosswind component results in drifter displacements whose angle with respect to the forcing wind is significantly greater than 0° and significantly less than 45° to the right (i.e., ∼30°).

Details of the flow obtained while the drogues were still below the deepening mixed layer suggests the presence of an Ekman-like spiral in the velocity vector beneath the mixed layer. Subsequent to all the drogues being in the mixed layer, the behaviour of all the drifters with respect to the local wind stress vector was essentially the same (i.e., independent of their drogue depth).

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W. B. WHITE and A. E. WALKER

Abstract

The meridional atmospheric teleconnections over the central North Pacific Ocean during winter are investigated by correlating the year-to-year fluctuations in the convective activity at the Equator, mid-latitude pressure/wind system, and subtropical pressure/wind system from 1950 to 1972. To establish fluctuations in these climatological systems, we consider the 23-yr time sequences of the maximum strength of the subtropical ridge, maximum intensity of the Aleutian Low, and strength of convective activity at the Equator, the latter inferred from the rainfall and sea-surface temperature at Canton Island (2°48′S, 171°43′W) observed by numerous authors to be representative of rainfall and sea-surface temperature over the entire central and eastern equatorial Pacific Ocean.

Upon cross correlation of these time sequences, we find that, throughout the two decades from 1950 to 1972, the intensity of the subtropical ridge and of the Aleutian Low were significantly correlated with one another (−0.50) as were the absolute magnitudes of their associated wind systems, the westerlies and northeast trades (0.83). Only during 1964–72, however, was the intensity of the Aleutian Low correlated (0.62) with fluctuations in the convective activity of the Equator. This indicates that the anomaly activity in the mid-latitude and subtropical pressure/wind systems and in the convective activity at the Equator were closely coupled during this time period. Conversely, during 1950–63, the lack of correlation between the convective activity at the Equator and the mid-latitude pressure/wind system suggests that the anomalies in these two systems were decoupled, at least in a linear way.

Bjerknes has found that fluctuations in the convective activity along the Equator in the Pacific were in association with the global tropical “southern oscillation” from 1955 to 1967. This suggests that, during the period of our study prior to 1964, the fluctuations in the mid-latitude pressure/wind system were independent of the fluctuations of sea-level pressure in the tropical latitudes of the world. However, the results of this paper together with the work of Bjerknes suggest that, after 1963, the mid-latitude pressure/wind system fluctuated in concert with the fluctuations of sea-level pressure over the tropical latitudes of the world. Speculation as to the causes for the approximate decadal variation in the meridional atmospheric teleconnections over the North Pacific is presented in terms of the decadal fluctuations found in the long-term trends of the major atmospheric systems.

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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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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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W. B. White and J. P. McCreary

Abstract

The El Niño studies carried out by Bjerknes indicate that the anomalous warming of the eastern boundary tropical ocean in 1939 and 1958 extended 1000 km into the interior ocean, corresponding with a diversion of the Humboldt Current offshore south of Peru. Furthermore, these features were associated with a large-scale weakening of the Southern Hemisphere wind systems. This suggests that major changes occurred in the wind-driven eastern boundary circulation during this time. To see if this is plausible from a theoretical viewpoint, we consider a mathematical model of large-scale ocean spin-down, induced by a reduction in the mean strength of the large-scale wind systems. This model shows that spin-down of the interior ocean is intensified along the eastern boundary with de-intensification propagating in time toward the west and extending out to 1000 km after one year. The interior portions of the ocean circulation are only weakly affected. Moreover, the spin-down is asymmetric, with greater de-intensification in the equatorial eastern boundary than in the poleward regions, making it appear as though the Humboldt Current is directed off-shore just south of Peru. This asymmetric aspect of gyre spin-down can be explained in terms of non-dispersive Rossby waves propagating energy [Cg=−β/(f 2/gH] to the west at a faster rate near the equator than near the poles.

The results of this study have direct application to El Niño. First, the general decrease in equatorward transport of the Humboldt Current off the coast of Peru allows for the anomalous increase in temperature simply by reducing the advection of cold subtropical waters. Moreover, the asymmetric intensification of eastern boundary spin-down has the effect of lowering the dynamic height of the sea surface in the eastern equatorial region. This situation is conducive for the cross-equatorial transport of warm water to the south; possible mechanisms for the anomalous warming have been proposed by Bjerknes and Wyrtki. The advection of warm interior water to the east does not seem to be allowed by the spin-down process and therefore, cannot be considered as a source of warm El Niño waters.

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W. B. White and T. P. Barnett

Abstract

During the autumn and winter seasons, large amounts of heat are given up to the atmosphere at the subarctic frontal zone off the east coast of Asia. According to Fisher, the Laplacian of this heat flux (∇2 Q) is related to increases in the intensity of the relative vorticity in the westerly wind regime. This increase is related to a similar increase in strength of the wind stress curl, which thereby increases the Sverdrup transport of the subarctic and subtropical gyres. The increase in transport in turn intensifies ∇2 Q at the subarctic frontal zone via geostrophic adjustment. This coupling of atmospheric relative vorticity, Sverdrup transport, and ∇2 Q results in the intensification of the relative vorticity of both fluid media that can be checked only by an instability in either one or the other media. This mutual interaction of the ocean and atmosphere is termed a servomechanism, the natural time scales of which are determined by a mathematical development wherein the vertically integrated vorticity equations of the ocean and atmosphere are coupled by their interaction at the naviface. This coupling leads to a single wave equation for the ocean/atmosphere system, the solutions of which are Rossby waves modulated by exp[(1+it)], where α depends upon the coupling parameters. Normal values of α are found to produce an e-folding increase in the vorticity of the ocean/atmosphere system in less than two months. For anomalously high values of α, the increase in vorticity can be extreme, possibly leading to the formation of a barotropic instability in the atmospheric medium. These theoretical results are illustrated using geophysical data from 1950–60 and are used to explain the events that triggered the unusual ocean/atmospheric vorticity state that existed in the North Pacific between 1956 and 1958.

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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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W. B. White and N. E. Clark

Abstract

Monthly mean atmospheric data taken over the North Pacific during the period 1950–70 are used to investigate blocking ridge activity over the central ocean. The blocking ridge is observed to be a finite-amplitude, quasi-stationary long wave, most often centered over the North Pacific at 170W, superimposed upon the quasi-zonal mid-latitude westerlies. The dominant length scale is 7000 km, the same dimensions as the width of the mid-latitude ocean. The growth time scale is 1–2 weeks, with the duration of blocking activity rarely exceeding 2 months in any given year. The blocking activity is confined almost exclusively to the autumn/winter months, where block development is closely coupled with the sensible heat transfer from the underlying ocean (anomalously small beat transfer under the ridge and anomalously large heat transfer under the associated troughs). Year-to-year variability in blocking ridge activity is found to have a dominant time scale of approximately 5 years from 1950–70 and to be inversely correlated (−0.79) with the strength of the autumn/winter mean mid-latitude westerlies (the mean formed using months not containing blocking activity). Further analysis shows that both blocking ridge activity and the strength of the westerly winds fluctuate together with the Southern Oscillation over this time period.

These space/time scale considerations suggest that this regional blocking activity owes its existence to the marine environment. To test this idea, appeal is made to some theoretical work by Haltiner, where the baroclinic instability process was modified by sensible heat transfer from the ocean to the atmosphere. Haltiner found that for normal winter values of the background flow, the otherwise stable stationary long wave became unstable when sensible heat transfer was allowed. The wavelength for the unstable stationary wave was 7000–8000 km with a growth time scale of approximately 2 weeks. The scales are similar to that of blocking ridge activity over the North Pacific.

In addition to good scale agreement with observations, Haltiner's theory is able to explain both the seasonal and year-to-year variability in blocking activity in terms of corresponding fluctuations in sensible heat transfer and the strength of the mean westerly winds.

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