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  • Author or Editor: S. P. Hayes x
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S. P. Hayes

Abstract

The low-frequency (<0.25 cph) velocity and bottom pressure variability were studied in the northeast Gulf of Alaska from March to August 1976. Measurements of velocity in 100, 185 and 250 m of water showed a contrast between the flow at the shelf break and that on the shelf. The former circulation had a weak mean alongshore flow (5 cm s−1), but large anticyclonic low-frequency fluctuations. On the shelf the flow was almost entirely alined along isobaths. The anticyclonic shelf break fluctuations did not propagate onto the shelf. Bottom pressure variations measured at four locations showed little variation along the shelf and a linear decrease in bottom pressure variance across the shelf. Correlations of bottom pressure gradient with velocity indicate much of the alongshore flow was consistent with barotropic quasi-geostrophic dynamics. Cross-shelf flow could not be related to the pressure gradients. Examination of the pressure field response to the wind showed that the nearshore sea level setup accompanied onshore winds, whereas in the outer shelf the setup accompanied alongshore winds.

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S. P. Hayes

Abstract

Vertical and horizontal structure of near-bottom currents at two locations in the eastern tropical Pacific (9°26′N, 151°17′W; 14°38′N, 125°29′W) have been studied. Low-frequency currents at these sites, located in a region of abyssal hills, increased between 500 m and 50 m above bottom and had small horizontal scale. Below 50 m frictional boundary-layer effects which were consistent with a simple, steady-state Ekman-like layer were apparent. Total veering within the boundary layer was, 10° counterclockwise (looking down) at both locations. High-frequency internal wave motions were consistent with a modified Garrelt and Munk internal wave spectrum. Energy correspondence calculations for several high-frequency bands showed some evidence for benthic internal wave generation by means of mean flow topography interaction. At the western site the near-inertial energy-time dependence and near-bottom enhancement both indicated local production in this band. Below 50 m frictional effects were again important. The observations showed veering and attenuation which depended on frequency and polarization in a manner consistent with time-dependent Ekman dynamics.

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S. P. Hayes
and
D. Halpern

Abstract

Upper-ocean current and temperature records from neat 0°, 110°W are compared with Galapagos Island sea level measurements (0°3′S, 91°28′W) for the period March 1980 to July 1981. Low-frequency (periods greater than 30 days) near surface zonal currents (20 and 50 m) were well correlated with local wind, but deeper currents (100 and 150 m) and sea level were not. Response to the local wind was studied in terms of linear dynamics. Periods where zonal acceleration was in phase with zonal wind stress were found; however, overall these two series were uncorrelated. The direct relation of low-frequency wind stress and surface current (rather than acceleration) suggests a quasi equilibrium response on these time scales. The response at deeper levels was investigated by comparing the zonal transport per unit width (vertically averaged zonal Velocity) in the upper 200 m with sea level. These series were significantly correlated (with 95% confidence) at a lag of 10.5 d indicating eastward phase propagation at a speed of 2.3 m s−1 which is consistent with a first baroclinic-mode Kelvin wave. This interpretation also explains the relative amplitudes of sea level and transport fluctuations as well as the correlation of zonal velocity and vertical displacement (estimated from temperature time series) at 100 m. This first vertical-mode Kelvin wave signal was dominated by event-like structures (high sea level, eastward velocity) which occurred in boreal spring or the two years studied. The residual near-surface zonal velocity was also maximum in spring. Thus, the annual cycle in the eastern Pacific appears to have at 1east two components with different vertical structures.

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S. P. Hayes
and
H. B. Milburn

Abstract

We report observations of the vertical structure of horizontal currents on the equator at 110°W in the eastern Pacific Ocean. Profiles indicate high, vertical-mode, deep currents with zonal velocities of up to 20 cm s−1 at a depth of 1500 m. The general similarity between our measurements and those reported in other equatorial regions suggest that such vertical structure is a ubiquitous equatorial phenomenon.

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S. P. Hayes
,
J. M. Toole
, and
L. J. Mangum

Abstract

An analysis of nine hydrographic sections collected in 1979–81 along 110°W in the equatorial Pacific Ocean is presented. Sections typically sampled the upper 500 m of the water column from 10°N to 3°S. Analysis concentrated on the repeated sections north of the equator. Examination of the variability of eastward transport indicates that the North Equatorial Countercurrent (NECC) and the Northern Subsurface Countercurrent (NSCC) cannot be distinguished solely on the basis of water-mass structure. However, using a potential density surface (σθ = 25.0) as a current boundary we find that on average the NSCC transports 13.7 × 106 m3 s−1 compared to only 8.3 × 106 m3 s−1 for the NECC. The NSCC flow is sufficiently stable so that meridional surface dynamic-height gradient remains a good index of zonal transport fluctuations. Variations in surface dynamic height observed in our data and in the EASTROPAC data indicate a seasonal cycle to the surface topography with large values for the equatorial and countercurrent depressions in boreal autumn and small values in spring. Broad meridional correlation scales for surface dynamic height were found; equatorial fluctuations were significantly positively correlated with variability at latitudes out to 5°N and significantly negatively correlated with variability at 9–10°N. The meridional and vertical structures or vertical displacement were reduced to two empirical orthogonal function (EOF) modes which contained 78% of the variance. These modes did not suggest simple dynamical interpretation in terms of first-vertical-mode linear waves.

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C. C. Eriksen
,
M. B. Blumenthal
,
S. P. Hayes
, and
P. Ripa

Abstract

Sea-level fluctuations during 1978–80 at equatorial Pacific islands separated by as much as one-quarter of the earth's circumference are coherent at periods of 1–6 weeks with phases implying eastward propagation. Eastward speeds are 16 ± 7.5% higher than expected for a linear, first-baroclinic-mode Kelvin wave (based upon hydrography). Zonal winds in the western Pacific exhibit variation on meridional scales comparable to those of equatorial-ocean baroclinic motions. Roughly one-quarter of sea-level variance in the 1–6 week period range can be explained by local zonal wind alone. The observed admittance magnitude, O[0.1 cm sea level per (m s−1)2 zonal wind pseudo-stress], and phase lag (a few days, sea level lagging wind) can be accounted for in a linear model of baroclinic equatorial Kelvin waves generated by a crudely idealized wind patch of 1000 km zonal scale. Zonal winds at the equator excite, among other things, low-mode Kelvin waves which are recognizable O(10000 km) to the east of the forcing.

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