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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

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 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, H. B. Milburn, and E. F. Ford

Abstract

A free-fall instrument, TOPS, measures vertical profiles of horizontal ocean velocity, conductivity and temperature. Profiling capability extends throughout the full water column (6000 db pressure limitation). Larger vertical wavelength (water depth > λ ≳ 20 m) velocity fluctuations are resolved by acoustically tracking TOPS relative to an array of bottom moored transponders. Shorter vertical wavelength velocity fluctuations (1000 m ≳ λ > 0.2 m) are resolved by an onboard acoustic velocimeter, which measures ocean velocity relative to the profiler. Motions of the profiler are monitored with a two-axis accelerometer and fluxgate compass. The instrument, data acquisition system and processing are described. In order to interpret the onboard velocimeter measurements, a planar, irrotational flow model is developed that describes the response of TOPS to an arbitrary oceanic shear profile. The model is verified using measured velocity and acceleration and by comparing oceanic velocity computed from the onboard velocimeter with that obtained from the acoustic tracking system. The two velocity profiles, smoothed with a 25 m half-width Gaussian filter, had a rms difference over a 1000 m depth interval of only 2 cm s−1. Accelerometer measurements, interpreted through the response model, are also used to obtain ocean velocity over wavelengths longer than 10 m. These estimates agree with those based on the velocimeter to within 1.5 cm s−1. Results of our model are compared with other models that describe similar free-fall dropsondes. In addition, TOPS measured velocities are compared with independent measurements taken with another profiler. The agreement found in these internal and external comparisons gives confidence in the accuracy of the TOPS model and the ability to obtain full water column high-resolution velocity profiles in the manner described.

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L. J. Mangum, S. P. Hayes, and L. D. Stratton

Abstract

Moored wind measurements at near-equatorial locations along 110°W, 125°W, 140°W, 170°W, and 165°E are used to investigate the space-time variability of the tropical Pacific wind field. These measurements complement previous studies that relied on island winds in the central Pacific or a few moored measurements in the eastern Pacific. Results indicate that the energetic portion of the zonal and meridional wind is significantly coherent over meridional scales of about 200 km and zonal scales of 1500 km. Even at these separations the estimated coherence often accounts for less than 50% of the variance. Temporal subsampling indicated (in agreement with previous studies) that at least ten samples per month were required to resolve monthly wind speed to within 1 m s−1 in the eastern equatorial Pacific. West of the date line and in the intertropical convergence zone (ITCZ), nearly daily sampling was required. Investigation showed that little error in the daily average of derived quantities such as wind speed and stress was associated with computing these variables from daily vector averages of the wind components rather than from hourly values of the components that were subsequently averaged.

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P. S. Hayes, L. A. Rasmussen, and H. Conway

Abstract

Precipitation in the central Cascades of Washington correlates well over 1966–96 with wind and moisture in twice-daily upper-air soundings at a radiosonde station near the Pacific coast, 225 km away. A simple model estimates precipitation by using the component of the wind roughly normal to the north–south range of the Cascade Mountains, which it raises to a power and scales by the relative humidity. Values at 850 mb are taken as an index of the total moisture flux. Thresholds are imposed for the wind component and the relative humidity to reduce the likelihood of estimating precipitation from weak onshore flow on dry days. A split-sample analysis indicates that the model parameters are highly robust against sampling error. This moisture flux model estimates precipitation over 5-day periods with the coefficient of determination r 2 ≈ 0.55, which increases for precipitation aggregated over 30-day periods to 0.75 and decreases to 0.30 for daily precipitation. Results over a 4-month period in the winter of 1996/97 were comparable to those from an advanced mesoscale precipitation model. The model estimates water-year (October–September) runoff for the period 1966–96 from two drainage basins in the region with r 2 ≈ 0.8, and on 1 May forecasts subsequent May–September runoff with r 2 ≈ 0.4.

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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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S. P. Hayes, M. J. McPhaden, and J. M. Wallace

Abstract

Temporal correlations between near-equatorial surface wind and sea-surface temperatures (SST) at 11°W in the eastern Pacific Ocean are investigated using data from an array of moored sensors between 5°N and 5°S. The signature of tropical instability waves with periods of 20–30 days is apparent in time series of SST and both the meridional and zonal wind components. Results indicate the existence of a band of pronounced horizontal divergence in the surface wind field associated with the large meridional SST gradient (equatorial front) normally located just north of the equator. Perturbations of the equatorial front by the instability waves induce fluctuations in the overlying winds. Evidence of the air-sea coupling is stronger in time series of the meridional gradients of wind and SST than between time series of the variables themselves. The meridional differencing serves as a high-pass filter in the space domain, which removes planetary-scale wind fluctuations that are unrelated to the local SST perturbations. The wind fluctuations observed in association with tropical instability waves are on the order of 1–2 m s−1.

These results indicate that SST variability on weekly to monthly time scales forces perturbations in the surface wind field. It is suggested that the principal coupling mechanism in this region is the modification of the atmospheric boundary layer stratification. Over the equatorial cold SST tongue the vertical wind shear within the lowest 100 m of the atmosphere is strong and the surface winds are conspicuously weak. As the air flows northward across the equatorial front the boundary layer becomes destabilized, momentum is mixed downward, and the surface winds increase.

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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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