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P. van Meurs and P. P. Niiler

Abstract

Data from Argos-tracked mixed layer drifters in fall and winter 1987 (49 drifters) and 1989 (16 drifters) are used to investigate the differences in the large-scale surface velocity and eddy activity in the northeast Pacific. The velocities were corrected for wind-induced slippage and corrected for wind-driven (Ekman) flow by matching an Ekman model to the observed currents. The model, which explains 15%–30% of the variance, indicates that the currents are at 60° to the right of the wind. The magnitude of the currents is 30% of the magnitude of the wind stress. In 1987–88, the geostrophic motion in the region from 46.5° to 48.5°N, 142° to 133°W was characterized by an eastward flow of 0.9 (±0.4) cm s−1 and a northward flow of 0.7 (±0.4) cm s−1. In 1989–90, for the same region, the geostrophic eastward component was 3.8 (±0.5) cm s−1, more than four times as large as in 1987–88, and the northward component was 0.3 (±0.5) cm s−1. In this region ageostrophic contributions to the velocities are small.

In 1987–88 the drifter tracks reveal evidence of the presence of several persistent, warm core mesoscale eddies. In 1989–90 there is no evidence of any significant eddy activity. The mean speed of the drifters in 1987–88 was 7.0 (±0.3) cm s−1 and in 1989–90 was 6.5 (±0.4) cm s−1. So, although the average speed is the same, drifters in 1987–88 take a longer time to travel eastward because of the significant north–south excursions due to the mesoscale eddies. Data from two drifter experiments have shown that the variability of mesoscale eddies can result in large interannual differences in estimates of mean velocity.

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P. P. Niiler, E. Simco, and R. Larue

Abstract

A two-layer model of a mid-oceanic thermocline is developed, and a comparison of the depth and temperature of the thermocline in this model is made with a root-mean-square description of the hydrographic structure of the North Atlantic. It is shown that the Sverdrup balance is maintained within the rms errors, and the entire estimated heat flux from the atmosphere is used to produce the observed density changes along the path of persistent flow.

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C. J. Koblinsky and P. P. Niiler

Abstract

Observations of currents collected at the POLYMODE array III cluster C (16°N, 54°W) are compared with radiosonde winds measured at Barbados (13°N, 62°W) using a linear response analysis. The winds and the currents are coherent throughout the water column (5400 m) over the subinertial frequency range of 0.025 to 0.25 cpd. The coherence is highest between the east-west wind stress and north-south currents over smooth topography. The wind-related ocean currents have a rms of 2.5 cm s−1 at 500 m and 1.1 cm s−1 at 4000 m and account for approximately one-third of the total eddy kinetic energy. The wind-current phase is depth independent and does not vary significantly over the 200 km horizontal scale of the array. The response amplitude is surface intensified and increases with decreasing frequency which is consistent with a deterministic theoretical model. The coherence values and response estimates suggest an off-resonant barotropic response to large-scale wind forcing.

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José Ochoa and Peter P. Niiler

Abstract

The Agulhas Current flows poleward along the western boundary of the southeastern Indian Ocean where, at the southernmost latitude of the African continent, it executes a dramatic anticyclonic turn, or retroflection, to the east. Since 1978, a large number of drifting buoys have passed through this eastward-flowing Agulhas Return Current (ARC), or the zonal frontal boundary between subtropical and subpolar waters of the south Indian Ocean. The spatial distribution of the ensemble-averaged near-surface velocity along the ARC axis reveals a series of steady-state meanders of 700-km wavelength and amplitudes that decrease from 170 km in the first meander to 50 km in the following four meanders. Here an analysis of vorticity balance of the meandering ARC speed axis is presented that demonstrates a balance between the β term and advection of curvature vorticity. This balance implies that the ARC axis, or frontal region, is horizontally nondivergent in agreement with the other observations of flow in the surface layers of near-zonal oceanic fronts.

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John Kroll and Pearn P. Niiler

Abstract

We know that long-period (>1 day) and long-wavelength (>100 km) topographical Rossby waves can be generated by a wind acting directly on a continental shelf (Adams and Buchwald, 1969). Here we examine the characteristics of them waves which can also be produced off the shelf by wind and current eddies and can propagate up to and onto the shelf. We use a shelf model which varies in depth in one direction only and assume that a shelf can be approximated by at most two breaks with the depth varying exponentially. We assume velocity-dependent bottom friction to determine the effect of frictional dissipation. The following results are derived by our analysis. The regression angle of scatter plots for topography-dominated waves should be small and the preponderant direction of the waves determined by the sign. The group velocity directed up the slope possesses an absolute maximum which occurs at a relatively short period. The ability of a wave moving up a slope to overcome friction correlates with this group velocity. The energy flux transmission across one and two breaks can be determined. It is suggested that the product of this flux transmission coefficient and the group velocity component up the shelf be the criterion to determine which wavelengths and frequencies penetrate nearest to shore. It is found, however, that the energy from off the shelf is likely to he decayed completely in bottom depths ≲25 m. A comparison of some results with data for the New England and west Florida shelf shows a general agreement.

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T. K. Chereskin, P. P. Niiler, and P. M. Poulain

Abstract

The shape and slip of freely drifting, two-dimensional, flexible weighted drogues tethered to a surface buoy in a specified upper-ocean velocity profile are examined numerically. A simple analytic solution for a drogue in a linear shear flow, in the limit of small deviations from a straight vertical configuration, is used to identify the parameters of the problem and to predict the functional dependence of the slip and shape of the drogue on those parameters. The numerical computations, using a finite elements static equilibrium model, confirm the functional dependence predicted by the analytic solution and estimate the parametric dependences. However, a linear shear is not the “worst case” shear one needs to design for. In optimizing a drogue for linear shear, one can make use of the symmetry of the velocity profile to minimize the slip. The design problem arises from not knowing a priori the shear for which one is designing (especially since a drogue eventually moves far from its deployment site) and from asymmetric shear (i.e., the “worst case” shear is one with a bias). The final computations examine three different drogue configurations in a series of profiles that model the diurnal cycle of the mixed layer (a diurnal jet) overlying a linear shear. The best design is found to be one that maximizes the drogue over the depth interval of interest, while minimizing the drag area of the tether. The drogue length needs to be larger than the depth interval of interest to account for the rise and tilt of the drogue in shear flow, but not so large that it averages too far outside the interval. For the practical cases considered, a drogue length that was twice the averaging interval gave the best results.

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Sean C. Kennan and Pearn P. Niiler

Abstract

Simultaneous moored temperature, salinity, velocity, and wind measurements from the equator at 157.5°E, during 10 May–21 December 1992, are combined with a no-stress-level boundary condition in the Equatorial Undercurrent core to estimate the total zonal pressure gradient force and subgrid-scale residuals of the momentum balance. Estimates are made of the depth of wind stress penetration, momentum depth, distribution of subgrid-scale stresses, and balance of forcing terms in the surface layer and pycnocline. Westerly winds of greater than 5 m s−1 in September 1992 coincided with the appearance of an eastward surface Yoshida jet and subsurface westward (Hisard) jet on the equator. The momentum depth increased with successive wind events, eroding the shallow halocline until it merged with the permanent thermocline. Wind-induced stresses were not restricted to the depth of density homogenization. The record-length-averaged pressure gradient force was westward and was balanced by downstream accelerations and stress drag. However, time-dependent accelerations were balanced by vertical divergence of the stresses. The pressure gradient dominated decelerations of the surface flows and played a lesser role in accelerating subsurface currents. The force balance was consistent with the concept of wind-driven surface flow above the momentum depth; in the pycnocline it implied forcing of the mean zonal currents via the eddy and turbulent momentum flux divergences. The results indicate that steady-state theories do not explain the existence of subsurface zonal currents on the equator. Time-dependent forcing in the equatorial pycnocline includes significant transfers of zonal momentum by submesoscale processes.

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W. Timothy Liu and Pearn P. Niiler

Abstract

A simple statistical technique is described to determine monthly mean marine surface-layer humidity, which is essential in the specification of surface latent heat flux, from total water vapor in the atmospheric column measured by space-borne sensors. Good correlation between the two quantities was found in examining the humidity sounding from radiosonde reports of mid-ocean island stations and weather ships. The relation agrees with that obtained from satellite (Seasat) data and ship reports averaged over 2° areas and a 92-day period in the North Atlantic and in the tropical Pacific. The results demonstrate that, by using a local regression in the tropical Pacific, total water vapor can be used to determine monthly mean surface layer humidity to an accuracy of 0.4 g kg−1. With a global regression, determination to an accuracy of 0.8 g kg−1 is possible. These accuracies correspond to approximately 10 and 20 W m−2 in the determination of latent heat flux with the bulk parameterization method, provided that other required parameters are known.

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Pearn P. Niiler and Melinda M. Hall

Abstract

A current meter mooring maintained for over three years at 28°N, 152°W, in the eastern North Pacific has yielded velocity and temperature data throughout the water column, with particularly good thermocline resolution The flow is characterized by weak primarily westward mean velocities, with a superimposed eddy field having rms velocities ranging from 10 cm s−1 in the upper thermocline to 3 cm s−1 at 1000 m depth. The eddy energy is divided into two main bands: the low frequency eddies have spatial scales of 250–300 km and periods of 100–200 days, propagate southwestward, and have slightly more zonal than meridional energy. The high frequency eddies also propagate southwestward, have spatial scales of 150–175 km and periods of 40–80 days, and are strongly meridionally oriented. Vertical EOF structure calculated in the frequency domain suggests that the low frequency eddies are more wavelike (linear) in nature than are the high frequency. The entire band appears to derive energy baroclinically from a secularly varying background flow; as a function of time, the eddy heat flux tends to be down the very low frequency varying temperature gradient. Some interesting points of comparison are found with eddies in a three-layer nonlinear model of the eastern North Pacific recently described by Lee.

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Sean C. Kennan and Pearn P. Niiler

Abstract

A new method for diagnosing the zonal gradient of sea level on the equator from in situ data is proposed and validated using satellite altimetry. A no-turbulent-stress-level boundary condition in the Pacific Equatorial Undercurrent core constrains the average flux of zonal momentum above, uniquely determining the zonal sea level gradient. The method is applied to simultaneous data from Tropical Atmosphere–Ocean Array moorings spanning 154°–165°E and ADCP moorings in the vicinity of 157.5°E from 6 October 1992 to 21 December 1993. An independent estimate of sea level slope is obtained from contemporaneous daily equatorial crossings of the first European Remote Sensing Satellite (ERS-1) and Ocean Topography Experiment (TOPEX)/Poseidon altimeters. The slope is also estimated by assuming no pressure gradient at 500-m depth. When accelerations and the pressure gradient can be estimated at the same location, 5-day averages of the no-stress-level estimate explain 59% of the variance in the altimetry data (correlation 0.77) with an rms difference of 2 × 10−8 and no significant mean offset. The variance explained improves to 66% with 7-day averages. In the absence of velocity and salinity gradient observations, 58%–75% of the signal can be captured with a bias of 0.5 × 10−8 on timescales greater than 10 days. Assuming no pressure gradient at 500 m cannot explain more than one-half of the variance on any timescale and is biased by −3 × 10−8 to 2 × 10−8, depending on the time period and timescale. Estimating the salinity gradient using temperature–salinity relations worsens the results. Error analyses of the observed data indicate that the daily sea level slope can be accurately determined to within less than 1.5 × 10−8 when comprehensive in situ data are available. Daily altimetry estimates from equatorial crossings determine the slope to within 2 × 10−8. Both methods are superior to assuming no pressure gradient at depth and improve upon previous comparisons between remotely sensed and in situ observations of sea level that have succeeded only for monthly and longer timescales.

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