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Peter M. Saunders

Abstract

Employing one million ship reports gathered in the years 1941–72 seasonal averages of the wind stress and its standard deviation have been computed for the shelf region of the eastern North American continent (out to a depth of 200 m). A drag coefficient is assumed which increases with wind speed, from 1.0×10−3 at 5 m s−1 to 2.3×10−3 at 25 m s−1. Atmospheric stratification is taken into account but its effect is shown to be small.

In the summer season the 32-year climatological wind stress is toward the northeast, having a magnitude close to 0.25 dyn cm−2 throughout the entire shelf region. In the three other seasons the stress is directed toward the south and east being strongest in winter (1–1.5 dyn cm−2) and weakest in fall (0.25–0.5 dyn cm−2). In addition to the expected increase in magnitude with increasing latitude remarkable small-scale variability occurs. An offshore increase in stress is widespread and dominates the mid-Atlantic Bight; in winter the stress there increases from 0.5 to 1.0 dyn cm−2 in going 200 km offshore. In the Gulf of Maine and especially in the Gulf of St. Lawrence local maxima occur; the tall of the Grand Banks 500 km from shore shows a minimum. Probably much of this variation is associated with the intensity (and frequency) of cyclonic activity rather than directly with changes in friction at the underlying surface. Some oceanographic consequences are commented on but the computations are principally intended as a data source for further research.

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Peter M. Saunders

Abstract

An experiment to measure near-bottom currents on the Madeira Abyssal Plain is described. The moorings placed near 33°N, 22°W were separated by 5–40 km with instruments at 10, 100 and 600 m above the bottom (depth ∼5300 m). Rotor stalling occurred ¼ to ⅓ of the time but does not hinder the analysis which separated currents into tidal (3 cm s−1, inertial (1.3 cm s−1) and low frequency (2.5 cm s−1) components. The M2 tide is found to be principally barotropic with magnitude, ellipticity, orientation and phase adequately predicted by the tidal model of Schwiderski (1979). Oscillations of near-inertial frequency are found to be bottom intensified, have a wavelength of 100 km directed nearly due south and 3 km vertically: their phase velocity is directed downward suggesting the bottom as the source. The vertical group velocity is estimated at ∼150 m day−1 upward and corresponds to the 4–6 day lag observed between 10 and 600 m for the envelope of inertial amplitude.

Low-frequency statistics are presented also revealing bottom intensification. The array mean current can not be distinguished from zero; the kinetic energy, 3.6 cm2 s−2 at 10 m, decreases to 1.6 cm2 s−2 by 600 m, and rms east currents exceed rms north currents by 50%. Despite the dominance of long periods, 50–150 days, the integral time scale derived from the array autocorrelation data is 7 ± 2 days. By assuming equality of Eulerian and Lagrangian time scales we estimate the abyssal (horizontal) diffusivity as 2.5 × 106 and 1.5 × 106 cm2 s−1, east and north, respectively.

Spatial correlations are calculated for the array, and the transverse velocity function is found to have a zero-crossing near 35 km. streamfunction maps are made at 10-day intervals by the method of optimum linear interpolation. trajectories calculated over a 6–12 day period agree well with the drift of two groups of four neutrally buoyant floats at depths 4600–4900 m despite very weak flows, circa 1.5 cm s−1. Trajectories have also been calculated of the behavior of 170 pairs of particles at initial separations 1–20 km for up to 10 days. Their statistics reveal mean dispersion rates which yield pair diffusivity estimates of 5 × 103 cm2 s−1 at 1 km separation increasing to 2 × 106 cm2 s−1 at 20 km separation. Over this range the average time for the squared separation of particles to double is 10–15 days. The float cluster growths are quite comparable. The merits and limitations of the methods employed are discussed.

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Peter M. Saunders

Abstract

Small developing shower clouds were studied from a Caribbean island using visual and radar techniques. Within as little as 25 minutes of the formation of a cloud, precipitation had developed in its summits to an estimated diameter of ½ mm. The subsequent growth of these particles, giving rise to an intensification of radar reflectivity at the rate of an order of magnitude per 60 to 200 seconds is shown to be consistent with growth by gravitational coalescence. In small clouds (tops between 3 and 5 km) the peak reflectivity factor attained in a shower increases rapidly with the height of its summits but in clouds that extend above the freezing level the peak reflectivity factor does not exceed a value of about 3×105 mm6/m3.

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Peter M. Saunders

Abstract

An observational study of cumulus is made from time-lapse film records, and laws describing the growth of cloud thermals (discrete masses of buoyant air) are deduced.

The diameter of isolated thermals emerging from the summits or on the flanks of a cumulus is found to have a well-defined upper limit which is a simple linear function of height. It is inferred that, in the saturated interior of a cloud, thermals broaden linearly with height incorporating into themselves the residues of earlier (now decaying) thermals. Their rate of broadening with height, 0.40 ± .04, is found insensitive to variations of the stability and humidity of the cloud layer and to the presence or absence of precipitation.

From the observed broadening, the buoyancy of cloud thermals can be deduced if a knowledge of the properties of decaying thermal residues is given. A probable upper limit to thermal buoyancy is computed and is found to be some fraction of the adiabatic value.

Observations of the rate of rise and the diameter of the most vigorous thermals emerging from the cloud interior and estimates of their buoyancy permit a test of a proposed simple relationship between these properties. Fair agreement is obtained in the statically unstable part of the cloud layer. Thermals with smaller rates of rise are deduced to have smaller buoyancy due to their growth in less favorable environment.

The relation between the properties of cloud thermals and the history of the cloud is indicated.

Both the quantitative and qualitative results of the investigation establish the ‘thermal theory’ of cumulus convection on a firmer basis than hitherto.

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Peter M. Saunders

Abstract

A conversion formula between pressure and depth is obtained employing the recently adopted equation of state for seawater (Millero et al., 1980). Assuming the ocean of uniform salinity 35 NSU and temperature 0°C the following equation is proposed, namely, z = (1-c 1)pc2p 2. If p is in decibars and z in meters c 1 = (5.92 + 5.25 sin2ϕ) × 10−3, where ϕ is latitude and c 2 = 2.21 × 10−6. To take account of the physical conditions in the water column a dynamic height correction is to be added but for many purposes this may be ignored.

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Peter M. Saunders

Abstract

From the period commencing midsummer 1990 and lasting for 13 months, 13 current meters were deployed on seven moorings at depths between 1000 and 2300 m southeast of Iceland (63°09′N, 17°18′W to 61°44′N, 15°24′W). The purpose of the array (designated WOCE ACM8) was to measure the overflow of dense water, downstream of the source in the Faeroe Bank Channel and downstream of any sources on the Iceland–Faeroe Ridge. In the depth range 1300–2100 m three of the moorings showed a bottom intensified current that was persistent and directed to the southwest; at the other moorings the current was erratic and weak. The records were combined to form a transport time series for water denser than σ0>27.8 kg m−3 in the direction 240°. A mean value of 3.2 ± 0.5 Sv was found with considerable variability near a period of seven days, but there was no convincing evidence for a seasonal signal. The amount of pure Norwegian Sea water present in the overflow was estimated at 1.6 ± 0.15 Sv, close to previous determinations and comparable to that measured in the Faeroe Bank Channel. A small contribution is therefore deduced as crossing the Iceland–Faeroe Ridge. A general freshening of the water column in the northern Iceland Basin in the period 1960–1990 is demonstrated.

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Peter M. Saunders

Abstract

Near-bottom currents at depths in exceeds of 5000 m have been measured in the Great Meteor East study area (near 31°30′N, 25°W) over a 3 year period. The sites selected were on top of a small abyssal hill, on its flank, and on the abyssal plain 30 km distant from the hill. The magnitude of the mean current 10 m above the seabed was 1–2 cm s−1 but its direction was quite different at the three sites and reflected the presence of a clockwise vortex trapped over the hill. On the plain the mean flow direction was to the west and directly opposed to that furnished by a β-spiral analysis of the density field. It is suggested that time dependent variations in the large-scale density field are more important than hitherto supposed.

For periods greater than 120 days the variance of the current on the plain is concentrated in the east component, and for periods 50–120 days the variance is concentrated in the north component. Fluctuations propagate westward at speed 1–10 cm s−1 but are more complex than barotropic planetary waves. From estimates of the integral time scale of these motions (6–14 days) horizontal diffusivities of between 2 and 5 (×102 m2 s−1) have been deduced.

Estimates of the abyssal vertical velocity on the flank and top of the hill reveal and influence of the slope of the local bottom; on the plain any signal is buried in measurement noise.

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Peter M. Saunders

Abstract

An experimental study is made of a baroclinic vortex produced in a two-layer fluid. After the equivalence between the experiment and the heated annulus experiment of Hide is established, it is argued that the Potential energy of the stratification is the source for wave-like instabilities which develop and split the vortex into separate, distinct circulations. The stability of sonic mesoscale oceanic vortices is examined.

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Peter M. Saunders

Abstract

A simple theory is presented to account for the difference between the temperature at the ocean-air interface and that of the water at a depth of about one meter. Except in very light winds and intense solar radiation the mean temperature difference ΔT is expected to be of the formwhere q is the sum of the sensible, latent, and long-wave radiative heat flux from ocean to atmosphere and τ/ρw is the kinematic stress. No data are available to test this prediction.

The influence of slicks and solar insolation on interface temperature is also briefly discussed.

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Sheldon Bacon and Peter M. Saunders

Abstract

An analysis is made of data from 30 Aanderaa recording current meters (RCMs) set on nine moorings located east of Cape Farewell, the southern tip of Greenland. The purpose of the measurements was to allow for the estimation of transport in the deep western boundary current (DWBC) below a depth of about 1500 m. The records commenced in September 2005 and lasted from 9.5 to 11.5 months. After calibration of the raw data, 12-h averages of temperature and current were derived and the latter employed to estimate the flow across and along the array direction. The 9.5-month average transport of water colder than 3°C was found to be 7.8 Sv (1 Sv ≡ 1 × 106 m3 s−1) with a standard error of 0.8 Sv. For water denser than σθ = 27.85 kg m−3, the transport is calculated as 4.5 Sv. Whether either of these values is significantly different from comparable measurements made 500 km upstream cannot be determined. In marked contrast, for σθ > 27.8 kg m−3, the transport is estimated as only 9.0 Sv, smaller than the widely accepted value of 13 Sv for nearby measurements made in 1978. A reevaluation of the calculations and assumptions made then allows one to determine the uncertainty of the earlier estimate and thereby conclude that the difference between the previous and present measurements is significant, that is, that the transport has decreased between 1978 and 2005–06. A weakening of the transport during the 9.5-month period is also observed, along with a warming and an increase in salinity in the core of the DWBC. These latter changes are shown to be consistent with interannual variability rather than a long-term trend.

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