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L. J. Zedel and John A. Church

Abstract

Computationally simple criteria for the identification of spurious Doppler profiler velocity estimates are described and evaluated. Their effectiveness is determined by changes in the confidence intervals and mean values of Doppler shear profiles. Our tests show that indicators based on measures of acoustic signal quality (signal-to-noise ratio, and spectral width) do not provide a mean reduction in confidence intervals. We show that a combination of velocity quality screening criteria can reduce the confidence intervals of velocity estimates made with a 150 kHz profiler by as much as 25 percent. After screening, both components of horizontal currents are about as accurate, almost independent of depth, to at least 240 m. More importantly, data screening also removes a depth-dependent bias (as large as 0.05 m s−1) from velocity profiles. For the indicators tested, we recommend threshold values for our system configuration; these values can serve as guidelines to suitable settings in other applications. To obtain maximum accuracy of shear profiles (and absolute profiles when accurate navigation data are available) some form of data screening is essential.

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John A. Church and Howard J. Freeland

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The sea level on the southern Australian coast is examined for the source of the coastal-trapped wave energy observed during the Australian Coastal Experiment. Sea level, adjusted for atmospheric pressure, and atmospheric pressure are observed to propagate eastward at about 10 m s−1. At the lowest frequency examined (24-day period), some energy travels south along the west coast of Tasmania, but does not reach the east coast of mainland Australia, while some energy travels through Bass Strait to reach the east coast of mainland Australia. At the most energetic frequency (8-day period), adjusted sea levels are coherent over the 3700 km of coastline from southern Australia to the east coast, and much of the wind-forced coastal-trapped wave energy appears to travel through Bass Strait to the mainland east coast. We have not identified a mechanism for energy transfer through Bass Strait, and we do not know what fraction of the coastal-trapped wave energy incident on western Bass Strait actually reaches the east coast. It is suggested that at low frequencies the long wavelength waves are not affected by relatively small gaps in the coastline, but that at higher frequencies the wavelength is smaller and breaks in the coastline become more important. The first and second coastal-trapped wave modes observed at Cape Howe during the Australian Coastal Experiment are most coherent with the sea level at Lakes Entrance at the eastern edge of Bass Strait. It is suggested that these coastal-trapped wave modes are generated when the east-west flow through Bass Strait has to adjust to the narrow shelf of the east Australian coast and that the second mode is preferentially generated because its length scale (k−1) more closely approximates the north-south extent of this east–west flow.

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J. A. Church and F. M. Boland

Abstract

A hydrological section adjacent to the Great Barrier Reef (GBR) was completed several times during 1980 and 1981. The data indicate a maximum southward current near the surface and a northward undercurrent at depths of 300–900 m. A detailed section indicates a double cell structure in the northward undercurrent. The cell structure in the geostrophic velocity is also clearly evident in the dissolved oxygen distribution where distinct core layers can he seen. Such a current offers considerable scope for the recycling of biological material along this section or the GBR.

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Trevor J. McDougall and John A. Church

Abstract

Current numerical models of ocean circulation parameterize diffusion using a diagonal diffusivity tensor in a horizontal/vertical coordinate system rather than in the isopycnal/diapycnal directions. It is pointed out that this procedure introduces a fictitious flux of density in the horizontal direction. A solution to this problem is available in the work of Redi. Also we show that care must be used in simple models of the deep ocean not to confuse the diapycnal velocity with the vertical velocity which simply occurs as a component of along isopycnal motion.

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John A. Church, Howard J. Freeland, and Robert L. Smith

Abstract

The currents observed over the shelf and slope during the Australian Coastal Experiment (ACE) are used to determine the amplitudes (as functions of time) of the first three coastal-trapped wave (CTW) modes at three locations along the southeast coast of Australia. A statistical “eddy” mode is included to minimize contamination of the coastal-trapped wave currents from East Australian Current eddies. The first three CTW modes account for about 65% of the observed variance in the alongshelf currents on the shelf and slope at Cape Howe, about 40% at Stanwell Park, but only about 24% at Newcastle. Currents associated with the East Australian Current dominate the observations offshore from Newcastle. CTWs account for all but 10%, 37% and 27% of the currents observed at the most nearshore locations on the shelf at Cape Howe, Stanwell Park and Newcastle. The first two coastal-trapped wave modes propagate at close to the appropriate theoretical phse speeds, but the third coastal-trapped wave mode and the eddy mode are not coherent between the three current meter sections along the coast. Surprisingly, mode 2 carries a greater fraction of the coastal-trapped wave energy than does mode 1 at two of the sections. Modes 1 and 2 are coherent with each other at the 95% significance level. The major energy source for the CTWs is upstream (in the CTW sense) of the first line of current meters.

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C. B. Moore, J. R. Smith, and D. A. Church

Abstract

No Abstract Available.

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John A. Church, Allan J. Clarke, Neil J. White, Howard J. Freeland, and Robert L. Smith

Abstract

No abstract available.

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John A. Church, Neil J. White, Allan J. Clarke, Howard J. Freeland, and Robert L. Smith

Abstract

The Australian Coastal Experiment (ACE) was designed to test coastal-trapped wave (CTW) theory and the generation of coastal-trapped waves by the wind. For the ACE dataset, we use CTW theory to attempt to hindcast the observed alogshelf currents and coastal sea levels at locations remote from the upstream (in the CTW sense) boundary of the ACE region. Local (in the ACE region) wind forcing is responsible for only about a quarter of the CTW energy flux at Stanwell Park (the center of the ACE region), and the remainder enters the ACE region from the south and propagates northward through the ACE region. Including the second-mode CTW improves the correlation between the hindcast and the observed near-bottom currents on the upper slope at Stanwell Park, but the use of the third-mode CTW cannot be justified. A linear bottom drag coefficient of r = 2.5 × 10−4 m s−1 works better than a larger drag coefficient, and simplifying the CTW equations by assuming the modes are uncoupled does not detract from the quality of the hindcasts. The hindcast and observed coastal sea levels are correlated at greater 2 than the 99% significance level. For the nearshore locations at Stanwell Park, the hindcast and observed alongshelf currents are correlated at greater than the 99% significance level, and the CTW model can account for about 40% of the observed variance. On the shelf at Stanwell Park, we find the hindcasts agree with the observations only if direct wind forcing within the ACE region and the correct (nonzero) upstream boundary conditions are included. However, even after attempting to remove the effects of the eddies and the East Australian Current, the CTW model is not useful for predicting the currents on the slope at Stanwell Park and on the shelf and slope at Newcastle (the northern boundary of the ACE region). The currents at these locations are dominated by the effect of the East Australian Current and its eddies.

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M. A. Hemer, X. L. Wang, J. A. Church, and V. R. Swail

Abstract

No Abstract available.

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John A. Church, J. Stuart Godfrey, David R. Jackett, and Trevor J. McDougall

Abstract

Warming of the atmosphere as a result of an increased concentration of greenhouse gases is expected to lead to a significant rise is global sea level. We present estimates of the component of this sea level rise caused by thermal expansion of the ocean. These estimates are based on the idea that the upper layers of the main gyres of the ocean are ventilated by the subduction of water at higher latitudes and its subsequent equatorward and downward flow into the main thermocline along surfaces of constant “density”. In this mechanism, heat enters the ocean by an advection process rather than by vertical diffusion, as in previous estimates of the component of sea level rise that is caused by thermal expansion. After the heat initially enters the subtropical gyres by subduction, it is then redistributed to preserve gradients of the depth-integrated pressure field, by an adjustment involving low vertical-mode baroclinic waves. Estimates of historical sea level rise based on this simple ventilation scheme, when combined with estimates of nonpolar glacial melt, are about equal to the observed sea level rise. For a global mean 3.0°C (1.5°C, 4.5°C) temperature rise by 2050 (and with the spatial distribution predicted by three climate models), we estimate the component of sea level rise that is caused by thermal expansion to be about 0.2 to 0.3 m (0.1 m, 0.4 m) by 2050. Low-mode internal Rossby and Kelvin waves appear to be quite efficient at distributing the sea level rise evenly over the earth without major distortions to the thermocline. A delayed warming, as suggested by transient coupled ocean-atmosphere models, can be simulated by using a smaller temperature rise, say 1.5°C rather than 3.0°C, by 2050. Changes in sea level arising from variations in the wind field could be estimated, but so far our calculations are based on the assumption that the wind stress field does not change from its present value. We estimate the maximum rate of sea level rise caused by changes in deep water formation is 0.1 meter per century. Contributions from the cryosphere reported in the literature range from near zero to about 0.35 m. When added to the thermal expansion components, our total sea level rise scenario for 2050 for a temperature rise of 3.0°C (1.5°C to 4.5°C) is about 0.35 m (0.15 and 0.70 m).

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