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Numerical models are used to test whether the sea surface height (SSH) can be used as an indicator for the variability of Atlantic meridional oceanic mass transports. The results suggest that if the transports over the western boundary current region and those in the eastern part of the basin are considered separately, significant correlations (0.3–0.9) are found between zonal SSH differences and the meridional transports in the top 1100 m. Much weaker correlations are found for the basinwide transport, which corresponds to the surface branch of the meridional overturning circulation (MOC). For the eastern and western branches of the meridional transport, combining the SSH signal with the baroclinic structure obtained from Rossby wave theory enables calculation of a quantitative estimate of the transport variability in the top 1100 m. The results of the method are less convincing for the variability of the MOC. The reason for this is that even small relative errors in the variability of the eastern and western branches can be large compared with the MOC variability. These errors project onto the sum of the eastern and western transports and therefore onto the surface branch of the MOC. Nevertheless, being able to infer transport anomalies from SSH signals in the eastern and western parts of the Atlantic might prove useful in interpreting MOC observations from the U.K. Natural Environment Research Council Rapid Climate Change (RAPID) mooring array at 268N, which show a large subannual variability that is mainly due to changes at the western boundary. Transports inferred from the SSH could help to identify the origin of this variability and whether transport anomalies propagate into the western boundary region from the basin interior or from other latitudes.

This paper examines the representation of eddy fluxes by bolus velocities. In particular, it asks: Can an arbitrary eddy flux divergence of density be represented accurately by a non-divergent bolus flux which satisfies the condition of no normal flow at boundaries? If not, how close can such a representation come? If such a representation can exist in some circumstances, what is the size of the smallest bolus velocity which fits the data? We find, in agreement with earlier authors, that the answer to the first question is no, although under certain conditions which include a modification to the eddy flux divergence, a bolus representation becomes possible. One such condition is when the eddy flux divergence is required to balance the time mean flux divergence. The smallest bolus flow is easily found by solving a thickness-weighted Poisson equation on each density level. This problem is solved for the north Pacific using time-mean data from an eddy-permitting model. The minimum bolus flow is found to be very small at depth, but larger than is usually assumed near the surface. The magnitude of this minimum flow is of order one-tenth of the mean flow. Similar but larger results are found for a coarse resolution model.

We seek to explain on what basis people choose to tell stigmatizing information about themselves to others. In particular, are there any rules governing how such decisions are made? We asked 70 HIV-positive individuals whether they knew various items of knowledge about their network members, and vice versa. These items range from things which might be known easily (e.g., marital status), things which are more difficult to know (e.g., blood type), to potentially stigmatizing information such as criminal record and HIV status. The information that one person knows about another may predict whether the latter’s HIV status is also known. We examine this question using a combination of ethnography and decision trees. Even an apparently simple decision – whether or not to tell someone that you are seropositive – turns out to be complicated; yet the complexity can be extracted from open-ended interviews.

This article estimates the variation in personal network size, using respondent data containing two systematic sources of error. The data are the proportion of respondents who, on average, claim to know zero, one, and two people in various subpopulations, such as "people who are widows under the age of 65" or "people who are diabetics". The two kinds of error—transmission error (respondents are unaware that someone in their network is in a subpopulation) and barrier error (something causes a respondent to know more or less than would be expected, in a subpopulation) — are hard to quantify. The authors show how to estimate the shape of the probability density function (pdf) of the number of people known to a random individual by assuming that respondents give what they assume to be accurate responses based on incorrect knowledge. It is then possible to estimate the relative effective sizes of subpopulations and produce an internally consistent theory. These effective sizes permit an evaluation of the shape of the pdf, which, remarkably, agrees with earlier estimates.

Existing solutions for exchange flow through straits rely upon the decomposition of the flow into a finite number of layers which have constant density. In this paper we provide a solution to inviscid steady exchange flow between continuously stratified reservoirs, where it is assumed that the flow in each direction is independently self-similar. The solution requires knowledge only of the two reservoir stratifications and an imposed net barotropic throughflow. The solution includes regions of stagnant fluid which separate two counter-flowing, stably stratified layers, with the provision that the two active layers may touch at no more than one point. Comparison of the theoretical solution with numerical simulations indicates that the assumption of self-similarity is reasonable, and that the disparity between the theoretical and simulated flows can be attributed to the inclusion of diffusion and viscosity in the numerical model.

We examine the evidence for global propagation of planetary wavelike features in sea-surface chlorophyll. Over much of the midlatitude ocean, westward propagating signals are seen that travel at the same speed as that predicted for long planetary waves. We then test three mechanisms for production of this signal. These are: horizontal (passive) north-south advection by the wave against a mean background gradient; vertical upwelling of nitrate, which is converted into chlorophyll; and vertical upwelling of chlorophyll itself. The tests involve comparisons of the amplitude and phase of the predicted signal with observations. The horizontal advective process predicts an amplitude for chlorophyll fluctuations that is in fair agreement with the data, though both overestimating and underestimating in places. The predictions for the phase difference between the chlorophyll and sea surface height signatures are in good agreement with the data. The upwelling biological mechanism could potentially give a large signal in the chlorophyll field, but the predicted amplitude patterns and the predicted phase difference (which is everywhere negative) are not in accord with the observations. Except in a few regions, the amplitude predicted by upwelling of chlorophyll is small compared with the horizontal advection mechanism. We conclude that over most of the ocean, the chlorophyll signal is well explained by horizontal advective processes, although we cannot rule out that there exist locations where additional biological mechanisms may be responsible for at least part of the signal.

Some requirements on eddy parameterisation are discussed, especially the implications of expressing them in terms of the quasi-Stokes velocity and the modified mean, rather than Eulerian mean, density. The difference between the two means is second-order in perturbation amplitude and thus small in the fluid interior (where formulae to connect the two exist). Near horizontal boundaries, the differences become first order, and so more severe. Existing formulae for quasi-Stokes velocities and stream function also break down here. The layer in which the largest differences between the two mean densities occur is the vertical excursion of a mean isopycnal across a deformation radius, at most about 20 m thick. Most climate models would have difficulty in resolving such a layer. It is shown that extant parameterisation appear to reproduce the Eulerian, and not modified mean, density field and so do not yield a narrow layer at surface and floor either. Both these features make the quasi-Stokes stream function appear to be non-zero right up to rigid boundaries, so that we must query what are the relevant surface and floor quasi-Stokes stream function conditions, and what are their effects on the density fields. To answer this, a variety of eddy parameterisation is employed for a channel problem, and the time-mean density is compared with that from an eddy-resolving calculation. The parameterization were only successful if the vertical component of the quasi- Stokes velocity vanished at top and bottom as in current practice, but all were almost equally successful given proper tuning. One parameterisation, based on linear instability theory, is extended to a global geometry. In low and mid-latitudes, because the predominant orientation of the instability wavevector is north-south, the main quasi-Stokes flow is east-west, only becoming the more traditional north-south at higher latitudes., Original contains color plates: All DTIC reproductions will be in black and white. Presented at the 'Aha Huliko'a Hawaiian Winter Workshop (12th) held in Honolulu, HI on 16-19 Jan 2001, p17-28. This article is from ADA412459 From Stirring to Mixing in a Stratified Ocean. Proceedings Hawaiian Winter Workshop (12th) Held in the University of Hawaii at Manoa on January 16-19, 2001