Winter 1999

The EPSONDE-Glider is a tethered free-fall glider designed to provide quasi-horizontal profiles of microstructure and turbulent shear near the ocean surface. A series of tests were performed with EPSONDE-Glider on the CSS Parizeau from June 17-30, 1996. The experiment site was located at Emerald Bank on the Scotian Shelf in a relatively flat area of 100 m deep water. As a complement to glider tests, the following measurements were also performed: 1) EPSONDE vertical profiles of ocean microstructure, 2) air-sea flux measurements with a bow anemometer system, 3) boundary layer meteorological data collected with a minimet buoy, 4) wave spectra from a wave rider buoy, 5) ADCP profiles, 6) CTD profiles, and 7) wave measurements using a ship-mounted radar. Results of recent tests carried out in Bedford Basin in July 1998 will also be discussed.

Edge waves are one of the different types of motion which, over a wide range of frequencies, play an important role in nearshore hydrodynamics and sediment transport. In spite of earlier studies, it is still unclear how edge waves are forced.

To test different forcing mechanisms, a linear model has been developed. The forced shallow water equations are solved numerically for edge wave solutions with longshore wavenumber, k. A periodic structure of form exp(iky) is assumed in the longshore, y, direction and this allows also for shear wave solutions. This results in a set of partial differential equations in eta, u, and v (respectively surface elevation, cross-shore and longshore velocities) functions of the cross-shore variable, x, and time, t, and solved using a finite difference scheme. Stochastic forcing terms for the momentum equations are specified at each cross-shore grid points. The equations are linear and therefore solutions for different wavenumbers, k, can be summed to retrieve the full spectrum of solutions.

The model allows for any cross-shore beach profiles h(x) and arbitrary longshore current profiles V(x) as well. The dynamical matrix is used to calculate the spectral transfer function between the forcing and the response. Analysis in the time domain and the frequency is applied to the outputs of the model to understand the edge wave response to different type of forcing mechanisms.

In overcoming some minor difficulties in the SCAMP software, we have developed a simple technique for fitting spectra that should be more broadly used. All one needs is a functional form for the theoretical spectrum, and an estimate for the instrumental noise spectrum. The method has several advantages over other fitting techniques:

1. It is unbiased in comparison with other least-squares or cost function approaches.

2. It is robust, i.e., insensitive to dips and wiggles in the spectrum. This is because the built-in noise model tells the routine to ignore the spectrum as it gets down towards the noise level.

3. Error bars on the fitted parameters. There is a theoretical estimate for the variance of the estimated Batchelor wavenumber, based on how broad or narrow the likelihood function peak is.

4. We calculate statistical quantities that indicate how well the observed spectrum fits the theoretical form. This is extremely useful in automating analysis software, to get the computer to automatically ignore "bad" fits.

The method is demonstrated using SCAMP data, compared to the SCAMP-generated fits and other least-squares techniques, and tested against pseudo-data generated by Monte-Carlo techniques.

A possible application of the method to the EPSONDE data analysis system is described.

First, we take a close look at the relationship between the SST gradient and wind speed. The climatological analysis is confined to the frontal zones of the Newfoundland Basin and Kuroshio areas. Two independent data sets are used in the study: COADS and the data set collected on 83 Russian cruises to the Newfoundland Basin in 1980-1991. Our findings are shown to differ for both regions and different data sets. The inferred component of thermally driven wind is compared to the Hsu, 1984, estimate. In the Newfoundland Basin, the thermal wind circulation is simulated better than in the Kuroshio area. We discuss possible reasons for this.

We also infer some estimates of quite an interesting atmospheric phenomenon, one type of marine fog, known as sea smoke, and describe the conditions in which it occurs. This is done for the Gulf Stream area only. In terms of SST gradients, there is some resemblance between the occurrences of sea smoke and thermally driven winds. The physics of the process is discussed.

Finally, we furnish some estimates of wind-driven drift of the surface water in the Gulf Stream area. The process is fairly weak in the frontal zone and becomes more intensive as one moves away from the main jet of the current.

Although Coriolis terms strongly inhibit convection on scales > 10 km, contra-diffusive advection effects that occur in subgrid scale plume ensembles can be can be emulated using practicable resolution for basin- and global- scale models, by using a vorticity-selective filter (thus reducing the Coriolis constraint on the smallest resolved scales). This strongly favors models having low numerical dissipation, such as the DieCAST model used in these numerical experiments.

Implications for modeling intermediate and deep watermass formation, a major ocean climate modeling issue and a focus of the Dynamo Project, are discussed.

Amplitudes of SST and AT annual cycles are the highest near the western boundaries of the oceans. SST and AT annual phases increase toward the eastern tropical parts of the oceans, revealing southeastern propagation of annual cycle over the Northern Hemisphere oceans. AT annual cycle leads one of SST by 1 to 3 weeks. The highest phase differences are observed along the western coasts of the North Pacific and the North Atlantic in the regions of western boundary currents. This is consistent with spatial patterns of integral air-sea heat fluxes.

A belt of low amplitudes of SLP annual harmonic stretches along the Equator (0-10°N) in both oceans. There are three distinct areas of high annual amplitudes of SLP in the North Pacific ocean: Asian, Aleutian and Californian, but only one in the North Atlantic, centered to the west of Iceland. A remarkable feature in the climate of the North Pacific is a maximum of semiannual SLP amplitudes, centered near 40°N and 170°W. It is also an absolute maximum in the entire Northern Hemisphere. Analysis of phases of harmonics of SLP seasonal cycle revealed trajectories of propagation of annual and semiannual cycles. Basing on analysis of semianual to annual amplitudes ratio the regions of semiannual cycle dominance are defined.

to restrain the seaward expansion of the the offshore front of the estuarine plume, and therefore form a large-scale cyclonic motion over this region.

The nested model covers the Gulf of Saint Lawrence and the Scotian Shelf and parts of the adjacent open ocean (roughly from 40N to 52 N). The ocean model is based on POM (Blumberg and Mellor, 1987), three-dimensional, non-linear, prognostic model that uses sigma-coordinates in vertical. Density fields were obtained from the archives of the Bedford Institute of Oceanography and gridded seasonally. At present the model is run in diagnostic mode, i.e. T and S fields are held fixed in time (season). We use a historic data set to validate the forecast scheme on the Scotian Shelf during winter 1996.

The optimal solution from the full assimilation approach has root-mean-square (rms) distance misfits of 3.5 cm and 1.2 cm/s for elevation and current, respectively (in terms of distances on the complex plane), compared to overall rms amplitudes of 30cm and 6cm/s. These misfits are reductions by over 40% and 70% from those in the conventional solution. Formal confidence limits for the optimal solution can be estimated, but depend on assumptions about the spatial covariance of the observational residuals. The partial-assimilative sensitivity cases provide quantitative indications of the importance of the quantity and location of the observational data. In particular, the inclusion of a fraction of the velocity data in the assimilation results in a significant improvement in the model fit to the velocity observations.