R.F. Marsden

Physical processes within the North Water (NOW) Polynya

ATMOSPHERE-OCEAN 39 (3) 2001, 163–166

A Proposal for a Neutral Regression

Abstract In cases of modest correlation, parameters calculated from a standard least squares linear regression can vary depending on the selection of dependent and independent variates. A neutral regression that addresses this problem is proposed. The eigenvector corresponding to the smallest eigenvalue of the cross-correlation matrix of the two variates is used as a set of regression coefficients. Error bars are calculated for the eigenvalues and eigenvectors by means of a perturbation expansion of the cross-correlation matrix and are then verified by Monte Carlo simulation. A procedure is suggested for extension of the technique to the multivariate case. Examples of a linear fit for low-correlation and a quadratic fit for high-correlation cases are given. Conclusions are presented regarding the strengths and weaknesses of both the least squares and the neutral regression.

Are polynyas self‐sustaining?

Abstract In this paper, 441 Conductivity Temperature Depth (CTD) casts from the North Water (NOW) Polynya study were used to calculate geostrophic currents between the 10 and 200 dbar surface during April, May and June 1998. Results for April and May indicated a surface intensified southward flow of 10 to 15 cm s–1 with a small return flow along the Greenland coast in agreement with inferred currents described by Melling et al. (2001) and surface ice drifts found by Wilson et al. (2001). Southward transports at this time were 0.4–0.55 Sv in April and May. In June, however, surface currents diminished markedly: southward transports declined to 0.1–0.35 Sv, coincident with a decrease in directly measured winds over the polynya and in the surface barometric pressure difference between Grise Fjord and the Carey Islands that was used as a surrogate for the local north wind speed. There was no evident decrease in air pressure difference between Resolute and Grise Fjord, indicative of the strength of the north wind over the eastern Arctic in general. The results are consistent with present thinking that the NOW Polynya is primarily a latent heat polynya, forced by dominant north winds. The idea, broached here, is that the polynya creates its own microclimate which sustains the polynyas ice‐free condition after its initial formation. The mechanism is identified by an anomalous low pressure region associated with surface buoyancy flux in the polynya and is pursued through the application of a simple geostrophic adjustment model that suggests two self‐sustaining mechanisms. Firstly, the frontal intrusion of the cold ambient terrestrial air mass drives a significant surface wind that transports frazil ice to the edge of the polynya before it can congeal. Secondly, rotation at these high latitudes restricts the penetration of the front into the polynya, essentially insulating the centre from freezing temperatures.

An Analysis of the Dissolution of Ice in Nares Strait using AVHRR Imagery

Abstract The North Water Polynya is the largest polynya in the Canadian Arctic. Its northern boundary is defined by a blockage, or ice bridge, that spans Smith Sound. The maintenance of the ice bridge, and the polynya itself, is contingent upon the southward flow of ice from the Lincoln Sea through Nares Strait. This paper analyzes the dissolution of ice in Nares Strait using Advanced Very High Resolution Radiometer (AVHRR) data. From March to August 1998, 1440 images were downloaded by a satellite receiver at Canadian Forces Station (CFS) Alert on the northern tip of Ellesmere Island. A preliminary evaluation of the data included a visual assessment of over 300 cloud‐free images. Of most interest were 42 images that revealed a rapid dissolution of ice in Nares Strait during mid‐June. In a four‐day span the area of open water in Nares Strait increased from 400 km2 to 1200 km2. Subsequent analysis of selected scenes included the application of both a sea and ice surface temperature algorithm. Based on the satellite imagery and archived weather data from CFS Alert, the ice in Nares Strait initially began to weaken and break‐up due to in situ melting. The significant reconfiguration of ice observed during mid‐June was the result of high winds funnelling southward through the channel.

Improved Active Sonar Performance Using Costas Waveforms

Costas waveforms are a class of waveforms having the form of frequency-hopped pulse trains. When used as a transmit waveform in an active sonar system they may provide superior performance to conventional waveforms such as continuous-wave (CW) and frequency-modulated (FM) pulses, as each Costas waveform provides both range and speed information for a target echo. Matched-filtering identifies individual Costas waveforms from among a set of Costas waveforms that are received. This characteristic allows for more frequent ensonification of a water mass for targets at moderate ranges, giving a higher rate of return echoes for improved target tracking. Alternatively, it offers the potential for overlapping bandwidths and waveform types to be used in a multisonar environment. In this work, the Costas wideband ambiguity, cross- and autocorrelation functions are derived. The range and speed resolutions of Costas waveforms are compared to CW and FM pulse resolutions both via a broadband pulse propagation model and using data obtained during the Defence Research And Development Canada-Atlantic (DRDC Atlantic) Towed Integrated Active-Passive Sonar (TIAPS) deep sea trials in September 2003. Costas waveforms are shown to provide simultaneous range and speed resolution in deep- and shallow-water environments, and Costas waveforms are shown to be individually identifiable at the receiver. The performance of Costas waveforms in reverberation is also briefly examined, and found to be similar to that of FM pulses.

Surface pulses in the lower St. Lawrence Estuary

Abstract Temperature, salinity, sea level and meteorological data from the ‘couplage entre les processus physiques et biogeochimiques’ (COUPPB) study of 1990 were examined to determine the forcing of fresh water pulses in the lower St. Lawrence Estuary. Anchor stations, during and after the passage of a pulse event, indicated that profound changes occurred in the hydrography at the head of the Laurentian Channel. A factor analysis of rotated eigenmodes of surface temperature and salinity indicated three co‐varying groups ‐ the first, on the north shore of the river, the second, on the south slope of the Laurentian Channel and the third in the middle of the estuary. A multivariate regression was used to relate salinity and temperature variations to forcing variates. It was found that sea level elevation and local winds accurately predicted fluctuations on the north shore. Salinity and temperature fluctuations on the south shore were best explained by propagation. In the middle of the estuary, salinity fluc...

A Method for Correcting Vertical Velocities Measured from a Vessel-Mounted Acoustic Doppler Current Profiler

Abstract Minor alignment errors (∼1°) can contaminate acoustic Doppler current profiler measurements of vertical velocities taken from a ship traveling at speed. The signature is consistently large positive or negative vertical velocities, recorded at all depths. A technique is proposed to postcorrect contaminated data in cases of tidally dominated flow over a flat bottom. The eigenmodes of the cross-correlation matrix of the depth-averaged velocities are found, and the data are rotated to align along the eigenvector corresponding to the smallest eigenvalue that is equivalent to the physical vertical direction. An example of corrupted data is presented, and corrections for the pitch and roll directions are found. The corrected flow field is shown to be physically plausible over both flat and sloping bottom sections of the cruise track.