Richard Pawlowicz

Acoustic travel‐time perturbations due to shallow‐water internal waves and internal tides in the Barents Sea Polar Front: Theory and experiment

During August 1992, a combined acoustics/physical oceanography experiment was performed to study both the acoustical properties and the ocean dynamics of the Barents Sea Polar Front in the region near Bear Island. Oceanographic observations from shipboard hydrography and moored sensors allowed the construction of the internal wave frequency spectrum for the area. A rapidly sampled tomographic section from a 224‐Hz, 16‐Hz‐bandwidth acoustic source to a 16‐element vertical receiving array enabled the monitoring of travel‐time fluctuations over the internal wave frequency band. To describe the measured acoustic fluctuations, theoretical expressions have been developed for the travel‐time variances which are functions of the internal wave oceanographic field, the local acoustic propagation characteristics, and the acoustical system’s properties. Both ray and mode theory expressions are generated, as the experiment was performed in shallow water and both ray and mode arrivals were resolvable. Comparison of the...

The Barents Sea Polar Front in summer

In August 1992 a combined physical oceanography and acoustic tomography experiment was conducted to describe the Barents Sea Polar Front (BSPF) and investigate its impact on the regional oceanography. The study area was an 80 × 70 km grid east of Bear Island where the front exhibits topographic trapping along the northern slope of the Bear Island Trough. Conductivity-temperature-depth, current meter, and acoustic Doppler current profiler (ADCP) data, combined with tomographic cross sections, presented a highly resolved picture of the front in August. All hydrographic measurements were dominated by tidal signals, with the strongest signatures associated with the M2 and S2 semidiurnal species. Mean currents in the warm saline water to the south of the front, derived from a current meter mooring and ADCP data, were directed to the southwest and may be associated with a barotropic recirculation of Norwegian Atlantic Water (NAW) within the Bear Island Trough. The geostrophic component of the velocity was well correlated with the measured southwestward mean surface layer flow north of the front. The frontal structure was retrograde, as the frontal isopleths sloped opposite to the bathymetry. The surface signature of the front was dominated by salinity gradients associated with the confluence of Atlantic and Arctic water masses, both warmed by insolation to a depth of about 20 m. The surface manifestation of the front varied laterally on the order of 10 km associated with tidal oscillations. Below the mixed layer, temperature and salinity variations were compensating, defining a nearly barotropic front. The horizontal scale of the front in this region was ∼3 km or less. At middepth beneath the frontal interface, tomographic cross sections indicated a high-frequency (∼16 cpd) upslope motion of filaments of NAW origin. The summertime BSPF was confirmed to have many of the general characteristics of a shelf-slope frontal system [Mooers et al., 1978] as well as a topographic-circulatory front [Federov, 1983].

Thermal evolution of the Greenland Sea Gyre in 1988–1989

Slice inverses of temperature and heat content from the 1988–1989 Greenland Sea Tomography experiment and other observations, including standard conductivity-temperature-depth stations, moored thermistors, surface meteorological variables, and surface ice cover are combined to better understand the thermodynamics of the Greenland Sea Gyre. Thermal evolution of the gyre center seems to divide naturally into the following three periods: a preconditioning phase (November–January), during which surface salinity is increased by brine rejection from ice formation and by entrainment but in which the mixed layer deepens only slowly to a depth of some 150–200 m; a deep mixing phase (February–March) during which the surface mixed layer deepens rapidly to approximately 1500 m in the gyre center purely under the influence of local surface cooling; and a restratification phase during which the products of deep mixing are replaced by Arctic Intermediate Water flowing in from the gyre edges. The onset of the deep mixing phase occurs after ice formation in the gyre center stops, resulting in an area of open water where large heat fluxes can occur. In surrounding regions, including the Odden region to the south, ice is still being formed. To the north and west, closer to the steep topography of the continental shelf, the inverse results show significant variability due to advection, and large temperature and heat content fluctuations with a period of about 50 days are seen.

A note on seasonal cycles of temperature and salinity in the upper waters of the Greenland Sea Gyre from historical data

Monthly climatologies of temperature and salinity for two regions in the Greenland Sea are created by combining historical archives of hydrographie stations with observations from recent cruises. The first region corresponds to the gyre center, where deepwater formation occurs and a large bay forms in the winter ice pack (“Nordbukta”), and the second region lies directly to the south, in the area usually covered by an ice tongue (“Odden”) in February and March. Surface temperature responds primarily to local surface heat fluxes, consistently warming in the summer to about 4°C and cooling in the fall to the freezing point. Interannual variability appears to be greater for salinity than for temperature. This, coupled with the sparse data coverage, makes it more difficult to define a seasonal salinity cycle. However, in July and August, surface salinity in both regions appears to decrease drastically, with freshening being more pronounced in the southern region. In the fall this mixed-layer fresh anomaly is removed, probably by brine rejection due to the local formation of ice and its subsequent removal by northerly winds. Eventually, ice formation causes the surface waters to overturn and entrain the warmer water below, inhibiting further ice formation. The fresh anomaly is smaller in the gyre center than in the Odden region, so that ice formation will end there earlier, resulting in the appearance of a bay to the north of an ice tongue. Since the disappearance of ice is a necessary precursor to mixing deeper than about 100 m, this suggests that large areal changes in ice cover may be used as a proxy for identifying the strength and onset of deep convection.

Shallow‐water receptions from the transarctic acoustic propagation experiment

In April 1994 a long‐range, low‐frequency acoustic propagation experiment took place in the Arctic to test the feasibility of using acoustic methods to observe large‐scale thermal variability. Here the characteristics of the received signals at a vertical hydrophone array and a horizontal geophone array located at the edge of the continental shelf in the Lincoln Sea, about 1 Mm from the source, are discussed with respect to the ‘‘forward’’ problem of understanding and correctly modeling propagation. Both cw (tonal) and tomographic M‐sequence transmissions are analyzed. It is found that phase stability is very good, and that phase changes are almost entirely due to source/receiver motions. Travel times are not quite as stable, but are consistent with the phase observations, showing that phase can be used to measure travel time changes very accurately. Modal decomposition of M‐sequence transmissions received by the vertical array shows an arrival structure in rough agreement with predictions from a coupled‐...

Internal wave generation from tidal flow exiting a constricted opening

The southern Strait of Georgia, British Columbia, often contains packets of large, near-surface internal waves. Wave crests at the leading edge of the packet, spaced a few hundred meters apart, can have a longitudinal extent of more than 10 km. It has long been assumed that these waves are generated by tidal flow through narrow passages and channels at the Straits southern boundaries, but no actual link has ever been made between these waves and a specific passage or generation mechanism. Here we identify the location and extent of a number of these large packets at specific times using mosaics of photogrammetrically rectified oblique air photos. Wave speeds are determined by analyzing a time sequence of images, with water column measurements used to subtract effects of tidal advection. The location and extent of these internal waves are then compared with the predicted location and extent of hypothetical waves generated in different passages, at different stages of the tide, which are then propagated through a predicted time-varying barotropic flow field. It is found that the observed waves are most likely generated near or after the time of the peak flood tide, or peak inflow into the Strait. They are therefore inconsistent with generation mechanisms involving the release and upstream propagation of waves by the relaxation of an ebb tide. Instead they are probably associated with the nonlinear adjustment of conditions at the edge of an inflowing injection of relatively weakly stratified water.

Evolution of the large‐scale temperature field in the Greenland Sea during 1988–89 from tomographic measurements

Moored thermistor, hydrographic, and tomographic measurements have been combined using least‐squares inverse methods to study the evolution of the 40 km and larger three‐dimensional temperature field in the Greenland Sea during winter 1988–89. In February, the sub‐surface temperature maximum at around 200‐m depth disappears over a large area. Upper waters warm around this time, while intermediate waters cool, consistent with vertical mixing. A chimney structure reaching depths in excess of 1000 m is observed to the southwest of the gyre center during March. The chimney has a spatial scale of about 50 km, a time scale of about 10 days, and breaks up in about 3–6 days. A one‐dimensional vertical heat balance adequately describes changes in total heat content in the chimney region from autumn 1988 until the time of chimney break‐up. A simple one‐dimensional mixed layer model is successful in reproducing fall to winter bulk temperature and salinity changes, as well as the observed evolution of the mixed layer...

Collection and processing of shipboard ADCP velocities from the Barents Sea Polar Front Experiment

Abstract : The Barents Sea Polar Front Experiment was a combined physical oceanography and acoustic tomography field study which took place from 6-26 August 1992. Both shipboard and moored data were collected in a 80 x 70 km experimental region on the south flank of Spitsbergen Bank about 60 km east of Bear Island. Of principal interest in this report are the data from an Acoustic Doppler Current Profiler (ADCP) which was operated continuously during the experimental period as a part of the shipboard instrumentation aboard the USNS BARTLETT. The data from eight current meters deployed on three moorings in the experimental region are used to supplement the ADCP analysis. Preliminary results showed that velocities in the experimental region were dominated by semi-diurnal tides. The strong tidal oscillations dictated the use of a tide removal scheme to extract a steady flow component from the space time grid of ADCP velocities. This report describes the configuration and operation of the ADCP, the space time sampling grid on which the data were collected, the determination of absolute velocity from the ADCP measurements, and the application and results of a tide removal technique which allowed estimation of the subtidal flow.

Physical oceanographic and acoustical results from the 1988–89 Greenland Sea tomography experiment

Analyses of the acoustical tomography data taken in the Greenland Sea in 1988–1989, when combined with inputs from other environmental measurements made in the region, have yielded a number of insights into both the physical oceanographic and acoustical properties of that polar sea. The formation of Greenland Sea deep water (GSDW), which is of importance to the global heat engine and climate studies, was imaged tomographically, allowing us to make significant new statements about the mechanisms, rates, and amounts of GSDW formation. Studies of the late arriving acoustic energy have allowed both detailed studies of the evolution of the surface mixed layer as well as a novel acoustical study of the temporal dispersion of acoustic normal modes by that layer in the marginal ice zone. Rough surface scattering processes in ice and open water conditions, as well as ambient noise, have also been studied using the tomography data set.

Greenland Sea moored tomography experiment

As part of the Greenland Sea Project, a six‐element acoustic tomography array was deployed in the Greenland Sea during the period September 1988–September 1989. The purpose of the array was (1) to understand the circulation of the gyre, (2) to study the formation of the Greenland Sea Deep Water, and (3) to examine the features of acoustic propagation in an MIZ region over a yearly cycle. Supplementing the acoustic data, a number of other measurements were made. A year‐long time series of wind stress over the region was produced by the British Meteorological Office. Satellite SSM/I and AVHRR measurements produced information on ice parameters. A main component of the Greenland Sea Project was a seasonal series of CTD surveys, which the tomography effectively interpolates. The data analysis has already shown a number of interesting preliminary results. Examination of the wind stress data allows us to predict, on the basis of circulation models, what vorticity our array should measure—the agreement will be a...