bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress‐dependent permeability and the formation of seafloor event plumes
Authors: Germanovich Leonid N. , Lowell Robert P. , Astakhov Dmitriy K.
Published: September 2002

We address the question of formation of event plumes following dike emplacement in a hydrothermal upflow zone at a mid‐ocean ridge. We assume a preexisting low‐ to moderate‐temperature single‐pass hydrothermal system and suggest that dike emplacement provides a damaged zone of high permeability along its margins as well as the heat required to drive the event plume. We also consider the role of thermoelastic stresses in limiting the heat output of the event plume. Our calculations show that event plumes can result from dike emplacement into a preexisting, moderate‐temperature hydrothermal system, provided the local permeability generated by the dike is ∼10−8‐10−10 ° m2. These values are consistent with our limited field observations from the Susanville ophiolite, suggesting that the permeability near dike margins attributed to dike emplacement results mainly from open fractures mostly aligned parallel to the margins of dike. We estimate the porosity in the damaged zone to be of the order 0.1–1%. If the high‐permeability zone has relatively low porosity (∼0.1%), thermoelastic stresses close the fractures sufficiently to reduce the heat output, giving rise to a weak event plume (∼1014 J). If the porosity is higher (≥ 1%), however, thermoelastic stresses become unimportant, and a similar dike can generate an event plume of ∼1017 J. Following event plume emission, the circulation decays rapidly to its original temperature; however, the heat output from the chronic plume is greater because of the increased permeability resulting from dike emplacement. The decay of heat output to preevent plume levels requires that the newly created permeability be sealed, perhaps as a result of chemical precipitation in the cracks.
Crustal deformation associated with glacial fluctuations in the eastern Chugach Mountains, Alaska
Authors: Sauber Jeanne , Plafker George , Molnia Bruce F. , Bryant Mark A.
Published: September 2002

The changes of the solid Earth in south central Alaska in response to two major glacial fluctuations on different temporal and spatial scales have been estimated and we evaluated their influence on the stress state and ongoing tectonic deformation of the region. During the recent (1993–1995) Bering Glacier surge, a large transfer of ice from the Bagley Ice Field to the Bering Glacier terminus region occurred. We estimated the elastic displacement of the solid Earth due to ice mass redistribution from Global Positioning System (GPS) measurements at sites near the surging glacier. We can account for these displacements by transfer of an ice volume of about 14 km3 from the surge reservoir area to the terminus region. We examined the background seismicity (ML ≥ 2.5) before, during, and after the surge. We found that the occurrence of small earthquakes (ML ≤ 4.0) in the surge reservoir region increased during the surge time interval possibly in response to a decrease in ice mass. This suggests that a small decrease in the vertical stress, σ3, could be enough to modulate the occurrence of small, shallow earthquakes in this dominantly thrust fault setting. During this century the southern Alaska coastal glaciers have been undergoing an overall decrease in volume. Based on our compilation of changes in the extent and thickness of the coastal glaciers between the Malaspina and Bering, we calculated surface displacements due to the Earth's viscoelastic response to annual thinning and to the cumulative retreat over the last 100 years. The uplift of the region due to an average annual thinning rate of 1–6 m/yr in the ablation region is 1–12 mm/yr. For our reference model with a viscosity of 5×1019 Pa s for depths between ≈ 40 and 200 km the total viscoelastic response due to the retreat over the last century may be as much as a couple of meters within the coastal ablation zone near Icy Bay. The maximum decrease in σv between 0 and 10 km was ≈ 1.0 MPa, which is significant in relation to the stress drops in recent earthquakes (≈ 2 to 10 MPa) but small in relation to the estimated tectonic stress magnitude. Therefore the occurrence of an earthquake such as the St. Elias (1979, MS = 7.2) may have been advanced in time; however, most of the ongoing stress accumulation would be primarily due to tectonic forces.
Crustal thickness and structure along three contrasting spreading segments of the Mid‐Atlantic Ridge, 33.5°–35°N
Authors: Hooft E. E. E. , Detrick R. S. , Toomey D. R. , Collins J. A. , Lin J.
Published: September 2002

The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid‐Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along‐axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6±0.5 km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6±0.4, 5.7±0.4 and 5.1±0.3 km). Near the segment ends, crustal thickness is 2.5 to 5±0.5 km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (∼3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ±0.6 km s−1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along‐axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH‐1) has an unusually thick crustal root (excess thickness of 2–4 km and along‐axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid‐Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH‐1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.
Subtropical Shelf Front off eastern South America
Authors: Piola Alberto R. , Campos Edmo J. D. , Möller Osmar O. , Charo Marcela , Martinez Carlos
Published: September 2002

Historical hydrographic data from the continental shelf off eastern South America are used to examine the thermohaline properties of the water masses in the region between 20°S and 40°S. The continental shelf water masses are originated by dilution of open ocean waters of the western boundary currents of the South Atlantic Ocean. On the basis of temperature‐salinity relation, two distinct water masses are identified, namely, the Subantarctic Shelf Water and the Subtropical Shelf Water. Subantarctic Shelf Water originates by dilution of Subantarctic Water, primarily in the southeast Pacific, due to excess precipitation and continental runoff and enters the continental shelf near 55°S. The Subtropical Shelf Water is modified South Atlantic Central Water diluted by continental runoff from the coast of Brazil. In addition, substantial dilution of the upper shelf waters takes place at the mouth of Río de la Plata (approximately located at 36°S) and, in a lesser extent, at the Patos‐Mirim Lagoon (at 32°S). The Río de la Plata and the Patos outflows form a low‐salinity tongue that caps the shelf water leading to a salinity decrease to values <30. The low‐salinity tongue extends northward over the shelf penetrating farther north in winter than in summer. The extent of the low‐salinity water has a strong impact on the vertical stratification and acts to limit winter convection to the layer above the halocline. There is little or no indication of mixing between Subantarctic Shelf Water and Subtropical Shelf Water. An intense temperature, salinity, and nutrient front separates these water masses. The front is oriented along the north‐south direction, located on average near the 50 m isobath at 32°S and extends southward toward the shelf break near 36°S. Between 32° and 34°S the Subtropical Shelf Front follows the 100 to 200 m isobaths and separates Subantarctic Shelf Water from the oceanic South Atlantic Central Water. On the basis of the temperature and salinity distributions, beneath the low‐salinity surface layer, the Subtropical Shelf Front appears as an extension of the Brazil‐Malvinas Confluence over the continental shelf of South America. Thus the location of the Subtropical Shelf Front may be linked to the migrations of the separation point of the Brazil‐Malvinas Confluence from the continental slope.
Wintertime dynamics of the Terra Nova Bay polynya
Authors: Van Woert Michael L.
Published: September 2002

Three variants of a one‐dimensional coastal polynya model, forced by data from a nearby automatic weather station, are used to examine wintertime ice concentration fluctuations in a coastal polynya at Terra Nova Bay, Antarctica. It has long been believed that the Terra Nova Bay polynya opens in response to strong offshore katabatic wind forcing. This study shows that the sensible and longwave heat fluxes share equally in controlling polynya extent fluctuations, and together, these two terms in the energy budget can explain up to ∼50% of the observed variance in open water fraction. Moreover, this study shows that the wind speed becomes important to the polynya dynamics through amplification of the longwave flux. Finally, this study suggests that local katabatic winds at Terra Nova Bay combined with regional ice conditions set up by winds blowing across the Ross Ice Shelf control the fluctuations in Terra Nova Bay polynya extent.
Spatial and temporal structures of suspension and transport over megaripples on the shore face
Authors: Vincent C. E. , Marsh S. W. , Webb M. P. , Osborne P. D.
Published: September 2002

Megaripples of 0.10–0.15 m height and 0.75–0.9 m wavelength occurred under swell wave activity on a macrotidal beach in southern Cornwall, England. Megaripples are relatively common features on the shore face in wave‐dominated environments, although their occurrence is not well predicted by bed form models; of the four bed form models examined [Clifton, 1976; Mogridge et al., 1994; Nielsen, 1981; Wiberg and Harris, 1994], only that of Clifton [1976] predicted the formation of megaripples. The average suspended sand concentration profiles (measured using acoustic backscatter) differed considerably, in absolute magnitude and vertical gradient, depending on whether the measurements were made over the crest or trough of the bed form. The temporal structure of the suspension also varied with position relative to the megaripple. Utilizing the motion of the bed form beneath the instruments, patterns of suspension over the megaripple were calculated as a function of the phase of the wave. Sand transport was computed from the cospectra of the suspended concentration with an electromagnetic current meter at 10 cm from the seabed. Transport was dominantly at the infragravity frequencies and was consistently directed offshore, while the transport associated with the swell waves was smaller and varied with position over the megaripple; the motion of the megaripple itself was toward the shore.
Wave‐wave interactions in finite depth water
Authors: Lin Ray Q. , Perrie Will
Published: September 2002

In this study we present a new formulation for the nonlinear wave‐wave interaction source function in finite water depth. The formulation, denoted the reduced integration approximation (RIA), is shown to compare well with published formulations, both for shallow water wave‐wave interactions [Hertench and Hasselmann, 1980; Polnikov, 1997; Hashimoto et al., 1998; A. Masuda and K. Komatsu, manuscript in preparation, 1998] and also for the asymptotic deep water limit: (1) the Hamiltonian formulation proposed by Lin and Perrie [1997], by (2) Hasselmann and Hasselmann [1981], and (3) the line integral transformation of Webb [1978] and Resio and Ferne [1991]. Of these deep water formulations, that of Lin‐Perrie generalizing the Hamiltonian representation of Zakharov [1968] to finite depth water, is notable for its simplicity, efficiency and its ability to apply to very shallow water (kh ≈ 0.3), and highly nonlinear (ε≤0.3) interactions. RIA is based on an analysis of the main resonance domain, which reduces the six‐dimensional integration to a quasi‐line integral to minimize computational time. In terms of computational time, RIA is a thousand times faster than the EXACT‐NL version formulated by Hasselmann and Hasselmann [1981], with similar accuracy. Thus RIA can be considered a candidate for operational forecasting in finite depth water, in the sense that the discrete interaction approximation was presented as a candidate for operational deep water wave forecasting by Hasselmann et al. [1988].
Intrusion of the Southwest Monsoon Current into the Bay of Bengal
Authors: Vinayachandran P. N. , Masumoto Yukio , Mikawa Tetsuya , Yamagata Toshio
Published: September 2002

The general eastward flow in the north Indian Ocean during summer, which is called the Southwest Monsoon Current (SMC), flows eastward south of India, turns around Sri Lanka, and enters the Bay of Bengal. The intrusion of the SMC into the Bay of Bengal is studied using expendable bathythermograph (XBT) observations along the shipping route between Sri Lanka and Malaca Strait, TOPEX/POSEIDON sea surface height anomalies, and an ocean general circulation model. The intrusion appears first as a broad northward shallow (confined to the upper 200 m) flow in the central part of the Bay of Bengal during May. As the summer season advances, it moves westward, intensifies, and becomes narrow. The mean seasonal (May‐September) transport of the SMC into the Bay of Bengal is about 10 Sv. The zonal variation of the geostrophic velocity across 6°N calculated using the XBT data compares well with that from TOPEX/POSEIDON altimeter data. However, the SMC in the XBT data is faster (40 cm s−1) than in the altimeter data or the numerical simulation (25 cm s−1). Harmonic analysis of the depth of 20°C isotherm together with a simple forced Rossby wave model demonstrates that the SMC east of Sri Lanka is forced by both Ekman pumping in the Bay of Bengal and Rossby wave radiation associated with the spring Wyrtki [1973] jet in the equatorial Indian Ocean. The initial intrusion of the SMC into the Bay of Bengal is attributed to this Rossby wave radiation from the eastern boundary.
Measurements of upper ocean heat and freshwater budgets near a drifting buoy in the equatorial Indian Ocean
Authors: Godfrey J. S. , Bradley E. F. , Coppin P. A. , Pender L. F. , McDougall T. J. , Schulz E. W. , Helmond I.
Published: September 2002

R/V Franklin followed a drifter for 8 days in the equatorial Indian Ocean to test the accuracy of mixed‐layer heat and freshwater budget closure. Four‐hourly triangles were repeated, towing a SeaSoar with a photometer and conductivity‐temperature‐depth (CTD) profiler. Currents relative to the buoy at one depth were obtained from (a) a current meter at 25 m below the buoy; (b) a Global Positioning System estimate; (c) a drag estimate. Acoustic Doppler current profiler (ADCP) shears provided currents at other depths, (b) and (c) agreed closely but differed from (a), possibly due to severe conditions at the current meter. Vertical advection was estimated from ADCP divergence around each triangle. Measurements on the buoy and the ship, calibrated earlier in the Coupled Ocean‐Atmosphere Response Experiment (COARE), provided bulk surface fluxes. Accuracy of net surface heat fluxes was estimated at ±10 W/m2. Rainfall differed considerably between the buoy and ship, but other quantities including shortwave radiation were very similar, even on cloudy days. A formalism for the budgets above a given isopycnal is developed, to clearly distinguish horizontal and vertical advection. When (b) and (c) above were used with the best estimate of surface fluxes, the 8‐day heat budget misclose for water above the 21.5 isopycnal was (−4.6±5) W/m2; the diurnal heating cycle was well resolved. The freshwater budget misclose was (0.9±3.5) mm/d. A small amount of vertical diffusion, in the ratio determined by the slope of the T‐S curve, will reduce both misclosures. Cruise design considerations for minimizing errors in the advection and storage components of heat and freshwater budgets are discussed.
Sensitivity of a global coupled ocean‐sea ice model to the parameterization of vertical mixing
Authors: Goosse H. , Deleersnijder E. , Fichefet T. , England M. H.
Published: September 2002

Three numerical experiments have been carried out with a global coupled ice‐ocean model to investigate its sensitivity to the treatment of vertical mixing in the upper ocean. In the first experiment, a widely used fixed profile of vertical diffusivity and viscosity is imposed, with large values in the upper 50 m to crudely represent wind‐driven mixing. In the second experiment, the eddy coefficients are functions of the Richardson number, and, in the third case, a relatively sophisticated parameterization, based on the turbulence closure scheme of Mellor and Yamada version 2.5, is introduced. We monitor the way the different mixing schemes affect the simulated ocean ventilation, water mass properties, and sea ice distributions. CFC uptake is also diagnosed in the model experiments. The simulation of the mixed layer depth is improved in the experiment which includes the sophisticated turbulence closure scheme. This results in a good representation of the upper ocean thermohaline structure and in heat exchange with the atmosphere within the range of current estimates. However, the error in heat flux in the experiment with simple fixed vertical mixing coefficients can be as high as 50 W m−2 in zonal mean during summer. Using CFC tracers allows us to demonstrate that the ventilation of the deep ocean is not significantly influenced by the parameterization of vertical mixing in the upper ocean. The only exception is the Southern Ocean. There, the ventilation is too strong in all three experiments. However, modifications of the vertical diffusivity and, surprisingly, the vertical viscosity significantly affect the stability of the water column in this region through their influence on upper ocean salinity, resulting in a more realistic Southern Ocean circulation. The turbulence scheme also results in an improved simulation of Antarctic sea ice coverage. This is due to to a better simulation of the mixed layer depth and thus of heat exchanges between ice and ocean. The large‐scale mean summer ice‐ocean heat flux can vary by more than 15% between the three experiments. Because of this influence of vertical mixing on Southern Ocean ventilation, sea ice extent, and ocean‐atmosphere heat fluxes, we recommend that global climate models adopt a sufficiently realistic representation of vertical mixing in the ocean.
Nonlinear evolution of shear instabilities of the longshore current: A comparison of observations and computations
Authors: Özkan‐Haller H. Tuba , Kirby James T.
Published: September 2002

The time dependent nearshore circulation field during 3 days of the SUPERDUCK field experiment is simulated. We consider the generation of nearshore currents due to obliquely incident breaking waves, damping effects due to bottom friction, and diffusion effects due to lateral momentum mixing caused by turbulence and depth‐varying current velocities. Because of uncertainties in the friction and lateral mixing coefficients, numerical simulations are carried out for a realistic range of values for these coefficients. The resulting shear instabilities of the longshore current exhibit unsteady longshore progressive vortices with timescales of O(100 s) and length scales of O(100 m) and longer. The time dependent flow involves the strengthening, weakening, and interaction of vortices. Vortex pairs are frequently shed offshore. During this process, locally strong offshore directed currents are generated. We find that a stronger mean current and faster and more energetic vortex structures result as the friction coefficient is decreased. However, the longshore length scales of the resulting flow structures are not altered significantly. An increase in the mixing coefficient causes relatively small variations in the propagation speeds. However, the resulting flow structures are less energetic with larger longshore length scales. Shear instabilities are found to induce significant horizontal momentum mixing in the surf zone and affect the cross‐shore distribution of the mean longshore current. Mixing due to the presence of the instabilities is found to be dominant over mixing caused by more traditional mechanisms such as turbulence. For values of the free parameters that reproduce the propagation speed of the observed motions, the frequency range within which shear instabilities are observed as well as the mean longshore current profile are predicted well.
Signatures of the Mediterranean outflow from a North Atlantic climatology: 1. Salinity and density fields
Authors: Iorga Michaela Ciobotaru , Lozier M. Susan
Published: September 2002

Using historical data from the National Oceanic Data Center, the climatology of the eastern North Atlantic basin has been investigated for the purpose of detailing the Mediterranean outflow water in terms of its salinity, density, and flow patterns. Part 1 of this work is a descriptive analysis of the fate of the Mediterranean Water once it flows out of the Strait of Gibraltar. Tracing the salinity and density signatures, high‐resolution maps of the climatological outflow are presented, with an emphasis on the continuity of the water from its source. From the climatological fields a continuous signal of Mediterranean Water is tracked northward to ∼50°20′N, yet its westward advection is limited to the Tagus Basin. Recirculations of Mediterranean Water in the Gulf of Cadiz and in the Bay of Biscay, deduced from property signals, are also detailed. Iorga and Lozier [this issue] present absolute velocity fields from a diagnostic model constrained by geostrophic dynamics, conservation of mass, and no‐flux boundary conditions.
Observations of sea ice ridging in the Weddell Sea
Authors: Granberg Hardy B. , Leppäranta Matti
Published: September 2002

Sea ice surface topography data were obtained by helicopter‐borne laser profiling during the First Finnish Antarctic Expedition (FINNARP‐89). The measurements were made near the ice margin at about 73°S, 27°W in the eastern Weddell Sea on December 31, 1989, and January 1, 1990. Five transects, ranging in length from 127 to 163 km and covering a total length of 724 km, are analyzed. With a lower cutoff of 0.91 m the overall ridge frequency was 8.4 ridges/km and the average ridge height was 1.32 m. The spatial variations in ridging were large; for 36 individual 20‐km segments the frequencies were 2–16 ridges/km and the mean heights were 1.16–1.56 m. The frequencies and mean heights were weakly correlated. The distributions of the ridge heights followed the exponential distribution; the spacings did not pass tests for either the exponential or the lognormal distribution, but the latter was much closer. In the 20‐km segments the areally averaged thickness of ridged ice was 0.51±0.28 m, ranging from 0.10 to 1.15 m. The observed ridge size and frequency are greater than those known for the Ross Sea. Compared with the central Arctic, the Weddell Sea ridging frequencies are similar but the ridge heights are smaller, possibly as a result of differences in snow accumulation.
The influence of tidal range on the exchange between San Diego Bay and the ocean
Authors: Chadwick D. Bart , Largier John L.
Published: September 2002

The tidal exchange between a bay and the ocean often results from a process termed “tidal pumping” caused by an asymmetry between the flow during the ebb and flood tides. For many bays the ebb flow is concentrated into a jet which occupies the region just offshore from the mouth of the bay, while the flood flow is drawn in rather radially from the region all around the mouth. A significant variation in tidal exchange with tidal range is based on the notion that the length scale of the ebb jet increases more rapidly than the length scale of the flood withdrawal zone as the tidal range increases. Exchange between San Diego Bay and the Pacific Ocean was quantified using a combination of flow measurements by acoustic Doppler current profiling and tracer measurements using a naturally occurring ultraviolet fluorescence tracer. We found that variations in exchange with tidal range could be isolated by separately evaluating the ebb and flood tidal transport budgets. The tracer transport during the ebb increased rapidly with tidal range, while during the flood tide, the transport increased more gradually. The resulting difference in tidal transport between the ebb and flood accounts for the exchange between the bay and ocean. For weak tides, the exchange tends to increase rapidly with increasing tidal range, while for stronger tides, the exchange is more constant. An empirical equation for the tidal exchange developed from the measurements is consistent with theoretical tidal pumping models and supports the concept that the exchange at the mouth of San Diego Bay is dominated by tidal pumping.
Microwave and acoustic scattering from parasitic capillary waves
Authors: Plant William J. , Dahl Peter H. , Keller William C.
Published: September 2002

We report simultaneous microwave and acoustic Doppler backscattering measurements made in a wind‐wave tank. The microwave system operated at 35 GHz (0.857 cm), while the acoustic system transmitted at 190 kHz (0.777 cm). The two systems were mounted to view the surface at the same incidence angle, which was varied. The measurements showed that when both systems looked upwind, horizontal transmit and receive polarization (HH) microwave backscatter from the rough water surface was 2 to 12 dB stronger than acoustic backscatter, depending on incidence angle and wind speed. When the acoustic system looked downwind, however, its backscattering level was consistently about 1 dB lower than that of the upward‐looking microwave system. We interpret these results to indicate that both the acoustic and microwave systems were scattering from parasitic capillary waves in addition to freely propagating, wind‐generated waves. The tilt of the parasitic capillary waves accounts for the observed differences in microwave and acoustic backscatter. We show that Bragg scattering theory predicts both the intensity and the Doppler shift of the microwave and acoustic signals very well using known properties of parasitic capillary waves. Spectral densities of the parasitic capillary waves derived from this Bragg scattering model are in good agreement with those predicted recently by Fedorov and Melville [1998] and observed by Fedorov et al. [1998].
A numerical study of the dynamics and energetics of cool filaments, jets, and eddies off the Iberian Peninsula
Authors: Røed Lars Petter , Shi Xiao Bing
Published: September 2002

Considered are possible links between mesoscale features and the presence of topographic and/or coastline irregularities off the Atlantic coast of the Iberian Peninsula, and what physical processes sustain the persistent filaments. The former is investigated by contrasting the results from two suites of three model runs differing only in the specification of topography and coastline irregularities and values assigned to key model parameters. In this a fairly simple 1½‐layer model is used. The latter is explored by use of an energy diagnostic scheme recently developed for multilayer models. On the basis of visual inspection and spectral analysis of the results, it is concluded that the physical processes leading to the formation, the separation distance, and the persistence of the filaments are largely independent of the presence of a variable topography and coastline irregularities. The role of the irregularities is to modify the qualitative appearance of the filaments and also to loosely anchor them to conspicuous topographic and/or coastline features. The variable topography alone modifies the appearance of the mesoscale structures and produces alongshore differences in the onset of instabilities, while coastline irregularities alone effectuate a delay in the onset of the initial small‐scale, linearly unstable waves. The energy diagnosis reveals that the eddy kinetic energy linked to the filaments and other mesoscale structures is steadily extracted from the mean available gravitational energy via a combination of frontal instability and mixed frontal‐conventional baroclinic instability. This cyclogenesis is made possible through the maintenance of a strong lateral front in both density and upper layer thickness close to the coast. The frontogenesis, in turn, is sustained by the persistent upwelling‐favorable wind forcing.
Shallow water currents during Hurricane Andrew
Authors: Keen Timothy R. , Glenn Scott M.
Published: September 2002

Oceanographic measurements are used in combination with a numerical model to examine the influence of stratification on shallow water currents during the directly forced stage of a tropical cyclone (Hurricane Andrew) on the continental shelf. The following stratification‐dependent coastal processes are examined: (1) turbulent mixing, (2) coastally trapped waves, (3) near‐inertial oscillations, and (4) upwelling and downwelling. Turbulent mixing was strong within 1 Rw (radius of maximum winds) of the storm track, and stratification was nearly destroyed. Turbulent mixing was weak at distances greater than 2 Rw. The dominant coastal wave was a barotropic Kelvin wave generated as the storm surge relaxed after landfall. Baroclinic near‐inertial oscillations were dominant at the shelf break and occurred along with a barotropic response on the middle shelf. Downwelling‐favorable flow developed east of the track prior to the storm peak, and upwelling‐favorable flow evolved west of the track as the eye crossed the shelf. The idealized storm flow was modified by local barotropic and baroclinic pressure gradients on the shelf. Ocean circulation during Hurricane Andrew was hindcast using both stratified and unstratified three‐dimensional numerical models. For areas within 1 Rw of the storm track, the unstratified model matched the observed currents better than the stratified model, partly because of errors in the initial stratification. At distances greater than 2 Rw the influence of stratification increases, and the unstratified model does not reproduce the observed upwelling‐favorable flow.
The role of snow on the thermal dependence of microwave backscatter over sea ice
Authors: Barber David G. , Nghiem Son V.
Published: September 2002

Our understanding of snow distribution in the polar regions is severely restricted owing to the heterogeneity, both in space and time, of this solid precipitate. Processes such as vapor and mass fluxes across the interface are, to a large extent, controlled by the presence and geophysical state of the snow cover on sea ice. Numerous studies have shown the importance of snow cover in ecosystem processes and particularly in photosynthetically active radiation extinction. Researchers are currently exploiting developments in electromagnetic interaction theory in an attempt to measure snow thickness distributions remotely. In this paper, we investigate the dependence of radar backscatter on snow thickness over smooth first‐year ice. We use data from the Seasonal Sea Ice Monitoring and Modeling Site located in the Canadian Archipelago. Results show that the thermodynamics of the snow cover affect wave propagation, attenuation, and scattering through the control that brine volume exerts on interfacial characteristics of the snow and ice layers. The effect is subtle and specific to certain ranges of salinity, surface roughness and thickness of sea ice. We describe the phenomenon responsible for this effect using a microwave model consistently for both complex effective permittivities and backscatter coefficients coupled to a one‐dimensional model. We validate the physical principles using in situ field data. We then discuss the potential of synthetic aperture radar in estimating snow thickness distributions under these specific conditions using both observed and modeled microwave scattering.
Coastal embayment circulation due to atmospheric cooling
Authors: Wang X. H. , Symonds G.
Published: September 2002

Observations of temperature and salinity distributions and currents have been combined with numerical simulations to investigate the response of a coastal embayment to atmospheric cooling during winter. The field experiment, including current meter mooring, conductivity‐temperature‐depth (CTD) surveys, and weather monitoring, was carried out in Jervis Bay, New South Wales, Australia, during the period of July to October 1996. The bay is small enough that a synoptic CTD survey can be achieved over a period of 6 hours but still large enough that Coriolis effects are important. During a cooling event, vertical convection and surface wind stress combined to produce a well‐mixed water column. Continued cooling produced cold, dense water in the shallow regions of the bay and could be identified as a tongue of cold bottom water flowing out of the bay onto the adjacent shelf. The cold outflow produced a surface inflow to the bay of warmer shelf water, causing the bay waters to restratify. The response has been modeled using the three‐dimensional Princeton Ocean Model with a prescribed surface heat flux based on meteorological observations. Following a period of cooling, the model produced a stronger anticyclonic gyre at the surface and a weaker cyclonic gyre nearer the bottom. As the cold bottom water flowed out of the bay, warm shelf water entered at the surface, and the anticyclonic gyre was replaced by two counter rotating gyres: cyclonic in the northern half and anticyclonic in the southern half of the bay. In order to achieve a quantitative agreement between the model and observations, including the restratification following the cooling event and flow reversal associated with the change from anticyclonic to cyclonic circulation in the northern half of the bay, the surface heat fluxes needed to be artificially increased to compensate for excessive mixing in the model. The model results predicted a flushing time of order 1 week, depending on the duration and magnitude of the surface cooling and the initial conditions in the bay.
Assimilation of TOPEX/Poseidon altimeter data into a global ocean circulation model: How good are the results?
Authors: Fukumori Ichiro , Raghunath Ramanujam , Fu Lee‐Lueng , Chao Yi
Published: September 2002

The feasibility of assimilating satellite altimetry data into a global ocean general circulation model is studied. Three years of TOPEX/Poseidon data are analyzed using a global, three‐dimensional, nonlinear primitive equation model. The assimilation's success is examined by analyzing its consistency and reliability measured by formal error estimates with respect to independent measurements. Improvements in model solution are demonstrated, in particular, properties not directly measured. Comparisons are performed with sea level measured by tide gauges, subsurface temperatures and currents from moorings, and bottom pressure measurements. Model representation errors dictate what can and cannot be resolved by assimilation, and its identification is emphasized.
The effects of Halmahera on the Indonesian throughflow
Authors: Morey Steven L. , Shriver Jay F. , O'Brien James J.
Published: September 2002

The pathways of the Pacific to Indian Ocean throughflow and the relative contributions of North Pacific (NP) and South Pacific (SP) water to the throughflow are examined using the Navy Layered Ocean Model. The roles of Halmahera Island in directing flow along the pathways and determining the composition of the throughflow are also studied. The global ocean simulations use a horizontal resolution of up to 1/4° between like variables and have a vertical resolution ranging from one and a half layer reduced gravity to six active layers with realistic bottom topography. All of the simulations are forced by the Hellerman and Rosenstein [1983] monthly wind stress climatology. The predominant throughflow pathway consists of NP water traveling through the Celebes Sea, Makassar Strait, Flores Sea, and to the Indian Ocean through the Timor, Savu, and Lombok Straits. Model results show that the island of Halmahera is responsible for preventing a flow of SP water into the Celebes Sea and for diverting some SP water southward through the Seram and Banda Seas. The island impacts the lower thermocline and intermediate water pathways throughout the entire year and affects the surface layer during the boreal spring through fall. To estimate the relative contributions of the NP and SP surface water to the throughflow, Lagrangian drifters are advected backward in time from near the exit to the throughflow region to their respective sources. By tracking these buoys, it is found that the presence of Halmahera changes the throughflow composition in the surface layer from ∼69% NP and 31% SP to 92% NP and 8% SP. Halmahera does not change the composition of the throughflow in the undercurrent layer, which is fed by the NP, or in the lower thermocline and intermediate water layers, which are fed by water from the SP.
Seasonal variability of yellow substance absorption in the surface layer of the Baltic Sea
Authors: Kowalczuk Piotr
Published: September 2002

The value of the yellow substance absorption coefficient in the visible‐light spectrum is a very important characteristic used in the optical classification of seawater. This quantity also needs to be included in algorithms for the remote detection of optically active seawater constituents. An extensive database of measurements of optical parameters has been collected during a series of cruises since 1993 in different water masses of the southern Baltic. The yellow substance absorption coefficient at a wavelength of 400 nm was measured and the yellow substance absorption spectrum slope coefficient calculated. The statistical distribution of values of these parameters in the three various water masses (bay waters, coastal, and open seawater) is presented. The seasonal variability of both parameters in the three regions is analyzed. Results are discussed and compared with the published record of yellow substance absorption properties and variability patterns of other optical parameters in the southern Baltic and other marine basins.
Seasonal cycles in two regimes of Arctic climate
Authors: Polyakov I. V. , Proshutinsky A. Y. , Johnson M. A.
Published: September 2002

The annual variability of the Arctic Ocean is characterized by a large seasonal cycle against a background of climatically significant interannual and decadal timescale variations. One of the most significant of these interannual variations is wind‐driven motion in the upper Arctic Ocean which alternates between anticyclonic and cyclonic regimes. This study focuses on differences in the seasonal cycle in years attributed to the cyclonic circulation regime (CCR), the anticyclonic circulation regime (ACCR), and the possible impact these seasonal anomalies have on fluctuations occurring over longer durations. Atmospheric, ice, and oceanic observational data and the results of numerical experiments with a coupled sea‐ice‐ocean model provide evidence that during the ACCR the arctic atmospheric pressure is higher and wind speed is lower compared with the CCR. A mean arctic ACCR winter is colder than a mean CCR winter. When the CCR dominates, precipitation increases over the ocean and decreases over the land. During the CCR, summer wind divergence effectively produces numerous sea‐ice openings in the central Arctic Ocean. Repetition of this cyclonic process over several years results in overall thinner ice in the central Arctic, compared with that during the ACCR. Under the CCR, more ice‐free summer areas lead to an accumulation of additional heat in the upper ocean, resulting in longer periods of ice melt, increases in fresher water content, and thinner ice. In CCR years both dynamical and thermodynamical factors cause excess ice and freshwater transport through Fram Strait from the Arctic into the Greenland Sea; the water balance in the Arctic Ocean is maintained via an increased inflow of the Atlantic water over the Barents Sea into the Nansen Basin. Finally, we stress that observed modifications of the Arctic Ocean thermohaline structure and Arctic Ocean‐North Atlantic interactions in the early 1990s are a manifestation of the cyclonic circulation regime.
A case study of air‐sea interaction during swell conditions
Authors: Smedman A. , Högström U. , Bergström H. , Rutgersson A. , Kahma K. K. , Pettersson H.
Published: September 2002

Air‐sea interaction data from a situation with pronounced unidirectional swell have been analyzed. Measurements of turbulence at three levels (10, 18, and 26 m above mean sea level) together with directional wave buoy data from the site Östergarnsholm in the Baltic Sea were used. The situation, which lasted for ∼48 hours, appeared in the aftermath of a gale. The wind direction during the swell situation turned slowly within a 90° sector. Both during the gale phase and the swell phase the over‐water fetch was >150 km. The wind speed during the swell phase was typically 4 m s−1. During the swell phase a wind maximum near or below the lowest wind speed measuring level 10 m was observed. The net momentum flux was very small, resulting in CD values ∼0.7 × 10−3. Throughout the lowest 26 m, covered by the tower measurements, turbulence intensities in all three components remained high despite the low value of the kinematic momentum flux
resulting in a reduction of the correlation coefficient for the longitudinal and vertical velocity from its typical value around −0.35 to between −0.2 and 0 (and with some positive values at the higher measuring levels), appearing abruptly at wave age c0/U10 equal to 1.2. Turbulence spectra of the horizontal components were shown not to scale with height above the water surface, in contrast to vertical velocity spectra for which such a variation was observed in the low‐frequency range. In addition, spectral peaks in the horizontal wind spectra were found at a frequency as low as 10−3 Hz. From a comparison with results from a previous study it was concluded that this turbulence is of the “inactive” kind, being brought down from the upper parts of the boundary layer by pressure transport.
Importance of ice‐ocean interactions for the global ocean circulation: A model study
Authors: Goosse H. , Fichefet T.
Published: September 2002

Numerical experiments are conducted with a coarse‐resolution global ice‐ocean model in order to determine to what degree the sea ice‐ocean exchanges of heat, salt/freshwater, and momentum control the general circulation of the world ocean on long timescales. These experiments reveal that the formation of North Atlantic Deep Water (NADW) in the model results from the strong heat losses that occur at the oceanic surface in the high‐latitude North Atlantic. The large‐scale ice‐ocean interactions have nearly no influence on this process. In particular, neglecting the freshwater flux associated with the southward ice transport at Fram Strait does not impact seriously on the salinity of the Greenland and Norwegian Seas. At equilibrium the absence of this freshwater flux is balanced by an enhanced oceanic freshwater transport from the Arctic. Furthermore, it appears that the model NADW formation does not critically depend on the media (ice or ocean) transporting the freshwater. Besides, both the salt/freshwater and heat exchanges between sea ice and ocean are crucial in the Southern Ocean for the deep water production, properties, and export. The large amount of brine released during ice formation on the model Antarctic continental shelf leads to very high salinities there. The resulting dense shelf waters are then transported toward great depths after some mixing with ambient waters, finally forming the Antarctic Bottom Water body. On the other hand, the net ice melting associated with ice convergence in some areas, such as the southwestern Pacific, stabilizes the water column and forbids deep mixing in these regions. Furthermore, the contact with the ice imposes that the polar surface waters must be maintained very close to their freezing point temperature. Our results suggest that this process takes an important part in increasing the density of the Antarctic Bottom Water. We also show that the modifications of the stress at the ocean surface induced by the internal ice stress have only a regional effect.
Modeling the impact of fronts and mesoscale circulation on the nutrient supply and biogeochemistry of the upper ocean
Authors: Mahadevan Amala , Archer David
Published: September 2002

A new model for three‐dimensional mesoscale flow is used to simulate the vertical transport of nutrients into the euphotic zone from deeper waters. We use the model to diagnose the effects of mesoscale motions on the upper ocean biogeochemical cycles of NO3, CO2, O2, sea surface exposure tracers that resemble dissolved organic carbon (DOC) and H2O2, and the corresponding rates of new production and gas exchange in regions near the Joint Global Ocean Flux Study (JGOFS) oligotrophic time series sites of Hawaii and Bermuda during late summer. The physical model is nonhydrostatic and designed to handle open boundaries without excessive damping as described by Mahadevan and Archer [1998]. It is initialized and driven at the open boundaries with flow fields from the 1/4° global circulation model of Semtner and Chervin [1992]. We examine the effect of model resolution and the attending increase in mesoscale and frontal‐scale motions on the nutrient and carbon chemistry of the upper ocean. New production, approximated and diagnosed as an exponential uptake of nutrients within the euphotic zone, increases with model resolution. Nutrient is supplied for new production where isopycnals from the subsurface outcrop at the base of the euphotic zone, primarily at fronts. The rate and spatial scale of upwelling and nutrient uptake are more sensitive to model resolution than are temperature or the more slowly responding geochemical tracers such as CO2.