bluuebunny/crossref_metadata_2025 huggingface dataset:
Ventilation of the Arctic Ocean estimated by a plume entrainment model constrained by CFCs
Authors: Anderson L. G. , Jones E. P. , Rudels B.
Published: September 2002

Intermediate and deep water formation rates in the Arctic Ocean are estimated using a plume entrainment model based on shelf‐slope processes and constrained by tracer distributions within the deep basin. Each plume is initiated by a fraction, rj, leaving the shelf break at 200 m, followed by an entrainment of rj for every 150 m depth the plume descends. The model is tuned by varying rj to achieve the transient tracer (CFC‐12 and carbon tetrachloride) distribution as measured in the Nansen, Amundsen and Makarov Basins during the Oden 1991 expedition, and the concentrations in the source waters are calculated assuming a water in 100% equilibrium with the atmosphere. The formation of water entering below 500 m is computed to be 1.5 and 1.9 Sv when constrained by CFC‐12 and CCl4, respectively, with a total uncertainty of ±0.45 Sv. Sensitivities of the model settings to the entrainment rate, degree of saturation of the transient tracer in the source waters, and age of the Atlantic Layer water are investigated. Processes in the Arctic Ocean contribute around 1/3 of the approximately 5.6 Sv that flows over the Scotland‐Greenland Ridge, with the rest likely attributed processes in the Greenland and Iceland Seas. We thus conclude that the Arctic Ocean has to be included in the discussion of the sensitivity of the Greenland‐Scotland overflow to a climate change.
The quality of sea ice velocity estimates
Authors: Thomas Don
Published: September 2002

Several aspects of observed and estimated velocities for Arctic sea ice are explored. The long‐term mean ice motion field and associated variance are derived using 15 years (over 85,000 observations) of daily buoy motions. The mean field is separated into the part due to long‐term currents and internal ice stress and the part due to mean winds. The velocity variance is partitioned into “linear” and “nonlinear” parts. The linear response has no trend over the 15 years examined. For a 5‐year subset of the buoy data, an ice‐ocean model with constant air‐drag parameters is used to estimate daily ice velocities at the buoy locations, and then the linear relationship between the modeled velocities and the geostrophic winds is examined. Compared with buoy velocities, the modeled velocities have a larger variance and larger linear response to the winds. In a parameter study, the effects of seasonally varying air‐drag coefficients, increased ice strength, and different ocean currents are examined. Seasonally varying air drag, with smaller average magnitude than the standard value, significantly improves the match of total, linear, and nonlinear variances between observed and modeled velocities. The model simulations with varying air drag also have a smaller root‐mean‐square daily velocity error. Comparison of daily large‐scale deformations shows that the day‐to‐day deformations are not modeled well but generally have means and standard deviations within ±50% of the observed values. No one version of the model is consistently better than the others at matching the means and standard deviations of the observed daily divergence, vorticity, and shear. Ice thickness is sensitive to the model parameters varied, with 5‐year average thicknesses ranging from 2.2 to 3.4 m for the various simulations.
Influence of the Rossby waves on the seasonal cycle in the tropical Atlantic
Authors: Döös Kristofer
Published: September 2002

The importance of the long “midlatitude” Rossby waves to the seasonal cycle in the Tropical Atlantic is investigated. An analytical model of the baroclinic Rossby waves is forced with the annual and semiannual components of the climatological wind stress in the reopica (20°S to 20°N), excluding the Equator. The Rossby waves are fast enough in the tropics to influence the ocean on the seasonal timescale. The seasonal migrations of the sea surface troughs (lows) and ridges (highs) are linked to the forced Rossby waves and to their latitudinal speed dependency. The sea surface height of the analytical Rossby wave solutions are compared to data from the TOPEX/Poseidon satellite, the Levitus et al. [1994] climatology, a high‐resolution ocean general circulation model, a baroclinic primitive equation model, and a numerical solution of the Rossby wave equation. The regions where the Rossby waves seem to play an important role are those with the strongest observed seasonal variability, in particular the region just north of Brazil. The forced Rossby wave is composed of one free Rossby wave part and one part due to the local forcing, both of equal importance in the whole region. The observed phase propagation is westward and decreases with latitude but not as much and as clearly as the theoretical Rossby waves do.
Interbasin deep water exchange in the western Mediterranean
Authors: Rhein Monika , Send Uwe , Klein Birgit , Krahmann Gerd
Published: September 2002

Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean. The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provençal Basin was postulated in the depth range 700–1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero‐Provençal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero‐Provençal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600–1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv = 106 m3 s−1), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero‐Provençal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero‐Provençal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior.
Tidal exchange at the bay‐ocean boundary
Authors: Chadwick D. Bart , Largier John L.
Published: September 2002

For bays and inlets with narrow entrances, tidal exchange with the ocean is often characterized by an ebb‐flood asymmetry in which the ebb flow exits the bay as a jet, while the flood flow is drawn in approximately as a radial sink, a process often termed “tidal pumping.” In this paper, the importance of tidal pumping in controlling the exchange of water at the bay‐ocean boundary is examined based on observations at the mouth of San Diego Bay. The observations are novel in combining the use of shipboard acoustic Doppler current measurements with a unique bay‐water tracer technique to provide visualization of the exchange as well as exchange budgets. Visualization of the tidal evolution of flow and tracers suggests that, in accordance with the classical tidal pumping model, the horizontal dynamics at the mouth of San Diego Bay are governed by an asymmetry between the jet‐like nature of the ebb flow and the sink‐like nature of the flood flow. This asymmetry results in strong tidal velocity and phase gradients similar to those observed in previous numerical modeling studies. A one‐dimensional trajectory model based on the observations indicates that the combined influence of the residual circulation and offshore gradients in the tidal velocity and phase provides the mechanism by which water from the bay is exchanged with water from the ocean. Both bottom friction and thermal gradients affect the dynamics of the vertical exchange. The resulting asymmetry in the offshore flow leads to a vertical mode of “pumping” similar to that observed in the horizontal view. This horizontal and vertical pumping of the offshore flow is shown to produce a strong correlation between the sectionally averaged velocity and bay‐water concentration, resulting in a significant net tidal exchange between the bay and ocean.
Numerical investigation of baroclinic instability in the Gaspé Current using a frontal geostrophic model
Authors: Reszka M. K. , Swaters G. E.
Published: September 2002

The Gaspé Current, situated in the western Gulf of St. Lawrence, is an intense, buoyancy‐driven jet, which is fed by freshwater runoff from the St. Lawrence River. This coastal current is known to exhibit frequent episodes of instability, culminating in backward breaking waves as well as filaments and eddies. We investigate the observed mesoscale variability in a series of numerical simulations, employing a two‐layer frontal geostrophic model that focuses on baroclinic dynamics. Unlike previous studies, we allow for large‐amplitude variations of the interface depth, with the possibility of outcroppings, as well as two‐way coupling of the dynamic pressures in each layer. It is found that the growth rates and length scales associated with the early evolution of disturbances compare favorably with those observed. Moreover, the mesoscale features resulting from highly nonlinear interactions closely resemble those seen in satellite data.
Tropical Pacific near‐surface currents estimated from altimeter, wind, and drifter data
Authors: Lagerloef Gary S. E. , Mitchum Gary T. , Lukas Roger B. , Niiler Pearn P.
Published: September 2002

Tropical surface currents are estimated from satellite‐derived surface topography and wind stress using a physically based statistical model calibrated by 15 m drogue drifters. The model assumes a surface layer dominated by steady geostrophic and Ekman dynamics. Geostrophy varies smoothly from a β plane formulation at the equator to an ƒ plane formulation in midlatitude, with the transition occurring at ∼2°–3° latitude. The transition is treated with a Gaussian weight function having a meridional decay scale that is found to be approximately the Rossby radius (∼2.2° latitude). The two‐parameter Ekman model represents drifter motion relative to wind stress, with downwind flow along the equator and turning with latitude. Velocities computed from satellite data are evaluated statistically against drifter velocities and equatorial current moorings. Examples of the geostrophic and Ekman flow fields in the western Pacific during a westerly wind burst in late December 1992 depict a strong eastward flow and equatorial convergence. A comparison between December 1996 and June 1997 illustrates the basin‐wide reversal of equatorial surface flow during the onset of the 1997 El Niño.
Influences of physical processes on the ecosystem in Jiaozhou Bay: A coupled physical and biological model experiment
Authors: Chen Changsheng , Ji Rubao , Zheng Lianyuan , Zhu Mingyuan , Rawson Mac
Published: September 2002

In this paper we have used a three‐dimensional coupled physical and biological model to examine the marine ecosystem of Jiaozhou Bay. Physical processes included (1) the M2 tide, (2) river discharges, and (3) winds. The biological model described a simple, phosphorous‐based, lower trophic food web system. The model results showed that tidal mixing had a direct impact on temporal and spatial distributions of nutrients and phytoplankton as well as on shellfish aquaculture. Nutrients and phytoplankton were well mixed vertically by tidal motion. Their concentrations were highest around northwestern and northern regions of the bay near river sources and decreased with water depth from the inner bay to the outer bay. A phytoplankton bloom can occur around the northwestern coast due to the “accumulation” of nutrients under southeasterly wind conditions. The flux analysis suggests that in summer the nutrients in Jiaozhou Bay was directly supplied and maintained by physical processes, but the temporal variation of phytoplankton was controlled dominantly by biological processes associated with nutrient uptake, grazing by zooplankton, and consumption by shellfish. Shellfish aquaculture can modify the bay‐scale balance of the ecosystem in Jiaozhou Bay. The loss of phytoplankton in shellfish aquaculture sites tended to be compensated by advection and diffusion from the surrounding waters. A large consumption of phytoplankton by shellfish can cause a net flux of phytoplankton from the Yellow Sea to Jiaozhou Bay, although there was a net nutrient flux flowing out of the bay. The physical mechanism for the water exchange between Jiaozhou Bay and the Yellow Sea was controlled mainly by a chaotic process associated with nonlinear interactions between oscillating tidal currents and double residual eddies.
West Florida continental shelf response to upwelling favorable wind forcing: 2. Dynamics
Authors: Li Zhenjiang , Weisberg Robert H.
Published: September 2002

Dynamics analyses are presented for the west Florida continental shelf response to upwelling favorable, alongshore or offshore winds using a three‐dimensional, time‐dependent, primitive equation model. These analyses complement the kinematics analyses of Li and Weisberg [1999]. Results are provided on four topics: (1) the response evolutions to quasi steady states, (2) the across‐shelf distributions of the vertically integrated alongshore and across‐shelf momentum balances, (3) the three‐dimensional structures of the terms composing the momentum balance closures, and (4) the model sensitivity to vertical friction parameterization. The response evolution starts with a local wind‐driven acceleration, and it transitions over the inner shelf into a primary balance between the pressure gradient, Coriolis, and vertical friction terms. Thus the inner shelf spin‐up takes a classical Ekman‐geostrophic route. The middle shelf takes longer because it also depends on coastal boundary effects, notably the partial closure of the domain in the south by the Florida Keys. The outer shelf never achieves a steady state because of topographic wave effects. Coastline and isobath geometries result in fully three‐dimensional flow fields. Thus a two‐dimensional model that ignores alongshore variations cannot account for the results obtained. The inner shelf is the transition region between Ekman and Ekman‐geostrophic balances where the surface slope results from mass adjustments through overlapping surface and bottom Ekman layers. Across the inner shelf the vertically integrated momentum balance shows a gradual trade‐off between the Coriolis, pressure gradient, and bottom friction terms, and with the inner shelf frictionally determined its scale is sensitive to vertical friction parameterization.
Overflow into the deep Caribbean: Effects of plume variability
Authors: MacCready Parker , Johns William E. , Rooth Claes G. , Fratantoni David M. , Watlington Roy A.
Published: September 2002

The deepest connection between the eastern Caribbean and the Atlantic is over the Jungfern‐Grappler Sill complex at 1815 m depth. Through these gaps flows the sole source of water for the deep Caribbean, presumably balanced by diffusively driven upwelling elsewhere in the basin. Fourteen‐month‐long moored observations at the sills in 1991–1992 reveal a mean transport of 0.11±0.05 Sv (1 Sv ≡ 106 m3 s−1) of Atlantic water colder than θ = 3.965°C flowing into the Caribbean, one quarter of which comes over the previously unmeasured Grappler Sill. This is about twice the transport seen in previous, shorter experiments. The overflow is highly episodic, with ∼10 “events” per year. The range of overflow density is comparable to the total vertical stratification of the deep Caribbean. A numerical stream‐tube model of the overflow plume is run with 13 different initial conditions, representing the observed range of overflow strength and density. Results indicate that much of the overflow only penetrates to 2200–3000 m depth and was dense enough to descend to the bottom of the Caribbean only ∼1% of the time. Combined plume model results are used to drive a model of the basin stratification. A steady balance is found to be possible with small diapycnal diffusivity in the basin, between 0.1 and 0.6 cm2 s−1. The reason for the small diffusivity is that the stratification is essentially built in place by the different detrainment depths, and so less diffusive transformation is needed.
Internal tide generation by seamounts, ridges, and islands
Authors: Holloway Peter E. , Merrifield Mark A.
Published: September 2002

A numerical investigation is made into the generation of semidiurnal internal tides by tidal flow over steep topographic features in the deep ocean. A fully three‐dimensional, free surface, nonlinear, hydrostatic model (the Princeton Ocean Model) is used with real stratification, representative tidal forcing, and Gaussian‐shaped seamounts, ridges, and islands for the topography. The efficiencies of the different topographies in extracting energy from the barotropic tide and in generating an internal tide are considered. Each topography produces an internal tide characterized by signals propagating away from the feature as beams that follow internal wave characteristic paths. The strength of the signals, however, varies markedly for different topographies. This is largely attributed to the way in which the barotropic tide interacts with the three‐dimensional topography. Internal wave generation requires a significant vertical displacement of stratified water by the barotropic tide as it flows over topography. It is found that for symmetric seamounts and islands the barotropic flow tends to go around the feature, producing only a weak internal tide. When elongated into a ridge, barotropic flow is forced across isobaths, generating an energetic internal tide. Changing the horizontal aspect ratio of the topography from 1:1 (a seamount) to 3:1 (a ridge) increases the resulting baroclinic energy flux by nearly an order of magnitude. The slope of the topography relative to that of the internal wave characteristics is also shown to be important with strongest generation occurring at regions of critical or supercritical topographic slope.
Are interdecadal sea level changes along the Indian coast influenced by variability of monsoon rainfall?
Authors: Shankar D. , Shetye S. R.
Published: September 2002

The Mumbai (Bombay) tide gauge data, the only century‐long record in the Indian Ocean, show that interdecadal changes in sea level mimic those in rainfall over the Indian subcontinent. We propose that the link between rainfall and sea level arises from changes in salinity in coastal waters. Rivers fed by southwest monsoon (June‐September) rainfall bring a large fraction of the runoff to the Bay of Bengal, from where it is transported to the west coast of India by an equatorward East India Coastal Current, which is triggered partly by the withdrawal of the southwest monsoon. The West India Coastal Current carries the low‐salinity water from the bay as well as the runoff from local rivers northward. The advection of the riverine inflow to Mumbai occurs within a season, but the slow mixing in the ocean forces changes in the cross‐shore density gradient on longer timescales. This density gradient forces a two‐layer geostrophic circulation, with a surface current, which flows with the lighter water on its right, and an undercurrent. Lower (higher) salinity at the coast implies higher (lower) coastal sea level and a rise (fall) of the pycnocline at the coast. Thus the interdecadal variability of sea level along the Indian coast can be linked directly to the variability of the monsoon, the major aspect of the climate of the region, but by a mechanism that is different from those generally proposed to link sea level to climate change; these hypotheses usually invoke a change in volume because of global warming.
Radar backscatter from stationary breaking waves
Authors: Ericson Eric A. , Lyzenga David R. , Walker David T.
Published: September 2002

Radar backscatter measurements from stationary breaking waves were used to examine how the surface roughness generated by wave breaking affects radar backscatter at moderate incidence angles. Stationary breaking waves were generated by submerging a stationary hydrofoil in a uniform flow. X band radar backscatter measurements were made at numerous streamwise positions along the stationary breaking waves from an incidence angle of 45° for horizontal transmit and receive polarization (HH) and vertical transmit and receive polarization (VV) looking both upwave and downwave. The radar returns increased substantially, and the HH‐to‐VV polarization ratio approached unity near the breaking crests. This radar signature is consistent with those observed in the field. Detailed optical measurements of the breaking surfaces revealed that the observed radar returns near the breaking crests were the result of increased incoherent backscatter from the small‐scale surface roughness generated by the breaking waves, although surface tilt effects also modified the radar return. Scattering models based upon the small perturbation solution performed well in the wake of the stationary breaking crest, but they significantly underestimated the HH‐to‐VV polarization ratio near the breaking crest. More advanced scattering solutions such as the integral equation method produced more accurate results in regions containing the largest surface roughness. These findings suggest that incoherent backscatter from surface disturbances produced by deep water breaking waves may be the source of the high radar returns and small polarization ratios observed from the ocean at moderate incidence angles.
An analysis of tropical instability waves in a numerical model of the Pacific Ocean: 2. Generation and energetics of the waves
Authors: Masina S. , Philander S. G. H. , Bush A. B. G.
Published: September 2002

The instability processes which generate unstable waves with characteristics similar to observed tropical instability waves in the Pacific Ocean are examined through a local energy analysis based on deviations from the time mean flow. Numerical experiments indicate that the waves develop preferentially in the eastern Pacific along the northern temperature front and have a westward phase speed and a structure with two peaks in amplitude: one located on the equator and the other a few degrees north of it. The energy analysis shows that the “two‐peak” structure of the eastern waves is explained by two different instability processes which occur at different latitudes. In the time mean sense the region north of the equator is baroclinically unstable, while barotropic instability prevails at the equator. The life cycle of the waves is revealed by the time evolution of the energetics. Baroclinic instability is the dominant triggering mechanism which induces growth of the waves along the northern temperature front. The eddy pressure fluxes radiate energy south of the equator where the meridional shear between the Equatorial Undercurrent and the South Equatorial Current becomes barotropically unstable. From the numerical simulations, there is evidence of a second unstable region in the central Pacific south of the equator where the instabilities have a lower phase speed. The energy analysis also shows that these waves grow from both barotropic and baroclinic conversions.
The bomb 14C transient in the Pacific Ocean
Authors: Rodgers Keith B. , Schrag Daniel P. , Cane Mark A. , Naik Naomi H.
Published: September 2002

A modeling study of the bomb 14C transient is presented for the Pacific Ocean. A primitive equation ocean circulation model has been configured for a high‐resolution domain that accounts for the Indonesian Throughflow (ITF). Four separate runs were performed: (1) seasonal forcing with 20 Sv of ITF transport, (2) seasonal forcing with 10 Sv of ITF transport, (3) seasonal forcing with no ITF transport, and (4) interannual forcing with 15 Sv of ITF transport. This study has two main objectives. First, it is intended to describe the time evolution of the bomb 14C transient. This serves as a tool with which one can identify the physical processes controlling the evolving bomb 14C distribution in the Pacific thermocline and thus provides an interpretive framework for the database of Δ14C measurements in the Pacific. Second, transient tracers are applied to the physical oceanographic problem of intergyre exchange. This is of importance in furthering our understanding of the potential role of the upper Pacific Ocean in climate variability. We use bomb 14C as a dye tracer of intergyre exchange between the subtropical gyres and the equatorial upwelling regions of the equatorial Pacific. Observations show that while the atmospheric Δ14C signal peaked in the early to mid‐1960s, the Δ14C levels in the surface water waters of the subtropical gyres peaked near 1970, and the Δ14C of surface waters in the equatorial Pacific continued to rise through the 1980s. It is shown that the model exhibits skill in representing the large‐scale observed features observed for the bomb 14C transient in the Pacific Ocean. The model successfully captures the basin‐scale inventories of bomb 14C in the tropics as well as in the extratropics of the North Pacific. For the equatorial Pacific this is attributed to the model's high meridional resolution. The discrepancies in the three‐dimensional distribution of bomb 14C between the model and data are discussed within the context of the dynamical controls on the Δ14C distribution of bomb 14C in the Pacific.
Field observations of wave setup
Authors: Lentz Steve , Raubenheimer Britt
Published: September 2002

Wave setup is assumed to be a balance between the cross‐shore convergence of the onshore flux of momentum (wave radiation stress Sxx) in the surfzone and a cross‐shore pressure gradient. Oceanic observations between the 2‐ and 8‐m isobaths near Duck, North Carolina, provide a test of the wave setup balance without assuming that wave height in the surfzone is proportional to water depth. Analysis of data from a cross‐shore array of 11 pressure gauges and 10 sonar altimeters deployed during the fall of 1994 indicates the wave setup balance holds to at least the accuracy of the pressure measurements (a few centimeters). The correlation between the two terms in the setup balance is 0.93, and the linear regression slope is 1.05±0.19. Accurate estimates of the cross‐shore pressure gradient require density measurements to adjust pressure measurements taken at different depths to the same level. The assumption that pressure and bathymetry are linear between the 2‐ and 8‐m isobaths (or the more common assumption that the height of normally incident, shallow water waves is proportional to the water depth) introduces errors of up to 6 cm for the conditions considered here. Given this assumption, 3.5 years of data from pressure gauges in 2 and 8 m of water indicate that the wave setup balance is valid for a wide range of conditions (correlation 0.71 and regression slope 0.98±0.08).
On subinertial flow in submarine canyons: Effect of geometry
Authors: Allen Susan E.
Published: September 2002

Shelf break canyons on the west coast of Canada and the United States have been observed to be regions of enhanced upwelling during southward currents compared to the surrounding shelf break. Most shelf break canyons from Oregon north cross only part of the continental shelf cutting from the shelf break toward the coast but end on the continental shelf well below the mixed layer. Juan de Fuca canyon, on the other hand, cuts the continental shelf from the slope to, and actually continues into, the Strait of Juan de Fuca. This difference in geometry has a very strong effect on the subinertial flow around the canyon. Model canyon shapes, which include convergent bathymetric contours, are constructed and motivated for Juan de Fuca canyon and a typical shelf break canyon. Geostrophic analytic solutions show that the in‐canyon flow in Juan de Fuca canyon is generated by first‐order geostrophic dynamics, whereas in the majority of canyons, of which Astoria is an example, in‐canyon flow is generated by higher‐order effects. This difference is postulated to lead to the observed, very deep upwelling over Juan de Fuca canyon compared to more moderate, episodic upwelling over Astoria canyon.
Shear stress model for the aqueous boundary layer near the air‐sea interface
Authors: Bourassa Mark A.
Published: September 2002

Two sets of data on currents in the near‐surface boundary layer are reanalyzed to more accurately determine shear stress and characteristics of the mean current profile (friction velocity and roughness length). The results are used to examine the mechanisms responsible for momentum transfer through the aqueous boundary layer (the top several meters of water). The relationship between friction velocity and roughness length found here is consistent with the prevailing concepts on wave‐generated mixing. This result conflicts with a commonly used parameterization of momentum transfer in the aqueous boundary layer, where molecular viscosity is treated as the dominant transfer mechanism. The shear‐related stress and current profiles are shown to follow Charnock's [1955] relationship, which indicates that the shear‐related mixing is due to gravity waves. Charnock's constant for water is found to be within an order of magnitude of 850 (105 times greater than that for the atmospheric boundary layer). Shear‐related stress in the aqueous boundary layer is compared to the wind stress: for conditions of near local wind‐wave equilibrium, the aqueous shear stress is ∼20% of the atmospheric stress, indicating that the majority of the vertical momentum flux is transferred through other mechanisms. These limited data sets suggest that this ratio increases (approaching 100%) for decaying wave fields.
Large‐scale drift of Arctic Sea ice retrieved from passive microwave satellite data
Authors: Martin T. , Augstein E.
Published: September 2002

A method of determining the large‐scale sea ice drift using 85.5 GHz Special Sensor Microwave Imager data are presented. A cross‐correlation method is applied to sequential images of gridded data covering the entire Arctic. Individual correlation results are validated with ice velocities derived from buoy data. The satellite‐derived mean drift values and the variabilities of the ice drift correspond closely with the buoy data. Similarly, time series of buoy data and associated satellite data are in good agreement even over large time periods. An example of a satellite‐retrieved 3 day mean drift field demonstrates the potential of the method for providing large‐scale ice circulation patterns. Mean drift fields of the winter periods 1987–1988 and 1992–1993 indicate a considerable interannual variability of the sea ice drift pattern in the Arctic Ocean. The Arctic region is divided into seven larger areas, and the area flux between these regions has been derived. The Kara Sea and the Laptev Sea show the largest area ice export with 0.02 and 0.015 km2 s−1, respectively. The central Arctic export through Fram Strait amounts to 0.12 Sv during the winter of 1992–1993 with a maximum of 0.15 Sv in January.
Sea ice thickness as a stochastic process
Authors: Thorndike Alan
Published: September 2002

Observed probability distributions of sea ice thickness are approximately exponential for thick ice. An explanation is given on the basis of simple representations of the thermal and mechanical processes. The argument predicts that the e‐folding scale for the exponential is proportional to the thermal equilibrium thickness. The constant of proportionality depends on the dimensionless ratio of the rates of the thermal and mechanical processes.
Impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break
Authors: Inall Mark E. , Rippeth Tom P. , Sherwin Toby J.
Published: September 2002

The vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the Malin Shelf has been determined using a combination of the free‐falling light yo‐yo profiler and acoustic doppler current profilers. Two distinct internal wave regimes were encountered: period I in which large‐amplitude high‐frequency nonlinear internal waves (NIWs) occurred (around neap tides) and period II in which the internal wave spectral continuum was not dominated by any particular frequency band (around spring tides). Empirical orthogonal function analysis shows that for the low‐frequency waves, 76% of the variance was described by mode 1, rising to 95% for the high‐frequency waves. During period I the dissipation and vertical mixing were characterized by the NIWs, and 70% of the dissipation occurred in the bottom boundary layer. During period II the depth‐integrated dissipation was more evenly distributed throughout the tidal cycle, whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification. During both periods I and II ∼30% of the total measured dissipation occurred within the thermocline when averaged over 12.4 hours; the remainder occurred within the bottom boundary layer(BBL). Tidal average values for depth‐integrated dissipation and vertical eddy diffusivity for period I (II) were 1.1×10−2 W m−2 (4.0×10−2 W m−2) and 5 cm2 s−1 (12 cm2 s−1), respectively. Decay rates and internal damping are discussed, and vertical heat fluxes are estimated. Observed dissipation rates are compared with a simple model for BBL dissipation.
Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate Water
Authors: You Yuzhu , Suginohara Nobuo , Fukasawa Masao , Yasuda Ichiro , Kaneko Ikuo , Yoritaka Hiroyuki , Kawamiya Michio
Published: September 2002

Recently obtained World Ocean Circulation Experiment (WOCE) sections and pre‐WOCE hydrography are used to study the water‐mass structure and formation and transformation of North Pacific Intermediate Water (NPIW). Five neutral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of ∼100 m between a pair of surfaces. NPIW is defined as a subtropical gyre salinity minimum which is well followed by a neutral density surface σN = 26.9. Formation and transformation of NPIW is examined by the mapped Turner angle on neutral density surfaces. Apparent diffusive double diffusion is found in the Alaskan gyre on σN = 26.5 neutral surface, in the northwest subpolar gyre and the Okhotsk Sea on σN = 26.9 neutral surface, and mainly in the Okhotsk Sea on the two deep neutral surfaces σN =27.2 and σN = 27.4. These diffusive regions indicate transformation sources for NPIW. Along with additional information of potential vorticity and stream function, it is found that there are two different NPIW formation sources: one in the Gulf of Alaska characterized by high potential vorticity and the other in the Okhotsk Sea characterized by low potential vorticity. The former lies shallower at σN =26.2–26.5, but its effect deepens to NPIW core density level at σN = 26.8 on the basis of potential vorticity distribution. The latter includes the influence of the northwest subpolar gyre and extends much deeper to σN = 27.4. We call them Gulf of Alaska Intermediate Water (GAIW) and Okhotsk Intermediate Water (OIW), respectively. GAIW contributes to NPIW in the eastern part of the subtropical gyre east of date line, whilst OIW dominates in the west and entire lower part of NPIW. Seasonal flow stream function mapped on neutral surfaces shows that the contribution of GAIW to NPIW occurs mainly in the wintertime, because in winter a significant northward shift of zero wind stress curl makes the Gulf of Alaska an additional source for NPIW.
Current distributions in the southern East China Sea in summer
Authors: Katoh Osamu , Morinaga Kenji , Nakagawa Norihisa
Published: September 2002

Current measurements using a shipboard acoustic Doppler current profiler (ADCP) and two satellite‐tracked buoys, together with conductivity‐temperature‐depth (CTD) casts, were carried out over the continental shelf in the southern East China Sea in July 1995. Two transects for ADCP measurements were defined and divided into segments of ∼56 km. In order to remove diurnal and semidiurnal tidal flows from observed flows, four round‐trip ADCP surveys were conducted along each segment over diurnal tidal periods of ∼24 hours and 50 min. The outflow through the Taiwan Strait, i.e., the Taiwan Current, with velocities of 35–40 cm s−1, was clearly observed to flow northeastward along the coast of China. The volume transport was estimated as at least 1 Sv. The Taiwan Current probably increased width and decreased velocity in the central East China Sea. Bifurcation into two branches of the Taiwan Current shown by previous studies was not observed. Another current entering onto the continental shelf northeast of Taiwan, i.e., the Kuroshio Branch, was found with velocities >15 cm s−1 in the surface, and it helped to make an anticyclonic eddy. The main portion of the Kuroshio Branch flowed northeastward along the 100 m isobath, with a volume transport of ∼0.3 Sv. The Taiwan Current was clearly separated from the Kuroshio Branch by the area of very weak or southward (reverse) flows.
Temperature anomalies beneath breaking waves and the decay of wave‐induced turbulence
Authors: Gemmrich Johannes R.
Published: September 2002

Temperature observations in the near surface layer of the ocean reveal brief temperature fluctuations O(20 mK) coincident with air entrainment due to wave breaking. These temperature fluctuations, typically confined to the upper ∼0.2 m, are interpreted as consequences of mixing of a surface layer of anomalous temperature. The thickness of this surface layer is estimated at ∼30–50 mm. Only ∼30% of breaking waves exhibit temperature signals, implying strong variability of the heat content of this layer. The evolution of the temperature profile in the upper 0.2 m is calculated according to a time dependent diffusion process which incorporates decay of wave induced turbulence. Combining the modeled temperature profile evolution with a simple one‐dimensional mixing model shows that our observations are consistent with the decay rate of the turbulent diffusivity kT∝t−1.85 inferred from laboratory measurements of stable stratified flows. Stratification in the oceanic near‐surface layer may be provided by microbubbles, suggesting that these bubbles play an important role in the dynamics of the near‐surface region.
Second‐moment closures and length scales for weakly stratified turbulent shear flows
Authors: Baumert Helmut , Peters Hartmut
Published: September 2002

For the special hydrodynamic situation of unbounded homogeneous shear layers, turbulence closure models of Mellor‐Yamada type (MY) and k‐ε type are put into a single canonical form. For this situation we show that conventional versions of MY and various k‐ε versions lack a proper steady state, and are unable to simulate the most basic properties of stratified shear flows exemplified in, for example, the Rohr et al. [1988] experiments: exponential growth at sufficiently low gradient Richardson number (Rg), exponential decay at sufficiently large Rg, and a steady state in between. Proper choice of one special model parameter readily solves the problems. In the fairly general case of structural equilibrium (state of exponential evolution) in weakly to moderately stratified turbulence (Rg ≲ 0.25), the ratio between the Thorpe scale (or Ellison scale) and the Ozmidov scale varies like the gradient Richardson number (Rg) to the power 3/4, and the ratio of the Thorpe scale to the buoyancy scale varies like Rg1/2. Length scales predicted by our current model are consistent with laboratory measurements of Rohr et al. [1988], with large‐eddy numerical simulations of Schumann and Gerz [1995], and with microstructure measurements from the 1987 Tropic Heat Experiment in the equatorial Pacific by Peters et al. [1995].
Ekman layers and two‐dimensional frontogenesis in the upper ocean
Authors: Thompson LuAnne
Published: September 2002

A two‐dimensional modified semi geostrophic model is used to study the evolution of oceanic fronts in the presence of vertical mixing. The parameterization used for mixing is an elevated constant value of vertical viscosity and friction; hence the Ekman layer acts as a surrogate mixed layer, with, however, vertical shear allowed everywhere. An initial condition representative of the observed fields in Frontal Air‐Sea Interaction Experiment (FASINEX) is used, and its modification in the presence of vertical mixing alone is investigated. Without external forcing an Ekman layer results because of the presence of vertical geostrophic shear at the surface. The maximum density gradient moves toward the dense side of the front driven by this flow. Convergence of Ekman flow results in downward bowing of the isopycnals beneath the surface expression of the front, a feature reminiscent of density sections taken during FASINEX. This feature is not evident when a barotropic convergence field is applied in the absence of mixing or in fully nonlinear simulations of the evolution of fronts with baroclinic instability. An analytic theory suggests that the maximum density gradient will increase over time when vertical mixing alone is present. In the presence of negative uniform wind stress in the direction opposite to the surface geostrophic flow the front moves toward the denser water, and the jet uniformly decreases in strength. In the presence of a positive uniform wind in the direction of the surface geostrophic flow the surface jet initially weakens but then strengthens again as the wind‐driven Ekman flow opposes the frictionally driven Ekman flow and the cross‐front density gradient increases.
Industrial emissions of trichloroethene, tetrachloroethene, and dichloromethane: Reactive Chlorine Emissions Inventory
Authors: McCulloch Archie , Aucott Michael L. , Graedel Thomas E. , Kleiman Gary , Midgley Pauline M. , Li Yi‐Fan
Published: September 2002

The identified emissions of the title compounds come predominantly from their use in industrial and commercial processes. Trichloroethene and tetrachloroethene have also been found as byproducts of gasoline and coal combustion; these sources were also considered but shown to be insignificant compared with industrial releases. Global emissions during 1990, amounting to 0.241±0.013 Tg of trichloroethene, 0.366±0.020 Tg of tetrachloroethene, and 0.583±0.032 Tg of dichloromethane (0.195±0.010, 0.313±0.017, and 0.487±0.027 Tg as chlorine, respectively) have been assigned to individual countries and thence to a 1° latitude × 1° longitude grid based on a combination of three data sets: regional sales data that were available on a continental scale; economic activity in the form of national Gross Domestic Products; and the population distribution within each country. For those countries where they were available, data for the quantities and locations of reported emissions were also incorporated. Uncertainty in the distributed emissions is ±4% relative to countries with the largest emissions. The results, which are complementary to the marine fluxes and releases from biomass burning reported by Khalil et al [this issue] and Lobert et al [this issue], respectively, are recorded here as maps and are also available from the Global Emissions Inventory Activity web site at http://groundhog.sprl.umich.edu/geia/rcei. While the industrial regions of North America, Europe, and Japan are the largest sites of anthropogenic emissions, there are also significant sources in the developing nations of Asia; in contrast, anthropogenic emissions within the southern hemisphere are much smaller and more widely dispersed. The total emissions of dichloromethane appear to match the observed atmospheric concentrations, but about 25% of the flux of tetrachloroethene calculated from observations remains unaccounted, and significant extra emissions of trichloroethene are necessary to effect a balance. The known sources have been examined thoroughly in this work, and so it is reasonably certain that the additional emissions are not a deliberate result of human activity; however, there is no means of discriminating their origin unequivocally, and the missing quantities may be inadvertent byproducts of anthropogenic activities.
Stable carbon isotopic composition of atmospheric methane: A comparison of surface level and free tropospheric air
Authors: Tyler Stanley C. , Ajie Henry O. , Gupta Mohan L. , Cicerone Ralph J. , Blake Donald R. , Dlugokencky Ed J.
Published: September 2002

We report CH4 mixing ratios and δ13C of CH4 values for remote air at two ground‐based atmospheric sampling sites for the period December 1994 to August 1998 and similar data from aircraft sampling of air masses from near sea level to near tropopause in September and October of 1996 during the Global Tropospheric Experiment Pacific Exploratory Mission (PEM)‐Tropics A. Surface values of δ13C‐CH4 ranged from −47.02 to −47.52‰ at Niwot Ridge, Colorado (40°N, 105°W), and from −46.81 to −47.64‰ at Montaña de Oro, California (35°N, 121°W). Samples for isotopic analysis were taken from 2° to 27°S latitude and 81° to 158°W longitude and from sea level to 11.3 km in altitude during the PEM‐Tropics A mission. They represent the first study of 13CH4 in the tropical free troposphere. At ∼11 km, δ13C‐CH4 was ∼1‰ greater than surface level values. Methane was generally enriched in 13C as altitude increased and as latitude increased (toward the South Pole). Using criteria to filter out stratospheric subsidence and convective events on the basis of other trace gases present in the samples, we find evidence of a vertical gradient in δ13C‐CH4 in the tropical troposphere. The magnitude of the isotopic shifts in atmospheric CH4 with altitude are examined with a two‐dimensional tropospheric photochemical model and experimentally determined values for carbon kinetic isotope effects in chemical loss processes of CH4 Model‐calculated values for δ13C‐CH4 in both the troposphere and lower stratosphere significantly underpredict the enrichment in 13CH4 with altitude observed in our measurement data and data of other research groups.
Measurements of acetone, acetic acid, and formic acid in the northern midlatitude upper troposphere and lower stratosphere
Authors: Reiner Thomas , Möhler Ottmar , Arnold Frank
Published: September 2002

We have measured acetone, acetic acid, and formic acid concentrations in the upper troposphere and lower stratosphere over Germany. The measurements were performed by ion molecule reaction mass spectrometry using new kinetic data on ion molecule reactions of formic and acetic acids with negative ions obtained at our laboratory. Mean volume mixing ratios between 384 and 832 parts per trillion (pptv) for acetone, 110 and 357 pptv for acetic acid, and 59 and 215 pptv for formic acid were obtained. The correlation between formic acid and acetic acid was very poor (r2 = 0.14). A better correlation could be observed for acetone and acetic acid, with a correlation coefficient r2 = 0.46 and a slope (acetic acid/acetone) of 0.31. For acetic acid a maximum around 9 km was observed. A significant fraction of the acetic acid observed in the lower stratosphere may be due to in situ photochemical production by reactions of HO2 and CH3O2 with peroxy acetyl radicals produced by the photolysis of acetone. In the upper troposphere, vertical transport is much more efficient, and significant acetic acid production is only possible if HOx concentrations are elevated, making the production of acetic acid fast enough to compete with vertical transport.