bluuebunny/crossref_metadata_2025 huggingface dataset:
What sets the mean transport through Drake Passage?
Authors: Gent Peter R. , Large William G. , Bryan Frank O.
Published: February 2004

Twelve experiments with two coarse resolutions of a global ocean model using a variety of surface forcings are analyzed to address the question of what sets the mean transport through Drake Passage. Seven of the experiments do not have an active sea‐ice model, but the remaining five do. Previous theories have suggested that the Drake Passage transport is governed by the Cape Horn Sverdrup transport or, alternatively, is proportional to the square root of the meridional Ekman transport at the latitude of Drake Passage. The results presented here do not support either of these theories. The Drake Passage transport depends quite strongly on the isopycnal diffusivity parameter in the model and less strongly on the background vertical diffusivity and horizontal viscosity parameters. However, when the magnitudes of these parameters are fixed, the results show a very strong correlation between Drake Passage transport and both the strength of the meridional Ekman transport at the latitude of Drake Passage and the thermohaline circulation off the Antarctic shelf. The relationships are monotonic, but not linear. The best estimate is that the meridional Ekman transport drives ∼100 Sv of Drake Passage transport, while the remaining 30 Sv are driven by the global thermohaline circulation.
Source of the baroclinic waves in the southeast Indian Ocean
Authors: Birol Florence , Morrow Rosemary
Published: February 2004

The dynamics of the seasonal and interannual sea level variability in the southeast Indian Ocean are investigated using a simple model of low‐frequency quasi‐geostrophic thermocline variability in order to determine whether the observed variability responds primarily to local or remote forcing. This region is important for climate studies, in relation to the interannual variations of the tropical oceans and atmosphere. The eastern Indian Ocean is directly forced by the strong seasonal monsoons as well as by a remote ocean forcing from the tropical Indian Ocean and by the western Pacific via the Indonesian Throughflow. As a result, the dynamics of the southeast Indian Ocean are unique, with unusually large variability and bands of energetic Rossby waves. The annual wave signal around 10°S is clearly marked and a band of propagating mesoscale variability between 20° and 35°S extends across the entire Indian Ocean, with characteristic timescales between 100 and 200 days. There is also strong interannual variability. To investigate the origin of the observed long baroclinic waves, we use a simple reduced gravity model allowing the radiation of long waves due to Ekman pumping and the radiation of long waves from the eastern boundary. Eastern boundary conditions are given by expendable bathythermograph data. In the band 10°–15°S the thermocline depth oscillation corresponds mainly to waves radiating from wind forcing in the east. Their amplitude is strongly damped west of 90°E. In the southeastern tropical Indian Ocean (STIO) the influence of free waves emanating from the eastern boundary is small but significant. In addition, a strong interannual signal appears to originate from Lombok Strait to the north and propagates southwestward into the STIO. In the band 20°–35°S the observed waves appear to be free waves generated by eastern boundary processes.
Asymmetry in the response of a stratified coastal embayment to wind forcing
Authors: Davidson Fraser J. M. , Greatbatch Richard J. , de Young Brad
Published: February 2004

We investigate mechanisms that lead to asymmetry in the response of a stratified coastal embayment following the onset of a uniform, steady wind that is blowing both along the axis and out of the bay. We focus on bays on the east coast of Newfoundland where the typical duration of wind events is 5 days and stratification representative of June conditions yields a first baroclinic mode wave speed of 0.51 m s−1. We use several numerical models ranging from a linear, reduced gravity model with a single baroclinic mode, to a nonlinear, prognostic, primitive equation model (CANDIE). We investigate the effect of factors such as continuous stratification, vertical mixing, nonlinearity, and realistic bottom topography. If the linear dynamics of only the first baroclinic mode is considered, the response of the idealized bay to 5 days of steady wind forcing is symmetric about the axis of the bay. Continuous stratification allows for higher‐order vertical modes. These slower modes increase the response time of the bay, yielding asymmetry in the circulation pattern after 5 days of constant wind forcing. Model results using realistic geometry demonstrate that realistic bottom topography has little effect on near‐surface circulation on the 5 day timescale. Adding nonlinearity allows a significant cross‐bay transport of upwelled water and leads to the characteristic along‐bay pattern of the surface isotherms evident in observations and can also lead to the separation of the coastal jet from the upwelling favorable shore.
Simulation of water mass formation processes in the Mediterranean Sea: Influence of the time frequency of the atmospheric forcing
Authors: Castellari S. , Pinardi N. , Leaman K.
Published: February 2004

Numerical experiments are conducted to investigate the interannual variability of water mass formation processes in the Mediterranean Sea. The experiments are conducted with a general circulation model which includes a set of calibrated air‐sea heat flux bulk formulae to account for the correct surface heat budget of the Mediterranean Basin. An analysis of the model response to different atmospheric forcing frequencies (monthly versus 12‐hour) for the period 1980–1988 is conducted in the areas of different water mass formation processes: the Western Mediterranean Deep Water (WMDW), the Eastern Mediterranean Deep Water (EMDW) (only the Adriatic component), the Levantine Intermediate Water (LIW), and the Levantine Deep Water (LDW). All the experiments are able to simulate well the LIW and LDW; however, the 12‐hour forced experiments (D experiments) tend to produce more LDW than LIW compared to the monthly forced experiments (M experiments). On the other hand, only the D experiments, with a salinity enhancement, are able in some years to model the WMDW deep convection. The WMDW and EMDW are formed approximately every 3 years depending on the model parameterizations; however, LIW is formed each year but with large‐amplitude fluctuations in the volume of water formed. LDW has a formation rate similar to that of deep waters.
Seasonal to interannual variability from expendable bathythermograph and TOPEX/Poseidon altimeter data in the South Pacific subtropical gyre
Authors: McCarthy Mary Cait , Talley Lynne D. , Roemmich Dean
Published: February 2004

Estimates of dynamic height anomalies from expendable bathythermograph (XBT) and TOPEX/Poseidon (T/P) sea surface height (SSH) measurements were compared along a transect at ∼30°S in the South Pacific. T/P SSH anomalies were calculated relative to a 5 year time mean. XBT dynamic height was calculated relative to 750 m using measured temperature and an objectively mapped climatological temperature‐salinity relationship. The anomaly was obtained by subtracting out an objectively‐mapped climatological dynamic height relative to 750 m. XBT temperature sections show evidence of a double‐gyre structure, related to changes in shallow isopycnals near the gyre's center. XBT dynamic height and T/P SSH anomalies compare well with an RMS difference of 3.8 cm and a coherence above 0.7 for scales larger than 300 km. The differences between the two measures of dynamic height yield systematic patterns. Time‐varying spatial averages of the differences are found to be related to changes in Sverdrup transport, zonal surface slope differences, and the 6°C isotherm depth. Higher zonally averaged altimetry SSH than zonally averaged XBT height and larger northward transport from altimetry SSH than from XBT height correspond to gyre spinup determined from Sverdrup transport changes. This implies mass storage during gyre spinup due to the phase lag between the Ekman pumping and the full baroclinic Sverdrup response. Increases in the spatially averaged differences and zonal slope differences, associated with gyre spinup, correspond to shoaling in the 6°C isotherm depth, requiring deep baroclinic changes out of phase with the 6°C isotherm depth changes.
Optical and physical variability on timescales from minutes to the seasonal cycle on the New England shelf: July 1996 to June 1997
Authors: Chang G. C. , Dickey T. D.
Published: February 2004

High‐resolution time series of physical and bio‐optical data were obtained using moored and bottom‐mounted instruments on the southern New England continental shelf during the Coastal Mixing and Optics experiment (CMO) from July 1996 through June 1997. The most prominent physical and bio‐optical signals observed during the experiment were associated with the seasonal variability. However, several important events interrupted the seasonal cycle. These episodic events appear to have had a great impact on biogenic and nonbiogenic matter. Hurricanes and storms passed over or near the CMO site, resulting in reduced stratification of the water column, particle redistribution, and sediment resuspension. Changing hydrographic conditions that resulted from the influence of several water mass intrusions greatly affected particle concentration on timescales of days to several weeks. The bottom boundary layer had an influence on particle movement in the water column and along the seafloor. The results suggest that there is likely considerable interannual variability in both the physics and bio‐optics because of active and diverse physical forcing. This experiment also sets the context for comparing our coastal ocean results with previous open ocean findings. Important differences arise because of coastal bottom boundary layer effects, large‐scale water mass intrusions, and the relatively greater role of tides on the shelf. Timescales of optical variability (e.g., changes in phytoplankton spectral shapes of absorption) are thus generally shorter for the coastal environment.
Integrated use of altimeter and in situ data for understanding the water exchanges between the Tyrrhenian and Ligurian Seas
Authors: Vignudelli Stefano , Cipollini Paolo , Astraldi Mario , Gasparini Gian Pietro , Manzella Giuseppe
Published: February 2004

In this paper we examine the relationship between the seasonal and interannual variability observed in the water flow through the Corsica Channel and the sea level difference between the Tyrrhenian and Ligurian Seas. The steric contribution to the sea level difference, computed from historical hydrological data, is in good agreement with the stable presence of the seasonal signal in the water exchanges. We obtain the maximum steric difference in winter (∼16 cm) and the minimum in summer (∼2 cm). These values are consistent with the corresponding estimates of water volume transport (0.8 and <0.1 Sv in winter and summer, respectively). Also, TOPEX/Poseidon (T/P) satellite altimetry is shown to be capable of capturing the sea level difference anomaly between the two basins at seasonal and interannual scales. Accuracy of altimeter data in the study region has been checked using measurements from a bottom pressure recorder deployed in Capraia Island (rms difference is found to be ∼2 cm after application of a 30‐day half‐amplitude Gaussian filter). Because of the lack of an accurate geoid, the total water transport cannot be adequately monitored by satellite altimetry alone. However, the recovered signal can be directly related to the water transport anomaly through the channel. The resulting T/P signal can be considered as representing, to a great extent, the real steric variation induced by the net sea surface heat fluxes. The high correlation found between the water transport and sea level difference anomalies confirms the results obtained by the climatological hydrography and provides new evidence that interannual differences in the sea level and water transport can be related to the different heat flux balance in the Tyrrhenian and Ligurian Seas. Moreover, it demonstrates, for the first time in the Mediterranean, the potential of satellite altimetry for long‐term monitoring of the water exchanges between adjacent basins.
Undertow over a barred beach
Authors: Faria A. F. Garcez , Thornton E. B. , Lippmann T. C. , Stanton T. P.
Published: February 2004

The spatial distribution of the mean cross‐shore flow (undertow) over a barred beach is examined with field data obtained on three energetic wave days during the Duck94 experiment. The vertical structure of the undertow is modeled using a turbulent eddy viscosity closure and includes the important effects of wave breaking (described using the roller concept) and convective acceleration of the current. Other than a more realistic description of observed turbulence variations, a depth‐dependent eddy viscosity (compared with a constant) does not improve the agreement between predicted and observed undertow profiles. The effect of using different boundary conditions is investigated by extending the formulations of Stive and Wind [1986] and Svendsen et al. [1987] to include random waves by ensemble averaging over the wave height distribution. The contribution of breaking wave rollers to the surface mass flux can be of the same order or greater than the contribution associated with the organized wave motion. The largest discrepancies between model predictions and observations occur over the sandbar, where the mass transport of the breaking waves appears to be underestimated.
Fronts, transport, and Weddell Gyre at 30°E between Africa and Antarctica
Authors: Park Young‐Hyang , Charriaud Edwige , Craneguy Philippe , Kartavtseff Annie
Published: February 2004

A detailed description of the frontal structure of major currents and estimates of transport between Africa and Antarctica at 30°E were made on the basis of a finely resolved hydrographic section made during the 1996 Civa‐2 cruise. Particular emphasis was put on a refinement of the eastern boundary of the Weddell Gyre by analyzing also supplementary hydrographic data from the 1993 Civa‐1 cruise and the best available historical hydrographic data in the Weddell and Enderby Basins. Contrary to the general belief, our results show that the Weddell Gyre extends far beyond 30°E, reaching at least as far east as the Enderby Land promontory (53°E). From this point, westward currents along the Antarctic continental margin constitute the gyre's southern boundary, thus closing the cyclonic circulation of the gyre at its easternmost end. As the most convincing evidence for this refined circulation scheme, significant westward transport south of 65°S has been identified from the Civa‐1 acoustic Doppler current profiler data. We have shown also, using an inertial jet model, that the blocked feature of the Weddell cold regime (∼57°S) to the west of 25°E is due to the topographic control of flow by local bathymetry. During the Civa‐2 cruise, ∼70% of the Antarctic Circumpolar Current (ACC) was concentrated near the Polar Front (51°S), with surface speeds reaching up to 70 cm s−1. The baroclinic current field in the subpolar area south of the Weddell Front (58°S for Civa‐2 and 57°S for Civa‐1) is featureless (<3 cm s−1), but yielding a nonnegligible eastward transport of ∼15 Sv (1 Sv = 106 m3 s−1). This bottom‐referenced baroclinic transport is entirely compensated by a substantial westward barotropic transport south of 65°S. The ACC transport across the 30°E section during the Civa‐2 cruise is estimated to be ∼160 Sv.
Spectral analysis of turbulent flow and suspended sediment transport over fixed dunes
Authors: Venditti Jeremy G. , Bennett Sean J.
Published: February 2004

Laboratory measurements of turbulent fluctuations in velocity and suspended sediment concentration were obtained synchronously over fixed two‐dimensional dunes in a sediment‐starved flow. Contour maps of turbulent flow parameters demonstrate that the flow separation cell and a perturbed shear layer are the main sources of turbulence production and that the distribution of suspended sediment is controlled by spatially dependent macro turbulent flow structures. Spectral analysis reveals that peak spectral energies generally occur at 1–2 Hz for the stream wise velocity component and 2–4 Hz for the cross‐stream and vertical velocity components. Spectra show larger and better defined energy peaks near the shear layer. Peak spectral energies for suspended sediment concentration occur near 1 Hz throughout the flow. Squared coherency values for cospectral analysis of velocity and sediment concentration are insignificant. Integral timescales for velocity range from 0.20 s for the streamwise component to 0.06 s for the cross‐stream and vertical components. Integral length scales for velocity range from 0.065 to 0.135 m for the streamwise component, which is comparable to flow depth, and from 0.020 to 0.030 m for the crossstream and vertical components, which is comparable to dune height. For suspended sediment concentration, integral timescales and length scales are similar to the streamwise velocity component.
Surface currents in the Canary Basin from drifter observations
Authors: Zhou Meng , Paduan Jeffrey D. , Niiler Pearn P.
Published: February 2004

Satellite‐tracked drifting buoys, deployed in the Canary Basin as part of the Subduction Experiment between July 1991 and October 1993 and the French Semaphore Experiment during October 1993, were used to obtain a description of surface currents and temperature in the Canary Basin. The study focuses on surface water convergence, eddy energy production, and heat transport. The Azores Current associated with the subtropical convergence zone is clearly visible at 34°N, and bifurcates around 22°W, with the major branch of the current circling the Madeira plateau and joining the Canary Current along the continental slope. Eddy kinetic energy maxima are found along the Azores Current. The mean current revealed a region of maximum convergence north of the Azores Current around longitude 29°W occurring with a negative heating anomaly and positive work done by the Reynolds stress. The southward meridional temperature fluxes in the Ekman layer (0–50 m) between 37°W and the African and European coast are estimated between −0.076±0.022×l015 W, produced by mean southward volume transport in our study area. The residual between local surface heat fluxes and horizontal convergence of heat implies a vertical heat convergence process associated with mesoscale temperature and flow fields.
Controls on floc size in a continental shelf bottom boundary layer
Authors: Hill Paul S. , Voulgaris George , Trowbridge John H.
Published: February 2004

Simultaneous in situ observations of floc size, waves, and currents in a continental shelf bottom boundary layer do not support generally accepted functional relationships between turbulence and floc size in the sea. In September and October 1996 and January 1997, two tripods were deployed in 70 m of water on the continental shelf south of Woods Hole, Massachusetts. On one a camera photographed particles in suspension 1.2 m above the bottom that had equivalent circular diameters larger than 250 μm, and on the other, three horizontally displaced acoustic current meters measured flow velocity 0.35 m above the bottom. The tripods were separated by ∼150 m. Typically, maximal floc diameter stayed relatively constant, around 1 mm, and it showed a dependence on turbulence parameters that was significantly weaker than that predicted by any model that assumes that turbulence‐induced stresses limit floc size. Occasionally, when waves and currents generated intense near‐bed turbulence, flocs were destroyed. These precipitous decreases in maximal floc size also were not predicted by conventional models. The correlation in time between episodes of floe destruction and elevated combined wave‐current stresses provides the first quantitative support for the hypothesis that floc size throughout bottom boundary layers can be controlled by breakup in the intensely sheared near‐bed region. These observations demand a reassessment of the forces limiting floc size in the sea, and they indicate the potential for significant simplifying assumptions in models of floc dynamics.
Ekman transports, tidal mixing, and the control of temperature structure in Australia's northwest waters
Authors: Godfrey J. S. , Mansbridge J. V.
Published: February 2004

Fifteen years of expendable bathythermograph (XBT) data from three lines that cross Australia's Northwest Shelf and the shallow part of the Arafura Sea, a region of broad continental shelf referred to below as “the Shelf,” have been used, with climatological salinity data, to estimate onshore geostrophic flow to the Shelf. A wind stress climatology is used for onshore Ekman transports. Onshore geostrophic flow in the top 200 m onto the whole Shelf has an annual mean of 3.1 Sv; it is weakest (0.8 Sv) in May–June and strongest (5.1 Sv) in January. The onshore Ekman transport has an annual mean of only −0.5 Sv, but its annual cycle almost completely cancels that of the geostrophic flow, so that the net onshore flow varies little through the year from its annual mean of 2.6 Sv. Most of this probably exits through the southern end of the shelf as the annual mean Leeuwin Current. The small annual cycle of the net onshore flow suggests that longshore flow at the northern end of the section has a similar seasonal cycle as flow at the southern end. We have not been able to test this idea or to obtain adequate mass budget closure owing to limitations and interannual variability in longshore current data. There is ∼1 Sv of onshore geostrophic flow below 200 m in winter onto the northern Shelf. We have used climatological heat fluxes and temperatures on the Shelf to estimate mean seasonal net heat export from the Shelf; export is positive throughout the year on the northern Shelf and is negative foi only 3 months or less on the southern Shelf. Despite the lack of mass closure we can infer (1) the Gentilli [1972] mechanism of alternating longshelf seasonal flow cannot explain all of the observations, (2) summer wind‐driven upwelling can export substantial amounts of heat from both the northern and southern Shelf, and (3) the deep winter inflow to the northern Shelf probably upwells into the Arafura Sea, where nutrient‐rich inflow has been observed [Rochford, 1966]. This flow has the right magnitude to supply westward Ekman transports in winter between Irian Jaya and Australia. We describe some striking observations of internal and bottom mixed layers; bottom mixed layer formation may permit the deep inflow to warm as much as 8°C before it flows off the Shelf. It may also permit upwelling of the Leeuwin Undercurrent.
A reconstruction of large‐scale circulation in the Pacific Ocean north of 10°N
Authors: Yaremchuk M. I.
Published: February 2004

A dynamically consistent steady state circulation pattern of the North Pacific (NP) ocean is presented. The fields describing oceanic state and atmospheric forcing are obtained by fitting a steady state large‐scale circulation model to the 1.0 release of World Ocean Circulation Experiment hydrology, da Silva atmospheric climatologies, TOPEX‐Poseidon altimetry, and Marine Environmental Data Service drifter data. Heat, salt, and mass transports of the major circulation features of the NP are presented. All the estimates are supplied with error bars obtained via the approximate inversion of the Hessian matrix. The optimized pattern shows, in particular, that NP is heated at an average rate of 11±7 W m−2 and loses 26±18 cm of fresh water from its surface per year. Zonal mean transport north of 20°N is characterized by an upwelling of 7±2 Sv (1 Sv = 106 m3 s−1) in the intermediate layers (500–3000 m) within the latitudes 30°–50°N. A weaker overturning cell characterized by the downwelling of 3±1 Sv at 55°–60°N contributes to the meridional circulation pattern north of 51°N. The net meridional advective heat transport in the midlatitudes is statistically undistinguishable from zero and amounts to −0.1±0.4 PW (1 PW = 1015 W). At 35°N, fresh water is transported southward at the rate equivalent to the average rainfall to the north of 35°N of 18±14 cm yr−1. Analysis of the water mass transformation rates reveals the major NP intermediate water formation site to be east of Hokkaido and South Kuril Islands. A much weaker source is detected in the Alaskan Gyre. The net production rate of the NPIW source water east of Kuril Islands is estimated as 2.5 Sv.
Flow through Denmark Strait
Authors: Käse Rolf. H. , Oschlies Andreas
Published: February 2004

On the basis of hydrographic observations taken in the vicinity of Denmark Strait, a primitive equation model is used to investigate physical mechanisms that control the exchange through the strait. The dense water transport is topographically controlled and predictions by Whitehead [1998] and Killworth and McDonald [1993] are consistent with numerical model results. The distribution of temperature and thickness of the modeled plume is in good agreement with the high‐resolution hydrographic data.
Tidal dynamics in the northern Adriatic Sea
Authors: Malačič Vlado , Viezzoli Dino , Cushman‐Roisin Benoit
Published: February 2004

Tides in the northern Adriatic Sea are investigated using two distinct numerical models. First, a two‐dimensional (2‐D) finite difference model is implemented with very high horizontal resolution (556 m) to simulate the northern Adriatic. After calibration of open boundary conditions the model gives very satisfactory results: The averaged vectorial difference between observed and simulated elevations is <1.3 cm for each of the seven major tidal constituents. Next, a 3‐D finite element model is applied to the entire sea in order to provide a better simulation of the tidal currents in the vicinity of the open boundary of the first model. Results show that the northern Adriatic behaves like a narrow rotating channel in which the instantaneous sea surface elevation (SSE) contours are aligned with the depth‐averaged velocity vectors and in which the SSE is always higher to the right of the local current. These features emphasize the rotational character that tides can exhibit in a relatively small basin. Wave fitting to the current elevation structure shows that semidiurnal tidal constituents are well represented with a system of two frictionless Kelvin waves (incident and reflected). In contrast, the diurnal constituents are best described as a topographic wave propagating across, not along, the basin, from the Croatian coast to the Italian shore. Despite this obvious disparity the semidiurnal and diurnal tides can be understood as distinct members of a single family of linear waves, which exist under the combined actions of gravity and topography.
Flow near submarine canyons driven by constant winds
Authors: She Jun , Klinck John M.
Published: February 2004

Circulation over coastal submarine canyons driven by constant upwelling or downwelling wind stress is simulated and analyzed with a primitive equation ocean model. Astoria Canyon, on the west coast of North America, is the focus of this study, and model results are consistent with most major features of mean canyon circulation observed in Astoria Canyon. Near‐surface flow crosses over the canyon, while a closed cyclone occurs within the canyon. Upwelling prevails within the canyon and is larger than wind‐driven upwelling along the adjacent shelf break. Water rises from depths reaching 300 m to the canyon rim and, subsequently, onto the adjacent shelf. Onshore flow within the canyon is driven by the onshore pressure gradient force, due to the free surface slope created by the upwelling wind, and is enhanced by the limitation to alongshore flow by the canyon topography. Density gradients largely compensate the surface slope with realistic stratification, but continual upwelling persists near the edges of the canyon. Within the upper canyon (50–150 m below the canyon rim) a cyclone is created by flow turning into the canyon mouth, separating from the upstream edge, and advecting toward the downstream rim. Below this layer the cyclone is created by vortex stretching due to the upwelling. Downwelling winds create nearly the opposite flow, in which compression and momentum advection create a strong anticyclone within the canyon. Momentum advection is found to be important both in creating strong circulation within the canyon and in allowing the surface flow to cross the canyon undisturbed. Model results indicate that Astoria‐like submarine canyons produce across shore transport of sufficient volume to flush a continental shelf in a few (2–5) years.
Thermohaline variability of the western tropical Pacific during 1995–1998: On the erosion/reconstitution of the fresh pool
Authors: Ioualalen Mansour , Hénin Christian
Published: February 2004

The thermohaline variability of the western tropical Pacific at an El Niño‐Southern Oscillation (ENSO) timescale is investigated for the 1995–1998 ENSO period along three transects, relying mostly on thermosalinograph and expandable bathythermograph data distributed along three cross‐equatorial shipping lines. The water masses crossing these transects are studied chronologically by using estimates of wind forcing and precipitation. Zonal advection tends to shift eastward (westward) the fresh pool of the western Pacific and the zonal salinity front at its eastern edge during an El Niño (a La Niña) event. Because of increased wind stress at the beginning of an El Niño episode, the fresh pool can be destroyed by upwelling of saltier water and increased evaporation. It is, however, reconstructed further on its way to the east, by more favorable wind and precipitation conditions. This erosion/reconstruction process is then discussed in relation to ENSO development.
A one‐dimensional percolation model of flooding and snow ice formation on Antarctic sea ice
Authors: Maksym Ted , Jeffries Martin O.
Published: February 2004

A one‐dimensional thermodynamic sea ice growth model is developed with emphasis on seawater flooding and snow ice formation on Antarctic sea ice. To examine the possibility that flooding is the result of brine percolation through a porous ice matrix, and the consequences of this percolation on the salinity structure and mass balance of the ice, flooding is modeled in a semiheuristic fashion by examining two possible regimes for seawater and brine infiltration through the porous ice into the snowpack. A standard model assumes that the upward flow of brine is horizontally homogeneous and brine flow is modeled using a Darcian scheme. A simple model assumes that the infiltrating brine does not interact with the internal ice brine network, but instead percolates upward through cracks, or is restricted in spatial extent. Results of simulations are shown for the climatology in the Ross Sea and compared to field data from one autumn and one winter cruise aboard the R/V Nathaniel B. Palmer in 1995. Sensitivity studies are performed to show the effects of variations in climate forcing. The dependence of brine percolation and snow ice formation on key thermophysical parameters is investigated. Results indicate that the mass balance depends very strongly on the brine flow regime. In the standard model, ice salinities and brine volumes are greatly affected by brine percolation, creating a strong feedback between flooding and future snow ice production. Snow ice formation for the simple model was much more predictable and depended primarily on snow load. Comparison of results with field data indicate that if brine percolation is the major mechanism for flooding and snow ice formation, then there must be either substantial convective redistribution of salt to maintain ice porosity, or the flow must be spatially inhomogeneous.
The role of the North Aegean in triggering the recent Eastern Mediterranean climatic changes
Authors: Zervakis Vassilis , Georgopoulos Dimitris , Drakopoulos Panos G.
Published: February 2004

Drastic changes have occurred in the vertical structure of the deep waters of the eastern Mediterranean in the early 1990s, as dense water of Aegean origin has displaced lighter waters of Adriatic origin at the bottom of the deep basins. This work suggests that the initiation of this process took place in the North Aegean in the winter of 1986/1987 and was intensified by another formation event in 1992/1993. The available observations from the North Aegean support such a scenario. Furthermore, we propose that the outflow of Black Sea waters into the Aegean through the Dardanelles could act as an insulator of the deeper layer from the atmosphere, thus absorbing a large part of the heat and salt exchange; despite this fact, the existence of the densest bottom water of the Mediterranean in the North Aegean, and the continuation of density increase for a large period of time, suggests that it is a region of formation, thus that the insulation layer may at times be penetrated. We suggest that reduced Black Sea outflow into the North Aegean could facilitate dense water formation during the passage of cold atmospheric fronts in the winter.
Near‐bottom flow and flow resistance for currents obliquely incident to two‐dimensional roughness elements
Authors: Barrantes Analía I. , Madsen Ole S.
Published: February 2004

Measurements of the three components of the velocity when a steady current is flowing over a rippled bottom are presented. The bottom consisted of equally spaced triangular bars placed at angles of 0°, 30°, 45°, and 60° to the incident flow. The experiments show that close to the bottom, there is a transverse component of the velocity, which, for the larger angles of incidence, can be of the same order of magnitude as the main flow. The spatial average velocity profiles obtained from the measurements performed between two ripple crests show that close to the bottom, there is a well‐defined logarithmic region. The bottom roughness obtained by fitting a logarithmic law in the near‐bottom region to the longitudinal component of the flow depends strongly on the angle of incidence. In contrast, the analysis of the near‐bottom velocity component in the direction perpendicular to the ripple axis shows that the bottom roughness is independent of the angle of incidence. These experimental observations support the concept of a direction‐dependent equivalent bottom roughness when currents are obliquely incident to two‐dimensional bottom roughness elements, such as wave‐generated ripples.
Response of a river plume during an upwelling favorable wind event
Authors: Fong Derek A. , Geyer W. Rockwell
Published: February 2004

The response of a surface‐trapped river plume to an upwelling favorable wind is studied using a three‐dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the cross‐shore Ekman transport. The thinned plume is susceptible to significant mixing because of the vertically sheared horizontal currents. The Ekman dynamics and shear‐induced mixing cause the plume to evolve to a quasi‐steady uniform thickness, which can be estimated by a critical Richardson number criterion. Although the mixing rate decreases slowly in time, mixing continues under a sustained upwelling wind until the plume is destroyed. Mixing persists at the seaward plume front because of an Ekman straining mechanism in which there is a balance between the advection of cross‐shore salinity gradients and vertical mixing. The plume mixing rate observed is similar to the mixing law obtained by previous studies of one‐dimensional mixing, although the river plume mixing is essentially two‐dimensional.
Flow of bottom water in the northwestern Weddell Sea
Authors: Fahrbach Eberhard , Harms Sabine , Rohardt Gerd , Schröder Michael , Woodgate Rebecca A.
Published: February 2004

The Weddell Sea is known to feed recently formed deep and bottom water into the Antarctic circumpolar water belt, from whence it spreads into the basins of the world ocean. The rates are still a matter of debate. To quantify the flow of bottom water in the northwestern Weddell Sea data obtained during five cruises with R/V Polarstern between October 1989 and May 1998 were used. During the cruises in the Weddell Sea, five hydrographic surveys were carried out to measure water mass properties, and moored instruments were deployed over a time period of 8.5 years to obtain quasi‐continuous time series. The average flow in the bottom water plume in the northwestern Weddell Sea deduced from the combined conductivity‐temperature‐depth and moored observations is 1.3±0.4 Sv. Intensive fluctuations of a wide range of timescales including annual and interannual variations are superimposed. The variations are partly induced by fluctuations in the formation rates and partly by current velocity fluctuations related to the large‐scale circulation. Taking into account entrainment of modified Warm Deep Water and Weddell Sea Deep Water during the descent of the plume along the slope, between 0.5 Sv and 1.3 Sv of surface‐ventilated water is supplied to the deep sea. This is significantly less than the widely accepted ventilation rates of the deep sea. If there are no other significant sources of newly ventilated water in the Weddell Sea, either the dominant role of Weddell Sea Bottom Water in the Southern Ocean or the global ventilation rates have to be reconsidered.
Temporal and vertical variability in optical properties of New England shelf waters during late summer and spring
Authors: Sosik Heidi M. , Green Rebecca E. , Pegau W. Scott , Roesler Collin S.
Published: February 2004

Relationships between optical and physical properties were examined on the basis of intensive sampling at a site on the New England continental shelf during late summer 1996 and spring 1997. During both seasons, particles were found to be the primary source of temporal and vertical variability in optical properties since light absorption by dissolved material, though significant in magnitude, was relatively constant. Within the particle pool, changes in phytoplankton were responsible for much of the observed optical variability. Physical processes associated with characteristic seasonal patterns in stratification and mixing contributed to optical variability mostly through effects on phytoplankton. An exception to this generalization occurred during summer as the passage of a hurricane led to a breakdown in stratification and substantial resuspension of nonphytoplankton particulate material. Prior to the hurricane, conditions in summer were highly stratified with subsurface maxima in absorption and scattering coefficients. In spring, stratification was much weaker but increased over the sampling period, and a modest phytoplankton bloom caused surface layer maxima in absorption and scattering coefficients. These seasonal differences in the vertical distribution of inherent optical properties were evident in surface reflectance spectra, which were elevated and shifted toward blue wavelengths in the summer. Some seasonal differences in optical properties, including reflectance spectra, suggest that a significant shift toward a smaller particle size distribution occurred in summer. Shorter timescale optical variability was consistent with a variety of influences including episodic events such as the hurricane, physical processes associated with shelfbreak frontal dynamics, biological processes such as phytoplankton growth, and horizontal patchiness combined with water mass advection.
Isotopic fractionation through water vapor condensation: The Deuteropause, a cold trap for deuterium in the atmosphere of Mars
Authors: Bertaux Jean‐Loup , Montmessin Franck
Published: February 2004

Recent observations of deuterium atoms at Lyman alpha with the Hubble Space Telescope have shown the puzzling result of a D/H ratio in the upper atmosphere of Mars to be 11 times smaller than the D/H ratio in HDO and H2O of the lower atmosphere [Krasnopolsky et al., 1998]. One factor to explain this vertical variation was proposed: the photo‐induced fractionation effect (PHIFE), due to a lower absorption cross section of solar UV of HDO compared to H2O [Cheng et al., 1999]. Here we suggest that in addition to PHIFE, this vertical variation of D/H ratio is also the result of preferred condensation of HDO in rising air, a process already documented in the Earth's upper stratosphere. Results of a cloud model, including condensation, sedimentation, and sublimation of icy particles in the atmosphere of Mars, are presented, supporting the efficiency of the condensation/evaporation fractionation effect (CEFE) as an important factor controlling the D/H ratio in the upper atmosphere of Mars. For a typical H2O profile, PHIFE provides a depletion factor of 2.5, while CEFE induces a factor of 3.5. The combined effects of PHIFE and CEFE (depletion factor of 9.5) can explain the paucity of D atoms in the upper atmosphere of Mars, implying a very low escape rate of deuterium at present and in the past and, correspondingly, a smaller quantity of H2O in the past.
Production of cometary X rays by electron impact and Bremsstrahlung processes
Authors: Krasnopolsky Vladimir A.
Published: February 2004

Bremsstrahlung and electron impact excitation are suggested in a few papers to be the main processes for generation of X rays in comets. To check this hypothesis, we determined a peak X‐ray volume emission rate in comet Hyakutake during the Roentgen satellite (ROSAT) observation. We found that scaling of the electron fluxes observed in comet Halley from the Vega probe to the known properties of the solar wind electrons agrees with theory of electron acceleration by lower hybrid waves in comets [Shapiro et al., 1999]. Using those scaled electron fluxes, we calculated Bremsstrahlung at the peak brightness in comet Hyakutake at a level of 0.2–0.3% of the observed peak emission. Bremsstrahlung in comet Hale‐Bopp constitutes 4% of the X‐ray emission observed with the EUVE, and the Bremsstrahlung spectrum, calculated for that comet by Shapiro et al. [1999], needs corrections. Emissions of the K‐lines of O 525 eV and C 277 eV, excited by energetic electron impact in comet Hyakutake, are at a level of 1.4–2.2% of the observed X‐ray emission. The calculation of energetic electron impact excitation in cometary oxygen by Shapiro et al. [1999] does not result in purely X‐ray emission but mostly results in visible and ultraviolet emission with just a small contribution at 525 eV which is equal to 1.7% of the total emission. With the Giotto electron fluxes the X‐ray emissions from Bremsstrahlung and electron impact would be smaller by orders of magnitude than the above values. The X‐ray luminosity associated with the energetic electron mechanism depends on the gas production rate and on the heliocentric distance in a different way than does the luminosity associated with the charge transfer mechanism. A test for these dependences using our EUVE observations rules out the energetic electron excitation mechanism on this basis. There are no papers that have adequately justified energetic electrons as the main source of X rays in comets.
Foreword [to Special Section on Insights From the Polar Spacecraft]
Authors: Giles Barbara L. , Hoffman Robert A.
Published: February 2004

The International Solar Terrestrial Physics (ISTP) program [Alexander and Nishida, 1984, p. 1] defined an ambitious goal: to develop a comprehensive, global understanding of the generation and flow of energy from the Sun through the Earth s space environment (geospace) and to define the cause‐and‐effect relationships between the physical processes that link different regions of this dynamic environment. Four ISTP spacecraft were launched to accomplish the objectives of the program, two into the solar wind, Solar and Heliospheric Observatory (SOHO) from the European Space Agency (ESA) [Fleck et al., 1995] and Wind [Russell, 1995] from NASA, and two within the magnetosphere, Geotail from the Institute of Space and Astronautical Science (ISAS) [Terasawa and Kamide, 1994] and Polar from NASA [Russell, 1995]. Within this fleet the Polar spacecraft has had the responsibility for measuring the entry of plasma into the Polar magnetosphere and the geomagnetic tail, the flow of plasma to and from the ionosphere, and the deposition of particle energy in the ionosphere and upper atmosphere [Hoffman et al., 1996]. Because of common science interests, the Polar Science Team looks with anticipation toward coordinated activities with the recently launched four Cluster spacecraft mission by ESA [Escoubet et al., 1997], which will become an integral part of the ISTP program.
Voyager and Pioneer spacecraft measurements of cosmic ray intensities in the outer heliosphere: Toward a new paradigm for understanding the global modulation process: 2. Maximum solar modulation (1990–1991)
Authors: Webber W. R. , Lockwood J. A.
Published: February 2004

In this paper we present interplanetary radial intensity profiles of five particle energies/species of cosmic rays measured by the IMP, Voyager 1, Voyager 2, and Pioneer 10 spacecraft at the time of maximum solar modulation in 1990–1991. These radial profiles are then compared with those measured in 1987 at a period of minimum modulation in order to examine the radial dependence of the 11‐year solar modulation effects. It is found that the radial intensity profiles observed in 1990–1991 can be described to first order by simple reduction factors of the radial profiles observed in 1987. These reduction factors range from ∼2.5 at the highest rigidities to ∼10 at the lowest rigidities measured here. These reduction factors describe the magnitude of the 11‐year modulation and imply that the intensities at the estimated location of the termination shock (TS), here taken to be at 88 AU, are reduced by a corresponding factor to those at the Earth at each energy. Therefore these TS intensities range from only 0.4 to 0.05 of the estimated IS intensities at the highest to lowest rigidities, respectively. This implies that most of the overall (11‐year and residual) modulation at this time and at these rigidities is occurring beyond the TS. Some of this 11‐year modulation (∼50% or less) occurs within this location. This part of the modulation is usually described in the present modulation models by invoking changing heliospheric current sheet tilt and changes in the drift and diffusion coefficients. However, overall, the 11‐year solar modulation, at least at the rigidities examined here, is most properly described as an outer heliospheric modulation phenomenon occurring mainly at or beyond the TS with a secondary (weaker) modulation occurring inside the TS. This is similar to the situation described by Webber and Lockwood [this issue], where the dominant modulation at times of minimum (residual) solar modulation also appears to occur in the outer heliosphere near to or beyond the TS. This large outer heliosphere modulation remains largely unexplained in present modulation theories. A new paradigm in solar modulation theory is required to more fully account for the large fraction of the 11‐year and/or residual solar modulation that is occurring in the outer heliosphere near to or beyond the TS.