bluuebunny/crossref_metadata_2025 huggingface dataset:
The thermal oceanographic signal of El Niño reconstructed from a Kiritimati Island coral
Authors: Evans Michael N. , Fairbanks Richard G. , Rubenstone James L.
Published: September 2002

Central equatorial Pacific sea surface temperature (SST) anomaly is a critical predictor of basin‐wide oceanographic and atmospheric effects of the El Niño‐Southern Oscillation (ENSO) phenomenon. We employ two geochemical thermometers measured on coralline aragonite to reconstruct an independent proxy‐based measure of central equatorial Pacific sea surface temperature anomaly. In addition, we assess the observational error associated with extraction of large‐scale SST anomalies from δ18O and Sr/Ca measurements. On the basis of paired data for the 1981–1987 period, RMS error for the estimation of SST from Kiritimati coral δ18O is about 0.4°C, assuming no seawater δ18O influence; for Sr/Ca, the observational error is about 0.5°C. Singular spectrum analysis of the δ18O time series suggests that 1/3 of the variance is explained by SST anomaly and that this variance may be separated from other signals in the frequency domain. The interannual component of the δ18O record shares 70% variance with the interannual component of local SST anomaly estimates (ρ=−0.84) and correlates as highly with NINO3 region (150°W–90°W, 5°N–5°S) SST anomaly estimates; comparison of annual averages of the δ18O data with analyzed SST for 55 years suggests that the error in deriving annually averaged SST anomalies at Kiritimati is about 0.4°C. A global SST correlation analysis suggests significant correlation with the rest of the Pacific Basin and the tropical Indian and Atlantic sectors. The ocean‐wide level of correlation achieved using the coral data is indistinguishable from that achieved using the NINO3 SST anomaly index and suggests that a few well‐located coral reconstructions with low observational error may be sufficient to reconstruct the global SST anomaly field associated with ENSO activity.
Coupled sea surface‐atmosphere model: 1. Wind over waves coupling
Authors: Makin V. K. , Kudryavtsev V. N.
Published: September 2002

A wind over waves coupling scheme to be used in a coupled wind waves‐atmosphere model is described. The approach is based on the conservation of momentum in the marine atmospheric surface boundary layer and allows to relate the sea drag to the properties of the sea surface and the properties of the momentum exchange at the sea surface. Assumptions concerning the local balance of the turbulent kinetic energy production due to the mean and the wave‐induced motions, and its dissipation, as well as the local balance between production and dissipation of the mean wave‐induced energy allow to reduce the problem to two integral equations: the resistance law above waves and the coupling parameter, which are effectively solved by iterations. To calculate the wave‐induced flux, the relation of Plant [1982] for the growth rate parameter is used. However, it is shown by numerical simulations that the local friction velocity rather than the total friction velocity has to be used in this relation, which makes the growth rate parameter dependent on the coupling parameter. It is shown that for light to moderate wind a significant part of the surface stress is supported by viscous drag. This is in good agreement with direct measurements under laboratory conditions. The short gravity and capillary‐gravity waves play a significant role in extracting momentum and are strongly coupled with the atmosphere. This fact dictates the use of the coupled short waves‐atmosphere model in the description of the energy balance of those waves.
Rafting and ridging of thin ice sheets
Authors: Hopkins Mark A. , Tuhkuri Jukka , Lensu Mikko
Published: September 2002

Rafting and pressure ridging are important processes in the deformation of sea ice that occur when two ice sheets are pushed together. In this study a two‐dimensional computer model of the rafting and ridging process is used to simulate a situation in which two identical ice sheets are pushed together at constant speed. Each model ice sheet is composed of two thicknesses of ice. The ratio of the thicknesses is varied to obtain degrees of inhomogeneity. The accuracy of the simulations is assessed by comparison with a series of similar physical experiments performed in a refrigerated basin. Following this comparison, the computer model is used to perform an extensive series of simulations to explore the effect of the thickness and the thickness inhomogeneity of the model ice sheets on the likelihood of occurrence of ridging and rafting. During the simulations the energy consumption and forces are explicitly calculated. The energy consumed during the simulations is used to demonstrate the smooth transition between ridging and rafting that occurs when the homogeneity of the sheets is varied.
Equatorial currents in the Pacific Ocean 1992–1997
Authors: Seidel Howard F. , Giese Benjamin S.
Published: September 2002

A medium resolution ocean general circulation model of the tropical Pacific Ocean is used to explore current structure and variability on the equator for the period from 1992 to 1997. The model assimilates surface and subsurface temperature data from expendable bathythermographs and the Tropical Ocean Global Atmosphere‐Tropical Atmosphere Ocean (TOGA‐TAO) moorings and altimetry data from the TOPEX/Poseidon satellite. Currents from the model are compared with current observations from the TOGA‐TAO moorings at 110°W, 140°W, and 165°E. Since current data are not assimilated, these data provide an independent verification of the model results. The comparison shows that the model produces accurate currents over a wide range of spatial and temporal scales. In particular, the model correctly resolves temporal variability from instability waves with periods of less than a month to interannual changes with periods of several years. The model results are used to explore zonal velocity changes associated with the onset of the 1997–1998 El Niño‐Southern Oscillation (ENSO) event. The event began with a series of equatorial Kelvin waves which were excited by westerly wind bursts in the western Pacific and propagated to the eastern Pacific. The passage of these waves is associated with an initial strengthening of the Equatorial Undercurrent (EUC) followed by a rapid shutdown of the EUC. The weakening of the Undercurrent during ENSO is consistent with previous descriptions and results from the collapse of the easterly trade winds in the western Pacific.
Air/sea momentum transfer and the microwave cross section of the sea
Authors: Plant W. J. , Weissman David E. , Keller William C. , Hessany Vahid , Hayes Kenneth , Hoppel K. W.
Published: September 2002

Measurements of atmospheric fluxes of heat, moisture, and momentum were made simultaneously and coincidentally with microwave backscatter measurements from an airship flown over the Pacific Ocean in 1993. The measurement technique was well suited to measure fluxes at very low wind speeds because the airship required an air speed near 10 m s−1 in order to maintain altitude. The measurements show that very low wind speeds are always associated with very low microwave cross sections and very high air/sea drag coefficients. The occurrence of regions of very low wind speed is not usually correlated with either the sea surface temperature or the air/sea temperature difference. Nevertheless, these regions can remain in place for time periods of several hours. The rate of increase of the microwave cross section at very low wind speeds agrees with that predicted by Donelan and Pierson [1987], but the absolute value of the threshold wind speed appears to be lower than their prediction. The high drag coefficient at low wind speeds is due to the fact that the friction velocity is nearly constant for wind speeds below 4–5 m s−1. Thus, at these wind speeds the increase of the microwave cross section follows the behavior of the wind speed rather than the wind stress. At higher wind speeds, however, the behavior is reversed with the cross section following the wind stress at a constant wind speed. We suggest that this behavior can be understood if momentum transfer across the air/sea interface is supported by both viscosity and the entire spectrum of waves on the surface, as many investigators have indicated.
Effect of barotropic tidal rectification on low‐frequency circulation near the shelf break in the northern Bay of Biscay
Authors: Pérenne Nicolas , Pichon Annick
Published: September 2002

Current meter data collected in the Bay of Biscay are used with a barotropic model to study the effect of the tidal rectification on the low‐frequency circulation near the shelf break. Data collected at two different locations are analyzed and show that the tidal forcing terms and the low‐frequency circulation are completely different at the two locations. In order to obtain a quantitative interpretation of these differences, a tidal forcing with both M2 and S2 waves is introduced in a barotropic tidal rectification model. We focus on the rectification process due to the “stretching of the water columns” alone, in the realistic case of large‐amplitude topography. A first theoretical result is that the bichromatic forcing generates a nonlinear variation of the residual current which depends on only one nondimensional parameter. This parameter is a function of the barotropic flow and of the wave frequencies. A second result is that the low‐frequency circulation generated by such a rectification process (including the two waves) is twice as large in spring tide than in neap tide. Finally, the model applied to both moorings confirms that the tidal rectification is much weaker at one mooring, the northwest one, than at the other, where the tidal rectification can explain 50% of the observed low‐frequency circulation.
Mesoscale eddies, satellite altimetry, and new production in the Sargasso Sea
Authors: Siegel David A. , McGillicuddy Dennis J. , Fields Erik A.
Published: September 2002

Satellite altimetry and hydrographic observations are used to characterize the mesoscale eddy field in the Sargasso Sea near Bermuda and to address the role of physical processes on the supply of new nutrients to the euphotic zone. The observed sea level anomaly (SLA) field is dominated by the occurrence of westward propagating features with SLA signatures as large as 25 cm, Eulerian temporal scales of roughly a month, lifetimes of several months, spatial scales of ∼200 km, and a propagation of ∼5 cm s−1. Hydrographic estimates of dynamic height anomaly (referenced to 4000 dbar) are well correlated with satellite SLA (r2 = 0.65), and at least 85% of the observed dynamic height variability is associated with the first baroclinic mode of motion. This allows us to apply the satellite observations to remotely estimate isopycnal displacements and the flux of nutrients into the euphotic zone due to eddy pumping. Eddy pumping is the process by which mesoscale eddies induce isopycnal displacements that lift nutrient‐replete waters into the euphotic zone, driving new primary production. A kinematic approach to the estimation of the eddy pumping results in a flux of 0.24 ± 0.1 mol N m−2 yr−1 (including a scale estimate for the small contribution due to 18° water eddies). This flux is more than an order of magnitude larger than the diapycnal diffusive flux as well as scale estimates for the vertical transport due to isopycnal mixing along sloping isopycnal surfaces. Eddy pumping and wintertime convection are the two dominant mechanisms transporting new nutrients into the euphotic zone, and the sum of all physical new nutrient supply fluxes effectively balances previous geochemical estimates of annual new production for this site. However, if biological transports (e.g., nitrogen fixation, etc.) are significant, the new nitrogen supply budget will be in excess of geochemical new production estimates. This suggests that the various physical and biological transport fluxes, as well as geochemical inferences of new production, still need to be reconciled and many outstanding questions remain.
Net sediment transport under wave groups and bound long waves
Authors: Deigaard Rolf , Jakobsen Jan Borup , Fredsøe Jørgen
Published: September 2002

The net sediment transport under regular waves and under wave groups with and without bound long waves is investigated by mathematical modeling. The bed is assumed to be plane (sheet flow conditions), which occurs for Shields parameters larger than about 1. The turbulent boundary layer flow is modeled by a mixing length model. The suspended sediment is modeled by the advection diffusion equation. The net sediment transport due to streaming, mass drift, and bound long waves is considered. It is found that the bound long waves cause an additional forward motion of the water near the bed. The bound long waves further give a negative contribution to the sediment transport because the backward motion in the long waves is coupled with the high waves and high sediment concentrations. The combined net sediment transport is found to depend on the grain size and the transport intensity: For very fine sand the mean water motion and the forward drift of the sediment dominate, giving a positive net transport. For coarser sand the coupling between backward motion and high sediment concentration dominates, giving a negative contribution. A parameter study is carried out to investigate the dependence of the net sediment transport on variations of the dimensionless parameters that characterize the condition.
Warm water formation and escape in the upper tropical Atlantic Ocean: 1. A literature review
Authors: Lee Sang‐Ki , Csanady G. T.
Published: September 2002

We review current understanding of the warm water formation and escape process in the tropical Atlantic Ocean and point out the physical aspects of the process not yet fully explored. From this review, we found a need for a simple model capable of describing the first‐order heat and mass balance in the tropical Atlantic Ocean. This line of work is continued in part 2 [Lee and Csanady, this issue].
Informational equivalence between synthetic aperture radar imagery and the thickness of Arctic pack ice
Authors: Kerman Bryan , Wadhams Peter , Davis Norman , Comiso Joey
Published: September 2002

The negative exponential form of the conditional probability of both differences in synthetic aperture radar (SAR) intensity and differences in thickness between neighbors is used as the basis of a representation of structural information in terms of textural information for sea ice. The analysis is based on a unique data set involving airborne SAR flights over the track of a submarine that simultaneously profiled ice thickness by means of an upwardly directed sonar. It is shown that both sets of data, namely, imagery and thickness, possess an identical conditional probability relationship. This probability is composed of both a fractal property for spatial separations and a simple negative exponential distribution for intensity or thickness differences for fixed separations. These properties allow for the derivation of two forms of information: textural and structural. An intercomparison of the structural information in terms of the textural information is offered for the same ice field as imaged by a SAR and measured from a submarine for its thickness. It is shown that the information curves for both the imagery and thickness have a similar geometrical form. It is demonstrated that three subranges within the information curves correspond to three visibly identifiable ice types and three thickness ranges associated previously with these ice types. It is suggested that an understanding of the transformation between the two sets of information states might provide estimated statistics of Arctic ice thickness from SAR imagery.
On the pressure gradient error in sigma coordinate ocean models: A comparison with a laboratory experiment
Authors: Kliem Nicolai , Pietrzak Julie D.
Published: September 2002

Boundary‐fitted vertical coordinates are frequently used in three‐dimensional primitive equation models. The sigma coordinate transformation, in which the water column is divided into a number of layers independently of the water depth, is commonly used. Unfortunately, it is well known that this transformation can produce an error in the calculation of the pressure gradient terms. The error can be significant in regions with steep topography and large density gradients. The importance of this error in simulations of coastal currents is difficult to test in reality. The competing effects of the models dynamics, its sensitivity, for example, to the chosen grid resolution or numerical techniques, are hard to separate. In the following we first identify the pressure gradient error and assess its magnitude in relation to two analytical reference solutions. A number of commonly used techniques to reduce the error are compared. Of these techniques, z‐level based pressure gradient calculations are shown to improve the simulation results. We then take a step in the direction of a more realistic simulation and use a laboratory experiment as our reference solution. The results demonstrate that a sigma coordinate model can be used in a region such as the Skagerrak where steep topography and large density gradients are present. Errors do exist which need to be taken into account when interpreting the model results. This is important, as the Skagerrak dominates the North Sea and Baltic Sea system, and models that run with a coarse grid resolution may lead to artificial flows.
Parameterizing particle dispersion in Langmuir circulation
Authors: Colbo Keir , Li Ming
Published: September 2002

Dispersion of neutrally buoyant and buoyant particles in Langmuir circulation is investigated using a numerical model. The crosswind diffusivity increases with decreasing Langmuir number (La) and approaches a constant at small La limit (i.e., high Reynolds number). Although Langmuir cells are more energetic at smaller values of La, the cells are less persistent: small cells are frequently regenerated and later merge with the existing ones. There is a stronger tendency for particles to move within individual cells, but this effect is offset by shorter lifetimes of the cells and frequent releases of particles for moving across the cells. The model results suggest a parameterization of the crosswind diffusivity that depends on the wind stress and Stokes drift current but is independent of the eddy viscosity. Buoyancy reduces the crosswind diffusivity, even for particles whose buoyancy rise speeds are significantly smaller than the downwelling velocity in Langmuir circulation.
Process studies on the Po River plume, North Adriatic Sea
Authors: Kourafalou Vassiliki H.
Published: September 2002

The coastal processes in the Adriatic Sea and in particular the development and evolution of the Po River plume are studied with a three‐dimensional, primitive equation model. Experiments are performed to examine the buoyancy‐driven flow and the interaction with topography, wind stress, and ambient stratification. The pathways of particles released from the Po are computed to simulate the transport of land‐drained materials. In the absence of wind forcing, the Po River plume consists of an offshore bulge that occupies most of the north part of the basin and a southward coastal current that is confined within the narrow coastal region along the Italian peninsula. When all Adriatic rivers are included, basinwide, buoyancy‐driven, cyclonic coastal flow is established that diminishes the seaward removal of Po waters and promotes their southward advection through the coastal current. Wind stress modifies the above. Winds that are downwelling‐favorable for the Po discharge site (like Bora) restrict offshore plume development, enhance the southward coastal current, and confine the plume within the narrow western Adriatic shelf. Conversely, upwelling‐favorable winds (like Scirocco) eliminate the baroclinic coastal current and advect low‐salinity Po waters toward the shallow parts of the Northern Adriatic. When preexisting stratification of basin waters is described, the density field is influenced by both temperature and salinity. During winter, when the coastal waters tend to be colder and fresher than offshore waters, the cyclonic circulation is sustained owing to input from rivers and particularly the Po. The intense spatial and temporal variability of wind stress and river runoff and the resulting variability in the circulation of the coastal Adriatic Sea are elaborated through a realistic simulation during January 1994.
Renewal and circulation of intermediate waters in the Canadian Basin observed on the SCICEX 96 cruise
Authors: Smethie William M. , Schlosser Peter , Bönisch Gerhard , Hopkins Tom S.
Published: September 2002

During the summer of 1996 the nuclear submarine USS Pogy occupied a line of stations extending through the middle of the Canadian Basin between about 88°N, 44°W (Lomonosov Ridge) and about 78°N, 144°W (center of the Canada Basin). CTD/Niskin bottle casts extending to 1600 m were carried out at eight stations, providing the first high‐quality temperature, salinity, CFC, tritium, and 3He data obtained from this region, although XCTD data had previously been collected in this region. These data, along with data from stations at the basin boundary to the south and west, reveal the presence of well‐ventilated intermediate water beneath the halocline in the center of the Canada Basin, indicating renewal times of the order of 1–2 decades. The least ventilated intermediate water was observed at the northern end of the Canada Basin along the southern flank of the Alpha Ridge. Intermediate water is derived from the Atlantic Ocean and enters the Arctic Ocean through Fram Strait and the Barents Sea. It flows around the Arctic basins in boundary currents and splits in the eastern Amundsen Basin with one branch crossing the Lomonosov Ridge and flowing along the East Siberian continental slope and the other flowing along the Eurasian flank of the Lomonosov Ridge. From the 1996 Scientific Ice Expedition (SCICEX 96) observations we conclude that the branch that flows along the East Siberian continental slope transports this water to the Chukchi Rise, where it apparently enters the central Canada Basin with some flow continuing along the boundary to the southern Canada Basin. The Fram Strait Branch Water mixes extensively with waters from the Canadian Basin during its transit along the East Siberian continental slope, being diluted by a factor of about 5 by the time it reaches the central Canada Basin. The Barents Sea Branch Water does not undergo such extensive mixing and is diluted by a factor of only about 2 when it reaches the central Canada Basin.
Convective heat transfer over wintertime leads and polynyas
Authors: Andreas Edgar L. , Cash Benjamin A.
Published: September 2002

Leads and polynyas are areas of open water or thin ice surrounded by thicker ice. In winter they are ideal natural laboratories for studying convective heat transfer from the ocean. First, the relevant length scales are much larger than those possible in the laboratory. Second, the large water‐air temperature difference provides a wide range of unstably stratified conditions. Third, the surrounding sea ice is a very stable platform on which to place turbulence instruments. Here we analyze three data sets obtained over Arctic leads and polynyas in winter. First, we compute the bulk aerodynamic transfer coefficient for sensible heat at neutral stability appropriate at a reference height of 10 m, CHN10. For fetches over the warm lead or polynya larger than ∼100 m, CHN10 has the value typically reported over the open ocean at lower latitudes, 1.00 × 10−3. At fetches much <100 m, CHN10 can be as large as 1.8 × 10−3. That is, smaller leads and polynyas transfer sensible heat more efficiently than large ones, presumably because the transfer is by mixed free and forced convection. In the second part of the study, we look at this transition from forced to free convection as parameterized by C* in the free convection relation Nu = C*Ra1/3, where Nu is the Nusselt number and Ra is the Rayleigh number. We demonstrate that δ/L, where L is the Obukhov length and δ is the fetch‐dependent height of the thermal internal boundary layer, is a useful free convection parameter. At large −δ/L, our data show that C* approaches the often quoted limit of 0.15. For −δ/L roughly <6, a flow is exchanging heat by mixed free and forced convection, and C* can be substantially >0.15. Because we have data over a wide enough stability range to evaluate how C* depends on δ/L, in effect, we develop a new algorithm for computing sensible and latent heat transfer in fetch‐limited, convective conditions. This algorithm could be especially useful for sensing the turbulent heat fluxes over leads and polynyas remotely because it depends only weakly on surface level wind speed, which is difficult to determine remotely over an ice‐covered ocean.
Evidence of episodic on‐shelf flow in the southeastern Bering Sea
Authors: Stabeno P. J. , Van Meurs P.
Published: September 2002

Trajectories from satellite‐tracked drifters released over the continental slope (depth 250–500 m) in the southeastern Bering Sea revealed an episodic event of on‐shelf flow. The first set of seven drifters followed the isobaths northwestward at speeds of 20–40 cm s−1; the second set, released 12 days later, moved 60 km onto the shelf at speeds of 10–15 cm s−1. Meanders or eddies were evident in the trajectories as the drifters were advected onto the shelf. An anticyclonic eddy formed at the shoreward termination of on‐shelf flow (120 m isobath). The eddy dissipated in 8–10 days. Such events can be important mechanisms in replenishing nutrients and introducing oceanic plankton and larvae onto the shelf.
Ice production and deep‐water entrainment in shelf break polynya off Enderby Land, Antarctica
Authors: Ushio Shuki , Takizawa Takatoshi , Ohshima Kay I. , Kawamura Toshiyuki
Published: September 2002

Aircraft observations off Enderby Land, Antarctica, verified that a coastal polynya forms even in severe winters and that new ice production continuously occurs there. In the polynya, water temperature profiles were measured for the first time using the aircraft‐launched expendable bathythermograph (AXBT). The AXBT data show that the thickness of the winter mixed layer is 350 m or more and that the layer temperatures are near the freezing point. According to the water mass analysis of the winter mixed layer the active haline convection by the high ice production in the winter polynya contributes to the formation of more homogeneous and saline mixed layer than that under the fast ice cover, where the ice grows slowly and convective mixing is calm. Furthermore, the active convection leads to entrainment of the oxygen‐poor deep water under Winter Water. Consequently, in the polynya located over the continental shelf break, the oxygen content of the mixed layer is somewhat lower than that in a coastal polynya that forms on the continental shelf, where the haline convection reaches to the sea bottom. The oxygen undersaturation of the thick mixed layer in the shelf break polynya suggests that the deep‐water entrainment rate is more intense than that under the pack ice region in the Weddell Sea with a relatively thin mixed layer. It is, therefore, important to consider such polynya processes which may contribute to water mass modification in the Antarctic coastal region.
Ocean heat transport in the South Atlantic in a coupled climate model
Authors: Banks Helene T.
Published: September 2002

Ocean heat transport in the South Atlantic of a coupled climate model has been diagnosed and compared with observational estimates. The coupled model overestimates the northward heat transport in the South Atlantic. This corresponds to an underestimate of the northward flux of bottom water across World Ocean Circulation Experiment (WOCE) section A11 (the model transports less than 1 Sv compared with climatological estimates of 6 Sv). The magnitude of the southward outflow of North Atlantic deep water agrees well with observational estimates while the northward flux of surface and intermediate waters is larger than observational estimates. We show that with the correct water mass properties a realistic northward flux of bottom water across 30°S is only possible, in the climate model, if the depths of the channels linking the Argentine and Brazil basins are properly represented. We find that an increase in the northward flow of bottom water corresponds to an increase in the southward flow of deep water, rather than a reduction in the northward flux of surface and intermediate waters. This indicates that the upper and lower limb of the overturning are decoupled in the model. As a consequence, increasing the northward transport of bottom water leads to a small reduction of the northward heat transport across 30°S. The magnitude of the heat transport from the Indian to the Atlantic ocean in the model (the warm water path) is surprisingly large for a noneddy resolving model. We find that the magnitude of the Indonesian Throughflow is important in climate resolution models for determining the strength of the warm water path. However, the relative strengths of the warm and cold water paths do not significantly change the heat transported across 30°S.
Response of the Weddell Sea pack ice to wind forcing
Authors: Uotila J. , Vihma T. , Launiainen J.
Published: September 2002

Sea ice drift in the Weddell Sea was studied on the basis of data from seven buoys deployed on ice floes in January–February 1996. Six of the buoys formed an array with initial mutual distances of 50 km, which by October 1996 were stretched to ∼400 km in an east–west direction. The differential kinematical parameters i.e., divergence, shearing rate, and vorticity, were estimated from the buoy array positions by applying a time‐dependent method. The large‐scale drift of the array was divergent until August 1996, but after the array came under the influence of the Antarctic Circumpolar Current the deformation was mainly shearing. Meteorological data from European Centre for Medium‐Range Weather Forecasts analyses and Special Sensor Microwave Imager derived ice concentrations were interpolated at the buoy sites. The drift velocity was highly wind‐dependent; it was also wind‐dependent in winter when the momentum balance was mainly between the internal ice stress and the air drag. The drift divergence and shearing rate were related to the wind forcing, while the array vorticity correlated better with the air pressure itself. We estimated the air‐ice drag coefficient to be 1.8×10−3 at a height of 10 m over the ice and the geostrophic drag coefficient to be 6.1 × 10−4. The stability effects on the coefficients were mostly felt in conditions of light winds. The average ice‐water drag coefficient based on 5 day periods was 2.1×10−3. The ice transport through a transect crossing the Weddell Sea was computed on the basis of the geostrophic winds, observed wind‐drift dependence, SSM/I‐derived ice concentrations, and the literature on ice draft statistics. The resulting annual mean net export in 1996 was 1600 km3 yr−1.
Oceanographic tracer release experiments using sulphur hexafluoride
Authors: Watson Andrew J. , Ledwell James R.
Published: September 2002

We review recent results of experiments using sulfur hexafluoride tracer releases to investigate ocean mixing, gas exchange, and response to iron fertilization. A release method has been devised that allows large‐scale mixing experiments to be initiated using ∼ 100 kg or more of tracer accurately targeted on a given density surface. Combined with the very low detection limit of SF6, this means that these experiments can last up to several years and cover thousands of kilometers. The experiments have revealed that in the open ocean pycnocline, rates of mixing are low (0.1–0.2 cm2 s−1 at 300 m in the subtropical North Atlantic, for example). An approximately inverse relationship between diapycnal mixing and buoyancy frequency is suggested by most (but not all) tracer investigations to date, though this may not hold in the abyssal ocean far from any boundary. Much smaller scale releases into surface waters have been used to obtain measurements of gas exchange and to enable biogeochemical studies such as iron fertilization experiments on well‐defined volumes of water. These experiments take advantage of the fact that in surface water, continuous rapid analysis of sulfur hexafluoride enables the tracer to be tracked and mapped by the observing ship. Recent gas exchange studies using the “dual‐tracer” method are summarized; we suggest that they may be reconciled to values based on global 14C exchange by appealing to the effect of organic films observed in coastal waters by Frew [1997]. The tracer technique makes possible biogeochemical “patch” studies such as the Ironex I and II experiments in the equatorial Pacific. The use of the tracer measurements to guide and normalize measurements of the effect of added iron is illustrated.
Instabilities in the Agulhas Retroflection Current system: A comparative model study
Authors: Wells N. C. , Ivchenko V. O. , Best S. E.
Published: September 2002

The Agulhas Retroflection Current system is analyzed in two ocean general circulation models, U.K. Fine Resolution Antarctic Model (FRAM) and U.S. Parallel Ocean Program (POP). It is shown that both models have two quasizonal jets, a northerly westward flowing jet (Agulhas Current) and a more southerly eastward flowing jet (Agulhas Return Current), which act as the entrance and exits of the Retroflection region, respectively. An instability analysis has been performed on both jets, and the unstable waves are shown to be baroclinicly unstable. It is shown that this instability process is well resolved in the POP model but is poorly resolved in FRAM. An energy analysis of POP shows that both barotropic and baroclinic instability processes play a major role in the current systems, with the former process concentrated in the surface layers between the two currents, and the latter process occurring below the two currents. In FRAM a regional energy budget suggests that barotropic instability is the main mechanism by which the eddies are generated, and a meridional cross‐sectional analysis shows that this occurs principally on the northern edge of the Agulhas Return Current.
Large‐scale comparison between buoy and SSM/I drift and deformation in the Eurasian Basin during winter 1992–1993
Authors: Geiger Cathleen A. , Zhao Yunhe , Liu Antony K. , Häkkinen Sirpa
Published: September 2002

A method for comparing sea ice velocity, divergence, and shear at the large‐scale between buoys and Special Sensor Microwave Imager (SSM/I) is presented. For initial testing, the method is applied in the Eurasian Basin because of its relatively simple circulation dominated by the wind. Using eight Argos buoys, 11 strain rate arrays 100–600 km in size are constructed. Daily 100 km resolution sea ice motion derived from SSM/I 85 GHz brightness temperatures is sampled 100–1000 km from the center of the buoy arrays. Over this range of possible scales, a minimum RMS difference (RMSD) for deformation is used to identify an optimal inclusion radius of 600 km corresponding to a length scale of 1000 km. This length scale is typical of local storms confirming a strong connection between wind and observed sea ice motion. On the basis of all 11 arrays, an average RMSD of 2.48±0.05 cm s−1 for velocity vector and 8.8±0.9×10−8 s−1 using all four deformation components (∂ui/∂χj∥ is found at the optimal inclusion radius corresponding to average correlation coefficients of 0.896±0.002 and 0.729±0.030, respectively. RMSD are found to scale with the temporal and spatial uncertainties of the SSM/I suggesting that even better results can be achieved with higher resolution instruments.
Seasonal variability of bottom water properties off Adélie Land, Antarctica
Authors: Fukamachi Yasushi , Wakatsuchi Masaaki , Taira Keisuke , Kitagawa Shoji , Ushio Shuki , Takahashi Akiyoshi , Oikawa Kohshiro , Furukawa Teruo , Yoritaka Hiroyuki , Fukuchi Mitsuo , Yamanouchi Takashi
Published: September 2002

The region off Adélie Land is considered as one of the sources of Antarctic Bottom Water. Hydrographic observations were carried out during two cruises in December 1994 and January 1995 and January and February 1996 in this region. Vertical sections along 140°E show that bottom water is colder and fresher than the water above. This bottom water also has higher dissolved oxygen and lower silicate concentrations. The saline bottom water that originated from Ross Sea is not found in these hydrographic data obtained west of 142°E. Current meter moorings were also carried out at three locations on the continental slope in this region. At one of these moorings (139°59′E, 65°10′S, 2665 m deep), data were successfully obtained from January 1995 to March 1996. Three current meters were deployed at 1075, 1778, and 2632 m deep in this mooring. The data show that the average current speed at the lower current meter is 16.2 cm s−1, and it is about 3 times larger than those at the upper two current meters. Also, variability of speed and temperature is largest at the lower current meter. In addition, seasonal variability of speed and temperature is evident only at this current meter. From August to December, speed is larger by 5.7 cm s−1 and temperature is lower by 0.27°C. Also, their variability is larger during the same period. This seasonal variability observed near the bottom suggests seasonal variability of bottom water formation in this region.
Vertical turbulent diffusion and upwelling in Lake Baikal estimated by inverse modeling of transient tracers
Authors: Kipfer R. , Hofer M. , Peeters F. , Imboden D. M. , Domysheva V. M.
Published: September 2002

Vertical turbulent diffusion coefficients, upwelling velocities, and oxygen depletion rates are estimated by inverse modeling of the concentrations of CFC‐11 (CCl3F), CFC‐12 (CCl2F2), 3H, 3He, and dissolved oxygen for the southern, central, and northern basin of Lake Baikal. A model is developed that considers two regions in each basin of Lake Baikal: (1) a surface mixed layer (SML) 400 m thick and (2) a deepwater column (DWC) below 400 m. The SMLs are assumed to be well mixed. In each of the DWCs, passive tracers are transported by vertical turbulent diffusion and upwelling. Upwelling is generated by a depth‐dependent source of water because of density plumes propagating from the SML downward to larger depths. This water is considered to contain the same tracer concentrations as the SML. The tracer concentrations in the SMLs of the three basins are coupled to the atmosphere by gas exchange (including water vapor transport) and precipitation to the catchment by river inflow and outflow and to the neighboring basins via diffusive exchange and advection. SMLs and DWCs of the same basin are connected by vertical turbulent diffusion, density‐driven water transport, and upwelling. Beginning at the turn of this century, the tracers CFC‐11, CFC‐12, 3H and 3He are modeled simultaneously to predict modern concentrations. On the basis of the tracer data the vertical diffusion coefficient K, is determined to be 4.6×10−4 m2 s−1±10% for the southern, 6.3×10−4 m2 s−4±10% for the central, and 1.7×10−4 m2 s−4±25% for the northern basin. The vertical advective flux of water at 400 m water depth is calculated as 110 km3 yr−1 in the southern, 70 km3 yr−1 in the central, and 290 km3 yr−1 in the northern basin. Concentration of dissolved molecular oxygen is modeled by using the estimated transport parameters and by fitting for the unknown consumption rate. Inverse modeling of oxygen suggests that O2 depletion in the DWC can be described by a volume sink of 44±3 mgO2 m−3 yr−1 combined with an areal sink at the sediment water interface of 17000±3000 mgO2 m−2 yr−1.
Seasonal sea surface height variability in the North Atlantic Ocean
Authors: Ferry Nicolas , Reverdin Gilles , Oschlies Andreas
Published: September 2002

We investigate the seasonal sea surface height (SSH) variability on large spatial scales in the North Atlantic by using both a numerical simulation and in situ data. First, an ocean general circulation model is run with daily forcing from the European Centre for Medium‐Range Weather Forecasts reanalysis. We evaluate the different contributions to the seasonal SSH variability resulting from the surface heat fluxes, advection, salt content variability, deep ocean steric changes, and bottom pressure variability. These terms are compared with estimates from in situ data. North of 20°N, there is an approximate balance between hQ, the air‐sea heat flux induced changes in steric height, and SSH variability. The next important component is the advection (its contribution to the annual amplitude is of the order of 1 cm except near the western boundary); other contributions are found to be smaller. Between 10°N and 10°S the advection variability induced by the seasonal wind stress cycle is the primary source of SSH variability. We then compare the sea surface height annual harmonic from TOPEX/Poseidon altimetry with the steric effect from the heat flux and with model and/or in situ estimates of the other terms. In many areas north of 20°N the balance between hQ and the altimetric SSH seasonal cycle is closed within the uncertainty limit of each of the terms of the SSH budget. However, hQ and the SSH do not balance each other in the eastern North Atlantic, and the results are sensitive to the choice of the heat flux product, suggesting that significant errors, typically 20–40 W m−2 for the seasonal cycle amplitude, are present in the meteorological model heat fluxes.
Changes of total CO2 and pCO2 in the surface ocean during the mixed layer development in the northern North Pacific
Authors: Sasai Yoshikazu , Ikeda Motoyoshi , Tanaka Noriyuki
Published: September 2002

A main objective is to simulate the seasonal variations in a carbon cycle during the mixed layer development from a fall through winer over the northern North Pacific. A bulk mixed layer model is employed, which is initialized with the historical data and forced by the observed atmospheric data. The model retains the following mechanisms: cooling and freshwater flux through the sea surface, kinetic energy input due to wind stresses, and air‐sea CO2 flux as well as turbulent vertical mixing of heat, salt, total CO2, and carbonate alkalinity at the bottom of the mixed layer. Total CO2, carbonate alkalinity, and pCO2 are related to each other under the assumption of chemical equilibrium. Since biological processes are secondary for the pCO2 evolution during winter, they are omitted. The evolution of pCO2 in the model mixed layer shows a rapid increase of pCO2 in the subpolar region due to mixing of deep water rich in CO2 before exceeding the atmospheric pCO2, and then the CO2 flux stays almost constant during the winter. In the subtropical region the CO2 is transported always from the atmosphere to the ocean, as the pCO2 decreases from cooling. The ocean is a CO2 sink (−0.18 GtC half‐year−1) over the entire model domain north of 30°N. However, in the subpolar region the ocean is a CO2 source (0.04 GtC half‐year−1). The CO2 flux varies from year to year, while there is no qualitative difference in the flux distribution.
Shallow and deep water global ocean tides from altimetry and numerical modeling
Authors: Tierney Craig C. , Kantha Lakshmi H. , Born George H.
Published: September 2002

We present here a 1/4° resolution near‐global barotropic tidal model designed specifically to provide more accurate tides in shallow water. The model assimilates tides derived from 4 years of TOPEX/Poseidon (T/P) altimetric data. Procedures are used that tend to preserve the small spatial scales in the tidal structure in shallow water. Data from coastal tide gauges are also assimilated into the model. The result is a tidal model that is useful in most of the shallow and deep areas of the global oceans. Some problems exist in regions where no data are available for assimilation, such as the Southern Ocean. Pelagic tide gauge comparisons show that in deep water the new model is comparable in accuracy to the best of the existing T/P‐based global tidal models. Comparisons to crossover differences in shallow water suggest improved performance. We found that more accurate accounting of the load tides in the ocean tide model using an iterative technique based on the Green's function formalism does not yield ocean tides that are significantly different from the simpler approaches used thus far by ocean tide modelers when data are assimilated. It is our hope that this tidal model will not only help advance our understanding of shallow water tidal processes around the globe but also extend the utility of altimetry to waters shallower than 1000 m.
Intrusion of the North Pacific waters into the South China Sea
Authors: Qu Tangdong , Mitsudera Humio , Yamagata Toshio
Published: September 2002

Water mass distribution was studied by analyzing historical hydrographic data in the South China Sea. Despite considerable modification of characteristics as a result of mixing, waters of both salinity maximum and minimum of the North Pacific origin were traced on the density surfaces around 25.0 and 26.73 σθ, respectively. In the salinity maximum layer, property distribution suggests an intrusion into the South China Sea all year‐round through the Luzon Strait. The seasonal variation of the intrusion contains a pronounced semiannual signal, with greater strength in winter and summer than in spring and fall. From spring to fall, the intrusion water from the Pacific is narrowly confined in the continental slope south of China; only in winter, when the northeast monsoon becomes fully developed, can it spread in the southern South China Sea. In the salinity minimum layer, water enters the South China Sea only in spring, when the intrusion in the salinity maximum layer is weakest. A combined use of the “island rule” with climatological data suggests a mean Luzon Strait transport of the order 4 Sv (1 Sv = 106 m3 s−1).
Rapid thermohaline transition in the Pacific western subarctic and Oyashio fresh core eddies
Authors: Rogachev Konstantin A.
Published: September 2002

Seven years of conductivity‐temperature‐depth data enable direct computations of volume transport and a description of the temporal changes of water properties in the western subarctic Pacific. The cooling of the western subarctic surface layer is shown through the observation of the thermohaline transition, which occured in the western subarctic during 1990–1997. Its main consequences were changes of the Oyashio path and restratification of the western subarctic water. The example presented here uses data obtained during the Canada/Russia International North Pacific Ocean Climate Study, as well as the Russian project Subarctic to show that a warming and ventilation of the midlayers of the western subarctic (200–1000 dbar) occurred because of the influx of warm subtropical waters to the Oyashio and Sea of Okhotsk and to the weak coastal Oyashio before the transition. This influx appears to have reversed in the early 1990s; now subarctic intermediate waters displace the remnants of subtropical waters that penetrated to the Sea of Okhotsk through the Soya Strait and create strong stratification in the Oyashio area. This transition appears to have begun during the reinforcement in the coastal Oyashio.