bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastically forced variability in the Antarctic Circumpolar Current
Authors: Weisse Ralf , Mikolajewicz Uwe , Sterl Andreas , Drijfhout Sybren S.
Published: September 2002

Interannual fluctuations in the Antarctic Circumpolar Current (ACC) were considered. In the present study we analyze a mode of variability in the Hamburg Large‐Scale Geostrophic ocean general circulation model which was driven by stochastic atmospheric forcing. The short‐term atmospheric weather fluctuations were represented by a number of spatially coherent patterns of momentum, heat, and freshwater flux which were superimposed onto the climatological fluxes. These patterns were derived from an experiment with an atmospheric general circulation model forced with observed sea surface temperatures, and they were chosen randomly at each month. We found anomalies which propagate along the ACC at an interannual timescale. They can be explained by the combined effects of anomaly advection with the mean ocean circulation and integration of the short‐term atmospheric weather fluctuations. Some similarities were found between our results and the concept of the Antarctic Circumpolar Wave, which was proposed recently to account for large‐scale anomalies which propagate along the ACC in both the atmosphere and the ocean.
Modeling of two‐layer eddies and coastal flows with a particle method
Authors: Esenkov Oleg E. , Cushman‐Roisin Benoit
Published: September 2002

An existing particle‐in‐cell (PIC) numerical method developed for the study of two‐layer mesoscale motions with outcropping pycnocline is applied to lens‐like anticyclonic vortices and buoyant coastal currents. From a first series of experiments investigating the evolution of an initially elongated lens‐like anticyclone, it is found that motions induced in the lower layer act only to increase the rotation of the vortex structure and do not appear to affect the process of eccentricity reduction (partial axisymmetrization). Eccentricity reduction, if any, produces a final vortex of aspect ratio between 1.8 and 1.9, a value that is very close to the stability threshold of large, reduced‐gravity lenses. A second series of experiments devoted to vortex mergers determines how the maximal separation distance for which two circular anticyclonic lenses merge spontaneously varies with vortex size (ratio of lens radius to deformation radius) and stratification (ratio of lens central thickness to ocean depth). A third series of experiments considers the interaction of a lens vortex in the upper layer with a potential‐vorticity anomaly in the lower layer. “Second‐hand” relative vorticity, generated in the lower layer under the action of vertical stretching induced by the movement of the upper‐layer vortex, interacts with “first‐hand” relative vorticity, created by the existing potential‐vorticity, to create effects similar to those predicted by studies of two‐layer point vortices (hetons). Finally, the PIC method is generalized to simulate the finite‐amplitude instability of a buoyant geostrophic/hydrostatic intrusion flowing along a vertical coastal wall. Those results, however, are reported here more as a demonstration on how the PIC method can be extended to include coastal boundaries than as a thorough investigation of coastal‐current instabilities.
Deep jets in the equatorial Atlantic Ocean
Authors: Gouriou Y. , Bourlès B. , Mercier H. , Chuchla R.
Published: September 2002

Deep velocity profiles taken in the equatorial Atlantic Océan show equatorially trapped deep jets with similar features to those of the Indian and Pacific Oceans: a zonal velocity of the order of 10 to 20 cm s−1 and a meridional scale of 1°. In the Pacific and Indian Oceans the zonal extent of the jets is at least 15° of longitude. Owing to the lack of synoptic measurements, we have no information on the zonal scale in the Atlantic Ocean, but we present here zonal velocity profiles, made at a 16‐month interval, that have identical baroclinic structure in the western (35°W) and central basin (13°W). The Atlantic jets have a vertical scale larger (400–600 m) than those observed in the Pacific Ocean (250–400 m). Our measurements confirm the opposite directions of the jets for different seasons in the Atlantic Ocean. Furthermore, for a given season, the vertical profiles of zonal velocity at 35°W–0° are astonishingly similar at a 5‐year interval. As in the Pacific and Indian Oceans, the jets are embedded in a large‐vertical‐scale current that changes direction with time, The few profiles available in the equatorial Atlantic Ocean suggest a seasonal reversal of the jets, but neither this nor the temporal variability of the large‐scale current has been adequately resolved.
Wavenumber‐frequency structure of infragravity swash motions
Authors: Holland K. T. , Holman R. A.
Published: September 2002

Alongshore‐separated time series of natural swash motions were obtained over a range of environmental conditions using a video technique. Although the frequency spectra and normalized wavenumber spectra for these motions were particularly bland, wavenumber‐frequency spectra of these data showed clear partitions of infragravity band energy levels associated with various wave types. For the frequencies 0.025 < ƒ ≤ 0.05 Hz, 45 ± 13% of the shoreline variance was, on average, associated with high‐mode (n≥ 2) edge waves and/or leaky waves, while approximately half that amount was associated with low‐mode edge waves. Gravity wave motions (comprising both edge and leaky modes) were typically dominant in a lower‐frequency band (0.001 < ƒ ≤ 0.025 Hz). A substantial portion of the variance in this band (21 ± 10%, with a maximum of 38%), however, was identified as a nondispersive waveform with wavenumbers well outside of the wavenumber‐frequency bounds for gravity waves. Surprisingly, this nongravity swash variance showed no significant dependence on mean alongshore current strength or mean alongshore current shear as measured in the surf zone trough separating the shoreline from an offshore bar. In addition, the celerities of these swash zone nondispersive waves were found to differ in magnitude, and in one instance, sign, from celerities of similarly structured waves measured farther offshore in the surf zone. These unexpected observations with respect to low‐frequency, nongravity swash energy imply a strong decorrelation between trough and shoreline fluid motions.
The Siberian Coastal Current: A wind‐ and buoyancy‐forced Arctic coastal current
Authors: Weingartner Thomas J. , Danielson Seth , Sasaki Yasunori , Pavlov Vladimir , Kulakov Mikhail
Published: September 2002

We describe circulation and mixing in the Siberian Coastal Current (SCC) using fall shipboard measurements collected between 1992 and 1995 in the western Chukchi Sea. The SCC, forced by winds, Siberian river outflows, and ice melt, flows eastward from the East Siberian Sea. It is bounded offshore by a broad (∼60 km) front separating cold, dilute Siberian Coastal Water from warmer, saltier Bering Sea Water. The alongshore flow is incoherent, because the current contains energetic eddies and squirts probably generated by frontal (baroclinic) instabilities. These enhance horizontal mixing and weaken the cross‐shore density gradient along the SCC path. Eventually, the SCC converges with the northward flow from Bering Strait, whereupon it deflects offshore and mixes with that inflow. Deflection occurs where the alongshore pressure gradient vanishes. That location varies on synoptic and seasonal timescales, because this gradient depends on the winds, buoyancy fluxes, and the sea level difference between the Pacific and Arctic Oceans. Deflection usually occurs on the Chukchi shelf, but the SCC occasionally flows southward through Bering Strait. Such events are short lived (1–10 days) and occur mainly in fall and winter under northerly winds. SCC transport is likely small (∼0.1 Sv), but its dilute waters could substantially freshen the Bering Strait inflow and affect the disposition of Pacific waters in the Arctic Ocean. Arctic river outflows should preferentially form surface‐advected fronts rather than bottom‐advected fronts because vertical‐mixing energy is low on arctic shelves. Surface‐advected fronts are more susceptible to upwelling winds (and for the SCC, the pressure gradient between the Pacific and Arctic Oceans) than bottom‐advected fronts. The SCC never developed in fall 1995 because of anomalously steady upwelling winds. The western Chukchi shelf could have formed upper halocline source water in the winter of 1995–1996.
Dynamics of the equatorial undercurrent in a high‐resolution ocean model
Authors: Wainer Ilana , Bryan Frank O. , Soares Jacyra
Published: September 2002

In this study, we investigate the sensitivity of the equatorial circulation simulated with the high‐resolution World Ocean Circulation Experiment (WOCE) Community Modeling Effort (CME) model to the specification of eddy viscosity. In one of two simulations the coefficient of vertical viscosity is tripled. We focus on the dynamics associated with the anomalous eastward surface flow found in the simulation results. We find, in agreement with other studies, that close to the western boundary, the equatorial undercurrent is dominated by a large positive pressure gradient. In the top layer, for the low‐viscosity case, the balance in the western part of the basin is everywhere between the horizontal pressure gradient and vertical viscosity with the advective terms playing a secondary role. In the high‐viscosity zonal momentum budget analysis the advective terms are less significant in the surface layers in the west. Toward the east the advective terms become more important in the low‐viscosity experiment, whereas in the high‐viscosity case the balance is still predominantly between the horizontal pressure gradient and vertical diffusion terms. It is found that in the CME, horizontal viscosity plays a very minor role in the zonal momentum balance. Changes in the vertical viscosity lead to changes in the nonlinear advective terms. These, in turn, contribute to the unusual eastward surface flow.
Seasonal distributions of satellite‐measured phytoplankton pigment concentration along the Chilean coast
Authors: Thomas Andrew C.
Published: September 2002

Five years (1979–1983) of Coastal Zone Color Scanner satellite ocean color data are used to examine seasonal patterns of phytoplankton pigment concentration along the Chilean coast from 20°S to 45°S. Four kilometer resolution, 2–4 day composites document the presence of filaments of elevated pigment concentration extending offshore throughout the study area, with maximum offshore extension at higher latitudes. In three years, 1979, 1981, and 1983, sufficient data exist in monthly composites to allow recreation of portions of the seasonal cycle. Data in 1979 are the most complete. Near‐shore concentrations and cross‐shelf extension of pigment concentrations in 1979 are maximum in austral winter throughout the study area and minimum in summer. Available data from 1981 and 1983 are consistent with this temporal pattern but with concentrations approximately double those of 1979. Seasonal, spatial patterns within 10 km of shore and 50 km offshore indicate a latitudinal discontinuity both in absolute concentration and in the magnitude of the seasonal cycle at approximately 33°S in both 1979 and in the climatological time series. The discontinuity is strongest in fall‐winter and weakest in summer. South of this latitude, concentrations are relatively high (2–3 mg m−3 in 1979), a strong seasonal cycle is present, and patterns 50 km offshore are correlated with those within 10 km of shore. North of 33°S, concentrations are <1.5 mg m−3 (in 1979), and the seasonal cycle within 10 km of shore is present but much weaker and less obviously correlated with that 50 km offshore. The seasonal cycle of pigment concentrations is 180° out of phase with monthly averaged upwelling favorable winds. Noncoincident Pathfinder sea surface temperature data show that over most latitudes, coastal low surface temperatures lag wind forcing by 1–2 months, but these too are out of phase with the pigment seasonal cycle. These data point to control of pigment patterns along the Chilean coast by the interaction of upwelling with circulation patterns unconnected to local wind forcing.
Upper ocean variability between Iceland and Newfoundland, 1993–1998
Authors: Reverdin Gilles , Verbrugge Nathalie , Valdimarsson Hedinn
Published: September 2002

We investigate the variability of upper ocean temperature and surface salinity between Iceland and Newfoundland along World Ocean Circulation Experiment line AX2 in 1993–1998. North of 54°N, as well as close to Newfoundland, deviations from the seasonal cycle indicate upper ocean warming from 1994 to 1998, with largest anomalous warming during late 1995/early 1996 and during the winter 1997/1998. North of 54°N, surface salinity increases after mid‐1996. The winter of 1995/1996 is the first winter of weak westerlies after more than 20 years of strong westerly conditions in the subarctic gyre, and the ocean loses less heat during that winter than during a normal year. The magnitude of upper ocean heat content change is at least twice as large as what results from the change in air‐sea fluxes, which indicates a complementary contribution to heat content change by advection. Altimetric sea level data indicate also an increase of sea level of the order of 6.5 cm during that year, out of which more than 4 cm is contributed by the steric change associated with the temperature increase. A principal component analysis of the sea level data suggests that the increase sensed along line AX2 west of the Reykjanes ridge is found in most of the subpolar gyre, although to a lesser extent in most other areas.
Effects of locally enhanced vertical diffusivity over rough bathymetry on the world ocean circulation
Authors: Hasumi Hiroyasu , Suginohara Nobuo
Published: September 2002

Observations indicate that vertical diffusivity in the deep ocean is considerably enhanced over rough bathymetry and is very small elsewhere. Here we investigate effects of locally enhanced vertical diffusivity over rough bottom topography on the world ocean circulation by use of a coarse resolution ocean general circulation model. Vertical diffusivity is enhanced in the model, taking into account effects of internal tide breaking. For comparison, two cases of an experiment are carried out, where vertical diffusivity increases with depth without horizontal inhomogeneity. Horizontal distribution of deep upwelling is qualitatively different, depending on whether or not vertical diffusivity is horizontally inhomogeneous. Upwelling of Circumpolar Deep Water in the Pacific and Antarctic Bottom Water in the Atlantic is confined where vertical diffusivity is enhanced in the case of horizontally inhomogeneous vertical diffusivity, whereas it is horizontally uniform in the other cases. This difference leads to different three‐dimensional structure of the deep circulation.
A numerical model of the Weddell Sea: Large‐scale circulation and water mass distribution
Authors: Beckmann Aike , Hellmer Hartmut H. , Timmermann Ralph
Published: September 2002

A circumpolar model for regional studies of the wind‐driven and thermohaline circulation of the Southern Ocean including the major sub‐ice shelf areas is described. A first series of numerical experiments focusing on the Weddell Sea reveals a pronounced and persistent double‐cell structure of the Weddell Gyre with a maximum transport of ∼60 Sv, in agreement with observations. Experiments with artificial passive tracers point to the shallow shelf areas off the Filchner‐Ronne Ice Shelf as main locations for bottom water production. The trajectories of Lagrangian floats are used to determine the pathways and timescales of water mass spreading in the model. In addition, the effect of sub‐ice shelf forcing on the water mass characteristics is evaluated. It is shown that water modified in the sub‐ice cavities contributes significantly to the deep and bottom water formation along the continental slope, and affects the water mass characteristics throughout the Weddell Sea, by increasing the stability of the near‐surface stratification and preventing deep convection.
Natal pulses and the formation of Agulhas rings
Authors: van Leeuwen Peter Jan , de Ruijter Will P. M. , Lutjeharms Johann R. E.
Published: September 2002

Large solitary meanders form on the Agulhas Current at irregular intervals as it travels along the east coast of South Africa. These so‐called Natal pulses are thought to have a significant effect on the shedding of Agulhas rings downstream at the Agulhas Retroflection and thereby on the exchange of water properties between the Indian and the Atlantic Ocean. Data from the Geosat, ERS 1, and TOPEX/Poseidon satellite altimeters and Advanced Very High Resolution Radiometer (AVHRR) infrared imagery from the Pathfinder project are analyzed and show that this intuitive idea seems to be correct. Close to the coast, individual altimeter tracks are used to identify the cyclonic Natal pulses as depressions in the sea‐surface topography. Using different tracks, the pulses can then be followed from close to Durban to the Agulhas Bank. They show that each shedding of an Agulhas ring is preceded by the appearance of a Natal pulse close to Durban. A significant correlation is found with a time lag of 165 days. Interpolated topography maps are used to follow pulses along the Agulhas Bank to the ring‐shedding area. They indicate that sometimes pulses trigger ring shedding by themselves or by merging with Rossby wave‐like meanders in the Agulhas Return Current. Infrared imagery supports these interpretations.
Interannual variability in the South China Sea from expendable bathythermograph data
Authors: Wang Liping , Koblinsky Chester , Howden Stephan , Huang Norden
Published: September 2002

Using 29 years of expendable bathythermograph data, we studied the interannual temperature variability in the upper 300 m of the South China Sea (SCS). It is found that the temperature field has significant quasi‐penta‐annual variability throughout the 29 years. The interannual variability has a large decadal modulation. Although outside the SCS, in the Kuroshio, significant interannual variability extends to a much deeper layer beyond the 300 m depth, inside the SCS, especially in the central SCS, strong interannual variability is located near the 100 m depth and is capped by a surface layer of much weaker interannual variability. Similarities and differences in the temporal characteristics between the interannual variability inside and outside the SCS suggest that the interannual variability inside the SCS is primarily generated locally inside the SCS. In addition to the strong interannual variability, there exist strong decadal variability and a linear trend, which are also stronger in the subsurface than near the surface. It is found that below 100 m, the SCS has been cooling (linear trend) at a rate of ∼0.4°C per decade, decreasing from the eastern to the western basin.
Signatures of the Mediterranean outflow from a North Atlantic climatology: 2. Diagnostic velocity fields
Authors: Iorga Michaela Ciobotaru , Lozier M. Susan
Published: September 2002

Part 1 of this study is a descriptive analysis of the spreading of Mediterranean Water based on high‐resolution maps of salinity and density in the eastern basin. In this second part of our study, velocity fields for two representative isopycnal surfaces of the Mediterranean outflow (σ0.5 = 29.70 and σ0.5 = 29.90) are estimated from a diagnostic model that combines climatological hydrographic data from the National Oceanic Data Center with long‐term direct measurements of water exchange through the Strait of Gibraltar. The model is constrained by geostrophic dynamics, conservation of mass, no‐flux conditions at the continental shelf, and specified flow through the Strait of Gibraltar. Our principal data source is a recently assembled database of the North Atlantic that consists of climatological mean property fields averaged on isopycnal surfaces. The mean fields are based on more than 80 years (1909–1990) of data and have a nominal horizontal resolution of 0.5°. To provide boundary conditions at the Strait of Gibraltar, we use the results of a model developed from data collected during the Gibraltar Experiment in 1985. The estimated velocity fields show Mediterranean Water exiting the Strait of Gibraltar, following the southern Iberian coast, and then entering the Tagus Basin, where it turns anticyclonically to create a reservoir of this water mass. The flow continues northward along the eastern boundary, penetrating into the Rockall Channel. Finally, the model flow fields do not show a significant westward advection of Mediterranean waters into the subtropical gyre.
Variability of surface ocean radiocarbon and stable isotopes in the southwestern Pacific
Authors: Druffel Ellen R. M. , Griffin Sheila
Published: September 2002

We present high‐precision radiocarbon (Δ14C) results and stable isotope (δ18O and δ13C) records for a coral from Heron Island (23°S, 152°E) and new stable isotope (δ18O and δ13C) records for annual coral bands from Abraham Reef (22°S, 153°E) in the southern Great Barrier Reef studied earlier [Druffel and Griffin, 1993]. These tracers provide unique information on the regional water mass history, and together these data allow us to constrain the variability of circulation in the upper Pacific over the past four centuries. First, we observe decreases in δ18O of coral from Abraham Reef and Heron Island, indicating an increase in sea surface temperature and/or a decrease in surface salinity since 1850. Second, the small Suess effect value (Δ14C decrease from 1880 to 1955, due mostly to fossil fuel CO2) observed previously at Abraham Reef[Druffel and Griffin, 1993] is confirmed in the measurements reported here from the Heron Island coral. This value is low compared to those observed in other areas of the ocean [Druffel, 1997; Druffel and Linick, 1978; Nozaki et al., 1978] between 1880 and 1955. Third, we report alterations in the correlation between El Niño events and the occurrence of low Δ14C, which is indicative of long‐term change(s) in circulation in the SW Pacific. The Δ14C shifts reported here are not large, but even small temporal changes in prebomb Δ14C suggest that important changes in the large‐scale state of the ocean have occurred, such as a temporal change in circulation.
Numerical study on signatures of atmospheric convective cells in radar images of the ocean
Authors: Ufermann Susanne , Romeiser Roland
Published: September 2002

Current and wind variations at the ocean surface can give rise to a modulation of the sea surface roughness and thus become visible in radar images. The discrimination between radar signatures of oceanic and atmospheric phenomena can be quite difficult, since signatures of different origin can have very similar shapes and magnitudes and are often superimposed upon each other. In this work we employ a numerical radar imaging model for an investigation of typical properties of radar signatures of atmospheric convective cells and of theoretical differences between such atmospherically induced radar signatures and those of oceanic phenomena. We show that main characteristics of observed multifrequency/multipolarization radar signatures of atmospheric convective cells over the Gulf Stream are reproduced quite well by the proposed model. This encourages us to vary wind and radar parameters systematically in order to get a general overview of the dependency of atmospherically induced radar signatures on these parameters. Finally, we compare typical characteristics of radar signatures of atmospheric and oceanic phenomena, and we present simulated radar images of a scenario of superimposed atmospheric convective cells and oceanic internal waves. We show that the proposed model supports the experimental finding that radar signatures of oceanic phenomena are stronger at horizontal (HH) than at vertical (VV) polarization, while atmospherically induced radar signatures are better visible at VV polarization.
Oceanic wave dynamics and interdecadal variability in a climate system model
Authors: Capotondi Antonietta
Published: September 2002

Western boundary disturbances detected in the first multicentury integration of the National Center for Atmospheric Research Climate System Model are analyzed, and the role they play in the model interdecadal variability is investigated. The boundary signals propagate southwestward along the coast of North America. At depth they reach the equator and propagate along the equator but with decreasing amplitude away from the western boundary. Their southwestward propagation is associated with the evolution of the western boundary current system (the Gulf Stream system in the upper part of the water column and the Deep Western Boundary Current at depth) from weaker than average to stronger than average conditions. Thus the boundary disturbances seem to play an important role in the model thermohaline circulation variability at interdecadal timescales. They show similarities with the boundary “waves” found in ocean‐only models exhibiting interdecadal variability, and their role in the model's response to changes in the rate of sinking appears to be in agreement with the theory of the spin‐up of the deep ocean circulation. The main result of this study is that the similarities with ocean‐only results and theories support the idea that interdecadal timescales of variability originate from the ocean dynamics.
On the dynamics of suspended matter transport in the tidal river Elbe: Description and results of a Lagrangian model
Authors: Rolinski Susanne
Published: September 2002

A numerical model for the transport of suspended particulate matter (SPM) in the Elbe Estuary is presented. The physically based SPM model applies the Lagrangian concept and includes advective as well as diffusive transport, interaction with the bottom sediment in terms of deposition and resuspension of SPM, and sinking of material. Three different SPM concentration peaks, which are caused by different mechanisms, form from an initially homogeneous concentration. One is the generally accepted concept of estuarine circulation, leading to an accumulation through upstream residual currents near the bottom of the mixing region. The remaining two peaks occur in the freshwater region and are therefore attributed to the barotropic velocities.
A global El Niño‐Southern Oscillation wave in surface temperature and pressure and its interdecadal modulation from 1900 to 1997
Authors: White Warren B. , Cayan Daniel R.
Published: September 2002

Zonal wavenumber frequency spectra of sea surface temperature (SST) anomalies along the equator in the Indo‐Pacific basin for the 98 years from 1900 to 1997 and of surface temperature (ST) and sea level pressure (SLP) anomalies extending around the globe along 10°N for the 48 years from 1950 to 1997 display significant peak spectral energy density for standing and eastward propagating waves of 3–7 year periods and 120°–360° zonal wavelengths. The global standing wave is the familiar Southern Oscillation, but the global propagating wave represents a new paradigm for the El Niño‐Southern Oscillation (ENSO). Global distributions of the phase velocities for this global ENSO wave finds covarying SLP and ST anomalies propagating eastward along the mean path of the Intertropical Convergence Zone (ITCZ), with the global zonal wavenumber 1 (2) component taking ∼4 (6) years to cross the tropical Indian, Pacific, and Atlantic Oceans at a zonal average speed of 90° (60°) longitude per year. Along this path the interannual SST and SLP anomalies are directly out of phase. Since thermocline depth anomalies underneath the ITCZ in the Pacific Ocean propagate westward [White et al. 1985], we view the global ENSO wave as a slow coupled SST wave trapped onto the ITCZ. Separating the global ENSO wave from the Southern Oscillation using complex empirical orthogonal function analysis finds the amplitude of the propagating wave to be half that of the standing wave, with the former (latter) accounting for one third (two thirds) of the interannual variability in Niño‐3 SST and SLP indices during the 1980s. The global ENSO wave is shown to be responsible for the eastward propagation of covarying zonal surface wind and thermocline depth anomalies across the equatorial Pacific Ocean and through this mechanism is able to influence both the phasing and intensity of El Niño. Examining the persistence of the global ENSO wave from 1900 to 1997 finds it and the intensity of El Niño in the eastern equatorial Pacific Ocean modulated by interdecadal change. Both were strong (weak or absent) during decades of global tropical cooling (warming).
On the uncertainty of the mass, heat, and salt budgets of the Arctic Ocean
Authors: Goldner Daniel R.
Published: September 2002

Estimates published between 1975 and 1997 of mass, heat, and salt fluxes into and out of the Arctic Basin are statistically combined into aggregate estimates for the purpose of determining the uncertainty of the budgets. Because they include information from many previous estimates, the aggregate estimates are consistent with previous results but with smaller uncertainty. The sums of the aggregate estimates are nonzero but within two standard errors of zero. Systematic adjustment of the aggregate estimates in proportion to their uncertainties gives two new sets of flux estimates: one set that conserves volume (as an approximation to mass), and another that conserves volume, heat, and salt. The conservation requirements greatly reduce the uncertainty of the most uncertain terms in the budgets. The budgets indicate the necessity, in steady state, of a net average flux of 1.4±0.3 × 106 m3 s−1 from the Arctic shelves to the Arctic basin, consistent with previous estimates. The budgets also imply a net freshwater flux from the shelves into the basins: the freshening effect of surface transport is larger than the salinating effect of dense shelf overflows. The uncertainties derived here are lower‐bound estimates as they do not consider the effects of aliasing or other undersampling errors.
Assimilating altimetric data into a South China Sea model
Authors: Wu Chau‐Ron , Shaw Ping‐Tung , Chao Shenn‐Yu
Published: September 2002

Sea surface heights from the TOPEX/Poseidon altimeter are assimilated into a three‐dimensional primitive equation model to derive the circulation in the South China Sea. With data assimilation the model resolves not only the basin‐wide circulation but also a dipole off Vietnam and a low/high feature near the Luzon Strait. Mesoscale features are missing in the simulation without data assimilation because of poor resolution in the wind field and inadequate knowledge of the transport through the Luzon Strait. Compared to the case without data assimilation, data assimilation reduces the root mean square error between the simulated and observed sea surface heights by a factor of 2–3. Circulation derived from data assimilation under climatological conditions is contrasted with that during El Niño. In the normal winter of 1993–1994, flow at 50 m depth is strong and cyclonic. Flow at 900 m depth is cyclonic as well. The deep cyclone persists into the following summer. During the 1994–1995 El Niño winter, features in the flow field at 50 m depth either weaken or disappear, and circulation at 900 m depth is anticyclonic. In the summer of 1995 the dipole and the eastward jet off Vietnam at 50 m depth are missing, and the anticyclonic circulation at 900 m depth persists. Temperature at 65 m shows significant warming from fall 1994 to summer 1995. A weakened flow field and warming in the upper ocean are consistent with findings from earlier El Niño events.
Solidification of leads: Theory, experiment, and field observations
Authors: Wettlaufer J. S. , Worster M. Grae , Huppert Herbert E.
Published: September 2002

Thin sea ice plays a central role in the surface heat and mass balance of the Arctic Ocean. In order to develop understanding of these balances we describe and analyze highly resolved temperature data taken through the air/sea/ice interface during the transition from an ice‐free to an ice‐covered Arctic Ocean surface. The data were taken to observe the thermodynamic evolution of a lead, a process that has previously only been accessible to measurement techniques confined to the lead edge. Our detailed analysis of the field data is guided by recent theoretical and experimental advances in understanding the phase dynamics of directionally solidified alloys. Because of the dearth of direct observations we also present time series of the relevant heat fluxes inferred from our data and demonstrate the controlling influence that the internal phase evolution has on these quantities. We have previously examined the stability of the brine trapped in a growing sea ice matrix both theoretically and experimentally and now find that haline convection, driven from within the growing layer, is consistent with this previous work and with the nature of direct turbulence measurements. The importance of this process is that although ice growth is continuous, the local brine flux commences abruptly, only after some time, in contrast to what had previously been supposed. Hence the ice growth process itself is a source of intermittency in oceanic boundary layer turbulence. Furthermore, we find that in this particular situation the sea ice growth is not simply a square root function of time, in contrast to the model typically used in numerical simulations. By far the most practical methods of studying lead convection are numerical simulations and laboratory models, and a strong conclusion of this study is the importance of the proper treatment of the boundary conditions describing the buoyancy flux.
Observations and modeling of a California undercurrent eddy
Authors: Cornuelle B. D. , Chereskin T. K. , Niiler P. P. , Morris M. Y. , Musgrave D. L.
Published: September 2002

A deep, nonlinear warm eddy advecting water that was also anomalously saltier, lower in oxygen, and higher in nutrients relative to surrounding waters was observed in moored current and temperature measurements and in hydrographic data obtained at a site ∼400 km off the coast of northern California. The eddy was reproduced using a nonlinear quasi‐geostrophic model, initialized by an iterative procedure using time series of 2‐day averaged moored current measurements. The procedure demonstrates how a data assimilative technique synthesizes and enhances the resolution of a relatively sparse data set by incorporating time‐dependence and model physics. The model forecast showed significant skill above persistence or climatology for 40 days. Our hypothesis, that the eddy was generated at the coast in winter and subsequently moved 400 km offshore by May, is consistent with the eddy movement diagnosed by the model and with the observations and coastal climatology. The model evolution significantly underpredicted the temperature anomaly in the eddy owing in part to unmodeled salinity compensation in trapped California Undercurrent water. Together, observations and model results show a stable nonlinear eddy in the California Current System that transported water and properties southwestward through the energetic eastern boundary region. Coherent features such as this one may be a mechanism for property transfer between the eddy‐rich coastal zone and the eddy desert of the eastern North Pacific Ocean.
Blending HF radar and model velocities in Monterey Bay through normal mode analysis
Authors: Lipphardt B. L. , Kirwan A. D. , Grosch C. E. , Lewis J. K. , Paduan J. D.
Published: September 2002

Nowcasts of the surface velocity field in Monterey Bay are made for the period August 1–9, 1994, using HF radar observations blended with results from a primitive equation model. A spectral method called normal mode analysis was used. Objective spatial and temporal filtering were performed, and stream function, velocity potential, relative vorticity, and horizontal divergence were calculated over the domain. This type of nowcasting permits global spectral analysis of mode amplitudes, calculation of enstrophy, and additional analyses using tools like empirical orthogonal functions. The nowcasts reported here include open boundary flow information from the numerical model. Nowcasts using no open boundary flow information, however, still provide excellent results for locations within the observation footprint. This method, then, is useful for filtering high‐resolution data like HF radar observations, even when open boundary flow information is unavailable. Also, since the nowcast velocity gradient fields were much less noisy than the observations, this may be an effective method for preconditioning high‐resolution observation sets for assimilation into a numerical model.
Effects of ambient stratification and shelfbreak topography on offshore transport of dense water on continental shelves
Authors: Gawarkiewicz Glen
Published: September 2002

The transport of dense water offshore from an idealized coastal polynya is examined using a primitive equation numerical model. First, the prior scaling of the equilibrium density of water produced within the polynya is extended to include the effects of linear bottom slope. Next, ambient linear stratification is added. The equilibrium density is modified slightly by the density at the bottom at the initial time. Next, shelfbreak topography is included with both weak and strong ambient stratification. For the case of weak stratification, dense water carried to the shelfbreak within eddies moves rapidly downslope and is trapped near the bottom. The along‐shelf scale of the dense water cascading down the slope is set by the diameter of the dense water eddies that form along the periphery of the coastal polynya. For the case of strong stratification, dense water is carried offshore across the shelfbreak at an intermediate depth, injecting water originally within the polynya across the shelf and over the continental slope. A bottom‐trapped current flowing near the shelfbreak in the direction of coastal‐trapped wave propagation also forms. These results suggest that water formed within coastal polynyas over Arctic shelves may be capable of ventilating the continental slope regions and possibly central basins of the Arctic Ocean via eddy fluxes.
Current observations on the development of the Yellow Sea Warm Current
Authors: Teague W. J. , Jacobs G. A.
Published: September 2002

Historically, the climatological mean northward flow in the Yellow Sea trough has been referred to as the Yellow Sea Warm Current (YSWC) since it was thought to originate as a branch of the Tsushima Current which transports warm Kuroshio waters. However, the Tsushima Current does not penetrate into the Yellow Sea interior during winter. The onset of what is referred to as the YSWC during winter is directly observed using measurements of current profiles, pressure, and temperature, and a wind climatology for the time period July 1995 through January 1996. The YSWC is a result of weak southerly winds during summer. During fall transition and winter monsoon periods, strong northerly wind bursts drive a north‐to‐south rise in pressure extending from the trough over to the Korean coast and force a northward flow in the Yellow Sea trough. The rise in pressure and corresponding north wind‐forced YSWC begin at the end of October for this measurement period and are observed into January. These measurements show that the YSWC has different seasonal forcing mechanisms. Direct current measurements are also presented which suggest a current flow north of Cheju, sometimes referred to as the Cheju Warm Current.
Microwave polarimetric measurements of the sea surface brightness temperature from a blimp during the Coastal Ocean Probing Experiment (COPE)
Authors: Trokhimovski Y. G. , Irisov V. G. , Westwater E. R. , Fedor L. S. , Leuski V. E.
Published: September 2002

Experimental data obtained with a microwave dual‐frequency radiometer at 23.87 and 31.65 GHz and a polarimeter at 37.0 GHz installed on a blimp during the Coastal Ocean Probing Experiment in 1995 are analyzed. Observations of the downwelling atmospheric radiation as a function of angle were used to achieve a highly accurate absolute calibration. The dependence of the sea surface brightness temperature on nadir and azimuthal angles is considered. The experimental data are compared with theoretical calculations based on several models of sea wave spectra. Models proposed by Romeiser et al. [1997] and Elfouhaily et al. [1997] fit well to the COPE azimuthal measurements and data averaged over azimuthal angle are in between of values predicted by spectrum models by Apel [1994] and Elfouhaily et al. [1997]. The conclusion is made about high sensitivity of the polarized radiometric measurements to parameters of gravity‐capillary spectrum.
Modeling Arctic sea ice with an efficient plastic solution
Authors: Zhang Jinlun , Rothrock Drew
Published: September 2002

A computationally efficient numerical method is developed for solving sea ice momentum equations that employ a nonlinear viscous‐plastic rheology. The method is based on an alternating direction implicit (ADI) technique that involves a direct solution of the momentum equations. This method is therefore more computationally efficient than those employing an iterative procedure in solving the equations. The ADI method for modeling sea ice dynamics is dynamically consistent since it rapidly approaches a viscous‐plastic solution described by the sea ice rheology. With different model configurations of varying spatial resolutions and decreasing time step intervals the ADI method converges to the same viscous‐plastic solution as another numerical method that uses a line successive relaxation procedure to solve the ice momentum equations. This indicates that the ADI method is also numerically consistent. The approximateness of numerical solutions of sea ice, resulting from coarse model resolutions in time, is addressed. It is found that a significant bias, up to 10% or more, in the solution is likely to occur for a typical but coarse time step interval. This indicates that an assessment of the numerically created bias from a crude time integration may be necessary when model data comparisons are performed. In addition, suggestions are given for selecting appropriate time step intervals to enhance numerical accuracy in model applications.
A model of sediment resuspension and transport dynamics in southern Lake Michigan
Authors: Lou Jing , Schwab David J. , Beletsky Dmitry , Hawley Nathan
Published: September 2002

A quasi‐three‐dimensional suspended sediment transport model was developed and generalized to include combined wave‐current effects to study bottom sediment resuspension and transport in southern Lake Michigan. The results from a three‐dimensional circulation model and a wind wave model were used as input to the sediment transport model. Two effects of nonlinear wave‐current interactions were considered in the sediment transport model: the changes in turbulence intensity due to waves and the enhancement of induced bottom shear stresses. Empirical formulations of sediment entrainment and resuspension processes were established and parameterized by laboratory data and field studies in the lake. In this preliminary application of the model to Lake Michigan, only a single grain size is used to characterize the sedimentary material, and the bottom of the lake is treated as an unlimited sediment source. The model results were compared with measured suspended sediment concentrations at two stations and several municipal water intake turbidity measurements in southern Lake Michigan during November–December 1994. The model was able to reproduce the general patterns of high‐turbidity events in the lake. A model simulation for the entire 1994–1995 two‐year period gave a reasonable description of sediment erosion/deposition in the lake, and the modeled settling mass fluxes were consistent with sediment trap data. The mechanisms of sediment resuspension and transport in southern Lake Michigan are discussed. To improve the model, sediment classifications, spatial bottom sediment distribution, sediment source function, and tributary sediment discharge should be considered.
A view of the 1993–1994 California Current based on surface drifters, floats, and remotely sensed data
Authors: Brink K. H. , Beardsley R. C. , Paduan J. , Limeburner R. , Caruso M. , Sires J. G.
Published: September 2002

Near‐surface drifters were deployed in the California Current System during the period 1993–1994 in order to study mean and eddy currents. The deployment scheme allowed a 2 year period where data density was high enough to make meaningful estimates of eddy statistics and of seasonal mean currents and variances on a 5° grid. Eddy variability, which tended to be isotropic, was higher near the coast (standard deviations around 0.2 m s−1) and decreased offshore (down to ∼0.1 m s−1). Seasonal mean surface flow west of California was southward during all seasons: if a Davidson Current existed during 1993–1995, it was probably either short‐lived or only found closer to the coast than our study area, i.e., within 100–150 km of shore. In some cases, drifter populations within a discrete feature were high enough to allow some detailed resolution of its behavior. We note a tendency for individual eddies (which can last for up to 3–5 months) to drift westward at rates of O(5 km day−1). Not all eddies in the region are surface‐intensified. There is clear evidence for at least the occasional presence of a thin, meandering southward jet in the region that can be 1000 km or more long. This jet can be identified with that found in the Coastal Transition Zone program in association with cool filaments. The jet appears to become broader and slower south of Point Conception. We hypothesize (on the basis of others' results as well as ours) that the eddies that are strongest at the surface result primarily from instabilities on a southward alongshore mean current and that eddies that are maximal at depth are spawned by the northward undercurrent.
Self‐adapting open boundaries for a sigma coordinate model of the eastern North Atlantic
Authors: Penduff Thierry , Barnier Bernard , de Verdière Alain Colin
Published: September 2002

A regional non‐eddy‐resolving configuration of the SPEM5.1 primitive equation model is used to study the dynamics of the eastern North Atlantic. Along most of the domain perimeter, extensive open boundaries (OBs) evacuate outgoing perturbations, while the outer ocean is simulated by lateral relaxation of some of the model variables. Unlike in existing regional models, this lateral constraint only concerns the tracers and the baroclinic velocities, for which good climatological estimates exist. Along the OBs, the barotropic stream function is entirely determined by the model response to the inner dynamics, without relaxation to any questionable a priori estimate. The adjustment processes and their interaction with the open boundaries are described. The final solution is remarkably stable and quite realistic. The net transports crossing the boundaries are prescribed; we therefore investigated the impact of these parameters. We first show that the prescribed mass exchange between the basin and the rest of the Atlantic has almost no influence on the interior circulation since the western and southern lateral conditions naturally adjust to the interior dynamics. The net circulation constrained around Iceland, however, has large‐scale consequences: Through the intensity of the overflow above the Iceland‐Scotland Ridge (ISR), this parameter affects the stratification north and south of the Rockall Plateau, the subpolar dynamics, and finally the North Atlantic Current path up to the Charlie Gibbs Fracture Zone. This slowly propagating dynamical effect, already noticed in previous studies, is shown to be forced baroclinically near the ISR.