bluuebunny/crossref_metadata_2025 huggingface dataset:
Convergence fronts in tidally forced rotating estuaries
Authors: Handler Robert A. , Mied Richard P. , Evans Thomas E. , Donato Timothy F.
Published: February 2004

In situ observation and remote sensing imagery reveal the presence of longitudinal velocity convergences over bathymetric channels in tidal estuaries. We present the results of numerical experiments designed to investigate the cause of these convergences for channels possessing shallow shoal regions and a deeper central region. The equations of motion for a homogeneous fluid on a rotating Earth are solved using a fully spectral code in the across‐estuary (i.e., the vertical or x‐z) plane, while no along‐estuary flow variations (in the y direction) are permitted. A Gaussian‐shaped bottom bathymetry is chosen. In the along‐channel (y) direction we impose a pressure gradient which is the sum of constant and fluctuating parts to simulate the steady and tidally oscillating parts of the estuarine flow. The details of the transient response can be complicated, but we observe that for most (∼80%) of the tidal cycle there exists a cross‐estuary recirculation cell colocated with a localized along‐channel jet. Both of these are situated over the bottom bathymetric groove; the circulation is always clockwise when facing down current. This feature results from the generation of stream‐wise vorticity through the tilting of planetary vorticity by the vertical shear of the along‐estuary flow. A surface convergence‐divergence pair is associated with the flow. The maximum value of each is seen to occur on the edge of the bathymetric feature but may migrate toward or away from the center as long as the current continues in the same direction. When the tide reverses, the feature reappears on the opposite shoal, and the migration of the convergence and divergence extrema begins again. We also find that the responses are qualitatively similar for all bathymetric grooves, even asymmetrically situated ones, provided that the estuary width‐to‐depth ratio is of order 100 or larger, the Rossby numbers are of order unity, and the Ekman layer thickness‐to‐channel‐depth ratio is greater than ∼0.65.
Modeling seasonal phosphate export and resupply in the Subantarctic and Polar Frontal zones in the Australian sector of the Southern Ocean
Authors: Wang Xiujun , Matear Richard J. , Trull Thomas W.
Published: February 2004

We developed and applied a one‐dimensional (z) biophysical model to the Subantarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) to simulate seasonal phosphate export production and resupply. The physical component of our model was capable of reproducing the observed seasonal amplitude of sea surface temperature and mixed layer depth. In the biological component of the model we used incident light, mixed layer depth, phosphate availability, and estimates of phytoplankton biomass from the Sea‐viewing Wide Field‐of‐view Sensor to determine production and tuned the model to reproduce the observed seasonal cycle of phosphate. We carried out a series of sensitivity studies, taking into account uncertainties in both physical fields and biological formulations (including potential influence of iron limitation), which led to several robust conclusions (as represented by the ranges below). The major growing season contributed 66–76% of the annual export production in both regions. The simulated annual export production was significantly higher in the PFZ (68–83 mmol P m−2) than in the SAZ (52–61 mmol P m−2) despite the PFZ's having lower seasonal nutrient depletion. The higher export production in the PFZ was due to its greater resupply of phosphate to the upper ocean during the September to March period (27–37 mmol P m−2) relative to that in the SAZ (8–15 mmol P m−2). Hence seasonal nutrient depletion was a better estimate of seasonal export production in the SAZ, as demonstrated by its higher ratio of seasonal depletion/export (64–78%) relative to that in the PFZ (34–47%). In the SAZ, vertical mixing was the dominant mechanism for supplying phosphate to the euphotic zone, whereas in the PFZ, vertical mixing supplied only 37% of the phosphate to the euphotic zone and horizontal transport supplied the remaining 63%.
A numerical study of the upwelling circulation off central Chile
Authors: Mesias Jorge M. , Matano Ricardo P. , Strub P. Ted
Published: February 2004

We present a modeling study of the upwelling ocean circulation off central Chile (34°–40°S). Using a primitive equation model, we make a numerical simulation of the ocean circulation for summer of 1993, a year characterized by moderate but persistent equatorward winds. The results indicate the formation of an eastern boundary current system that shows classical Ekman and geostrophic dynamics. A strong equatorward coastal jet develops during intense upwelling activity, induced by the prevailing equatorward winds. The spatial and temporal variability of the upwelling‐favorable winds strongly controls the variability in the oceanic circulation. Upwelling activity in the coastal areas occurs during periods of growth, peak, and decay. The circulation in the region is divided into two distinct dynamical areas that extend north and south of a prominent cape (Punta Lavapie), located in the center of the model domain. During the peak upwelling stage, the equatorward jet separates from the coast at Punta Lavapie and remains separated in the northern region. This detachment is followed by the formation of cyclonic eddies and decreased upwelling during periods of wind relaxation. The northern area is affected by advection processes and weaker local winds. In contrast, the equatorward flows in the southern region are persistently attached to the coastline and are more coherent with the stronger local winds.
Mesoscale characteristics
Authors: Jacobs G. A. , Barron C. N. , Rhodes R. C.
Published: February 2004

The spatial length, time, and propagation characteristics of the ocean mesoscale variability are examined throughout the globe. Sea surface height (SSH) variations from a combination of the Geosat Exact Repeat Mission, ERS‐1, ERS‐2, and TOPEX/Poseidon altimeter satellites are used to compute the observed covariance of the mesoscale. The mesoscale is defined as the residual SSH after removing a filtered large‐scale SSH having length scales greater than 750 km zonally and 250 km meridionally. From the observed binned covariance function, an objective analysis computes characteristics of the climatological mesoscale variability. Westward propagation is dominant throughout the globe. The Antarctic Circumpolar Current appears to affect the zonal propagation, as the propagation direction is eastward throughout this current. Length scales and propagation speeds generally decrease away from the equator and are slightly larger in the west basin areas than in the east basin areas. Zonal propagation speeds are roughly in agreement with speeds based on linear quasi‐geostrophic dynamics. The eddy field diverges from the equator and converges at 15° in the Northern and Southern Hemispheres. Requirements for synoptic mesoscale observation are examined.
Current measurements in the Halmahera Sea
Authors: Cresswell George R. , Luick John L.
Published: February 2004

A mooring with current meters at 400, 700, and 900 m at the 950‐m isobath south of the 700‐m‐deep sill across the Halmahera Sea revealed many signals between June 1993 and July 1994. Strong tidal currents of 50 cm/s dragged the mooring down by as much as 80 m on occasions when the lunar perigees corresponded with new or full moons. At 400 m the nontidal currents were southward at up to 25 cm/s from October to April and northwestward at up to 20 cm/s at other times. At 700‐m depth there was a near‐continuous nontidal southward flow of 9 cm/s across the sill into the Halmahera Basin, which accords with findings by earlier Dutch and Indonesian workers. The current meter at 900‐m depth showed the nontidal flow to be weak (∼1 cm/s) to the west. The southward transport between 350 and 700 m was inferred to reach a maximum of 5 Sv during the NW monsoon, with the annual mean being 1.5 Sv. There was a spring‐neap effect on the nontidal currents at 400 m that was most pronounced in the last few months of the mooring's life: these currents changed from ∼10 cm/s to the east during neap tides to ∼20 cm/s to the NNW during spring tides. Temperature and salinity profiles suggest that the waters of the Halmahera Sea are derived in part from the New Guinea Coastal Undercurrent.
Current measurements in the Maluku Sea
Authors: Luick John L. , Cresswell George R.
Published: February 2004

A current meter mooring was deployed in the eastern Indonesian archipelago, in the Maluku Sea, between the islands of Sulawesi and Halmahera, to contribute to estimates of the volume of the Indonesian Throughflow, for 13 months (June 1993‐July 1994), with meters located at 740, 1250, 1750, and 2240 m below the surface. Hydrocasts for salinity and temperature were made at the time of deployment. The current meter and hydrographic data indicate a predominantly southward flow at the upper three meters throughout the year and a steady northward flow at the bottom meter. T‐S and oxygen properties below 1800 m indicate a clockwise circulation in the Maluku Sea, with a vertical velocity of about 2 m/d. If the currents at the mooring, which is located on the western side of the Maluku Sea, are typical of the entire basin width, the average southward through flow between 740 m and 1500 m below the surface, 7 Sv (1 Sv = 106 m3/s ), is comparable in magnitude with estimates of upper layer transport and should be accounted for in estimates of the Indonesian Throughflow.
Theoretical expression for an ocean internal soliton synthetic aperture radar image and determination of the soliton characteristic half width
Authors: Zheng Quanan , Yuan Yeli , Klemas Vic , Yan Xiao‐Hai
Published: February 2004

This paper deals with the development of techniques for satellite synthetic aperture radar (SAR) ocean image interpretation. We derived a theoretical model of a radar image for a Korteweg‐de Vries type ocean internal soliton and validated the model using ocean internal wave signals taken from ERS‐1 SAR and RADARSAT SAR images. The results indicate that the model perfectly simulates ocean internal soliton signatures with double‐sign variations of radar backscatter. On the basis of the model, we developed the curve fitting method and the peak‐to‐peak method for determining the internal soliton characteristic half widths, which then were used to calculate the internal soliton amplitudes. The test results indicate that ocean internal soliton amplitudes derived by the two methods agree with in situ data acquired on the Portuguese Continental Shelf and in the South China Sea with reasonable accuracy. The role that wind fields play in ocean radar remote sensing was also analyzed. Finally, the modulation ratio of ocean internal waves on radar images was quantitatively estimated.
Satellite observations of small coastal ocean eddies in the Southern California Bight
Authors: DiGiacomo Paul M. , Holt Benjamin
Published: February 2004

This study describes the characteristics of extensive small‐scale coastal ocean eddies in the Southern California Bight. These surface features were primarily detected by using ERS‐1 and ERS‐2 synthetic aperture radar (SAR) satellite imagery from 1992 to 1998. The eddies, predominantly cyclonic in their rotation, appeared to result from several forcing mechanisms. They were mainly observed within the Santa Barbara Channel and the Santa Monica‐San Pedro Basin regions and appeared to be seasonal in their distribution. Observed eddy diameters were all less than 50 km, with over 70% less than 10 km. The SAR data were complemented by sea surface temperature measurements derived from advanced very high resolution radiometer satellite imagery, as well as in situ data from moorings and drifters that provided substantial verification of the small‐scale eddies. These findings are significant in that the eddies were, in general, smaller in size and more abundant than previously reported. Additionally, these results provide further evidence of the complex near‐surface circulation within the Southern California Bight, with important implications for nutrient flux, productivity, plankton patchiness, larval transport and recruitment, and dispersal of pollutants.
Mechanisms for the variation of sea ice extent in the northern hemisphere
Authors: Kimura Noriaki , Wakatsuchi Masaaki
Published: February 2004

Using daily sea ice data derived from satellite‐borne sensors and atmospheric data, we examined processes controlling the variation of sea ice extent in the Northern Hemisphere. The daily ice motion field was computed from imagery of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) for seven winters (December to April) from 1991/1992 to 1997/1998, by employing the maximum cross‐correlation method. In order to examine mechanisms of the temporal variation of the ice extent, we analyzed 50 specified lines across the daily ice edge. Although a high correlation between the ice motion and the geostrophic wind speed was observed in all the ice edge areas, the degree for correlation between the speed of the ice edge displacement and the wind speed varied with region. The degree for response of the ice edge speed to the wind speed largely depended upon that of the ice edge speed to the ice motion. The following mechanisms controlling the variation of ice extent for regions in the Northern Hemisphere were anticipated. In the Barents Sea, Bering Sea, and the Sea of Okhotsk the ice extent advances by wind‐driven ice advection and the daily scale variation of the ice extent were also controlled by the variation in wind speed. In contrast, the ice extent in the Labrador Sea and the Greenland Sea seemed to be considerably affected by oceanographic factors such as the location of the thermal front and was not related to the variation in wind speed. The regional difference of the variation mechanism was also reflected in the interannual variation in maximum ice extent.
Modeling the high‐frequency barotropic response of the ocean to atmospheric disturbances: Sensitivity to forcing, topography, and friction
Authors: Hirose Naoki , Fukumori Ichiro , Zlotnicki Victor , Ponte Rui M.
Published: February 2004

This study examines high‐frequency sea level variations forced by changes in surface atmospheric pressure and wind and their sensitivity to different forcing mechanisms, bottom topography resolution, and amount of friction in a barotropic ocean model. Optimal model performance, defined in terms of the explained variance in satellite altimeter and bottom pressure data, is found when using relatively strong friction, equivalent to a damping timescale of only a few days over the deep ocean, and topography with minimal smoothing. Spatial variations of the optimal friction parameter seem to reflect the roughness of bottom topography. The model demonstrates skill in simulating the wind‐driven response as well as the nonequilibrium response to atmospheric pressure variations.
Temporal variation in natural methane seep rate due to tides, Coal Oil Point area, California
Authors: Boles J. R. , Clark J. F. , Leifer I. , Washburn L.
Published: February 2004

Two large steel tents (each 30 m by 30 m), open at the bottom to the seafloor, capture ∼16,800 m3 d−1 (594 MCF) of primarily methane from a large natural hydrocarbon seep, occurring a kilometer offshore in 67 m of water. The gas is piped to shore where it is metered and processed. The seep flow rate was monitored hourly for 9 months. Our results show that the tidal forcing causes the flow rate to vary by 4–7% around the mean. These results are the first quantitative documentation of the effect of tides on natural gas seepage in relatively deep water. Time series analyses of the 9 month record clearly show four principal tidal components with periods of 12.0, 12.4, 23.9, and 25.8 hours. High tide correlates with reduced flow, and low tide correlates with increased flow. The correlation indicates that each meter increase of sea height results in a decrease of 10–15 m3 hr−1 or 1.5–2.2% of the hourly flow rate. The observed changes are best accounted for by a pore activation model, whereby gas is released from small pores at low pressures but is inhibited at higher pressure. Pressure‐dependent gas solubility changes are a less likely cause of flow variation. Our study implies that sea level differences, on a tidal timescale, can significantly change the gas seepage rate from sediments. Lower sea level in the last hundred thousand years would presumably allow higher gas loss from the sediment, assuming sufficient gas present, because of reduced hydrostatic pressure at the sediment‐sea interface. The magnitude of this long‐term change cannot be extrapolated from our tidal data.
Ocean wave height spectra computed from high‐altitude, optical, infrared images
Authors: Gelpi C. G. , Schuraytz B. C. , Husman M. E.
Published: February 2004

Calibrated ocean wave height spectra are derived from Airborne Visible/Infrared Imaging Spectrometer images. A modulation transfer function that relates the ocean surface wave slopes to the at‐sensor radiance is developed by extending the Cox and Munk [1954] model to account for image‐resolvable waves. Parameters in the transfer function include the atmospheric transmittance and the small‐scale wave slope statistics. These parameters are estimated from the change in at‐sensor radiance with respect to view angle. The derived transmittance and atmospheric path‐scattered radiance are consistent with results obtained from a radiative transfer calculation using a reasonable model for the aerosol loading. The optically measured wave heights agree well with National Oceanic and Atmospheric Administration buoy measurements extrapolated to the image region.
High‐resolution simulations of the turbulent flow in the vicinity of an Arctic lead
Authors: Weinbrecht Sonja , Raasch Siegfried
Published: February 2004

A lead (a break in the Arctic ice) allows contact between the warm ocean and the cold atmosphere. Because of its importance for the energy budget of the atmosphere many numerical simulations have been made so far to get detailed information about the vertical heat flux and the atmospheric turbulence induced by leads. Nevertheless, the spatial resolution of these simulations is still too coarse to resolve the turbulence directly above the lead, where the convective boundary layer (CBL) may increase to maximum heights of only a few meters. Whereas in the past spatial resolution was restricted due to insufficient hardware resources, today high‐performance computer architectures allow significant increases in model resolution. In this study a large‐eddy simulation model developed for a massively parallel computer is used to show that an increased model resolution leads to significant changes in the simulation results, which are in contrast with earlier studies of flows above and on the leeward side of a single lead. The better the model resolution is, the earlier the convection starts, and this affects the overall CBL structure. Organized roll‐like structures on the downstream side of the leads that appeared in earlier studies are shown to be completely caused by insufficient grid resolution. The effect of model resolution on the simulation results becomes even greater for higher wind speeds because with increasing geostrophic wind speed (and mean wind speed) the boundary layer depth decreases and an even finer grid becomes necessary.
Spatial and temporal variability of absorption by dissolved material at a continental shelf
Authors: Boss Emmanuel , Pegau W. Scott , Zaneveld J. Ron V. , Barnard Andrew H.
Published: February 2004

Optical properties of dissolved (colored dissolved organic material (CDOM)) and particulate matter and hydrographic measurements were obtained at the Mid‐Atlantic Bight during the fall of 1996 and the spring of 1997 as part of the Coastal Mixing and Optics experiment. To assess the temporal and spatial variability, time series were obtained at one location and cross‐shelf transects were carried out. On short timescales, variability in the vertical distribution of the dissolved fraction was mostly due to high‐frequency internal waves. This variability was conservative, resulting in no changes on isopycnals. Over longer periods and episodically, CDOM variability was dominated by storms. The storms were associated with sediment resuspension events and were accompanied by an increase in the absorption by the dissolved materials. Data from spatial transects show that near the bottom, over the shelf, and in both spring and fall, increased particulate absorption and increased CDOM absorption co‐occur. These data support the hypothesis that bottom sediments can act as a source of dissolved organic carbon during sediment resuspension events.
On modeling the anisotropic failure and flow of flawed sea ice
Authors: Hibler W. D. , Schulson Erland M.
Published: February 2004

The failure and flow of sea ice on scales small and large is characterized by the propagation of oriented leads and cracks. In this paper a theory for the dynamical treatment of anisotropic oriented flaws in sea ice is developed and used to examine the interaction of these oriented flaws under idealized stress forcing. The essential idea of the theory is to take one or more oriented weak leads imbedded in thick ice. A constitutive law for both the thin and thick ice is taken to be similar to laboratory observations, which are consistent with Mohr Coulomb‐like failure. Application of normal continuum mechanics boundary conditions leads to anisotropic flow and failure characteristics of this anisotropic composite under both near‐ and far‐field forcing. The local failure characteristics show that there is a preferred orientation for failure with the intersection between “leads” increasing as confinement increases. For spatially separated interacting flaws the theory predicts a preference for the weakening of flaws in a narrow range of angles of ∼10°–20° relative to the principal far‐field stress. Imbedding isolated flaws leads to simulated damage directions consistent with local failure characteristics. In the case where the ice is considered to have available flaws in all directions, fracture propagation is found to proceed by picking out oriented flaws alternating in direction along large‐scale damage strikes with individual flaw alignments of ∼20° relative to far‐field principal stresses. The mechanisms responsible for these results and how this anisotropic sea ice rheology might be implemented in current dynamic/thermodynamic sea ice models are discussed.
Role of surface films in ERS SAR signatures of internal waves on the shelf: 3. Mode transitions
Authors: da Silva J. C. B. , Ermakov S. A. , Robinson I. S.
Published: February 2004

It was shown recently that three different types of short‐period internal wave (IW) synthetic aperture radar (SAR) signatures occur on the Iberian Shelf depending on parameters of surface active films. Large‐scale modulation of film characteristics by currents associated with internal tidal waves can also produce a signature in ERS SAR images. On the western Iberian Shelf short‐period IWs typically propagate in groups of packets that are believed to be generated by the evolution of internal tidal waves over the shelf. Here onshore propagating short‐period IWs can exhibit different SAR signatures within the same wave packet according to their position relative to the phase of the internal tidal wave. These transitions of signature mode can be explained by large‐scale variations of film parameters produced by the internal tidal waves. At low‐to‐moderate winds, leading waves in the IW packets usually exhibit double sign signatures, which evolve to negative sign signatures for the subsequent waves in the packets. At very low, near‐threshold winds the IW signatures can be in the form of the double sign type undergoing transition to positive sign signatures when compared to the background behind the IW packet. This is a consequence of different threshold wind speeds for excitation of Bragg waves at different film pressures. An increase of unperturbed film pressure in the direction of the ranking order of solitons in a packet of short‐period IWs, which is predicted by a theoretical model and in agreement with measurements of film elasticity, is demonstrated. Observations of SAR signature mode transitions are concluded to be consistent with previous model results, and a diagram describing mode transitions of the IW signatures in relation to film pressure and wind speed is presented.
Effect of the Earth's rotation on the circulation in regions of freshwater influence
Authors: Kasai Akihide , Hill A. Edward , Fujiwara Tateki , Simpson John H.
Published: February 2004

Recent surveys in various regions of freshwater influence have shown considerably different flow patterns in each region. An analytical model including viscous effects and the Earth's rotation is proposed to examine the along‐channel flow pattern and to explain the differences. Model results show that the flow pattern is strongly dependent on the Ekman number E. With a large Ekman number (E > 1) the system is governed by gravitational circulation, and thus horizontal density gradients in along‐channel direction are important. The whole water column is in the Ekman layer and consequent jet inflow penetrates the surface over a deep depression if the bottom topography varies in a cross‐channel direction. With an intermediate Ekman number (E ∼ 0.1) this jet inflow concentrates in the lower layer in which the viscosity still plays an important role. The flow in the upper layer is, on the other hand, determined by the geostrophic balance. The contribution by the geostrophic flow becomes larger so that the cross‐channel density gradients are important when die Ekman number is small (E < 0.01). Since the Ekman layer is clung to the bottom, the jet inflow exists only in the thin bottom layer. The hydrographic and acoustic Doppler current profiler surveys were conducted in Ise Bay, Japan. Both the density structure and the flow pattern were different from those observed in many drowned river valleys. A strong jet inflow existed in the lower layer over the depression while the flow in the upper layer suggested anticyclonic circulation. The estimated Ekman number is 0.07 in the bay and thus the observed pattern is consistent with the model result when E = 0.1.
Global high‐resolution mapping of ocean circulation from TOPEX/Poseidon and ERS‐1 and ‐2
Authors: Ducet N. , Le Traon P. Y. , Reverdin G.
Published: February 2004

This study focuses on the improved estimation of mesoscale surface ocean circulation obtained by merging TOPEX/Poseidon (T/P) and ERS‐1 and ‐2 altimeter measurements between October 1992 and May 1998. Once carefully intercalibrated and homogenized, these data are merged through an advanced global objective analysis method that allows us to correct for residual long wavelength errors and uses realistic correlation scales of ocean dynamics. The high‐resolution (0.25°×0.25°) merged T/P+ERS‐1 and ‐2 sea level anomaly maps provide more homogeneous and reduced mapping errors than either individual data set and more realistic sea level and geostrophic velocity statistics than T/P data alone. Furthermore, the merged T/P+ERS‐1 and ‐2 maps yield eddy kinetic energy (EKE) levels 30% higher than maps of T/P alone. They also permit realistic global estimates of east and north components of EKE and their seasonal variations, to study EKE sources better. A comparison of velocity statistics with World Ocean Circulation Experiment surface drifters in the North Atlantic shows very good agreement. Comparison with contemporary current meter data in various oceanic regimes also produces comparable levels of energy and similar ratios of northward and eastward energy, showing that the maps are suitable to studying anisotropy. The T/P + ERS zonal and meridional components of the mapped currents usually present comparable rms variability, even though the variability in the Atlantic is more isotropic than that in the Pacific, which exhibits strong zonal changes. The EKE map presents a very detailed description, presumably never before achieved at a global scale. Pronounced seasonal changes of the EKE are found in many regions, notably the northeastern Pacific, the northeastern and northwestern Atlantic, the tropical oceans, and the zonally extended bands centered near 20°S in the Indian and western Pacific Oceans and at 20°N in the northwestern Pacific.
Rossby waveguides in high‐latitude shear flows with boundaries
Authors: Harlander Uwe , Schönfeldt Hans‐Jürgen , Metz Werner
Published: February 2004

This study investigates the dynamics of trapped Rossby waves at high latitudes in zonal ocean currents with linear and jet‐like shears. As integration domain, we use a modified β plane (with a rigid boundary at the poleward side), where the second derivative of the Coriolis parameter with respect to latitude is taken into account (δ term). We distinguish between trapped modal Rossby waves and trapped Rossby wave packets. The former exhibit a monochromatic wave structure alongshore but have a modal structure in the direction perpendicular to the boundary. In contrast, trapped wave packets form a local disturbance and consist of a superposition of a range of different Fourier modes rather than a single wave. We study the propagation and the structural change of barotropic quasi‐geostrophic Rossby wave packets by means of a Wenzel, Kramers, and Brillouin (WKB) method and consider also the amplitudes and wave numbers of the corresponding trapped modal waves. The amplitude equation is solved either by the WKB method or by analytical and numerical techniques. Basic flows and wave frequencies are prescribed in such a way that no critical lines can occur. We compare analytical, asymptotical, and numerical solutions of trapped modal waves and discuss the influence of the Earth's sphericity on the wave packet propagation and on the eigenvalues and eigenfunctions of the trapped modal waves. Dispersion diagrams of the coastally trapped modal Rossby waves in the considered shear flows are also presented. The dispersion curves show that trapped modal Rossby waves exist for a wide range of different basic flow amplitudes, shear strengths, and frequencies. It further turns out that for a high‐latitude β plane, the δ term should definitely be taken into account. We suggest that such a selective Rossby waveguide may play a role in the dynamics of the Antarctic Circumpolar Current.
A semiannual Indian Ocean forced Kelvin wave observed in the Indonesian seas in May 1997
Authors: Sprintall Janet , Gordon Arnold L. , Murtugudde Ragu , Susanto R. Dwi
Published: February 2004

Recent observations within the Indonesian exit passages and internal seas highly resolve the arrival and passage of a semiannual Kelvin wave. In mid‐May 1997, surface and subsurface currents were to the southeast at a mooring located south of Java in the South Java Current, while local wind forcing was northwestward. Subsequent northward fluctuations in the geostrophic current through Lombok Strait and in observed currents from two moorings located in Makassar Strait are commensurate with the speed and passage of a Kelvin wave through the region. The Kelvin wave was due to westerly wind forcing in the remote equatorial Indian Ocean during the semiannual April/May monsoon transition period. This was confirmed through a simple remote wind‐forced analytical Kelvin wave model of velocity at the South Java Current mooring location and sea level in Lombok Strait and also in the numerical general circulation model of Murtugudde et al. [1998]. Warm temperature anomalies measured at the south Java mooring and within Makassar Strait are associated with the passage of the Kelvin wave. Salinity anomalies measured at the south Java mooring are consistent with an Indian Ocean source. The observed passage of the Kelvin wave during May 1997 unambiguously demonstrates for the first time that equatorial Indian Ocean remote wind forcing may on occasions influence the internal Indonesian seas
An optimal definition for ocean mixed layer depth
Authors: Kara A. Birol , Rochford Peter A. , Hurlburt Harley E.
Published: February 2004

A new method is introduced for determining ocean isothermal layer depth (ILD) from temperature profiles and ocean mixed layer depth (MLD) from density profiles that can be applied in all regions of the world's oceans. This method can accommodate not only in situ data but also climatological data sets that typically have much lower vertical resolution. The sensitivity of the ILD and MLD to the temperature difference criteria used in the surface layer depth definition is discussed by using temperature and density data, respectively: (1) from 11 ocean weather stations in the northeast Pacific and (2) from the World Ocean Atlas 1994. Using these two data sets, a detailed statistical error analysis is presented for the ILD and MLD estimation by season. MLD variations with location due to temperature and salinity are properly accounted for in the defining density (Δσt) criterion. Overall, the optimal estimate of turbulent mixing penetration is obtained using a MLD definition of ΔT =0.8°0, although in the northeast Pacific region the optimal MLD criterion is found to vary seasonally. The method is shown to produce layer depths that are accurate to within 20 m or better in 85% or more of the cases. The MLD definition presented in this investigation accurately represents the depth to which turbulent mixing has penetrated and would be a useful aid for validation of one‐dimensional bulk mixed layer models and ocean general circulation models with an embedded mixed layer.
Processes that influence sea surface temperature and ocean mixed layer depth variability in a coupled model
Authors: Alexander Michael A. , Scott James D. , Deser Clara
Published: February 2004

A 50‐year coupled atmosphere‐ocean model integration is used to study sea surface temperature (SST) and mixed layer depth (h), and the processes which influence them. The model consists of an atmospheric general circulation model coupled to an ocean mixed layer model in ice‐free regions. The midlatitude SST variability is simulated fairly well, although the maximum variance is underestimated and located farther south than observed. The model is clearly deficient in the vicinity of the Gulf Stream and in the eastern tropical Pacific where advective processes are important. The model generally reproduces the observed structure of the mean h in both March and September but underestimates it in the North Atlantic during winter. The net surface heat flux strongly regulates both the mean (¯) and the anomalous (') SSTs throughout the year. The entrainment heat flux, which is proportional to the product of the entrainment rate (We) and the temperature jump at the base of the mixed layer (ΔT), influences SSTs in summer and fall, especially north of ∼35°N (45°N) in the Pacific (Atlantic).
is more important for the development of SST' in fall compared to
, which is larger in summer. The entrainment rate is dominated by wind‐induced mixing in summer and surface buoyancy forcing in winter; the density jump at the base of the mixed layer is of secondary importance. In addition, anomalies in h have a significant impact on the heat balance of the mixed layer during spring and summer. Deep winter mixed layers and the storage of thermal anomalies beneath the shallow mixed layer in summer leads to large winter‐to‐winter persistence of SST anomalies in the far North Atlantic, in accord with observations and stochastic climate theory.
Tidal energy fluxes and dissipation on the European continental shelf
Authors: Davies Alan M. , Kwong Simon C. M.
Published: February 2004

The spatial distribution of the energy flux, energy dissipation, current magnitude, and surface elevation amplitude of the major tidal constituents over the northwest European shelf are examined in detail. These distributions are obtained from the harmonic analysis of a 6 month three‐dimensional simulation of 28 tidal constituents. The model currents are validated by a comparison of computed tidal harmonics against those derived from a harmonic analysis of up to 278 current time series. A similar comparison is performed for tidal elevations based on 257 tide gauges. Calculations show that for the most significant tidal constituents (e.g., M2, S2 and K2) there is a major energy flux in the deep water along the shelf edge off the northwest of Scotland, with some energy leaking onto the shelf and into the North Sea. A second source of energy is across the shelf edge at the southern end of the Celtic Sea, with this energy flux propagating into the Irish Sea and southern North Sea, where the majority of the energy is dissipation in the shallow regions. Significantly different distributions are found for the diurnal and shallow water constituents, and the spatial distributions of energy flux and dissipation of the various constituents are considered. The accuracy of separating tidal current harmonics using model data of less than a synodic period is examined with reference to the S2 and K2 tide. Calculations suggest that the accuracy of computed currents comparable to those obtained from observations can be obtained from an analysis of a 60 day period compared with the synodic period for S2 and K2 of 182 days, a significant saving in computer time.
Importance of peakedness in sea surface slope measurements and applications
Authors: Chapron B. , Kerbaol V. , Vandemark D. , Elfouhaily T.
Published: February 2004

We recall the simple statistical concept that non‐Gaussian distribution peakedness results from the compounding of random processes. This idea is applied to observations and analysis of sea surface slopes as inferred using optical and microwave‐scattering measurements. Our study emphasizes the importance of identifying and quantifying the distribution variance and kurtosis from observations. Data are shown to indicate consistently non‐Gaussian peakedness, to indicate the need to report at least two parameters in an even order analysis, and to indicate near equivalence between radar and optical data. Physical interpretation for observed infrequent steep slopes is given via compounding statistical processes where normally distributed short‐scale waves are modulated because of random fluctuations mainly associated with the underlying long wave field. Implications of non‐Gaussian peakedness are provided for altimeter backscatter theory and for modeling wave‐breaking probability.
Propagation of swell across a wide continental shelf
Authors: Herbers T. H. C. , Hendrickson E. J. , O'Reilly W. C.
Published: February 2004

The transformation of ocean swell across a wide, shallow (nominal depths 25–50 m) continental shelf is examined with data from a 100 km long transect of bottom pressure recorders extending from the shelf break to the beach at Duck, North Carolina. The analysis is restricted to periods with light winds when surface boundary layer processes (e.g., wave generation by wind and wave breaking in the form of whitecaps) are expected to be relatively unimportant. The majority of the observations with low‐energy incident swell conditions (significant wave heights <1 m) shows weak variations in swell energy across the shelf, in qualitative agreement with predictions of a spectral refraction model. Although the predicted ray trajectories of waves propagating over the irregular shelf bathymetry are quite sensitive to the deep water incident wave directions, the predicted spatial energy variations for broadbanded wave fields are small and relatively insensitive to incident wave conditions, consistent with the observations. Whereas swell dissipation on the shelf appears to be insignificant in low‐energy conditions, strong attenuation of swell energy levels (a factor 4 between the shelf break and nearshore sites) was observed in high‐energy conditions (significant wave height 2.5 m). This decay, not predicted by the energy‐conserving refraction model, indicates that dissipative bottom boundary layer processes can play an important role in the transformation of swell across wide continental shelves.
Salt conservation, free surface, and varying levels: A new formulation for ocean general circulation models
Authors: Roullet Guillaume , Madec Gurvan
Published: February 2004

In order to clarify the link between ocean salt content (OSC) conservation and the freshwater flux formulation in ocean general circulation models (OGCMs), a varying level thickness, nonlinear free surface version of the OPA is presented. Linear/nonlinear free surface equations are solved using an original approach based on an explicit damping of fast external gravity waves. The method leaves both the potential vorticity equation and the equilibrium state unchanged. Its numerics and cost are quite similar to those of implicit schemes. When nonlinearities are kept, a variable first level thickness is required. Its discretization is determined by volume and energy constraints. The OSC conservation depends on the surface kinematic equation used. Four formulations are presented: (1) virtual salt flux (fixed ocean volume and no volume flux), (2) natural (fixed ocean volume and volume flux), (3) linear free surface (fixed volume and volume flux computed from a linear free surface equation), and (4) assumption free (variable volume computed from a nonlinear free surface equation). Their impact is illustrated in 25 year low‐resolution global OGCM simulations. In all cases the first‐order ocean response is quite similar, as the concentration‐dilution effect always exists. Formulations 4 and 2 ensure a strict conservation of the OSC. Nevertheless, the difference in formulation 3 is not strong enough to play a significant role: the conservation is almost perfect. Only formulation 1 neglects fresh water‐driven surface pumping. This mainly modifies the sea surface salinity of the ocean basin where river runoffs are strong. No significant difference is found between the other formulations as a large time step dampens high‐frequency free surface motion. The best compromise for climate is the linear free surface formulation. It allows a nearly exact OSC conservation, introduces the fresh water‐driven pumping, and runs faster than all the other formulations.
Implications of the deep circulation and ventilation of the Indian Ocean on the renewal mechanism of North Atlantic Deep Water
Authors: You Yuzhu
Published: February 2004

In the Indian Ocean sector of the “great ocean conveyor” scheme the North Atlantic Deep Water (NADW) replacement is realized by the transformation of the sinking Circumpolar Deep Water (CDW) from the Southern Ocean to a prevailingly upward flow in the northern Indian Ocean. This water‐mass transformation scenario has been studied by using hydrographie data including potential temperature, salinity, dissolved oxygen, and nutrients in a water‐mass mixing model and by calculation of dianeutral velocity. The model comprises a mixing system of three major deep water masses, NADW, CDW, and North Indian Deep Water. This third water mass is introduced as a virtual water mass to represent the aged CDW. The assumption is feasible as the artificial water mass can be eliminated on the deepest σN = 28.12 neutral surface, characterized by isopycnal mixing. The experiment is fulfilled when a conservative variable, initial phosphate PO40, is introduced and all conservative parameters are used in the mixing scheme. The σN = 28.12 surface provides a major isopycnal transition path of CDW to the northern Indian Ocean, and thus CDW is transformed to an upward flow. Dominant dianeutral up welling is detected on all four neutral surfaces north of 30°S, showing increasingly strong dianeutral velocity toward shallower surfaces. With the water‐mass mixing pattern and spreading path, dianeutral circulation, and dynamical information of the acceleration potential (10 m2 s−2) mapped on neutral surfaces a schematic of deep water circulation and ventilation of the Indian Ocean emerges from this study. The contribution of NADW is largely limited to the southwest Indian Ocean. The northward CDW is found mainly in two paths: one in the western Indian Ocean (apparent between 2000 and 3000 m) through the western Crozet and Madagascar Basins, and farther north into the Somali Basin and Arabian Basin and another in the Central Indian Basin (apparent below 3000 m where ridge‐blocking becomes effective in the west and NADW salinity reaches a maximum). While the major path switches from west to the central basin in the deeper layer (3000–3500 m), the flow in the western Indian Ocean reverses southward. North of the Southeast Indian Ridge, the deep path bifurcates, from which point the eastern branch feeds the deep western boundary flow in the West Australian Basin. Except for part of the transformed CDW, which is contributed to the upward flow in the north, the return flow of CDW is found on the eastern side of these basins.
Baroclinic transport variability of the Antarctic Circumpolar Current south of Australia (WOCE repeat section SR3)
Authors: Rintoul Stephen R. , Sokolov Serguei
Published: February 2004

Baroclinic transport variability of the Antarctic Circumpolar Current (ACC) near 140°E is estimated from six occupations of a repeat section occupied as part of the World Ocean Circulation Experiment (WOCE section SR3). The mean top‐to‐bottom volume transport is 147±10 Sv (mean ±1 standard deviation), relative to a deep reference level consistent with water mass properties and float trajectories. The location and transport of the main fronts of the ACC are relatively steady: the Subantarctic Front carries 105±7 Sv at a mean latitude between 51° and 52°S; the northern branch of the Polar Front carries 5±5 Sv to the east between 53° and 54°S; the southern Polar Front carries 24±3 Sv eastward at 59°S; and two cores of the southern ACC front at 62° and 64°S carry 18±3 and 11±3 Sv, respectively. The variability in net property transports is largely due to variability of currents north of the ACC, in particular, an outflow of 8±13 Sv of water from the Tasman Sea and a deep anticyclonic recirculation carrying 22±8 Sv in the Subantarctic Zone. Variability of net baroclinic volume transport is similar in magnitude to that measured at Drake Passage. In density layers, transport variability is small in deep layers, but significant (range of 4 to 16 Sv) in the Subantarctic Mode Water. Variability of eastward heat transport across SR3 is significant (range of 139°C Sv, or 0.57×1015 W, relative to 0°C) and large relative to meridional heat flux in the Southern Hemisphere subtropical gyres. Heat transport changes are primarily due to variations in the westward flow of relatively warm water across the northern end of the section. Weak (strong) westward flow and large (small) eastward heat flux coincides with equatorward (poleward) displacements of the latitude of zero wind stress curl.