bluuebunny/crossref_metadata_2025 huggingface dataset:
Iceberg trajectory modeling and meltwater injection in the Southern Ocean
Authors: Gladstone Rupert M. , Bigg Grant R. , Nicholls Keith W.
Published: February 2004

This is the first large‐scale modeling study of iceberg trajectories and melt rates in the Southern Ocean. An iceberg model was seeded with climatological iceberg calving rates based on a calculation of the net surface accumulation from each snow catchment area on the Antarctic continent. In most areas, modeled trajectories show good agreement with observed patterns of iceberg motion, though discrepencies in the Weddell Sea have highlighted problems in the ocean general circulation model output used to force the iceberg model. The Coriolis force is found to be important in keeping bergs entrained in the coastal current around Antarctica, and topographic features are important in causing bergs to depart from the coastal regions. The modeled geographic distribution of iceberg meltwater joining the ocean has been calculated and is found in many near‐coastal regions to be comparable in magnitude to the excess of precipitation over evaporation (P‐E).
Generating deep water three‐dimensional waves by coupling four‐wave and five‐wave interactions
Authors: Lin Ray Q. , Su Ming‐Yang
Published: February 2004

Su [1982a, b] reported the results of experiments that demonstrate the occasional predominance of three‐dimensional waves in deep waters. Three‐dimensional waves require the interaction of five‐ (or more) wave modes. However, both instability analysis [MacLean, 1982] and numerical simulation [Lin and Perrie, 1997a, b] demonstrate that five‐wave interactions have nonlinear transfer rates 2 orders of magnitude less than those for four‐wave interactions in deep waters. Su and Green [1984] suggested that two‐dimensional instabilities generate waves whose steepening beyond a critical point then triggers three‐dimensional instabilities, a phenomenon known to occur in shallow water. We report the results of simulations that confirm the coupling of four‐ and five‐wave interactions in deep water but show that this coupling occurs in response to decreases in spectral width rather than increases in wave steepness. In contrast to the shallow water scenario, the wave steepness never reaches critical values.
Importance of solar subsurface heating in ocean general circulation models
Authors: Rochford Peter A. , Kara A. Birol , Wallcraft Alan J. , Arnone Robert A.
Published: February 2004

The importance of subsurface heating on surface mixed layer properties in an ocean general circulation model (OGCM) is examined using attenuation of solar irradiance with depth below the ocean surface. The depth‐dependent attenuation of subsurface heating is given by global monthly mean fields for the attenuation of photosynthetically available radiation (PAR), kPAR. These global fields of kPAR are derived from Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) data on the spectral diffuse attenuation coefficient at 490 nm (k490), and have been processed to have the smoothly varying and continuous coverage necessary for use in OGCM applications. These monthly fields provide the first complete global data sets of subsurface optical fields that can be used for OGCM applications of subsurface heating and bio‐optical processes. The effect on global OGCM prediction of sea surface temperature (SST) and surface mixed layer depth (MLD) is examined when solar heating, as given by monthly mean kPAR and PAR fields, is included in the model. It is found that subsurface heating yields a marked increase in the SST predictive skill of the OGCM at low latitudes. No significant improvement in MLD predictive skill is obtained when including subsurface heating. Use of the monthly mean kPAR produces an SST decrease of up to 0.8°C and a MLD increase of up to only 4–5 m for climatological surface forcing, with this primarily confined to the equatorial regions. Remarkably, a constant kPAR value of 0.06 m−1, which is indicative of optically clear open ocean conditions, is found to serve very well for OGCM prediction of SST and MLD over most of the global ocean.
Modeling the upper ocean dynamics in the Subantarctic and Polar Frontal zones in the Australian sector of the Southern Ocean
Authors: Wang Xiujun , Matear Richard J.
Published: February 2004

A one‐dimensional (1‐D) mixed layer model (the Chen scheme) was applied in the Subantarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) to simulate the upper ocean dynamics. The model was forced with 4 years data of the heat fluxes, freshwater fluxes, and wind stresses from the National Centers for Environmental Prediction. In both the SAZ and PFZ, the 1‐D model was capable of reproducing the amplitude of the seasonal sea surface temperature (SST) and the seasonally of the mixed layer depth (MLD). The shallowest MLD was found in January‐February (20 m in the SAZ, 35 m in the PFZ), and the deepest MLD was found between August and October (600 m in the SAZ, 160 m in the PFZ). The summer MLD was shallower in the SAZ than in the PFZ due to the lower wind stress. However, the shallower winter MLD in the PFZ than in the SAZ was due to the strong stratification in the water below the mixed layer. In the SAZ, variability in the wind stress was the dominant term driving the fluctuation in MLD in the summer, but variability in the heat flux was the major factor controlling the timing of the deepening and shoaling of the mixed layer in the winter. In the PFZ both the variability in the wind stress and the heat flux dominated the variability of the MLD in both the summer and the winter.
Dynamical interpolation of mesoscale flows in the TOPEX/Poseidon diamond surrounding the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time‐series study site
Authors: McGillicuddy D. J. , Kosnyrev V. K.
Published: February 2004

An open boundary ocean model is configured in a domain bounded by the four TOPEX/Poseidon (T/P) ground tracks surrounding the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time‐series Study (BATS) site. This implementation facilitates prescription of model boundary conditions directly from altimetric measurements (both T/P and ERS‐2). The expected error characteristics for a domain of this size with periodically updated boundary conditions are established with idealized numerical experiments using simulated data. A hindcast simulation is then constructed using actual altimetric observations during the period October 1992 through September 1998. Quantitative evaluation of the simulation suggests significant skill. The correlation coefficient between predicted sea level anomaly and ERS observations in the model interior is 0.89; that for predicted versus observed dynamic height anomaly based on hydrography at the BATS site is 0.73. Comparison with the idealized experiments suggests that the main source of error in the hindcast is temporal undersampling of the boundary conditions. The hindcast simulation described herein provides a basis for retrospective analysis of BATS observations in the context of the mesoscale eddy field.
A quasi‐one‐dimensional coupled climate‐change cycle model: 1. Description and behavior of the climate component
Authors: Harvey L. D. Danny , Huang Z.
Published: February 2004

A quasi‐one‐dimensional, coupled climate‐carbon cycle model is presented, which consists of two polar domains and one nonpolar domain. The model simulates the distribution of dissolved inorganic carbon (DIC), alkalinity, phosphate, dissolved oxygen, and temperature and contains a biological pump with production of organic tissue, calcite, and arogonite. Bottom water is conditioned in one polar domain through interaction with the atmosphere and convective mixing and is injected into the lower portion of the nonpolar domain. Bottom water formed in the downwelling polar domain largely upwells in the nonpolar domain, although a portion upwells from intermediate depth into the other polar domain. In this paper the climate component of the coupled model and its behavior are described, while the carbon cycle component is documented by Harvey [this issue], hereafter referred to as part 2. We develop a simple physical basis for determining the relative magnitudes of the effective vertical diffusion coefficient (kv) for different tracers in a one‐dimensional (1‐D) model and find that kv is smallest for temperature, intermediate for carbon, and largest for dissolved oxygen. We deduce a substantially smaller kv for temperature in the upper ocean than previously used in 1‐D models (∼0.2 cm2 s−2 rather than 0.6–1.0 cm2 s−1) and a smaller peak upwelling velocity (2 m yr−1 rather than 4 m yr−1). The explicit representation of convective mixing has a significant effect on the model surface temperature transient response and sea level rise when the intensity of the thermohaline changes. As a result, the transient temperature response and sea level rise obtained here when the thermohaline circulation intensity decreases is significantly different from that of the classical 1‐D upwelling‐diffusion model.
SeaCluse: Numerical simulation of evaporating sea spray droplets
Authors: Van Eijk Alexander M. J. , Tranchant Benoît S. , Mestayer Patrice G.
Published: February 2004

The SeaCluse computer code calculates the nonlinear interactions between sea spray droplet concentrations and the scalar fields of water vapor concentration and temperature in the marine atmospheric surface layer. The code simulates most of the dynamics of the evaporating sea spray droplets in the turbulent air flow over a wavy surface, the thermodynamic transformations of the spray droplets, and the influence of the droplets on the structure of the lower marine atmosphere. The main processor computes along the vertical the horizontally averaged budgets of droplet concentration, water vapor concentration, and sensible heat, including the dynamic and thermodynamic air‐droplet interactions. This paper presents the first simulations of evaporating sea spray droplets made with the SeaCluse model. It is shown that common assumptions such as exponential vertical concentration profiles and the evaporation of all droplets to their equilibrium radius do not necessarily hold for droplets with a radius larger than ∼30 μm. The present results also demonstrate the importance of the nonlinear interactions between sea spray droplets and the scalar fields of water vapor and temperature. Most droplets evaporate close to the surface, in a layer of ∼2–3 times the wave height. The impact of evaporating sea spray droplets increases rapidly with increasing wind speed.
Tropical ocean response to atmospheric forcing at kilometer scales with light precipitation
Authors: Lebedev Ivan , Redelsperger Jean‐Luc , Tomczak Matthias
Published: February 2004

The response of the oceanic mixed layer to atmospheric forcing during a 6 day period of weakly precipitating tropical atmospheric convection is investigated. A simulation of a cloud‐resolving model is used to provide ground level fields of atmospheric variables that drive the upper ocean model. The effect of kilometer scale oceanic variability on the area average air‐sea heat flux is assessed and found negligible. Spatial wind variability is found to be the dominant factor leading to temperature anomalies at the sea surface. A simple parameterization of this effect for weakly precipitating conditions is suggested.
Influence of precipitation minus evaporation and Bay of Bengal rivers on dynamics, thermodynamics, and mixed layer physics in the upper Indian Ocean
Authors: Han Weiqing , McCreary Julian P. , Kohler Kevin E.
Published: February 2004

A 4½‐layer model with active thermodynamics and mixed layer physics is used to examine how salinity distributions forced by precipitation P minus evaporation ε and by river runoff in the Bay of Bengal affect dynamics, thermodynamics, and mixed layer physics in the upper Indian Ocean. Each of the four active layers represents a distinct water mass type: the surface mixed layer, the seasonal thermocline (barrier layer in the tropics), the thermocline, and upper intermediate water. Waters are allowed to transfer between layers by interfacial velocities w1, w2, and w3. Velocity w1 parameterizes entrainment and detrainment from the surface mixed layer, and it is determined largely by Kraus and Turner [1967] physics. Velocity w2 is primarily a parameterization of subduction. In regions where precipitation is strong enough for P − ε > 0, forcing by P − ε thins the surface mixed layer (layer 1) because of decreased entrainment, and thus thickens the seasonal thermocline (layer 2, a barrier layer). Additionally, surface currents generally strengthen, T2 warms considerably, and sea surface temperature (SST) increases somewhat, resulting in temperature inversions at some locations in the southern bay and eastern equatorial ocean. This forcing also causes large temperature changes in the thermocline (layer 3), primarily because of heating or cooling by anomalous subduction. During the Southwest Monsoon, forcing by inflow from Bay of Bengal rivers increases SST by 0.5°–1°C along the northeast coast of India. This is because coastal Kelvin waves driven by the Ganges‐Brahmaputra River inflow suppress coastal upwelling there. During the Northeast Monsoon, fresh river water is carried southward by the East India Coastal Current (EICC), raising sea level along the coast and strengthening the EICC by 10 cm s−1. The river water decreases entrainment around the perimeter of the bay during winter, thereby thinning the surface mixed layer, increasing T2 and resulting in temperature inversions in the northwestern bay. River inflow also causes significant temperature anomalies in layer 3 by affecting subduction.
A model for estimating bulk refractive index from the optical backscattering ratio and the implications for understanding particle composition in case I and case II waters
Authors: Twardowski Michael S. , Boss Emmanuel , Macdonald Jacob B. , Pegau W. Scott , Barnard Andrew H. , Zaneveld J. Ronald V.
Published: February 2004

A model based on Mie theory is described that estimates bulk participate refractive index
from in situ optical measurements alone. Bulk refractive index is described in terms of the backscattering ratio and the hyperbolic slope of the particle size distribution (PSD). The PSD slope ξ is estimated from the hyperbolic slope of the particulate attenuation spectrum γ according to the relationship γ ≈ ξ − 3, verified with Mie theory. Thus the required in situ measurements are the particulate backscattering coefficient, the total particulate scattering coefficient, and the particulate attenuation coefficient. These parameters can be measured with commercially available instrumentation with rapid sampling rates and real‐time data return. Application of the model to data from the Gulf of California yielded results that agreed with expectations, e.g., predicted
was 1.04–1.05 in the chlorophyll maximum and 1.14–1.18 near sediments. Below the chlorophyll maximum in case I type waters, predicted
values were between 1.10 and 1.12, suggesting the presence of a significant inorganic mineral component in the background or detrital organic particles with low water content.
Improved neural network scatterometer forward models
Authors: Cornford Dan , Nabney Ian T. , Ramage Guillaume
Published: February 2004

Current methods for retrieving near‐surface winds from scatterometer observations over the ocean surface require a forward sensor model which maps the wind vector to the measured backscatter. This paper develops a hybrid neural network forward model, which retains the physical understanding embodied in CMOD4, but incorporates greater flexibility, allowing a better fit to the observations. By introducing a separate model for the midbeam and using a common model for the fore and aft beams, we show a significant improvement in local wind vector retrieval. The hybrid model also fits the scatterometer observations more closely. The model is trained in a Bayesian framework, accounting for the noise on the wind vector inputs. We show that adding more high wind speed observations in the training set improves wind vector retrieval at high wind speeds without compromising performance at medium or low wind speeds.
Offshore propagation of eddy kinetic energy in the California Current
Authors: Haney Robert L. , Hale Robert A. , Dietrich David E.
Published: February 2004

Low‐pass‐filtered velocities obtained from surface drifters and surface geostrophic velocities estimated from TOPEX/Poseidon altimeter data have recently revealed a clear and robust seasonal cycle in the surface eddy kinetic energy (EKE) in the California Current (CC) [Kelly et al., 1998; Strub and James, 2000]. The seasonal cycle begins in spring when a surface‐intensified baroclinic equatorward jet develops next to the coast in response to strong upwelling favorable winds. This jet, and a developing eddy field, then moves offshore during summer and fall. The EKE maximum associated with the jet progresses only as far as 127°W, beyond which it decreases rapidly. This is a robust characteristic of the seasonal cycle that has been previously attributed only to an unspecified dissipation process. To investigate this aspect of the surface EKE, a multiyear simulation of the CC is carried out using the Dietrich/Center for Air‐Sea Technology primitive equation regional ocean model [Dietrich, 1997]. The simulation accurately reproduces many aspects of the observed annual cycle, including the offshore propagation of the EKE at the surface. The model results indicate that the decrease of surface EKE west of 127°W in the simulation is not due to dissipation but rather is caused by the vertical redistribution of EKE to the deep ocean. This redistribution occurs through the transformation of kinetic energy from the vertical shear flow to the vertical mean flow. The transformation is a nonlinear process inherently associated with the life cycle of baroclinically unstable waves, and in the CC, it effectively energizes the deeper ocean at the expense of the upper ocean. The process is also known to be important in the atmosphere [Wiin‐Nielsen, 1962]. Taken together, the recent California Current observations and the new model results strongly suggest that the CC regularly supplies EKE to the deep waters of the eastern North Pacific.
Eastern North Pacific Subtropical Mode Water in a general circulation model: Formation mechanism and salinity effects
Authors: Hosoda Shigeki , Xie Shang‐Ping , Takeuchi Kensuke , Nonaka Masami
Published: February 2004

The Eastern North Pacific Subtropical Mode Water (ESTMW) is a water mass of low potential vorticity (PV) and appears as a weak pycnostad or thermostad. Distinct from other subtropical mode waters, it forms in the absence of a deep winter mixed layer. The formation mechanism of this ESTMW is investigated using an ocean general circulation model that is forced by monthly climatological temperature, salinity, and wind stress at the sea surface. An equation based on the ventilated thermocline theory is used to diagnose the initial PV of a water mass right after its subduction. In this equation, three factors affect the initial PV: the spacing of density outcrop lines, the mixed layer depth gradient, and the vertical velocity at the bottom of mixed layer. Among them the wide spacing between outcrop lines is the most important for ESTMW's low PV instead of the deep mixed layer, which is most important for classical mode waters. It is found that weak gradients in both sea surface temperature and salinity in the direction of mixed layer flow are important for the low PV formation. A low‐salinity tongue that extends southeastward off North America is responsible for the small surface density gradient in the eastern North Pacific and contributes to the formation of the ESTMW. An additional experiment forced with observed freshwater flux demonstrates that the southward advection of fresher water from the high latitude along the eastern boundary is the cause of this low‐salinity tongue.
Warm and saline water transport to the North Pacific subarctic region: World Ocean Circulation Experiment and Subarctic Gyre Experiment data analysis
Authors: Ueno Hiromichi , Yasuda Ichiro
Published: February 2004

A warm and saline water transport to the North Pacific subarctic region which could feed the mesothermal (intermediate temperature maximum) water is investigated through the analysis of World Ocean Circulation Experiment and Subarctic Gyre Experiment hydrographic data. The analysis of individual cruise data supports the hypothesis that warm and saline water is transported from the area east of Japan to the Gulf of Alaska and also indicates that the isopycnal mixing is active in the area east of 165°E. The transport route obtained from individual cruise data mostly agrees with that based on climatological data. As a whole, isopycnal potential temperature and potential vorticity are conserved along the transport route using both the climatological and individual data excepting that the potential vorticity at 26.8σθ rapidly increases along streamlines in the Gulf of Alaska when the climatological data are used. This discrepancy might be attributed to the existence of the strong potential vorticity front in the Gulf of Alaska and excessive smoothing in the climatological data.
Transport estimates in the Strait of Gibraltar with a tidal inverse model
Authors: Baschek Burkard , Send Uwe , Lafuente Jesus Garcia , Candela Julio
Published: February 2004

To estimate the volume transport through the Strait of Gibraltar and to study the spatial structure of the time‐variable flow, a varying number of current meter moorings were maintained at the eastern entrance of the strait between October 1994 and April 1998, and was complemented with intensive shipboard measurements during the European Union project Canary Island Azores Gibraltar Experiment (CANIGO). A tidal inverse model is used to merge these data sets in order to investigate the flow at the eastern entrance of the strait. The two‐dimensional structure of the tidal flow was described by simple analytical functions. Harmonics with the seven most important tidal frequencies were used as temporal functions. With this model, the tidal currents can be predicted for any time and location at the eastern entrance of the strait, and more than 92% of the variance of the lower layer flow is explained. It was used to remove the tidal currents from the individual measurements and to calculate the mean flow through the strait from the residuals. Combined with a similar inverse model for determining the depth of the interface between Mediterranean and Atlantic water, the volume transport was estimated to be 0.81±0.07 Sv for the upper layer and −0.76±0.07 Sv for the lower layer. The correlation of the tidal currents and the fluctuations of the interface accounts for ∼7% of the transport at the eastern entrance.
Intensive shipboard observations of the flow through the Strait of Gibraltar
Authors: Send Uwe , Baschek Burkard
Published: February 2004

During the European project Canary Island Azores Gibraltar Experiment (CANIGO), intensive shipboard observations were carried out in April 1996 and October 1997 in order to observe the spatial and temporal variability of the flow and of the water mass structure in the Strait of Gibraltar. At the sill and the eastern and western entrances to the strait, repeated cross‐strait sections and station time series of the flow and of T‐S profiles were measured using vessel‐mounted and lowered acoustic Doppler current profilers (ADCP) and conductivity‐temperature‐depth probes (CTD)/expendable bathythermographs (XBT), yielding new views of the rapid changes over tidal cycles and of approximate tidal means. It is argued that transport observations might be easier to carry out away from the sill, in the eastern part of the strait, even though maximum resolved shears were comparable in both places, 0.03–0.04 s−1 in the vertical and 0.001–0.016 s−1 in the horizontal. In the east, coordinated changes in the stratification and the flow field are documented via four time series over M2 tidal cycles, showing a sharpening/diffusing of the vertical gradients in the water masses and the flow. Maximum shear and maximum water mass gradients do not always coincide there, and both are much shallower (50–80 m) than the delimiter between inflow and outflow (120–150 m). The mean salinity of the outflow core decreases from 38.43 in the east to 38.17 west of the sills as a result of the mixing processes. The internal bore was followed and directly observed with rapid CTD‐yoyo stations and with XBT/vessel‐mounted ADCP measurements. It generates extreme changes in currents and shears on timescales of minutes, with directly measured vertical velocities reaching ±50 cm s−1. Patches of density inversions were observed as the bore passed by, consistent with active turbulent mixing along the interface. The time series of flow and CTD measurements allow the direct calculation of Froude numbers at various locations and over tidal cycles. These and along‐strait sections suggest that the exchange through the strait is maximal in April 1996 and submaximal in October 1997, supporting the expectations of Garrett et al. [1990].
Effects of intermittent entrainment of air bubbles by breaking wind waves on ocean reflectance and underwater light field
Authors: Stramski Dariusz , Tegowski Jarosław
Published: February 2004

Light‐scattering properties of air bubbles suspended in water and observational evidence of bubble entrainment by breaking wind waves indicate that bubble clouds may influence ocean reflectance and in‐water light field characteristics within the surface layer. We estimate potential changes in remote sensing reflectance and in‐water light field associated with a bubble entrainment event observed at a wind speed of 10 m s−1. Our approach combines acoustic measurements of bubble concentration as a function of time and depth and radiative transfer simulations of the light field within and leaving a water body. We show that the remote sensing reflectance can increase significantly (more than twofold) due to bubble entrainment, and these large variations occur over time periods on the order of minutes or less. The bubble clouds have a spectral effect on ocean reflectance such that the water patch containing bubbles will appear greener or more yellowish than the surrounding waters with no bubbles. These results are relevant to measurements of remote sensing reflectance made from just above the water surface with downlooking radiance meters whose spot size at the surface is of O(0.1–1) m. The light field characteristics within the bubble layer are also significantly affected, with most pronounced effects seen in the profiles of upwelling irradiance and upwelling radiance. Therefore the bubble entrainment can be a source of error in the estimation of reflectance from extrapolation of underwater measurements at a depth up to and across the surface. While our radiative transfer simulations were made using a model with a plane‐parallel geometry, future efforts to examine the effects of spatial distribution of bubble clouds and their properties on ocean reflectance from spatially integrating [O(10–1000) m] satellite measurements will have to be based on a three‐dimensional approach.
On internal waves near the high‐frequency limit in an enclosed basin
Authors: Antenucci Jason P. , Imberger Jörg
Published: February 2004

Observations of the internal wave field using thermistor chains in Lake Kinneret (Israel) showed a substantial energy peak at frequencies just below the maximum buoyancy frequency. The waves were of the first vertical mode, occurred in groups, and resulted in isotherm displacements of up to 4 m in 22 m of water. The appearance of this high‐frequency internal wave energy was closely related to the strength of the wind field. A weaker relationship was found with the phase of the basin‐scale internal waves, with more energy observed under the same wind conditions when the metalimnion was closer to the surface. The relationship of the high‐frequency waves to the wind and the basin‐scale internal waves implies a generation mechanism associated with shear in the surface layer (observed to be approximately 0.08 s−1), similar to that responsible for generating high‐frequency internal waves in the equatorial Pacific. A linear stability analysis of the mean state during periods of high‐frequency wave activity revealed the existence of unstable modes, whose period and frequency bandwidth closely matched field observations.
Internal tides and baroclinicity in the Southern Weddell Sea: 2. Effects of the critical latitude and stratification
Authors: Robertson Robin
Published: February 2004

Tidal dynamics affect Antarctic Bottom Water and Ice Shelf Water production in the southern Weddell Sea and the general circulation and mixing. In this region, tidal dynamics were found to be extremely sensitive both to the proximity of the continental shelf‐slope break to the critical latitude ψcrit and to the existence of a shelf break front. If the continental slope was not in the vicinity of ψcrit or a shelf break front was absent, generation of internal tides was negligible. Generation of internal tides was greatest when both a shelf break front was present and the continental slope was near and equatorward of ψcrit. When a shelf break front was present and the continental slope was near but poleward of ψcrit, a strong benthic response occurred. This response included development of benthic inertial oscillations at the continental shelf‐slope break, increased benthic shear, a thicker benthic boundary layer, and increased benthic dissipation. A shelf break front with even a slight horizontal density gradient was found to significantly enhance internal tide generation and/or the benthic effects.
Seasonal spreading of the Persian Gulf Water mass in the Arabian Sea
Authors: Prasad T. G. , Ikeda M. , Kumar S. Prasanna
Published: February 2004

The characteristics of the subsurface salinity maximum associated with the Persian Gulf Water mass (PGW) are used to quantify the spreading and mixing of PGW in the thermocline of the Arabian Sea based on a bimonthly climatology of temperature and salinity. Examination of the seasonal cycles of heat and freshwater fluxes in the Persian Gulf region indicates that PGW forms as a result of elevated evaporative cooling in conjunction with reduced insolation during winter. Maps are presented of the distributions of depth, salinity, and geostrophic flow on σθ = 26.5, which nearly coincides with the core of the PGW. After intense mixing in the Strait of Hormuz, the property fields suggest that warm (>17°C) and high‐salinity (>36.2 psu) PGW enters the Arabian Sea to form a subsurface salinity extremum between 200 and 300 m. We have found variability in the distribution of PGW in the Arabian Sea associated with monsoonal changes in the Arabian Sea circulation. During the winter monsoon, there is southward spreading of PGW along the western boundary; during summer it is not present. Lateral mixing with low‐salinity water from the Bay of Bengal in the region south of 10°N and along the west coast of India during winter accounts for changes in the characteristics of PGW along these paths. Associated with the Findlater Jet during summer, the entire thermohaline structure is vertically displaced along the coasts of Somalia and Arabia. Ekman convergence in the central Arabian Sea accounts for deepening of the PGW. Either lateral or vertical mixing would cause changes in PGW properties in these regions. During this time, PGW spreads predominantly southward along the central Arabian Sea, as indicated by a tongue of high salinity.
Comparison of near‐surface mean and eddy flows from two numerical models of the South Atlantic Ocean
Authors: Penduff Thierry , Barnier Bernard , Béranger Karine , Verron Jacques
Published: February 2004

The near‐surface mean and eddy flows simulated by two eddy‐permitting models of the South Atlantic (S‐coordinate Primitive Equation Model (SPEM) and Ocean Parallel (OPA) model) are compared. These models are intrinsically different by virtue of their vertical discretizations (σ versus geopotential coordinates, respectively), so that their solutions differ essentially through the representation of current‐topography interactions. The path of the Agulhas Current, the generation process, the characteristics, and the subsequent trajectory of the Agulhas rings are differently affected by topography in the two models. Along the American western boundary, SPEM traps a larger part of the Antarctic Circumpolar Current (ACC) around the Falkland Plateau and consequently produces a stronger Malvinas Current, which overshoots far to the north, inshore of a rather realistic Brazil Current and Confluence regime. In contrast, OPA allows a large part of the ACC to flow across the Scotia Sea, produces a weaker Malvinas Current, traps the Brazil and Malvinas Currents along the American coast, and tends to distort the complex dynamics of the Confluence region and the Zapiola anticyclone. Some of these differences can be attributed to the particularities of each vertical coordinate system and to the representation of topographic slopes (staircases in OPA and facettes in SPEM). More generally, the topographic constraint (trapping of boundary currents and interaction of mean and eddy flows with bathymetry) is stronger in SPEM than in OPA. The topographic smoothing usually performed in sigma coordinate models removes mesoscale topographic roughness and reduces topographic slopes. This treatment is probably responsible for several of the differences observed in the numerical solutions.
Maud Rise revisited
Authors: Muench R. D. , Morison J. H. , Padman L. , Martinson D. , Schlosser P. , Huber B. , Hohmann Roland
Published: February 2004

An oceanographic field program called the Antarctic Zone Flux experiment was carried out in the eastern Weddell Sea during austral winter (July–September) 1994. Data from a drift buoy array were used in concert with shipboard observations to provide exceptionally high horizontal resolution of upper ocean hydrographic parameters near Maud Rise. Chemical and tracer data were obtained from the ship. We identify a “warm pool” southwest of the rise as a dynamically necessary region of positive (cyclonic) vorticity that is associated with a Taylor column over the rise. Both a warm “halo” surrounding the Taylor column and the warm pool are associated with thermocline shoaling that is a necessary condition for high upward heat fluxes to occur. These features extend the influence of Maud Rise bottom topography on upper ocean heat flux over a region that is larger, by a factor of at least 2, than the area directly overlying the rise. Areal mean upward heat fluxes of about 25 W m−2 are derived using both upper ocean T (“instantaneous”) values and tracer data (“integrated”) values. Fluxes derived over the warm halo and pool regions using only upper ocean T exceeded 100 W m−2 at specific sites. Elsewhere in the region, the T‐derived heat fluxes varied widely from <10 to >50 W m−2, whereas the tracer‐derived heat fluxes showed a considerably more uniform distribution. Our mean values are similar to those that have been previously reported. Historical ice cover data have shown that the geographical region encompassed by Maud Rise and the warm pool area to the southwest is a preferred site for polynya formation, consistent with these findings. Time series analyses of the historical upper ocean data set suggest that conditions conducive to polynya formation are correlated with climate processes remote from the Southern Ocean.
Seasonal climatology of hydrographic conditions in the upwelling region off northern Chile
Authors: Blanco J. L. , Thomas A. C. , Carr M.‐E. , Strub P. T.
Published: February 2004

Over 30 years of hydrographic data from the northern Chile (18°S–24°S) upwelling region are used to calculate the surface and subsurface seasonal climatology extending 400 km offshore. The data are interpolated to a grid with sufficient spatial resolution to preserve cross‐shelf gradients and then presented as means within four seasons: austral winter (July‐September), spring (October‐December), summer (January‐March), and fall (April‐June). Climatological monthly wind forcing, surface temperature, and sea level from three coastal stations indicate equatorward (upwelling favorable) winds throughout the year, weakest in the north. Seasonal maximum alongshore wind stress is in late spring and summer (December‐March). Major water masses of the region are identified in climatological T‐S plots and their sources and implied circulation discussed. Surface fields and vertical transects of temperature and salinity confirm that upwelling occurs year‐round, strongest in summer and weakest in winter, bringing relatively fresh water to the surface nearshore. Surface geostrophic flow nearshore is equatorward throughout the year. During summer, an anticyclonic circulation feature in the north which extends to at least 200 m depth is evident in geopotential anomaly and in both temperature and geopotential variance fields. Subsurface fields indicate generally poleward flow throughout the year, strongest in an undercurrent near the coast. This undercurrent is strongest in summer and most persistent and organized in the south (south of 21°S). A subsurface oxygen minimum, centered at ∼250 m, is strongest at lower latitudes. Low‐salinity subsurface water intrudes into the study area near 100 m, predominantly in offshore regions, strongest during summer and fall and in the southernmost portion of the region. The climatological fields are compared to features off Baja within the somewhat analogous California Current and to measurements from higher latitudes within the Chile‐Peru Current system.
Impact of the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission on ocean circulation estimates: Volume fluxes in a climatological inverse model of the Atlantic
Authors: LeGrand Pascal
Published: February 2004

The Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission, by providing a precise estimate of the marine geoid height, will allow the determination of absolute geostrophic velocities at the surface of the ocean with unprecedented accuracy. The resulting impact on oceanic flux estimates is quantified within a climatological inverse model of the Atlantic in terms of reduction of uncertainties in volume transports. These uncertainty reductions are obtained by replacing the error spectrum of present‐day geoid models by the error spectrum expected for the GOCE mission. The impact is large in the Circumpolar Current, with relative uncertainty reductions reaching 50% in the upper layers of the ocean, and 40% in the whole water column. It is also large in regions of sharp oceanic fronts like the Gulf Stream or the Brazil Current, with uncertainty reductions reaching 60% in the upper layers of the ocean. The reduction in transport uncertainties is large enough in absolute terms to have a significant impact on estimates of important climate processes like the rate of overturning in the Atlantic or the exchange of water between the Circumpolar Current and the South Atlantic. The impact of the Gravity Recovery and Climate Experiment (GRACE) mission, estimated within the same inverse model, is on average less than half the impact of GOCE because of the lower precision of this mission at small spatial scales. The fact that uncertainties in the baroclinic component of the velocity field limit the impact of GOCE at depths points to the need for precise in situ observations to complement gravity and altimetric observations.
Numerical evidence against reversed thermohaline circulation in the warm Paleocene/Eocene ocean
Authors: Bice Karen L. , Marotzke Jochem
Published: February 2004

The question of whether deep water formation might have occurred in subtropical latitudes in the early Cenozoic is examined through use of a global ocean model forced by mixed boundary conditions. Zonal mean surface temperatures and wind stresses are derived from an atmospheric general circulation model (AGCM) simulation of the warm Paleocene/Eocene boundary interval (∼55 Ma) and are held constant for a series of sensitivity tests. The control case for moisture flux (evaporation minus precipitation,E‐P), also derived from the AGCM, is perturbed so that the subtropical evaporation increases and high‐latitude precipitation increases. A dramatic response is seen in the temperature and salinity structure of the model ocean, but the perturbation does not result in deep convection in subtropical latitudes. In all cases, bottom water is formed in the southern high latitudes, and the global meridional overturning is characterized by a strongly asymmetric circulation. No multiple equilibria have been found for any particularE‐Pconfiguration. In the most extreme case (5 times the controlE‐P) the model oscillates between meridional overturning circulation “on” and “off”. Shorter‐lived thermohaline slowing and reinvigoration are observed as a transient response under less extremeE‐Pperturbations. Despite the high evaporation implied in the perturbation experiments, mean mixed layer salinities in the subtropics do not rise much above the control case because of efficient removal of salt (and heat) through deepened subduction beneath the subtropical gyres. The sensitivity of the results to the parameterization of continental runoff and the specified diapycnal mixing coefficient (Kν) are also examined. Distributing runoff purely zonally, rather than globally, has approximately the same effect as a 50% increase in the strength of the hydrologic cycle. DecreasingKνto 0.3 cm2s−1from the standard value of 1.0 cm2s−1increases the sensitivity to an increased hydrologic cycle considerably, but in no case does low‐latitude deep water formation occur, indicating that subtropical bottom water formation is implausible in a model with some degree of realism. These experiments support changes in moisture flux as a mechanism for ocean warming (largely in the thermocline through intermediate water depths), but the process involved is deepened subtropical subduction and not subtropical deep water formation.
Dark production: A significant source of oceanic COS
Authors: Von Hobe M. , Cutter G. A. , Kettle A. J. , Andreae M. O.
Published: February 2004

Carbonyl sulfide (COS) in air and dissolved in seawater was determined during a cruise in August 1999 in the Sargasso Sea in the northwest Atlantic Ocean. Dissolved concentrations at the sea surface displayed only a weak diel cycle with a mean of 8.6±2.8 pmol dm−3 owing to low abundance of photochemical precursors and high temperatures causing rapid hydrolysis. Depth profiles measured over the oceanic mixed layer revealed significant vertical gradients of COS concentration with higher values at the surface, suggesting that the rate of photochemical production at the surface exceeds the rate of vertical mixing. The mean atmospheric mixing ratio was 486±40 ppt, and calculated sea‐air fluxes ranged from 0.03 to 0.8 g COS km−2 d−1. COS dark production, estimated from the predawn COS concentration at the surface and the hydrolysis constant, contributed significantly to the total amount of COS produced. A strong temperature dependence of the COS dark production rate q was found by comparing previously published values. The data further indicate an approximately first‐order relationship between q and chromophoric dissolved organic matter (CDOM) absorbance at 350 nm, a350, which is used as a proxy for the CDOM content of the water but is likely to covary with other parameters, such as biological activity, that could also affect COS dark production. Together with known functions for COS hydrolysis and solubility, the parameterization of dark production as a function of temperature and a350 allows for the prediction of COS concentrations and saturation ratios as a function of physical and optical seawater properties in the absence of photoproduction. This is used to estimate a lower limit of 0.056 Tg COS yr−1 to the annual COS flux from the ocean to the atmosphere.
Field observations of wave‐driven setdown and setup
Authors: Raubenheimer B. , Guza R. T. , Elgar Steve
Published: February 2004

Wave‐driven setdown and setup observed for 3 months on a cross‐shore transect between the shoreline and 5 m water depth on a barred beach are compared with a theoretical balance between cross‐shore gradients of the mean water level and the wave radiation stress. The observed setdown, the depression of the mean water level seaward of the surf zone, is predicted well when radiation stress gradients are estimated from the observations using linear theory at each location along the transect. The observed setdown also agrees with analytical predictions based on offshore wave observations and the assumption of linear, dissipationless, normally incident waves shoaling on alongshore homogeneous bathymetry. The observed setup, the superelevation of the mean water level owing to wave breaking, is predicted accurately in the outer and middle surf zone, but is increasingly underpredicted as the shoreline is approached. Similar to previous field studies, setup at a fixed cross‐shore location increases with increasing offshore wave height and is sensitive to tidal fluctuations in the local water depth and to bathymetric changes. Numerical simulations and the observations suggest that setup near the shoreline depends on the bathymetry of the entire surf zone and increases with decreasing surf zone beach slope, defined as the ratio of the surf zone‐averaged water depth to the surf zone width. A new empirical formula for shoreline setup on nonplanar beaches incorporates this dependence.
Sea ice, atmosphere and upper ocean variability in the Weddell Sea, Antarctica
Authors: Venegas Silvia A. , Drinkwater Mark R.
Published: February 2004

A frequency domain singular value decomposition is performed on 20 years (1979–1998) of monthly sea ice concentration, sea ice drift and sea level pressure data in the Weddell Sea, Antarctica. Interannual oscillations with periods of around 3–4 years are found to dominate the variability in this region. Anomalous atmospheric patterns periodically reach the Weddell Sea from the west and perturb the sea ice circulation and distribution in the Weddell Gyre through changes in the intensity and direction of the climatological winds. Sea ice accumulates in the southeastern Weddell Sea every 3–4 years owing to two atmospherically driven processes: (1) weak ice export to the north due to a weak northward branch of the gyre (driven by weak southerly winds) and (2) large ice import from the east due to a strong East Wind Drift (driven by strong easterly winds along the coast). The opposite situation gives rise to a depletion of sea ice in the same region half a cycle later. Sea ice anomalies are then advected north‐northeastward before turning eastward in the gyre circulation. The eastward propagation of ice anomalies along the ice margin accounts for the passage of the Antarctic Circumpolar Wave through the Atlantic sector of the Southern Ocean. A low frequency signal is also detected in the Weddell Sea variations, albeit rather speculatively in this 20‐year‐long record. Sea ice variability on this timescale appears to be associated with a change in the shape and characteristics of the Weddell Gyre circulation around 1990. This mode of variability implicates feedbacks between the gyre and Weddell Deep Water temperature variations, whose impact is observed near the Maud Rise topographic feature.