bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling dense water production and salt transport from Alaskan coastal polynyas
Authors: Signorini Sergio R. , Cavalieri Donald J.
Published: October 2002

A three‐dimensional primitive equation model was used to assess the effects of dense water formation from winter (1996/1997) polynyas on the ambient stratification, salt transport, and circulation in the vicinity of Barrow Canyon. The model, which includes ambient stratification and bottom topography, is forced by time‐varying surface heat flux, surface salt flux, and coastal flow. The influence of sea ice drift on the circulation and salt transport is also analyzed by prescribing ice water stress at the sea surface. The surface fluxes and ice drift are derived from satellite observations (Special Sensor Microwave Imager (SSM/I) and NASA scatterometer (NSCAT) sensors). The coastal flow (Alaska coastal current), which is an extension of the Bering Sea throughflow, is formulated in the model by using a wind‐transport regression. One set of experiments was forced by strong and persistent polynyas, simulated by 20‐day averaged heat and salt fluxes originating from the largest events. In this set of experiments both strong and weak steady coastal currents were imposed. The amount of salt exported from the generation area depended on the strength of the current. Another set of experiments was forced by weaker and less persistent polynyas using time‐varying forcing. The experiments with time‐varying polynya forcing were conducted with two ambient vertical stratifications, one representing fall conditions and one representing winter conditions. The amount of salt retained on the shelf was found to be quite sensitive to the initial stratification. Weaker vertical stratification promotes a deeper mixed layer, which develops 20 times faster than the horizontal advective timescale of the coastal current, thus increasing the residence time of the salt generated by the polynya on the shelf. The time‐varying northeastward coastal current, combined with the offshore Ekman transport, can export 29–73% of the salt produced by polynyas upstream of Barrow Canyon, depending upon the ambient vertical stratification. The inclusion of ice water stress in the model makes the coastal current much wider due to the resulting offshore Ekman transport and also doubles the amount of salt exported.
Observations and linear analysis of sill‐generated internal tides and estuarine flow in Haro Strait
Authors: Pawlowicz Rich
Published: October 2002

Current meter records from two sets of observations 20 years apart in Haro Strait, British Columbia, Canada, are analyzed. Haro Strait is a deep channel separated from larger bodies of water on either side by relatively shallow sills. The estuarine flow in this region is in approximate geostrophic balance and is apparently unaffected by the tidally driven spring/neap cycle in vertical stratification. Strong baroclinic variability with an amplitude 1 m s−1 (close to the amplitude of the barotropic tide) is present. A linear theory for internal tides in silled basins is developed using no‐bottom‐flow boundary conditions to represent sill effects. By comparison with observations it is concluded that an internal tide is generated at the seaward sill and propagates inshore. Although local damping appears to be weak, no return signal is found from the inshore sill, suggesting that either dissipation is strong in that area or that the internal tide is transmitted across that sill.
Dianeutral exchange between intermediate and deep water in the tropical Atlantic
Authors: You Yuzhu
Published: October 2002

In the tropical Atlantic the vertical distance between Antarctic Intermediate Water (AAIW) and upper North Atlantic Deep Water (uNADW) cores (defined by their salinity extrema) exceeds 1000 m. In this thick layer, two additional water mass signals, characterized mainly by temperature extrema, are identified in the western boundary region. They are the intrusive flows of upper Circumpolar Deep Water (uCDW) and shallow uNADW (suNADW). The latter is considered a water type of uNADW. Since uCDW is a northward flow as a lower phase of AAIW and suNADW is a southward flow as an upper phase of NADW, a reference of level of no motion (LNM) is defined between uCDW and suNADW at the neutral density surface σN = 27.70 (at about 1150 m). This surface is coincident with a minimum of dianeutral velocity and measured velocity by lowered acoustic Doppler current profiler. Dianeutral motion processes are examined across the five selected neutral surfaces σN = 27.45, 27.60, 27.70, 27.80, and 27.90 that span the depth range between AAIW and uNADW and follow the major water mass signals. Dianeutral transport shows a decreasing tendency toward the middle σN = 27.70 neutral surface and reaches a minimum of 0.6 Sv (1 Sv = 106 m3 s−1), integrated in the tropical region between 10°N and 10°S. Thus the weakening of dianeutral motion and required minimum of epineutral advection at LNM seem to be relevant.
Consolidation of first‐year sea ice ridges
Authors: Høyland Knut V.
Published: October 2002

Measurements of spatial and temporal temperature development, geometry morphology, and physical properties in three first‐year sea ice ridges at Spitsbergen and in the Gulf of Bothnia have been performed. The corresponding thickness and the physical properties of the surrounding level ice were also measured. The thickness of the consolidated layer was examined through drilling and temperature measurements: the temperatures gave a ratio of the thickness of the consolidated layer to the level ice thickness from 1.39 to 1.61, whereas the drillings indicated a ratio of 1.68–1.85. The measured consolidated layer appeared to be 28% thicker when based on drillings in comparison to temperature. Thus the result depended on the method of investigation; the drillings included a partly consolidated layer. However, the measured growth of the consolidated layer did not depend on the method of investigation. The scatter of the physical properties in the consolidated layer was higher than that of the level ice. The consistency of the unconsolidated rubble differed markedly at the two sites. It was soft and slushy at Spitsbergen and harder in the Gulf of Bothnia. Three possible explanations for these differences are discussed: surrounding currents, different keel shapes, and difference in salinity.
Pathways and modification of the upper and intermediate waters of the Arctic Ocean
Authors: Karcher Michael J. , Oberhuber Josef M.
Published: October 2002

The purpose of this study is to investigate the pathways and the ventilation of source water masses of the upper and intermediate waters of the Arctic Ocean. For the Arctic and subarctic domain a coupled ice‐ocean general circulation model is set up to be integrated for several decades. It is driven by a climatological seasonal cycle of monthly mean atmospheric data from 1980 to 1989 and by restored sea surface salinities. Passive tracers are used to visualize and interpret the modeled flow and to compare it with observations. The model is able to reproduce known features of the Arctic Ocean circulation like the inflow of two branches of Atlantic origin via the Fram Strait and the Barents Sea and their subsequent passage at middepths in several cyclonic circulation cells. The fate of these Atlantic source water masses, river water, and Bering Strait inflow water in the model are studied. The branch crossing the Barents Sea is subject to an intense heat loss and ice formation. As a result, water of this branch leaves the shelf toward the central part of the Arctic Ocean not only at the surface but also in denser varieties, which finally feed the central Arctic at halocline and middepths. The lightest part turns northward and finally westward joining the Transpolar Drift; the densest part (200–1000 m depth) moves eastward along the continental slope. A similar path is taken by the Atlantic water branch from the Fram Strait. The inflowing branch over the Barents Sea turns out to be the dominant source for the lower Atlantic Water layer in the Arctic Ocean in this investigation. Atlantic tracers starting in Fram Strait need 6 years to reach the northern Laptev Sea slope. Travel times to return to Fram Strait are 15–20 years along the Lomonossov Ridge and about 30 years along the continental slope of the Canadian Basin. Tracers that mark the Pacific Water or the Mackenzie River Water flow eastward and leave the Arctic mainly via the Canadian Archipelago. The Siberian River water tracers at the surface penetrate far into the Canadian Basin before they join the Transpolar Drift. The travel times of the river water from the river mouths are 2–3 years to the shelf edge and 12–14 years to Fram Strait.
Response in a regional model for the Southeastern Pacific Ocean to open boundary conditions
Authors: Williams Michael J. M.
Published: October 2002

An assessment of four open ocean boundary stream function conditions based on different data sources for a regional model of the Southeast Pacific Ocean (SEP; 103°W to the Chilean coast and 16.5°S to 54°S) is presented. The model run, with the boundary conditions which most successfully describes the SEP, is forced with a stream function extracted from a global ocean circulation model and is suitable for forming the basis of climate process studies in the SEP. Investigations of other model boundary conditions based on observational data, two using a Sverdrup balance argument to determine the stream function from two commonly used wind stress climatologies and a third using a geostrophic balance argument to determine the stream function from WOCE hydrographic data, were less successful. The main difficulty experienced in all of the model runs was the lack of formation of a sufficiently fresh salinity minimum at intermediate depths. This is linked to the inadequate production of Antarctic Intermediate Water (AAIW) within the model domain or insufficient AAIW propagating across the open boundaries. Comparisons between the model runs highlight the level of uncertainty in aspects of the oceanography of the SEP that govern water mass formation in the region.
Stability of the Mediterranean's thermohaline circulation under modified surface evaporative fluxes
Authors: Myers Paul G. , Haines Keith
Published: October 2002

A series of experiments with an ocean general circulation model of the Mediterranean forced by artificial (but realistic) surface fluxes of heat and fresh water is performed over 100 year periods. The model has a stable thermohaline circulation under the baseline fluxes. Small increases/decreases in excess evaporation (±8%) produce a linear strengthening/weakening of this thermohaline circulation with more/less water formation and strait transports. The water properties do not change much, and the responce is consistent with submaximal exchange at Gibraltar, where changes in volume and freshwater transport can be achieved mainly by changing the interface level between inflowing and outflowing waters. Larger changes in excess evaporation lead to nonlinear responses for increasing and decreasing values that are quite different. Reducing evaporation (−20%) causes both western and eastern deep water formation to cease, leaving a shallower intermediate circulation only. Increasing evaporation (+25%) initially strengthens the overturning but also produces a hydraulic jump‐like feature at Gibraltar, which mixes inflowing and outflowing waters, decreasing the salt flux out of the basin. As a result, the basin salinity increases greatly over 140 years, until the overturning collapses and the Gibraltar mixing ceases, leaving a shallow intermediate circulation above highly saline and stagnant deep waters. The possible relevance of these experiments to recent changes in Mediterranean fluxes and water properties is discussed.
The influence of ocean flow on newly forming sea ice
Authors: Feltham Daniel L. , Worster M. Grae , Wettlaufer J. S.
Published: October 2002

The heat and mass balance of the Arctic Ocean is very sensitive to the growth and decay of sea ice and the interaction between the heat and salt fields in the oceanic boundary layer. The hydraulic roughness of sea ice controls the detailed nature of turbulent fluxes in the boundary layer and hence is an important ingredient in model parameterizations. We describe a novel mechanism for the generation of corrugations of the sea ice–ocean interface, present a mathematical analysis elucidating the mechanism, and present numerical calculations for geophysically relevant conditions. The mechanism relies on brine flows developing in the sea ice due to Bernoulli suction by flow of ocean past the interface. For oceanic shears at the ice interface of 0.2 s−1, we expect the corrugations to form with a wavelength dependent upon the permeability structure of the sea ice which is described herein. The mechanism should be particularly important during sea ice formation in wind‐maintained coastal polynyas and in leads. This paper applies our earlier analyses of the fundamental instability to field conditions and extends it to take account of the anisotropic and heterogeneous permeability of sea ice.
Analysis of velocity field in the eastern Black Sea from satellite data during the Black Sea '99 experiment
Authors: Afanasyev Y. D. , Kostianoy A. G. , Zatsepin A. G. , Poulain P.‐M.
Published: October 2002

Maximum cross correlation (MCC) analysis of National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) imagery of the eastern Black Sea in late September 1999 has been used to reconstruct the velocity and vorticity fields of the upper layer of the sea. Analysis revealed the large‐scale dynamic features characteristic of the Black Sea, namely the Rim Current, as well as the detailed pattern of mesoscale vortical activity including meanders, eddies, and dipoles, which are often observed on satellite visible, infrared, or sea color imagery. The dynamical character of the pronounced dipole structure in the northeastern part of the Black Sea was determined by comparison with satellite IR images, geostrophic velocities calculated on the basis of conductivity‐temperature‐depth (CTD) casts performed and trajectories of six Argos‐tracked surface velocity program (SVP) drifters deployed during the Black Sea '99 expedition onboard R/V Akvanavt on 25–30 September 1999.
Mesoscale variability in time series data: Satellite‐based estimates for the U.S. JGOFS Bermuda Atlantic Time‐Series Study (BATS) site
Authors: Glover David M. , Doney Scott C. , Mariano Arthur J. , Evans Robert H. , McCue Scott J.
Published: October 2002

Objectively analyzed fields of satellite sea surface temperature (SST, advanced very high resolution radiometer (AVHRR) Pathfinder) and sea surface height anomaly (SSHA, combined TOPEX/Poseidon–ERS‐1/2) are used to characterize, statistically, the mesoscale variability about the U.S. Joint Global Ocean Flux Study (JGOFS) Bermuda Atlantic Time‐Series Study (BATS) site. These results are applied to the in situ BATS time series data and a local one‐dimensional (1‐D) physical upper ocean model to better understand the contribution of mesoscale eddies to the time series record and the model‐data mismatch. Using a low‐pass spatial filter, we decompose the anomalies from the seasonal cycle into two components: the large‐scale, regional climate variability and a mesoscale signal. The mesoscale SST and SSHA fields are positively cross‐correlated at a statistically significant level, consistent with near‐surface isotherm displacements for cyclonic and anticyclonic eddies. The results from time‐lagged cross‐correlation analysis show that detectable eddy signatures exist in the in situ SST data and that eddies are a noticeable (∼10%) but not dominant error source for the 1‐D model solution. Several factors may be at work: the 1‐D model captures a more regional signal, whereas the BATS in situ data include small‐scale spatial heterogeneity; the satellite data and 1‐D model are indirectly coupled via the National Centers for Environmental Prediction (NCEP) reanalysis forcing data; and the satellite‐based mesoscale variability estimates are also missing specific events because of the sparse space‐time sampling of a polar orbiting, visible/infrared wavelength sensor. The mesoscale eddy cross‐correlation signature did not show up clearly in a similar analysis conducted on the original anomaly fields, highlighting the fact that climate scale variability needs to be carefully removed to isolate the eddy signature.
Oscillatory flow across an irregular boundary
Authors: Pawlak Geno , MacCready Parker
Published: October 2002

Laboratory experiments examining oscillatory flow along a coastline or isobath with a series of headlands or ridges reveal a mechanism for transfer of momentum and mass from the boundary into the interior flow through the establishment of organized residual flow patterns. Particle imaging velocimetry is used to visualize the vorticity field and to quantify large‐scale residual circulations generated near the region of roughness. Two flow regimes are observed for differing ratios of along‐coast tidal excursion, L, to the length of the roughness region, D. For L/D > 1 a weak inflow is produced in the vicinity of the roughness region, with a corresponding alongshore flow away from the region. For L/D < 1 a strong outflow jet is established near the center of the roughness region. The two flow regimes result from different residual vorticity patterns and rely on eddies being long‐lived relative to the tidal period. Thus these results are relevant to flows in which dissipation is weak relative to the mean flow kinetic energy such as for tidal flow in deep water. Flow past a single roughness element is used as an analog to tidal flow past a headland and is represented by the high L/D limit.
Role of the western Pacific Ocean boundary conditions during 1980–1998 in the El Niño‐Southern Oscillation events simulated by a coupled ocean‐atmosphere model
Authors: Florenchie Pierre , Perigaud Claire
Published: October 2002

In situ and satellite sea level data sets over 1980–1998 are used to estimate interannual variations of the geostrophic zonal transport from the western Pacific Ocean into the Celebes Sea. Then the transport component due to the Pacific wind‐driven and fully reflected equatorial waves is removed. Finally, the residual variations, named Indonesian throughflow correction (ITFC), are used to correct for the model closed western boundary. ITFC inflows/outflows are leading the warm/cold events by a few months. They are then prescribed at the model western boundary over 1980–1998 to compare with closed boundary simulations. An ITFC inflow anomaly makes the eastern Pacific slightly warmer and trade winds in the central Pacific slightly weaker. Indeed, these quantities are simulated by both experiments in very good agreement with observations. More importantly, prescribing the ITFC greatly improves two characteristics of the model that play a key role in the coupled simulations. The first is the RMS variability of sea level versus longitude; the ITFC shifts to the west the position of the minimum. The second is the basin average sea level; the ITFC largely amplifies its fluctuations, stimulating the charge and discharge of the system. Consequently, the ITFC can have a very large impact on El Niño‐Southern Oscillation forecasts. For example, the 1982/1983 and 1997/1998 1‐year lead forecasts which fail to predict the warm events with closed boundaries are successful with the ITFC. The impact of the ITFC on coupled simulations is not linearly dependent on the ITFC value itself: In addition to the fluctuating conditions at the western boundary, results depend on how close to instability the system is at each time step.
Assimilating high‐resolution salinity data into a model of a partially mixed estuary
Authors: Xu Jiangtao , Chao Shenn‐Yu , Hood Raleigh R. , Wang Harry V. , Boicourt William C.
Published: October 2002

A three‐dimensional circulation model of the Chesapeake Bay is used to validate a simple data assimilation scheme, using high‐resolution salinity data acquired from a ship‐towed undulating vehicle (a Scanfish). The simulation period spans the entire year of 1995 during which the high‐resolution Scanfish data were available in July and October, lasting a few days each. Since Scanfish data were irregularly distributed in time and space, only salinity fields are nudged in the model for simplicity. Model improvements through data assimilation are evaluated from a pair of experiments: one with data assimilation and one without. Data from scattered Chesapeake Bay Program monitoring stations and a few stations maintained by the National Ocean Service inside the bay are used independently to check the model performance. In general, the simple assimilation scheme leads to visibly improved density structures in the upper and middle reaches of the bay. The improvement in the lower bay is equally pronounced after data assimilation but diminishes in a shorter timescale because of faster flushing from the adjacent coastal ocean. Moderate to weak nudging normally enhances the gravitational circulation. Strong nudging may produce transient overshooting, during which the gravitational circulation is renewed vigorously.
Turbulent stress at the ice/ocean interface and bottom surface hydraulic roughness during the SHEBA drift
Authors: McPhee Miles G.
Published: October 2002

Upper ocean turbulence and mean current data from the 1997–1998 Surface Heat Budget of the Arctic Ocean (SHEBA) drift in the western Arctic Ocean were used to estimate turbulent shear stress at the ice/ocean interface and hydraulic roughness of the ice undersurface. Techniques for determining interfacial stress from velocity covariance measurements in the ice/ocean boundary layer (IOBL) are complicated by buoyancy flux and the heterogeneity of the ice undersurface, where protrusions corresponding to relatively small upper surface features become major obstacles when compared to the scale of the IOBL. In addition, ice deformation forced relocation of the main SHEBA oceanographic program about midway through the experiment. Hence, there were two different measurement sites to consider. Three different methods were used to compute the undersurface roughness characterizing the undeformed, multiyear floe occupied by the SHEBA project. The first was straightforward (naive) application of the “law of the wall” to turbulent stress and mean velocity measurements from the turbulence instrument cluster nearest to the interface, assuming a logarithmic velocity profile and constant stress. For the second method, local turbulence closure was used to model the entire IOBL in order to estimate the impact of Coriolis stress attenuation and buoyancy forces on stress and shear near the boundary. A third method, proposed here for the first time, estimated mixing length and eddy viscosity from a scale inversely proportional to the wave number at the peak of the weighted vertical velocity spectrum. The last method appears most robust, in that it shows no overall difference in z0 between the two sites and only minor seasonal and directional differences. The best estimate for the undersurface roughness of undeformed, multiyear ice at the SHEBA site is 0.0048 m < z0 < 0.007, i.e., about 6 mm. Specification of a regional “aggregate” under‐ice roughness requires additional consideration of the added drag from isolated pressure keels and floe edges, along with reduced drag from smoother, newly frozen ice and open water.
Across‐shelf sediment transport: Interactions between suspended sediment and bed sediment
Authors: Harris Courtney K. , Wiberg Patricia
Published: October 2002

We use a two‐dimensional, time‐dependent sediment‐transport model to quantify across‐shelf transport, deposition, and sorting during wave‐driven resuspension events characteristic of those that dominate sediment transport on many continental shelves. Decreases in wave‐orbital velocities as water depth increases, and the resulting cross‐shelf gradient in bed shear stress favor a net offshore transport of sediment. On wide, flat shelves (slopes ∼0.1%percnt;), these gradients are low, and the depth to which the seabed is reworked depends mainly on bottom shear stress and local sediment availability. On narrow, steep shelves (slopes ∼0.5%percnt;), however, the gradient in bottom stress generates significant cross‐shelf suspended sediment flux gradients that create regions of net erosion and deposition. While the magnitude of waves generally determines the water depth to which sediment can be resuspended, erosional and depositional patterns on narrow shelves are sensitive to cross‐shelf gradients in wave energy, nonlocal sediment availability, and the direction and magnitude of the cross‐shelf current. During energetic waves, cross‐shelf divergence of suspended sediment flux can create a coarsened, erosional area on the inner shelf that abuts a region of fine‐grained sediment deposition on the mid‐to‐outer shelf. If currents are strongly shoreward, however, flux divergence leads to erosion over the entire shelf.
Effect of wind variability and variable air density on wave modeling
Authors: Abdalla Saleh , Cavaleri Luigi
Published: October 2002

The impact of wind gustiness on the evolution of wave fields is analyzed by superimposing to the nominal wind speed a fluctuation whose amplitude is related to the local air‐sea temperature difference. The use of fluctuations represented by a Gaussian process, characterized by coherence in time, produces realistic time series whose characteristics are compatible with those obtained from previous studies and open sea measurements. For the sake of forcing a wave model two more representations of gustiness have been used, a simple alternate oscillation of fixed amplitude around the mean value and a Gaussian process without coherence in time. The introduction of gustiness leads to an evident average increase of the resulting wave heights, larger in the Atlantic Ocean than in the Mediterranean Sea. The randomness of the wind and, hence, to a more limited extent, of the wave fields implies the possible occurrence of wave heights much larger than expected in a nongusty field. Besides two 6 month hindcasts two ensemble exercises have been done by forcing two storms with 50 different random realizations of the input wind fields. On the basis of the statistics of the outputs the authors suggest for practical applications the use of two runs, without and with noncoherent gustiness. This will provide information on the statistics of the possible significant wave heights. The effect of a variable air density on wave generation has been explored by repeating the hindcasts using air density values estimated from the output of a meteorological model. It is found that in the North Atlantic Ocean this leads to an increase of the wave heights during the winter storms. The effects are more limited in the Mediterranean Sea.
Impact of the Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission on ocean circulation estimates 2. Volume and heat fluxes across hydrographic sections of unequally spaced stations
Authors: Schröter Jens , Losch Martin , Sloyan Bernadette
Published: October 2002

Accurate geoids are expected to improve our knowledge of the dynamic sea surface height (SSH) as a mirror of the dynamic state of the oceans. The dedicated Gravity Field and Steady‐State Ocean Circulation Explorer (GOCE) mission will lead to a highly accurate geoid model with a resolution of degree and order 200. We examine the impact of this mission on the assessment of large‐scale oceanic mass and heat transports via its expected error characteristics. We do so applying a linear box inverse model and a nonlinear section inverse model to hydrographic data and to (synthetic) SSH data. The results are compared to those obtained when substituting the error estimates of the Gravity Recovery and Climate Experiment (GRACE) mission and the present‐day Earth Gravitational Model 1996. For the box inverse model we find an average reduction in transport uncertainties of about 9% for GRACE geoid error covariances and about 17% for GOCE over the “hydrography‐only” solution. In both GRACE and GOCE these average percentage improvements are significantly increased when model error is excluded. Summarizing our results and those of the companion parts of this study, we conclude that the GRACE mission reduces the marine geoid uncertainties such that altimetry may become useful for the study of the steady state ocean circulation. The GOCE mission will improve the accuracy of the circulation estimates significantly on the large scales and introduce higher accuracy on shorter wavelengths as well. Furthermore, it will enable us to study individual ocean currents.
Effect of horizontal grid resolution on the near‐equilibrium solution of a global ocean–sea ice model
Authors: Duffy P. B. , Wickett M. E. , Caldeira K.
Published: October 2002

We compare the near‐equilibrium solution of a global ocean/sea ice model at a horizontal grid resolution of 1° × 1° to near‐equilibrium solutions obtained in two configurations at 4° (longitude) × 2° (latitude) resolution. All simulations use realistic, smoothed topography and monthly averaged climatological forcings. Our comparison of the results emphasizes large‐scale features relevant to global climate change. Since neither the 1° × 1° simulation nor the 4° × 2° simulations resolves ocean eddies, our results do not address the possible importance of resolving eddies in ocean‐climate simulations. There are significant differences between the 1° × 1° and 4° × 2° simulations, most notably in the Arctic Ocean. However, the large‐scale features of the model solutions are very similar at the two resolutions and in many cases are more sensitive to a large difference in horizontal viscosity than to the difference in resolution. This suggests that other approaches to improving the solution of ocean‐climate models will be more effective than increases in horizontal resolution outside the eddy‐resolving regime.
Differences in heat budgets of the near‐surface Arabian Sea and Bay of Bengal: Implications for the summer monsoon
Authors: Shenoi S. S. C. , Shankar D. , Shetye S. R.
Published: October 2002

An analysis of the heat budgets of the near‐surface Arabian Sea and Bay of Bengal shows significant differences between them during the summer monsoon (June–September). In the Arabian Sea the winds associated with the summer monsoon are stronger and favor the transfer of heat to deeper layers owing to overturning and turbulent mixing. In contrast, the weaker winds over the bay force a relatively sluggish oceanic circulation that is unable to overturn, forcing a heat budget balance between the surface fluxes and diffusion and the rate of change of heat in the near‐surface layer. The weak winds are also unable to overcome the strong near‐surface stratification because of a low‐salinity surface layer. This leads to a shallow surface mixed layer that is stable and responds quickly to changes in the atmosphere. An implication is that sea surface temperature (SST) in the bay remains higher than 28°C, thereby supporting large‐scale deep convection in the atmosphere during the summer monsoon. The atmospheric heating associated with the convection plays a critical role in sustaining the monsoon winds, and the rainfall associated with it, not only over the bay but also over the Indian subcontinent, maintains a low‐salinity surface layer. In the Arabian Sea the strong overturning and mixing lead to lower SST and weak convective activity, which in turn, lead to low rainfall and runoff, resulting in weak stratification that can be overcome easily by the strong monsoon winds. Thus, in both basins, there is a cycle with positive feedback, but the cycles work in opposite directions. This locks monsoon convective activity primarily to the bay.
On the freshwater forcing of the thermohaline circulation in the limit of low diapycnal mixing
Authors: Keeling Ralph F.
Published: October 2002

A conjecture is offered on the stability characteristics of the thermohaline circulation in the limit of very low diapycnal mixing. In this limit the action of the winds on the Antarctic Circumpolar Current (ACC) can sustain a deep overturning pattern known as the “reconfigured conveyor,” consisting of upwelling around Antarctica and sinking in the North Atlantic, as shown by the work of Toggweiler and others. It is conjectured that in this limit, northern sinking should be stabilized in an “on” state because of the penetration of freshwater into the ocean interior via isopycnal layers that outcrop to the surface within and south of the ACC. This conjecture is supported by qualitative arguments and by a hydraulic model for the reconfigured conveyor. The hydraulic model takes into account the freshwater budgets of the Atlantic basin, Antarctic surface waters, and the remaining oceans. It also takes into account, in simple terms, wind‐driven Antarctic upwelling, eddy transports and mixing within the ACC, changes in pycnocline depth, the role of temperature forcing, and advective feedbacks on salinity. The hydraulic model suggests that multiple “on/off” states of the reconfigured conveyor are possible but only if the deep waters that form in the Northern Hemisphere are fresher than the intermediate waters that form in the vicinity of the ACC in the Southern Hemisphere, a condition that is not satisfied in the modern ocean.
Isopycnal empirical orthogonal functions (EOFs) in the North and tropical Atlantic and their use in estimation problems
Authors: Faucher P. , Gavart M. , De Mey P.
Published: October 2002

We investigated the vertical variability of the thermal/dynamical structure of the North and tropical Atlantic ocean from a set of historical hydrographic data [Reynaud et al., 1998]. The analysis was performed in 10° geographic bins in terms of vertical empirical orthogonal functions (EOFs) in isopycnal coordinates as first proposed by Gavart and De Mey [1997] (hereinafter referred to as GDM97). In the isopycnal coordinate system the state vector is made up of isopycnal displacements (η) and the compensated potential temperature along isopycnals (θ). GDM97 showed that in the Azores‐Madeira region the isopycnal EOFs consistently help to better separate physical processes in the water column and to better project the dynamic height anomaly downward in comparison with the classical EOF analysis in depth coordinates. In the present study, we extended their work geographically and the connection with state estimation and data assimilation was more formalized. From our results, smooth spatial variations of isopycnal EOFs were found in the North Atlantic and confirmed the large‐scale relevance of isopycnal EOFs in the ocean. North of 20°N the dominant isopycnal EOF was associated with a quasihomogeneous vertical displacement of the isopycnal surfaces (η ≈ 1) with a quasiconservation of compensated potential temperature on those surfaces (θ ≈ 0). That mode was strongly modified by thermocline‐intensified baroclinic effects along the path of the Gulf Stream up to 40°W. Some physical interpretations of the vertical EOFs were also suggested in several areas as far as the isopycnal formulation was concerned (e.g., some thermal fronts, the Mediterranean outflow, water lenses, or “meddies”). This work confirmed the validity of the ideas of GDM97 over the entire North Atlantic ocean. The potential use of the isopycnal EOFs in state estimation problems was discussed. They are possible candidates to reduce the order of state estimation problems provided that they have good observability properties. These observability properties were studied in the case of satellite altimetry; that is, we examined how the dominant isopycnal EOFs were connected to sea‐level anomalies. In most bins it was found that the order of an estimation problem could be truncated to one (primarily north of 20°N) or two modes. The dominant EOF was found to account for most of the surface dynamic height variability with a rather normally distributed error of only a few centimeters. Dynamically more robust results were also presented, performing a univariate isopycnal analysis in η, leaving coherent thermal effects out. As a result, the dominant character of mode 1 was further enhanced everywhere. These univariate isopycnal EOFs can be used for purely dynamical state estimation. More generally, we believe that our results can be used to set up reduced‐order data assimilation schemes in the North Atlantic or in other regions with good historical data.
Observations of the response of a buoyant estuarine plume to upwelling favorable winds
Authors: Hallock Z. R. , Marmorino G. O.
Published: October 2002

The response of the buoyant plume from the Chesapeake Bay to upwelling favorable winds and during conditions of low freshwater discharge is examined using an offshore array of bottom‐deployed acoustic Doppler current profilers (ADCPs) as well as ancillary data. During periods of northward (upwelling favorable) winds the observed response is strong offshore flow (U > 0.2 m/s) in a shallow layer of plume water and a rapid reversal of the near‐surface flow from the usual southward flow associated with inertial turning of the plume. The offshore migration of the plume front is deduced from sequential arrivals at the various mooring locations. The orientation of the plume front is approximately alongshore (north‐south), suggesting that the upwelling response is two‐dimensional. A plume thickness calculated from the ADCP data is relatively constant at about 4 m as the plume moves across the moorings. The offshore propagation speed of the plume front of about 0.20 m/s matched the advection speed predicted by Ekman theory using the calculated plume layer thickness. The mooring velocity profiles provide a view of the across‐plume structure in a reference frame fixed to the front. This reveals a pattern of circulatory motion with sinking of plume and ambient water at the front and an ascending motion occurring inshore, suggesting a recirculation between the seaward and inshore portions of the plume. These results are consistent with recent model simulations of the response of a river plume during upwelling favorable winds.
A northern boundary current along Australia's southern shelves: The Flinders Current
Authors: Middleton John F. , Cirano Mauro
Published: October 2002

South of Australia, the monthly mean wind stress curl is positive during both summer and winter and leads to Ekman pumping and downwelling throughout the region. Sverdrup dynamics indicates that this downwelling should lead to a northward transport of around 5–10 Sv (1 Sv = 10 m3 s−1). Classical arguments for western boundary currents are adapted to show that this transport should be deflected into an upwelling favorable boundary current that flows from east to west along Australia's southern shelves: the Flinders Current. Support for this proposition is obtained from results of the Ocean Circulation and Climate Advanced Modelling project (OCCAM), Sverdrup transports, and limited observations for the region. In addition, the OCCAM results show that the northward transport leads to upwelling at depths below 400 m and that the Flinders Current (1) intensifies from Victoria to Western Australia (transport ∼8 Sv with speeds up to 15 cm s−1), (2) can extend from the surface to depths of 800 m, and (3) is found during both summer and winter. During winter the winds are downwelling favorable and lead to a coastal current that flows from west to east and opposes the Flinders Current. Further support for the origin and nature of the Flinders Current is obtained from simple numerical experiments made using a rectangular domain and idealized representations of the summer and winter wind stress.
Freshwater and temperature transports through the Tsushima‐Korea Straits
Authors: Isobe Atsuhiko , Ando Mitsuru , Watanabe Toshiteru , Senjyu Tomoharu , Sugihara Shigehiko , Manda Atsuyoshi
Published: October 2002

Using acoustic Doppler current profiler (ADCP) and conductivity‐temperature‐depth (CTD) data sets obtained from 1991 to 1993, volume, freshwater, and temperature transports through the Tsushima‐Korea Straits are estimated. To remove tidal currents contained in raw ADCP data, tidal currents are calculated using harmonic constants obtained by ADCP surveys from 1998 to 1999. The annual mean and range of the volume transport through the straits are 2.4 Sv (1 Sv = 106 m3/s) and 0.8 Sv, respectively. These values are consistent with those of previous studies. The annually averaged freshwater transport is 33 × 106 kg/s, which is the same as the total river discharge around the Yellow and East China Seas. This suggests that the total river discharge and freshwater transport through the Tsushima‐Korea Straits are the prominent two components in the freshwater budget, averaged annually in the Yellow and East China Seas. The annually averaged temperature transport through the Tsushima‐Korea Straits is 0.17 PW.
Annual cycle of equatorial zonal currents in the Pacific
Authors: Keenlyside Noel , Kleeman Richard
Published: October 2002

Observational (Tropical Atmosphere‐Ocean array) data on the annual cycle of upper ocean zonal currents on the equator are analyzed using a simple dynamical ocean model in order to investigate underlying dynamics. The model, by treating linear and nonlinear terms semi‐independently, allows a separation of various linear and nonlinear effects. The model focuses on linear dynamics of low‐order baroclinic modes. By realistically simulating the vertical structure of annual cycle, the model shows that linear dynamics determines the vertical and meridional structure of the annual cycle. Nonlinearity is weak and only important in the undercurrent, where it provides a simple mechanism for the annual cycle: mean meridional advection of the annual cycle north of the equator onto the equator, with the boreal springtime surge in the undercurrent being a direct result of a surge centered at 2°N. Model results show that annual variations in zonal currents are out of phase across the equator, surging in the corresponding spring. This behavior is a response to trade wind variations, which are also equatorially antisymmetric, and is generated by the second meridional mode Rossby wave.
Wave response of an ice floe of arbitrary geometry
Authors: Meylan Michael H.
Published: October 2002

A fully three‐dimensional model for the motion and bending of a solitary ice floe due to wave forcing is presented. This allows the scattering and wave‐ induced force for a realistic ice floe to be calculated. These are required to model wave scattering and wave‐induced ice drift in the marginal ice zone. The ice floe is modeled as a thin plate, and its motion is expanded in the thin plate modes of vibration. The modes are substituted into the integral equation for the water. This gives a linear system of equations for the coefficients used to expand the ice floe motion. Solutions are presented for the ice floe displacement, the scattered energy, and the time‐averaged force for a range of ice floe geometries and wave periods. It is found that ice floe stiffness is the most important factor in determining ice floe motion, scattering, and force. However, above a critical value of stiffness the floe geometry also influences the scattering and force.
Coastal polynyas in the southern Weddell Sea: Variability of the surface energy budget
Authors: Renfrew Ian A. , King John C. , Markus Thorsten
Published: October 2002

The surface energy budget of coastal polynyas in the southern Weddell Sea has been evaluated for the period 1992–1998 using a combination of satellite observations, meteorological data, and simple physical models. The study focuses on polynyas that habitually form off the Ronne Ice Shelf. The coastal polynya areal data are derived from an advanced multichannel polynya detection algorithm applied to passive microwave brightness temperatures. The surface sensible and latent heat fluxes are calculated via a fetch‐dependent model of the convective‐thermal internal boundary layer. The radiative fluxes are calculated using well‐established empirical formulae and an innovative cloud model. Standard meteorological variables that are required for the flux calculations are taken from automatic weather stations and from the National Centers for Environmental Production/National Center for Atmospheric Research reanalyses. The 7 year surface energy budget shows an overall oceanic warming due to the presence of coastal polynyas. For most of the period the summertime oceanic warming, due to the absorption of shortwave radiation, is approximately in balance with the wintertime oceanic cooling. However, the anomalously large summertime polynya of 1997–1998 allowed a large oceanic warming of the region. Wintertime freezing seasons are characterized by episodes of high heat fluxes interspersed with more quiescent periods and controlled by coastal polynya dynamics. The high heat fluxes are primarily due to the sensible heat flux component, with smaller complementary latent and radiative flux components. The average freezing season area‐integrated energy exchange is 3.48 × 1019 J, with contributions of 63, 22, and 15% from the sensible, latent, and radiative components, respectively. The average melting season area‐integrated energy exchange is −5.31 × 1019 J, almost entirely due to the radiative component. There is considerable interannual variability in the surface energy budget. The standard deviation of the energy exchange during the freezing (melting) season is 28% (95%) of the mean. During the freezing season, positive surface heat fluxes are equated with ice production rates. The average annual coastal polynya ice production is 1.11 × 1011 m3 (or 24 m per unit area), with a range from 0.71 × 1011 (in 1994) to 1.55 × 1011 m3 (in 1995). This can be compared to the estimated total ice production for the entire Weddell Sea: on average the coastal polynya ice production makes up 6.08% of the total, with a range from 3.65 (in 1994) to 9.11% (in 1995).
Origin of waters observed along 137°E
Authors: Bingham Frederick M. , Suga Toshio , Hanawa Kimio
Published: October 2002

Using the World Ocean Atlas data set, we examine the origins and flow paths of subducted waters observed along the 137°E section in the western North Pacific. The waters studied consist mainly of the water mass known as North Pacific Tropical Water, but includes much of the water midlatitude thermocline. A method is developed to trace these waters from 137°E back through the subtropical gyre to their outcrops. Subducted waters are aged using this technique and found to be between 0.5 and 35 years old by the time they reach 137°E. For this subducted regime, waters on a given isopycnal observed along 137°E increase in age with decreasing latitude, with waters at the southern end of the section being 2–3 times older than waters at the northern end. This estimate of age is consistent with previous estimates calculated from chlorofluorocarbon measurements. It is found that subducted water masses are strongly homogenized by the time they reach 137°E. That is, the originally subducted waters have a wide variation in θ‐S characteristics, but by the time they reach 137°E, they form a coherent water mass with a tight θ‐S relation. It is shown that isopycnal mixing is not a plausible mechanism for this homogenization but that diapycnal mixing is a more likely process.