摘要:

Transient tracers and the closely related “age” tracers exhibit a rich physical and mathematical structure even for problems of one space dimension. This richness tends to make interpretation of observations, which are inevitably thin in both space and time, difficult, in contrast to the situation in modeling studies. At least six different timescales and corresponding space scales can appear in one‐dimensional problems. In higher dimensions the number of scales increases. Several examples of analytical and numerical solutions are explored for the light they cast on understanding a fluid flow. Boundary Green functions emerge as the fundamental physical/mathematical link between interior tracer distributions and surface and other boundary variations. With transient tracers in inverse calculations one should normally use the underlying tracer distributions to attempt to solve for fundamental fluid properties, such as the mixing coefficients, rather than ambiguous “ventilation” times, which among other problems, may be determined only by the detection threshold and are often mainly functions of the tracer decay constant rather than of fluid properties. Tracers that are transient only through stochastic boundary conditions show that large‐scale space/time patterns can emerge in the tracer field, having little or no clear connection to the underlying

ISSN:0148-0227    

DOI:10.1029/2001JC000797

年代:2002

数据来源: WILEY

摘要:

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 Fr

ISSN:0148-0227    

DOI:10.1029/2000JC000530

年代:2002

数据来源: WILEY

摘要:

Laboratory experiments were used to investigate the growth of intrusions due to internal‐wave reflection from a sloping boundary. When normalized by the incident energy density flux, the average intrusion spreading velocity was found to be a linear function of the frequency ratio ω/ωc, where ω is the frequency of the incident wave and ωcis the critical frequency, at which the wave characteristic has the same angle as the bottom slope. Evenly spaced layers, indicating thin perturbations in the background density gradient, developed within the mixing region and spread into the tank interior. The vertical spacing of these layers also bore a linear relationship to ω/ωc. A linear model of internal‐wave reflection suggests that these layers may be related to an isopycnal displacement, or overturn, scale. Intrusion growth occurred at a range around the critical frequency and was strongest at slightly supercritical conditions. A balance relating the spreading rate of intrusions to the divergence of energy density flux across the boundary layer is derived. Fitting the laboratory results to this theoretical prediction suggested a weak net buoyancy flux. This balance might be of use in predicting spreading rates of intermediate nepheloid layers generated by internal‐wave mixing at ocea

ISSN:0148-0227    

DOI:10.1029/2001JC000801

年代:2002

数据来源: WILEY

摘要:

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 primar

ISSN:0148-0227    

DOI:10.1029/2000JC000679

年代:2002

数据来源: WILEY

摘要:

Interaction between a vortex and a right vertical cylinder was investigated in the laboratory for both a self‐propagating vortex and one advected by a background uniform flow. In the former case, experiments were carried out with a sloping bottom in order to simulate the β plane. In the latter case the bottom was flat and a cylinder was towed, with a uniform speed, through a fluid otherwise at rest and into a stationary vortex. In both cases, after a cyclonic vortex came in contact with the cylinder, fluid peeled off the outer edge of the vortex and went around the cylinder with a counterclockwise velocity vsas predicted by the circulation equation. This fluid formed a new cyclonic vortex in the wake of the cylinder, and bifurcation of the original vortex into two vortices occurred provided 400 ≤Re≤ 1100, where the Reynolds numberRe= vsLmax/ν andLmaxis the larger of the vortex or the cylinder diameter. This result is in agreement with previous studies of uniform flow past a cylinder in a rotating environment, and therefore we suggest that the new vortex in the wake of the cylinder was formed like those in the well‐known Karman vortex street. Experiments have been carried out systematically by varyingD/d, the ratio of the cylinder diameter to the vortex diameter and the geometry of the encounter. The results suggest that the presence of a background flow enhances the bifurcation mechanism. A good agreement between the laboratory experiments and the observation of a meddy bifurcating after collision with the Irving Seamount in the Canary Basin, suggests that the oceanic vortex‐bifurcation process is similar to that observed in the laboratory

ISSN:0148-0227    

DOI:10.1029/2000JC000599

年代:2002

数据来源: WILEY

摘要:

Measurements taken by two instrumented tripods deployed on the Ebro delta inner shelf at 8.5‐ and 12.5‐m depth were used to characterize low‐frequency currents under the action of a moderate eastern storm. Results show that inner shelf currents respond very rapidly to wind action with along‐shelf currents dominating over the across‐shelf ones. Mean along‐shelf velocities of 0.35 and 0.24 m s−1at the inner and outer position, respectively, were recorded during the event, and they were highly correlated with the along‐shelf wind stress. The measured current profiles were used to derive wind drag coefficients, assuming a balance between the along‐shelf components of the wind and current bottom stresses. Since under eastern wind events, wave action in the Ebro inner shelf is significantly enhanced, wave‐current interaction processes controlling the current bottom stress and sediment mobilization can affect the current structure. During the event, depth‐averaged concentrations in the lower meter of the water column larger than 1 g L−1were recorded that were also accompanied by the generation of vertical concentration gradients larger than 1 g L−1m−1. When the bottom drag coefficient was obtained from velocity profiles without considering the presence of these gradients, the required wind drag coefficients to fulfil the along‐shelf balance were much higher than the ones derived from wind data, and they vary across the inner shelf. However, when bottom drag coefficients are obtained including stratification effects, the wind drag coefficients fulfilling the along‐shelf balance were of the same order of magnitude as the ones derived from wind data and without a

ISSN:0148-0227    

DOI:10.1029/2001JC000905

年代:2002

数据来源: WILEY

摘要:

Atmospheric and oceanic partial pressures of carbon dioxide and fluorescence were measured underway off the coast of northern Chile in January 1997. Seawater samples were taken for the analysis of nutrient and chlorophyll concentrations at 31 stations. The objectives were to improve the understanding of the biologically induced responses of upwelling processes off the coast of Chile and to assess the air‐sea exchange of CO2during the austral summer. The cruise sampled within a grid from 22°40S to 24°S extending offshore to 71°52′W during 10–16 January 1997 (survey 1) and 22–27 January 1997 (survey 2). Survey 1 measurements showed relatively uniform hydrographic and chemical properties, except south of the Mejillones peninsula, where colder and fresher water was observed. During the second survey, concurrent satellite imagery showed colder water in a narrow coastal band. The relatively cold and fresh water recorded there suggested that upwelled water originated mainly from subantarctic water rather than from the nutrient‐rich equatorial subsurface water. Onshore‐offshore variability dominated over longshore variability. The offshore subtropical water was characterized by a temperature of 23°C, a salinity of 34.9, and relatively uniform pCO2around 400 μatm that was mainly temperature driven. Although strong CO2drawdowns (pCO2as low as 200 μatm) were located close to the coast following the upwelling events, overall, the region was a source of CO2to the atmosphere. A pCO2algorithm was constructed using the cruise data of sea surface temperature (SST) and salinity (S). Synthetic salinity was calculated fromS‐SST relationships determined for survey 2 and used with advanced very high resolution radiometer SST to calculate pCO2over the cruise area, weekly, from January to March. European Centre for Medium‐Range Weather Forecasts wind speeds were used to calculate the CO2flux in the upwelling off Chile (22°S–29°S, 68°W–73°W). The region was a source of CO2flux of 1.6 mmol m−2d

ISSN:0148-0227    

DOI:10.1029/2000JC000395

年代:2002

数据来源: WILEY

摘要:

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

ISSN:0148-0227    

DOI:10.1029/2000JC000504

年代:2002

数据来源: WILEY

摘要:

High concentrations of chlorophyll are found in the California Current System over 300 km offshore, far from the productive coastal upwelling region, and between 150 and 250 m, well below the depth to which photosynthetically active solar radiation penetrates. This exceptionally deep chlorophyll feature is formed near the coast and transported offshore in the meandering California Current jet. Chlorophyll is forced downward along sloping density surfaces through conservation of potential vorticity along the meandering jet path. Thus mesoscale physical dynamics serve to inject large amounts of carbon, e.g., 2400 t as reported here, per event from regions of active coastal upwelling into the adjacent deep ocean, a process that must be considered when computing oceanic carbon budgets.

ISSN:0148-0227    

DOI:10.1029/2001JC000956

年代:2002

数据来源: WILEY

摘要:

A new concept, valid prediction period (VPP), is presented here to evaluate ocean (or atmospheric) model predictability. VPP is defined as the time period when the prediction error first exceeds a predetermined criterion (i.e., the tolerance level). It depends not only on the instantaneous error growth but also on the noise level, the initial error, and the tolerance level. The model predictability skill is then represented by a single scalar, VPP. The longer the VPP, the higher the model predictability skill is. A theoretical framework on the basis of the backward Fokker‐Planck equation is developed to determine the mean and variance of VPP. A one‐dimensional stochastic dynamical system [Nicolis, 1992] is taken as an example to illustrate the benefits of using VPP for model evaluat

ISSN:0148-0227    

DOI:10.1029/2001JC000879

年代:2002

数据来源: WILEY