摘要:

A variable‐density, 1½‐layer model is used to investigate the dynamics of the fresher‐water plumes generated by river outflow. Solutions are found in a north‐south channel, and the transportMrand salinitySrof the outflow are specified as boundary conditions along a 25 km‐wide segment of the western boundary. In most cases, the river water discharges into a preexisting, oceanic mixed layer with thicknessH1. WhenMris sufficiently low, plumes remain coastally trapped. Immediately after the outflow is switched on, a coastal Kelvin wave is excited at the river mouth that establishes a southward current of oceanic water along the right‐hand coast. In contrast, all the river water first bends to the left as it exits the river mouth, and the resulting plume advances northward along the left‐hand coast. At the plume nose, some of the fresher water reverses direction, and this water, together with some oceanic water, flows southward on either side of the offshore density front between the fresher and salty waters. Two processes cause this upstream movement: geostrophic adjustment generates the southward frontal current, and Kelvin‐wave propagation from the nose thins the layer within the plume thereby establishing the northward, geostrophic, coastal jet. WhenMris sufficiently high, coastally trapped plumes no longer exist. If the Rossby number of the outflow is also large enough, the river water flows directly offshore and only a portion of it recirculates to form a northward propagating coastal plume. The angle at which the outflow leaves the river mouth becomes more southward asMrandSrincrease and asH1decreases. The strength of the northward propagating plume is weakened asH1decreases andSrincreases. WhenH1=0 so that there is no ambient oceanic layer, there is no northward plume at all. Likewise, the plume is weakened when the model includes entrainment, a process that acts to prevent the layer thickness from thin

ISSN:0148-0227    

DOI:10.1029/97JC00985

年代:1997

数据来源: WILEY

摘要:

A three‐dimensional, limited‐area, ocean general circulation model is used to study the response of the coastal ocean to small‐scale wind forcing. The model considers an idealized forcing of a semi‐infinite, upwelling‐favorable wind with a large positive wind curl at one end. Such a wind pattern is commonly found in the vicinity of capes and points where the synoptic‐scale wind is disturbed by coastal mountain ranges. The model study includes both process and hindcast studies. The process study considers the upwelling spinup and relaxation, driven by a positive wind curl. It also examines the model sensitivity to wind patterns and an external pressure force. The model results show that a strong poleward alongshore pressure gradient is set up by a positive wind curl. This pressure force drives an inshore poleward current into the upwelling region during active wind forcing, and it causes a surge of warm water into the upwelling zone during wind relaxation. The hindcast study is applied to the 1983 Organization of Persistent Upwelling Structures (OPUS) observations near Point Conception off central California. The simulation uses an idealized geometry but with observed winds. During OPUS, a sudden reversal of the surface equatorward currents occurred in the midst of a steady upwelling‐favorable wind. This unexpected coastal flow feature was faithfully reproduced in the mod

ISSN:0148-0227    

DOI:10.1029/97JC00635

年代:1997

数据来源: WILEY

摘要:

A numerical model has been used to study separation/intrusion of an unsteady, baroclinic coastal jet, the Gaspé Current (GC) and its impact on dynamics and thermodynamics of the Baie des Chaleurs (BdC, Gulf St. Lawrence, Canada). The model has 2½ layers with primitive equation dynamics and an embedded bulk mixed layer (ML) model. It is forced with observed atmospheric fluxes, as well as the GC. The simulations show that the variability in the Baie des Chaleurs is controlled by the characteristics of the unsteady GC separation/intrusion. On the basis of the dynamic and vorticity balance analyses, it is found that the separation is related to the adverse pressure gradient force which is induced by the ageostrophic terms in the momentum equations and to local vorticity intensification due to the inertial effect in the flow. The simulations demonstrate characteristics of unsteady separation. The presence of separation in an accelerating current, as well as in a linear current, with strong deceleration when momentum advection is neglected, gives new insight into the mechanisms of unsteady boundary current separation. Unsteady separation or intrusion occurrence depends on the magnitude of the GC transport, as well as its rate and duration of deceleration or acceleration. The GC intrusion generates cyclonic circulation in the BdC. Prevailing westerly winds reduce the cyclonic circulation inside the bay but have little effect on separation/intrusion near the entrance. The GC either intrudes along the coastline (attachment) or after separation (reattachment). Increasing vertical shear of the GC and offshore movement of the GC axis reduces the tendency to separate. The finding of an asymmetric response of GC separation/intrusion to symmetric GC forcing explains the monthly mean features seen in observations taken in the ba

ISSN:0148-0227    

DOI:10.1029/97JC00589

年代:1997

数据来源: WILEY

摘要:

The wind‐induced, near‐inertial oscillations over the Texas‐Louisiana shelf were studied using a two‐dimensional primitive equation model forced by an impulse of the surface wind. Numerical experiments reproduced the cross‐shelf structure of observed near‐inertial oscillations. For a given spatially uniform impulse of the wind, the model predicted the large near‐inertial oscillations that were surface‐intensified and characterized by a first baroclinic mode in the vertical. The oscillations reached a maximum kinetic energy near the shelf break and decayed gradually toward the coast but rapidly offshore. The magnitude and cross‐shelf distribution of the wind‐induced, near‐inertial energy changed with wind direction. Effects of nonlinear advection on near‐inertial oscillations were largest near the shelf break and decreased both onshore and offshore where the bottom topography was gentle or flat. They were stronger during the wind event and weaker after the wind ceased. Nonlinear interaction of tides and wind was weak on the shelf. However, it tended to provide a positive net contribution to near‐inertial currents for both cases where tides were in and out of phase with winds. A simple theory suggests that the cross‐shelf variation of near‐inertial oscillations was controlled by the cross‐shelf gradient of surface elevation and the verti

ISSN:0148-0227    

DOI:10.1029/97JC00228

年代:1997

数据来源: WILEY

摘要:

The circulation system in an ice shelf cavity is driven by buoyancy fluxes due to melting and freezing of ice and horizontal pressure gradients at the interface between the cavity and the open ocean. Hence the inflow and outflow pattern and the hydrography in the open ocean influence the general hydrographic condition in the cavity, which at least provides the potential for melting and freezing processes. Applying a three‐dimensional ocean general circulation model to an idealized ice shelf cavity geometry coupled with an open ocean at a topographic ice shelf barrier, we found an important parameter controlling the interaction between these two systems. Idealized studies for different ice shelf and sea bottom topographies and forcing mechanisms for the open ocean show that the ice shelf edge represents a natural barrier for barotropic interaction, because of the sudden decrease in water column thickness. Since the water column thickness and the Coriolis force determine the characteristics for geostrophic flow, separated circulation systems arise for the open ocean and the ice shelf cavity. Only in areas where constant water column thickness and, from the oceanographic point of view, constant ƒ/Hcontours can be observed across the barrier, an increased barotropic current can surmount the ice edge and ventilate the water mass beneath the ice shelf. This is only the case at lateral sloping sidewalls or at deep depressions, which can be found, for example, in the southern Weddell Sea. In all other cases the circulation in the ice shelf cavity is closed and almost unaffected by the hydrography outside the barri

ISSN:0148-0227    

DOI:10.1029/97JC00891

年代:1997

数据来源: WILEY

摘要:

The two main diurnal (i.e., K1and O1) and semidiurnal (i.e., M2and S2) tidal waves have been computed from a hydrodynamic, finite element, spectral model. The spatial resolution of the model varies from a few kilometers at the coastline to 100 km in the deepest regions. An optimization of the open boundary conditions has been performed to improve the solution accuracy. The bottom topography used in this model has been originally extracted from the Earth Topography 5‐minutes Grid (ETOPO5) and corrected from various sources of data. The impact of the input parameters like the friction coefficient, the loading effects, and the parameterisation of the additional friction due to the Arctic ice cover is investigated. The tidal elevations are compared to a set of in situ observations and validated. They also are compared to theKowalik and Proshutinsky[1993] model. These comparisons mainly show good agreement between the two models. Nevertheless, significant differences are observed where small wavelength structures are produced by the topographic trapping of the tidal energy. The velocities associated with the elevations are also presented. The potential rate of work, the dissipation, and the energy fluxes are computed from these elevations and velocities. Because of the uncertainties of the velocities at the open boundaries, we are restricted to a qualitative analysis of the energy equation terms, which appears to be consistent with the previous studies on Arctic tide

ISSN:0148-0227    

DOI:10.1029/96JC02596

年代:1997

数据来源: WILEY

摘要:

Further details of the “warming” of bottom water flowing through the Vema Channel, first reported byZenk and Hogg[1996], are given. Because cross‐channel gradients of temperature are large and the channel is narrow, careful analysis is required to determine whether or not the bottom water temperature minimum was adequately sampled by a particular station or cruise. During a period from 1972 to 1996, at least 14 visits with quality hydrographic measurements have been made to the region. Of these, 11 were judged successful, and their data indicate an abrupt rise in potential temperature by 0.03°C from −0.18°C to −0.15°C in the early 1990s that has remained until the latest observation in the spring of 1996. Although this observation is consistent with the report of warming of Antarctic Bottom Water in the Argentine Basin byColes et al[1996], their associated conclusion that this water mass has also freshened by 0.008 psu (on potential density surfaces) is in contradiction with our finding of no measu

ISSN:0148-0227    

DOI:10.1029/97JC00591

年代:1997

数据来源: WILEY

摘要:

A sigma coordinate, free surface numerical model with turbulence dynamics has been implemented for the Atlantic Ocean and the Greenland Sea, from 80°S to 80°N. It is driven at the surface by monthly mean sea surface temperature and wind stress climatologies and is executed for 30 years. This is the first time that a model of this type, previously used mostly for coastal and regional simulations, has been implemented for the entire Atlantic Ocean and run for a long period of time. The model horizontal circulation, thermohaline overturning circulation, and meridional heat fluxes are described; the results are compared with observations and the results of other models. The model produces intense deep western boundary currents and complicated gyre structures associated with small‐scale topographic variations. The meridional overturning circulation consists of about 14 Sv (1 Sv=106m3s−1) of southward flowing deep water mass crossing the equator and a northward heat flux with a maximum value of more than 1 PW (1015W). Although the maximum meridional heat flux is comparable to estimates obtained from observations, the amplitude of the seasonal variations of northward heat flux across 26°N is underestimated in comparison with observations; it is similar to that obtained by other

ISSN:0148-0227    

DOI:10.1029/97JC00984

年代:1997

数据来源: WILEY

摘要:

To investigate the possibility of climatic influence on the marine biosphere, we analyzed the decadal time series of Zooplankton biomass and sea surface temperature (SST) measured at station P, Gulf of Alaska, during the period 1957–1980. A relationship between the two series is not apparent in the raw data. However, Fourier analysis revealed peaks at 28.8 months in both Zooplankton and SST spectra. This period belongs to the range of the quasi biennial oscillation (QBO). Such a signal has been found originally in atmospheric and later in physical oceanographic variables. However, it has not been reported previously in biological variables. Cross‐spectral and band‐pass filter analyses indicate that temperature and Zooplankton variations at this frequency may be related. Possible mechanisms are discussed. These results are important because they indicate that a climatic signal, the QBO, is present in the oceanic

ISSN:0148-0227    

DOI:10.1029/97JC00595

年代:1997

数据来源: WILEY

摘要:

The hypothesized “biological pump” mechanism for removing carbon from the euphotic zone to the deep ocean requires different rates of recycling of C, N, and P in sinking particles. At Station ALOHA in the subtropical North Pacific Ocean (22° 45′N, 158°W), the C/N and C/P ratios in sinking particles increase with depth, while the ratios in the upward mixing flux decrease. An exponential model of the sinking flux as a function of depth is used, which permits remineralization length and time scales (more properly called solubilization scales) to be estimated. Time scales (2–5 days) are consistent with microbiological decomposition, and length scales are greater for C (368 m) than for N and P (249 and 248 m), consistent with the existence of a biological pump for carbon. Assuming that the system is nutrient limited (total upward and downward fluxes of N or P are equal) permits us to estimate vertical eddy diffusivity (Kz) and the net export of carbon from the system. Values ofKzrange from 5×10−6to 4×10−5m2s−1and are consistent with independent estimates for similar environments. We estimate that approximately 20% of the air‐to‐sea carbon flux at Station ALOHA and a similar fraction of new carbon production are exported to the deep ocean (σθ>27) by particle sinking. Eddy diffusivities are lower and net carbon export greater if phosphorus is assumed to be the controlling element. This has implications for the role of dinitrogen fixation in the su

ISSN:0148-0227    

DOI:10.1029/97JC00369

年代:1997

数据来源: WILEY