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Patrick F. Cummins

Abstract

Numerical simulations of upstream propagating disturbances generated through the interaction of an inviscid, nearly two-layer flow past a two-dimensional obstacle are discussed. The experimental configuration is an appropriate one for flow past ice keels–the inverted ridges of submerged ice found in frozen seas. In the first series of simulations, a steady inflow is specified and properties of upstream propagating bores are compared with the predictions of hydrostatic theory. While the comparison shows a generally good agreement with theory, a dependence of the response an the height-to-width aspect ratio of the obstacle is identified. For obstacles with relatively large aspect ratios, bore amplitudes and propagation speed are overestimated by the theory as the Froude number becomes large. Conversely, bore amplitudes are slightly underestimated for obstacles with small aspect ratios.

In a second series of simulations, flow past the obstacle is modulated at the M2 frequency over one-half tidal cycle. In these cases, the response is characterized by the generation of an upstream propagating bore, which evolves into a packet of rank-ordered solitons as the tidal flow slackens. The results suggest ice keels as possible generation sites for internal solitons over Arctic shelves.

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Patrick F. Cummins

Abstract

A quasi-geostrophic regional model of the northeast Pacific is used to investigate the effects of bottom topography and seasonal wind forcing on the circulation of the Alaskan Gyre. In a numerical experiment with a flat bottom and steady forcing the most energetic signal is due to mesoscale eddies with a 100-day period associated with barotropic wave propagation along the Aleutian Island arc. Bottom topography eliminates this signal and causes the flow fluctuations to be of lower frequency and primarily in the first baroclinic rather than the barotropic mode. Experiments with a climatological seasonal cycle in the wind field show that bottom topography has an important influence in moderating the intensity of the seasonal response. It is suggested that the very substantial seasonal variations of the Alaskan Gyre transport obtained in previous numerical studies of the NE Pacific are due to the failure of these models to include or to resolve adequately the bottom topography.

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Patrick F. Cummins and Laurence Armi

Abstract

The time-dependent response of upstream undular bores and internal hydraulic jumps from initial formation to eventual release is documented. Two events, characterized by qualitatively different responses, are discussed. In the first case, an undular bore develops upstream of the sill crest. This disturbance remains upstream through the ebb tidal flow but is transformed to a hydraulic jump as its amplitude increases. Toward the end of ebb tide, it is released and subsequently disperses into a group of solitary-like waves. During the second event, an upstream jump also develops at an early stage of the tide. However, it is subsequently swept downstream by the tidal flow such that the upstream region then appears featureless. Approaching slack tide, as an exchange flow becomes established, a large bore or gravity current is emitted. The different responses seen in these two events are interpreted in terms of the Froude number associated with the near-surface stratification.

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Patrick F. Cummins and Greg Holloway

Abstract

A parameterization of eddy–topographic stress interactions has been proposed previously by Holloway, based on the statistical mechanical equilibria of unforced inviscid models. Tendency toward such equilibria may account for the presence of deep currents observed along oceanic continental margins. The parameterization includes simplifications of the statistical equilibria permitting a local implementation of these tendencies into coarse-resolution ocean models. The approximations involved in this parameterization are examined in simulations with an inviscid quasigeostrophic model with idealized topography representing a continental margin. Time-averaged flows are in qualitative agreement with the suggested pattern and are dominated by a large-scale cyclonic circulation along the basin margin. Results show that the parameterized equilibria can significantly overestimate the along-isobath transport due to neglect of the relative vorticity component of the potential vorticity.

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Diane Masson and Patrick F. Cummins

Abstract

A three-dimensional prognostic numerical model has been developed to study the ocean circulation around Vancouver Island, British Columbia. In a series of simulations, the model is applied to examine the role of buoyancy forcing in the dynamics of the summer coastal countercurrent found off the west coast of Vancouver Island. The forcing is provided by the Fraser River discharge into the Strait of Georgia. An estuarine circulation establishes itself in Juan de Fuca Strait, from which a distinctive right-bounded current is formed and advances along the coast. Sensitivity studies are conducted to determine the robustness of this current to initial conditions, opposing wind, enhanced vertical mixing, and grid resolution. Finally, various characteristics of the numerically modeled coastal flow are compared with observations.

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Patrick F. Cummins and Gary S. E. Lagerloef

Abstract

Low-frequency variability of the depth of the main pycnocline at Ocean Weather Station P and over the northeast Pacific is examined in terms of the one-dimensional response to local Ekman pumping according to the Hasselmann stochastic climate model. The model is forced with monthly wind stress curl anomalies derived from the National Centers for Environmental Prediction reanalysis for the period 1948–2000. An empirical orthogonal function analysis shows that the leading mode of the response bears the signature of the Pacific (inter) Decadal Oscillation (PDO) and that the associated principal component captures the “regime shift” of 1976/77. The correlation is 0.77 between annually averaged pycnocline displacement anomalies hindcast from the model and anomalies in the depth of the main pycnocline at station P (50°N, 215°E) observed over a 43-yr period. The comparison indicates that variability in the depth of the upper layer on interannual to interdecadal timescales at station P occurs largely as an integrated response to local Ekman pumping. In addition, the results suggest that the PDO mode dominates the observed variability.

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Patrick F. Cummins and Howard J. Freeland

Abstract

Velocity measurements from the interior of the Alaskan gyre are presented from a current meter mooring deployed in 4000 m of water at 49°33′N, 138°38′W, in the vicinity of Ocean Weather Station P. The mooring held five current meters, which spanned the depth of the water column. The data reveal surface-intensified motions with flow fluctuations in the upper layers of the water column characterized by long-period, O(100 d) time scales of variability. At abyssal depths, the flow displays shorter, O(20 d) time scales of variability. The data are compared with observations from one of the NEPAC moorings in the northeast Pacific (42°N, 152°W). Similar characteristics in kinetic energy levels, in the vertical structure of the flow, and in the vertical variation of eddy time scales are found at this location.

The current measurements are considered in terms of the linear theory of directly wind-driven variability of Müller and Frankignoul. A comparison with simulated currents from a quasigeostrophic numerical model demonstrates that stochastic atmospheric forcing of the ocean can account for the observed variability. The numerical experiments and a simple extension of the linear theory suggest that the presence of bottom topography is important for the partition of energy between vertical modes and for the vertical variation of the time scales of the flow.

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Patrick F. Cummins and Lie-Yauw Oey

Abstract

The tidal response of northern British Columbia coastal waters is studied through simulations with a three-dimensional, prognostic, primitive equation model. The model is forced at the boundaries with the leading semidiurnal and diurnal constituents and experiments with stratified and homogeneous fluid are compared. The barotropic response shows good agreement with previously published studies of tides in the region. A comparison with tide gauge measurements indicates that average relative rms differences between observations and the model surface elevation field are less than 5% for the largest constituents.

An internal tide is generated in cases where the model is initialized with a vertical stratification. Diagnostic calculations of the baroclinic energy flux are used to identify regions of generation and propagation of internal tidal energy. With a representative summer stratification, the integrated offshore flux is about 0.5 gigawatts, higher than previously estimated from theoretical models. Comparisons between observed and modeled M 2 current ellipses are discussed for several moorings and demonstrate the significant influence of the internal tide.

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Patrick F. Cummins, Greg Holloway, and E. Gargett

Abstract

A coarse resolution, primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability. A case with constant diffusivity is compared with a case in which the diffusivity is inversely proportional to the local Brunt-V frequency. The stability-dependent parameterization of vertical diffusivity yields a poleward heat flux similar to that of a small, constant diffusivity. However, this parameterization increases the mean temperature in the deep ocean by about 0.8°C and the strength of the meridional circulation by over 40%. In addition, the stability-dependent diffusivity is found to increase the stratification in the deep ocean. The experiments suggest that it may be possible to calibrate the rate of deep-water formation of general circulation models, without affecting the poleward heat transport, by varying the magnitude of the vertical diffusivity below the thermocline.

The explicit vertical diffusivity is further compared with the field of diapycnal diffusivity induced by horizontal diffusion in presence of sloping isopycnals. It is found that the induced and explicit diapycnal diffusivities are of comparable magnitude in some regions of the deep ocean, while exhibiting different spatial dependences.

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Patrick F. Cummins and Lawrence A. Mysak

Abstract

No abstract available.

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