Continental Geometry Research Articles

Internal variability of the North Atlantic wind-driven ocean circulation

A new approach to understand the physical processes that govern internal variability of the large scale North Atlantic ocean circulation is outlined and current methods and results are reviewed. In this approach, based on the theory of dynamical systems, internal variability is viewed as arising through successive transitions when parameters are changed. The potential of the approach is demonstrated through analysesof solutions of intermediate complexity models of the wind-driven ocean circulation in the North Atlantic. In a quasi-geostrophic modelfor the flow in a rectangular basin with idealized wind forcing, the basic transitions are already found and physical mechanisms at work can be described in detail. Qualitatively, this transition behavior remains robust in more realistic models, having shallow water dynamics, realistic wind forcingand continental geometry, although patterns and time scales changethrough the model hierarchy. The relevance of the results is discussed inrelation to those of observations and of ocean general circulation models.

The impact of paleogeographic evolution on the surface oceanic circulation and the marine environment within the Mid‐Cretaceous tethys

The concept of a stable, westward flowing circumglobal current throughout the Cretaceous Tethys has become the subject of extensive debate. Results from a series of oceanic general circulation model experiments, using the NCAR Parallel Ocean Climate Model, are presented which oppose this concept and suggest a more complicated circulation pattern dominated by a clockwise gyre in the Mediterranean Tethys. Although a narrow westward flowing current is simulated hugging the southern margin of Tethys, ocean model experiments reveal the large sensitivity of this current to paleogeographic evolution. Small modifications to continental geometry, representing sea level change or tectonic plate movement during the Cretaceous, alter the direction and strength of the Tethyan circulation and, consequently, lead to regional changes in seawater properties (i.e., temperature and salinity). These oceanic general circulation model experiments indicate that geographic evolution is an important mechanism of climatic and environmental change. Since the Cretaceous experienced rapid tectonic changes these results imply large surface circulation variations in the Tethys Ocean, supporting the idea of a dynamic tropical oceanic environment. In fact, a simulation of sea‐level rise during the mid‐Cretaceous reproduces the fluctuations in seawater properties recorded by fauna in the Caribbean Sea.

Westward Propagation of Latitudinal Asymmetry in a Coupled Ocean–Atmosphere Model

Abstract Hemispheric asymmetries of continental geometry have long been speculated to be the cause of the Northern Hemisphere position of the intertropical convergence zone over the central and eastern Pacific. It is unknown, however, how the effects of continental asymmetries are transmitted to and felt by the central Pacific thousands of kilometers away. This paper proposes a transmitter mechanism by investigating the response of a coupled ocean–atmospheric model to a symmetry-breaking force by the American continents. The model treats land forcing implicitly as an eastern boundary condition. In the absence of oceanic feedback, the model response to the eastern boundary forcing is tightly trapped and confined to a small longitudinal extent off the coast, whereas the climate over the interior ocean is symmetric about the equator. Ocean–atmosphere coupling greatly enhances the transmissibility of the effects of the land forcing, establishing large latitudinal asymmetry over a great zonal extent. A westwar...

The influence of the Bering Strait on the circulation in a coarse resolution global ocean model

An ocean general circulation model of global domain, full continental geometry and bottom topography, is used to study the influence of the Bering Strait on the general circulation by comparing equilibrium solutions obtained with and without a land-bridge between Siberia and Alaska. The model is integrated with restoring boundary conditions (BC) on temperature and salinity, and later, with mixed BC in which a restoring BC on temperature is maintained but a specified flux condition on salinity is imposed. In both cases, the effect of the Bering Strait is to allow a flow of about 1.25–1.5 Sv from the North Pacific to the Arctic Ocean and, ultimately, back to the North Pacific along the western boundary current regions of the Atlantic and Indian Oceans. When a restoring BC on salinity is used, the overturning associated with North Atlantic Deep Water and Antarctic Intermediate Water formation are increased if the Bering Strait is present in the model geometry. The result of switching to a specified flux BC on salinity is to cause a transition in the THC in which the overturning associated with North Atlantic Deep Water formation increases from about 12 Sv to about 22 Sv. This transition occurs in an essentially smooth fashion with no significant variability and is about 12% smaller in magnitude if the Bering Strait is present in the model geometry. Because the Bering Strait appears to exert some influence on the general circulation and the formation of deep water masses, it is recommended that this Strait be included in the geometry of similar resolution models designed to study the deep ocean and potential changes in climate.

Large igneous provinces: Crustal structure, dimensions, and external consequences

Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Such provinces do not originate at “normal” seafloor spreading centers. We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen‐Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts. Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (7.0–7.6 km s−1) compressional wave velocities. Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short‐lived pulses of increased global production; Ontong Java's rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid‐ocean ridge system. The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory. Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust. We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle. This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs. LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact. Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features.

Model simulations of Cretaceous climates: the role of geography and carbon dioxide

A general circulation model (GENESIS) with seasonally varying solar insolation and a mixed layer ocean is applied to assess the role of continental geometry and increased levels of carbon dioxide to explain the warmth of the Cretaceous period. Model experiments suggest that the role of geography is negligible, in contrast to early model studies with mean annual solar insolation and a simple energy balance ocean. Higher atmospheric carbon dioxide (4 times present) resulted in a 5.5°C globally averaged surface temperature increase, close to the lower limit required to explain the geologic record. Mid-Cretaceous carbon dioxide concentrations of 4-6 times the present day concentrations are a reasonable explanation of Cretaceous warmth if the GENESIS model provides an accurate estimate of climate sensitivity to geography and carbon dioxide.

Vorticity Dynamics of Seasonal Variations of the Antarctic Circumpolar Current from a Modeling Study

A one-layer numerical model was developed to analyze the vorticity dynamics of the seasonal variations of currents in the Southern Ocean. The model includes the continental geometry and bathymetry of the Southern Ocean and is forced by monthly climatological wind stress. Five cases are considered that compare (i) circulation over a flat bottom to that with bathymetry, (ii) effects of zonally averaged wind stress forcing versus the climatological forcing and (iii) anomaly wind stress (winds with the annual mean removed) versus the fun stress. The individual terms in the vorticity conservation equation are calculated from the model solution along two latitude lines; 57.5°S, which passes through Drake Passage, and 43.5°S, which is in the subtropical gyre. In the zonal part of the flat bottom simulation, the curl of the surface stress balances bottom stress curl. However, in Drake Passage, beta (advection of planetary vorticity) balances bottom loss—the western boundary balance. Such vorticity interactions depend on the partial barrier of South America and, thus, do not occur in zonal channel models. The removal of vorticity occurs throughout the Southern Ocean for the seasonally varying winds but the mean circulation is balanced mainly by losses near Drake Passage. The location of the Antarctic Circumpolar Current (ACC) is controlled by the lip of South America rather than the structure of the wind. The seasonal changes in the model surface elevation in Drake Passage occur largely in the southern part of the passage, in agreement with pressure observations. The calculated ACC transport is similar for climatological and zonally averaged winds but the structure of the forced circulation is rather different for the two cases. Bottom topography changes the vorticity interactions so that the largest effect occur where the flow is forced over bathymetry creating relative vorticity by stretching, which is then removed by bottom friction. The major loss in the model occurs near Drake Passage, although there are smaller losses at other locations along 57.5°S. Bathymetry provides a strong counterforce to the wind stress and the transport is reduced by a factor of ten compared to the comparable uniform depth simulation. Friction plays a secondary role by determining the width of the currents and the spinup time but has only a weak effect on the total transport.

Paleo-oceanographic Circulation as Determined from Radiolarian Populations in Humboldt Basin: ABSTRACT

Analyses of radiolarian assemblages from middle Miocene to Pliocene samples along a transect in the Humboldt basin were utilized to determine the presence of various water masses at key time planes off of the coast of northern California. Structural analyses indicate that the basin has acted as a structural unit throughout the Miocene to present. Therefore it was possible to infer paleophysiography by assuming that the past continental shelf geometry was similar to that of the present. Radiolarian fauna indicate that initial deposition was in a basin open to deep marine waters. Through time and in an east to west direction, the radiolarian populations exhibit increasingly shelfal characteristics, and from these data nearshore paleo-oceanic circulation was determined.

Continental distribution as a forcing factor for global-scale temperature

Since the advent of the continental drift hypothesis, changing continental geometries have been proposed as an explanation for long-term temperature variability1–9. The climatic influence of a few specific past geographies has been investigated quantitatively6,10–12, but these studies do not indicate the potential temperature variability due to continental positions. This problem has been examined only with simple climate models having limiting assumptions such as no cloud cover5. Here idealized continental geometries are used as boundary conditions in a simulation using a general circulation model (GCM) of the atmosphere. The range in model simulated globally-averaged surface temperature is 7.4 K with a difference in polar surface temperature of up to 34 K. The simulations suggest a substantial climatic sensitivity to continental positions with the coldest global climate when land masses are in high latitudes. Although the simulations have not captured theoretical limits of climatic variability due to continental positions, present-day geography is near the cold end of this spectrum.

Holocene sea level change: an interpretation

Interpretation of factors responsible for land-sea level change in areas such as the Great Barrier Reef involve an appreciation of not only the field evidence purporting to show change, but also the theoretical models which attempt to explain depth variations in shorelines of a given age. Relative movements in sea level in Holocene time may result from a number of factors operating either external to the study area (e.g. glacio-eustatic, and broad-scale hydro-isostatic deformation of the globe resulting from the last deglaciation and sea level rise), or those whose effects are essentially local (e.g. changes in circulation and tidal levels within partially enclosed water bodies induced by sedimentation or biogenic reef growth, meteorological changes affecting the magnitude and frequency of storminess, regional flexures and/or faulting, and hydro-isostatic deformation of shelves and adjacent coasts accompanying the Postglacial Transgression). In this paper, data from the northern Great Barrier Reef Province are evaluated in relation to various causes of sea level change. Emphasis is placed on explaining variations in relative sea level position by hydroisostatic theory. Deflexion in the ocean margin ‘hinge zone’ varies with continental shelf geometry and rigidity of the underlying lithosphere. The fact that the oceanic crust meets the continental crust quite abruptly east of the study areas, dictates that moderately strong flexures occur, and that variations in Holocene hydro-isostatic flexure in the Great Barrier Reef Province are partly explainable in these terms.