Search Results

You are looking at 1 - 10 of 28 items for

  • Author or Editor: J. Pedlosky x
  • All content x
Clear All Modify Search
J. Pedlosky

Abstract

The relationship between coastal upwelling and coastal long-short currents is studied in a simple model of a continuously stratified fluid in a closed basin on the f-plane. It is shown that the coastal long-shore current at any point on the basin's perimeter is determined primarily by the components of the long-shore stress with the largest long-shore scales, in particular from the perimeter average of the long-shore stress. In addition, the depth structure of the upwelling itself is shown to be dependent on the long-shore scale.

It is concluded that the relationship between upwelling and long-shore currents is non-local, that mass balance is not achieved locally in planes normal to the coast, and that the relationship between onshore flows and long-shore currents is scale-dependent. An intrinsic long-shore length scale is found separating two regimes of flow with distinctly different relations between the wind stress, onshore, and long-shore flows. Since both regimes are present simultaneously for a wind stress containing a variety of long-shore scales, the determination of the long-shors structure of the wind stress is essential to an understanding of the local oceanic upwelling and long-shore current structure.

Full access
J. Pedlosky

Abstract

The effect of shelf-like bottom topography on a steady, linear, stratified, three-dimensional model of coastal upwelling is examined. It is shown that the presence of the bottom slope 1) reduces the role of the lower Ekman layer in the upwelling mass balance, and 2) introduces a barotropic boundary layer which can, depending on a balance of driving mechanisms, give rise to a deep poleward undercurrent. The structure, amplitude and cause of the undercurrent are distinct from those of the surface equatorward flow.

Full access
J. Pedlosky

Abstract

The transmission of westward propagating baroclinic Rossby waves incident on a gappy meridional barrier is studied in the context of the two-layer, quasigeostrophic model. The meridional barrier models the presence of very steep topography such as the midocean ridge system or extensive island arcs.

The nature of the transmission depends strongly on the nature of the gaps in the meridional barrier. If the gaps extend throughout the depth of the fluid, the Rossby waves propagate through the barrier, as a consequence of Kelvin’s theorem, with no change in vertical structure. On the other hand, if the gaps in the barrier are partial and extend only over a single layer, there is a significant transformation of the vertical structure of the wave field as it traverses the barrier. In particular, waves of baroclinic vertical structure in the model are transformed on the western side of the barrier into barotropic waves that radiate from the segment of the barrier between two such gaps. Such segments act as antennae radiating barotropic energy into the western subbasin. It is suggested that recent observations of signal enhancement of Rossby waves at the midocean ridge system in the Pacific may be related to such transformation of wave structure.

The problems of free waves and forced waves in open regions and normal modes in closed basins are described.

Full access
J. Pedlosky

Abstract

The steady baroclinic flow in a basin containing a meridional barrier representing a midocean ridge is studied in the linear, quasigeostrophic limit of a two-layer model. Thermal damping and a simple friction provide dissipation of thickness (heat) and momentum. The ridge is pierced by two gaps in the upper layer but only a single gap in the lower layer. The flow in the model is forced by specified upwelling at the upper surface and by a specified cross-isopycnal velocity at the interface in addition to the autogenerated cross-isopycnal velocity associated with the thermal damping. The forcing may be either broad in longitude or narrowly confined.

The nature of the geometry of the model ridge mixes the baroclinic and barotropic response to the forcing, and this has profound consequences for the resulting circulation. In particular, when the baroclinic interaction of the two layers is strong, the recirculation region to the east of the ridge, previously discovered in earlier barotropic models of the circulation, grows in meridional extent so that the flow along the ridge segment may be unidirectional along the ridge. It is suggested that the theory may explain observations of such flow in the Angola Basin, which appeared previously to violate an application of Kelvin's theorem.

The theory also predicts zonal jets west of the gaps in the ridge, spreading meridionally with distance from the ridge. The jets are strongly barotropic whether the external forcing is baroclinic or barotropic.

Full access
J. Pedlosky

Abstract

No abstract available.

Full access
J. Pedlosky

Abstract

An a priori estimate is given of the effect of ventilation on the process of potential vorticity homogenization. Since the homogenization process depends on the presence of weak mixing, it is shown that only a small exposure to a zone of ventilation is required to lead to an arrest of the homogenization.

Full access
J. Pedlosky

Abstract

The evolution of longshore currents produced by upwelling on an f-plane over shelf-like bottom topography for times long compared to a barotropic spin-up time, but short compared to a diffusion time, reveals in a linear, time-dependent, three-dimensional model that:

  1. The topographic constraints yield a steady topographic boundary layer on these short time scales similar in structure to the layer found in an earlier steady-state model.
  2. Within a Rossby radius of deformation of the coast a swift equatorward longshore current with a pole-ward countercurrent is formed.
  3. Wind-stress forcing with large north-south scales are the most efficient in driving longshore currents, but do not effectively produce internal Kelvin waves, as do the shorter longshore scales of forcing.
Full access
J. Pedlosky

Abstract

The finite-amplitude dynamics of a weakly unstable baroclinic disturbance in an atmosphere with continuous shear and static stability is investigated. The effects of β (the planetary vorticity gradient), Ekman dissipation and thermal damping are included. The multi-scale analysis demonstrates the existence of two-dynamical regimes. First, under the influence of Ekman friction quasi-steady states are achieved after an oscillatory approach to the equilibrium state. It is shown that under the influence of friction long zonal waves possess a weak instability. Second, under the joint influence of Ekman friction and thermal damping (of the simplest Newtonian cooling model) a true asymptotic state is achieved. The amplitude is calculated and is shown to be independent of the thermal damping κ when κ is small. The rectified beat flux is calculated in the steady finite amplitude state. It is proportional to κ. The beat flux is proportional to the difference between the basic temperature gradient and a critical value which increases with both β and N, the Brunt-Väisälä frequency.

Full access
J. Pedlosky

Abstract

A simplified but nonlinear model of large-scale air–sea interaction is formulated which involves the interaction between a finite-amplitude cyclone wave, the large-scale atmospheric temperature field, and sea–surface temperatures. The system interacts through a simplified model of air–sea heat exchange and consequent alterations of wind-driven advection of the sea–surface temperature field. It is shown that in cases when there is a large heat release to the atmosphere and long-term storage of heat in the mixed layer that small “seed” anomalies can grow by a finite-amplitude feedback instability.

Full access
J. Pedlosky

Abstract

An analytical theory is presented for long-period pulsations of a finite-amplitude baroclinic wave. It is shown that for small dissipation a limit cycle is possible whether or not the steady wave regime is stable to infinitesimal disturbances. Moreover, the limit cycle is shown to be stable. A second limit cycle is shown to exist only when the steady wave regime is stable but in that case the second limit cycle is unstable.

Full access