A nonlinear, time-dependent thermocline theory

A noneddy resolving, time-dependent, nonlinear theory of the large-scale ocean circulation is presented. The variability in this theory occurs as a response to variability in forcing. Baroclinic and barotropic evolution is computed using a two-layer, quasigeostrophic, wind-driven model. Both analytical and numerical solutions are obtained. Attention is focussed on the low-frequency, basin scale fluctuations of the wind. Based on these restrictions, the various modes of response are separated by means of a multiple time scale analysis. The barotropic response is found to be effectively instantaneous, and a relatively simple advection equation is shown to govern the baroclinic response. Analytical solutions of the baroclinic equation are obtained under the assumption that the time scales of the wind variability are short compared to the cross-basin baroclinic wave propagation time. Numerical solutions are obtained in more general cases. The baroclinic large-scale response is fundamentally nonlinear in that baroclinic waves propagate in the presence of the Sverdrup flow, which is itself time dependent. This nonlinearity results in at least two effects. First, the characteristics of wave propagation are significantly altered from pure zonality. This leads to the formation of homogenized zones, within which directly forced thermocline variability vanishes. Second, thermocline fluctuations are produced which have variance at frequencies other than those of the forcing. Consequently, forcing the model with an annually varying wind stress yields contributions to the thermocline spectrum at one year and all its superharmonics (i.e., 6 months, 4 months, 3 months, etc.). The amplitude of the superharmonics increases with distance from the eastern boundary. Mean baroclinic circulation on the scale of the thermocline waves is also found. The above features are predicted by analytical theory and confirmed by numerical experimentation. Observations of the geographical distribution of thermocline variability in the North Pacific and North Atlantic Oceans and of first mode variance at inadmissible planetary wave frequencies are discussed in light of the theory

Topography and barotropic transport control by bottom friction

The problem of the stratified general circulation in the presence of topography is revisited. The novel effect examined here is that of localized, but large-scale, topographic anomalies on the wind-driven circulation, a problem whose relevance is found in the occurrence of many such features in the open ocean. Using the classical methods of homogenization theory, it is argued that the barotropic transport near topography can come under the direct control of bottom friction. This result differs substantively from either the well-known Sverdrup constraint (which applies to a flatbottomed ocean, or to one with a resting deep layer) or its recent extensions that allow for planar bottom topographic profiles. Bottom friction emerges as a controlling parameter roughly in the event that the topography forms closed f/(H – hb) contours, where H – hb is the total fluid depth, although the theoretical minimum requirements are somewhat looser than this. Our analytical predictions are supported by numerical experimentation with a multi-layer quasi-geostrophic model, and we examine some mean flow observations from the North and South Atlantic in light of the theory. In particular, the theory can rationalize the 100 Sv transport observed recently around the Zapiola Drift

Arrested fronts

A theory of fronts within the general circulation is discussed. These fronts and their associated jets differ from the more classical picture of boundary jets and their extensions (an example being the Gulf Stream system) in that they occur in the interior of the general circulation. The mechanism at the heart of the frontogenesis is nonlinear beta steepening, which arises from geostrophy in the presence of continuity. Such dynamics have been used in the past in a study of planetary shock waves. It is here argued that these dynamics result in stationary shock fronts in the general circulation. An equation governing the trajectory of the shock front is found. This equation represents a balance between the westward beta-driving of the shock front and the retarding influence of the Sverdrup flow. The fronts are shown to play a number of potentialIy important roles in the circulation. For example, they are capable of resolving conflicting information emanating from the boundaries, can determine subsurface potential vorticity in certain regions and can influence the positioning of outcrop trajectories. An examination of the thermocline structure at the latitude separating the subpolar and subtropical gyres, as a function of the complete range of possible boundary conditions, reveals that arrested front solutions dominate the possible solution types for the general circulation and the complete suite of such solutions is described. Lastly, an attempt to reproduce the thermocline parameters of the mid-ocean jet observed during the Local Dynamics Experiment is discussed

Eastern Boundary Effects on General Circulation Structure

My NASA proposal included plans to examine the dynamics of the eastern oceanic boundary, with a view towards those processes important to the interior. Several relevant tasks have been completed and either have appeared or will appear soon in the refereed literature

Umjetnost u medicini: retrospektiva anatomskih crteža Charlesa Bella

Perhaps best known for his discovery of the eponymous syndrome ‘Bell’s Palsy’, Charles Bell (1774-1842) made significant contributions to neuroscience, medical education and philosophy. Our aim was to examine his neuroanatomical drawings in the context of the era in which they were produced and their influence on future scholars. Emphasis is placed on analysing the artistic techniques employed and Bell’s unique manner of conveying both structure and function. The images discussed include those featured in his book entitled The Anatomy of the Brain: Explained in a Series of Engravings. These images can be viewed in parallel with his writing on the anatomy of the brain, in which he describes the usual manner of demonstrating neuroanatomy as ‘dull’ and ‘unmeaning’. His mastery of artistic technique complements his insightful descriptions of this prodigiously complex organ. The result is a more engaging account of neuroanatomy and an impressive display of his skill as an artist, anatomist and physician. Examining these expressive portrait-like diagrams provides greater insight into the mind of the pioneer of modern neuroscience.Iako je možda najpoznatiji po otkriću sindroma koji je po njemu nazvan Bellova paraliza, nemjerljiv je doprinos Charlesa Bella (1774.-1842.) neuroznanosti, medicinskoj izobrazbi i filozofiji. Cilj je bio istražiti neuroanatomske crteže u kontekstu vremena u kojem su nastali te njihov utjecaj na buduće znanstvenike. Naglasak je na analizi primijenjenih umjetničkih tehnika te na Bellovu jedinstvenom načinu prikazivanja strukture i funkcije. Raspravlja se o slikama koje se nalaze u njegovoj knjizi “Anatomija mozga: objašnjenje kroz niz gravura” (The Anatomy of the Brain: Explained in a Series of Engravings). Ove slike mogu se promatrati usporedno s njegovim tekstovima o anatomiji mozga u kojima on opisuje uobičajeni način prikazivanja neuroanatomije kao “dosadne” i “besmislene”. Njegova majstorska umjetnička tehnika dopunjava njegove pronicave opise ovoga silno složenoga organa. Rezultat je znatno privlačniji pogled na neuroanatomiju i dojmljiv prikaz njegove vještine kao umjetnika, anatoma i liječnika. Izučavanje ovih izražajnih dijagrama nalik portretima omogućava bolji uvid u um ovoga pionira suvremene neuroznanosti

Vorticity dynamics near sharp topographic features

In ocean models, the interaction with boundaries is often parameterized as it involves small-scale processes that are usually hard to capture in a large-scale model. However, such interactions can play important roles in the model dynamics. For example, the choice of boundary conditions (free-slip vs. no-slip) has a direct impact on the vorticity (enstrophy) budget: with no-slip boundary conditions, vorticity is injected into the system, whereas with free-slip boundary conditions, there should be no vorticity injection as long as the coastline is smooth. However, we show here that at boundary singularities (e.g., corners), vorticity is injected into the domain even for free-slip boundary conditions. In this article, we use North Brazil Current rings to better understand the dynamics of eddy-topography interaction. This complex interaction is first analyzed in terms of a point vortex interacting with a wall. Within this simplified framework, we can describe the vorticity generation mechanism as a pseudoinviscid process. To quantify this vorticity injection, we first consider the inviscid limit for which we can derive an analytical formula. This theoretical prediction is then evaluated in conventional gridded ocean models. In such models, the representation of such a viscous boundary interaction may be affected by the grid representation and the discretization of the advection and viscous operators

Submesoscale generation by boundaries

An important dynamical question involves how oceanic balanced flows lose energy. Recent numerical and analytical studies suggest topography catalyzes energy exchanges between balanced flows and a variety of unbalanced phenomena, which presumably leads to dissipation. We here develop a general theory of inviscid balanced flow interactions with walls that predicts submesoscale and unbalanced flow generation. Comparison with primitive equation-based numerical experiments supports the basic tenets of the theory

Mode waters and subduction rates in a high-resolution South Atlantic simulation

Water mass production and destruction in the subtropical South Atlantic gyre is studied. A high resolution numerical model is used to examine regional mode water formation and estimate the associated instantaneous and mean subduction rates. Primitive equation dynamics expressed in depth following (sigma) coordinates are employed. The main hydrographic and kinematic features of the South Atlantic are faithfully reproduced by the model. In particular, the principle current systems appear and the model exhibits a sequence of ventilated potential vorticity minima on density surfaces coinciding with those of observed South Atlantic mode waters. The formation sequence within the model of these mode waters is described. Net formation rates are estimated using a pseudo-Lagrangian method and by diagnosing the time history of subsurface water mass volumes. Maximum formation rates occur in the density bands of the mode waters. It is argued that the roots of the model mode waters are found along open ocean late winter outcrops, rather than in the waters entering the gyre from the Brazil Current/Malvinas Current Confluence region. Eddies generate interannual variability in mode water formation and precondition the waters in the outcrop regions for convection. On the other hand, the eddy kinetic energy of the Confluence region is too intense to permit a direct connection between deep convection cells in the western boundary current and those in the open South Atlantic that directly form mode water