David N. Straub

On the Transport and Angular Momentum Balance of Channel Models of the Antarctic Circumpolar Current

Abstract Angular momentum balances are discussed, both in general as well as in the context of simple channel models of the Antarctic Circumpolar Current (ACC). Particular emphasis is placed on the close relationship between the angular momentum balance and the meridional circulation. It is found that topographic form drag is established very early in the integration, whereas interfacial form drag can take much longer to develop. Restrictions on the geostrophic portion of the meridional circulation imposed by zonally reconnecting potential vorticity contours in the upper ocean allow derivation of an estimate for the steady-state transport. The estimate assumes there to be little or no circumpolar flow at great depth, an assumption that stems from the belief that the band of zonally reconnecting geostrophic contours in the Southern Ocean does not extend to the ocean floor. The predicted transport is proportional to the strength of the stratification and compares favorably with numerical results in the lite...

Wind Stress Dependence on Ocean Surface Velocity: Implications for Mechanical Energy Input to Ocean Circulation

Abstract It is pointed out that accounting for an ocean surface velocity dependence in the wind stress τ can lead to a significant reduction in the rate at which winds input mechanical energy to the geostrophic circulation. Specifically, the wind stress is taken to be a quadratic function of Ua − uo, where Ua and uo are the 10-m wind and ocean surface velocity, respectively. Because |Ua| is typically large relative to |uo|, accounting for a uo dependence leads only to relatively small changes in τ. The change to the basin-averaged wind power source, however, is considerably larger. Scaling arguments and quasigeostrophic simulations in a basin setting are presented. They suggest that the power source (or rate of energy input) is reduced by roughly 20%–35%.

Evidence for interannual and interdecadal climate variability in the South Atlantic

A singular value decomposition (SVD) analysis is used to determine the coupled modes of variability of monthly sea surface temperature (SST) and sea level pressure (SLP) data from the South Atlantic region, for the period 1953–1992. We find that the three leading SVD modes respectively account for 63%, 20% and 6% of the total square covariance. The first mode represents an approximately 15-year period oscillation in the strength of the subtropical anticyclone, accompanied by fluctuations of a north-south dipole structure in the SST. It appears to be linked to the global-scale interdecadal (15-year) joint mode in SST and SLP recently studied by Mann and Park. The second mode is characterized by east-west displacements of the anticyclone center, in association with strong 6 to 7-year period fluctuations of SST off the coast of Africa. The third mode is characterized by north-south displacements of the anticyclone and 4-year period fluctuations in the SST in a broad band across the central South Atlantic. This mode is strongly correlated with ENSO.

An interdecadal climate cycle in the South Atlantic and its links to other ocean basins

A singular value decomposition analysis and a combined complex empirical orthogonal function analysis are performed on 80 years of monthly sea surface temperature (SST) and sea level pressure (SLP) data from the South Atlantic region. The analyses reveal the existence of interdecadal fluctuations in the coupled atmosphere-ocean system with a period of around 20 years. The SST anomalies are observed to propagate anticyclonically around the South Atlantic basin following the subtropical gyre circulation. At the same time, a westward propagation of SLP anomalies across the basin generates changes in the atmospheric circulation that appear to reinforce such SST anomalies through anomalous exchanges of heat. It is thus proposed that the dominant physical processes involved in this interdecadal cycle include the horizontal advection of heat by the ocean currents and changes in the atmosphere-ocean heat fluxes through local air-sea interactions. The global SST and SLP patterns that accompany the different phases of the South Atlantic cycle are also presented. They show similarities with other well-known interdecadal signals observed by several investigators in other ocean basins (e.g., the low-frequency part of the North Atlantic Oscillation). This suggests that the South Atlantic signal described in this study may be a regional aspect of global interdecadal variability.

Spinup of Source-driven Circulation in an Abyssal Basin in the Presence of Bottom Topography

Abstract Spinup of the circulation driven by an inflow into an abyssal basin containing a simple large-scale topographic feature is studied, using an inverted one-and-a-half-layer shallow-water model. Two types of topography, one a plateau and the other a depression, are considered. In both cases, the topography gives rise to a region of closed geostrophic contours. The character of the adjustment and the resultant flow differ markedly between the closed geostrophic contour region and the region outside. Long-wave processes set up a modified Stommel–Arons flow outside the closed contour region, while inside the closed contour region “frictional spinup” leads to a vigorous recirculation. It is shown that the ratio of the velocity of the recirculation to that of the Stommet–Arons flow is equal to half the ratio of the radius of the closed contour region to the Stommel boundary-layer thickness. Moreover, it is shown that the recirculation inside the closed contour region is always cyclonic, whether the topog...

Basin and Channel Contributions to a Model Antarctic Circumpolar Current

The idea that basinlike dynamics may play a major role in determining the Antarctic Circumpolar Current (ACC) transport is revisited. A simple analytic model is developed to describe the relationship between the wind stress and transport. At very low-wind stress, a nonzero minimum is predicted. This is followed by two distinct dynamical regimes for stronger forcing: 1) a Stommel regime in which transport increases linearly with forcing strength; and 2) a saturation regime in which the transport levels off. The baroclinic structure of the Sverdrup flux into the Drake Passage latitude band is central to the analytic model, and the geometry of characteristics, or geostrophic contours, is key to predicting the transition between the two regimes. A robustness analysis is performed using an eddy-permitting quasigeostrophic model in idealized geometries. Many simulations were carried out in large domains across a range of forcing strengths. The simulations agree qualitatively with the analytic model, with two main discrepancies being related to zonal jet structures and to a western boundary inertial recirculation. Eddy fluxes associated with zonal jets modify the baroclinic structure and lower the saturation transport value. Inertial effects increase the transport, although this effect is mainly limited to smaller domains.

Simulation of the South Atlantic Ocean circulation and its seasonal variability

The high-resolution Princeton Ocean Model is used to simulate the circulation and seasonal variability of the South Atlantic Ocean. A diagnostic calculation, using the Levitus annual mean fields and forcing consisting of the climatological annual mean wind stress, produces a realistic steady circulation pattern. In particular, the meridional overturning cell shows equatorward flow in the surface and intermediate waters and poleward flow at depth. Associated with this circulation is a northward heat transport that reaches a maximum of 1 PW near 20°S. The model results are improved when the density field near the ocean bottom is allowed to deviate from the Levitus values. Using forcing consisting of observed monthly wind stress, heat flux and a prescribed seasonally varying Antarctic Circumpolar Current (ACC), a prognostic calculation is next carried out to study the seasonal variability in the South Atlantic Ocean. With a realistic heat flux, the model output for the seasonal thermal field compares well with that in the Levitus data. An analysis suggests that the seasonal cycle of the Brazil Current separation latitude, as well as the strength of the Falkland-Malvinas Current depends more strongly on the prescribed seasonal variability in the strength of the ACC than on the seasonal variations in wind and buoyancy forcing in the South Atlantic.

A Case Study of Downstream Baroclinic Development over the North Pacific Ocean. Part II: Diagnoses of Eddy Energy and Wave Activity

The sequential development of a western, and then an eastern, North Pacific cyclone is examined in terms of eddy energy and a phase-independent wave activity. Based on the propagation of both a contiguous wave activity center and eddy energy, the development of the western cyclone appears to influence its downstream neighbor. A quantitative comparison of these two diagnoses is made in terms of group velocity, and only minor differences are found during much of the initial evolution. It is only once the tropopause undulations lose their wavelike appearance (at which point, application of the group-velocity concept itself becomes quite tenuous) that the downstream propagation of eddy energy seems faster than that of wave activity. Conventional methods of tracking this wave packet are also briefly discussed.

Instability of 2D Flows to Hydrostatic 3D Perturbations

Abstract Considered here is the evolution of three-dimensional perturbations to the hydrostatic equations linearized about a two-dimensional base state U. Motivated by an argument by T. Warn, this study begins with the nonrotating, unstratified case, and draws analogies between the perturbation equations and equations describing evolution of material line elements and scalar gradients embedded in the same 2D flow. When U is chaotic, both scalar gradients and line elements are characterized by rapid growth, and this leads one to suspect that the perturbations behave similarly. A generalized Okubo–Weiss parameter is proposed, and it is argued that this gives a reasonable litmus test for identifying regions where growth is most probable. Rotation modifies the generalized Okubo–Weiss parameter and tends to curb growth of the perturbation fields, as expected. It is also pointed out that, in realistic geophysical settings, the stability parameter can be suggestive of growth locally, even when a globally defined...

A Simple Model of Mass-driven Abyssal Circulation over a General Bottom Topography

Abstract The classic Stommel–Arons problem is revisited in the context of a basin with a general bottom topography containing an equator. Topography is taken to be smoothly varying and, as such, there are no vertical side walls in the problem. The perimeter of the abyssal basin is thus defined as the curve along which the layer depth vanishes. Because of this, it is not required that the component of horizontal velocity perpendicular to the boundary curve vanish on the boundary. Planetary geostrophic dynamics leads to a characteristic equation for the interface height field in which characteristics typically originate from a single point located on the eastern edge of the basin at the equator. For a simple choice of topography it is possible to solve the problem analytically. In the linear limit of weak forcing, the solution exhibits an intensified flow on the western edge of the basin. This flow is pan of the interior solution and is thus not a traditional, dissipative western boundary current. When the ...