Ricardo P. Matano

A numerical study of the Southwestern Atlantic Shelf circulation : Stratified ocean response to local and offshore forcing

[1] This article discusses the results of a suite of numerical simulations of the oceanic circulation in the Southwestern Atlantic Shelf region that are aimed to characterize its mean circulation and seasonal variability and to determine the dynamical mechanisms controlling them. Our experiments indicate that south of 40S the mean circulation is dominated by a general northeastward flow in the southern portion of the shelf, which is controlled by the discharges from the Magellan Straits, tidal mixing, wind forcing, and the offshore influence of the Malvinas Current farther north. The region from 40 St o 33S presents the highest seasonal variability, with intrusions of cold sub-Antarctic waters

On the implementation of passive open boundary conditions for a general circulation model : the barotropic mode

The objective of this study is to evaluate the performance of several open boundary conditions applied to the Princeton ocean model. The focus is on passive open boundary conditions applied to the external mode, i.e., conditions that are applied when the mean flow at the open boundary is unknown and the values of the variables must be assumed or extrapolated from the interior solution. Three types of open boundary conditions (OBCs) are tested: (1) radiation conditions, (2) characteristic methods, (3) relaxation schemes. Numerical experiments are conducted in a zonal channel that include different forms of bottom topography. The experiments are designed to emphasize flow conditions dominated by direct wind forcing or wave radiation. Three sets of experiments are discussed: (1) forcing by a uniform, alongshelf wind stress; (2) the barotropic adjustment of an initial perturbation in the sea surface elevation; and (3) forcing by a traveling storm. The results of these experiments are compared with analytical solutions and the results of experiments using cyclic boundary conditions, or expanded domains. According to our results all radiation schemes of Orlanski/s type perform poorly in either flows with strong nonlinear components or when the model response is dominated by the propagation of dispersive wave packets. The characteristic method recommended by Roed and Cooper [1987] provided reasonable responses in most cases, but we found problems in handling dispersive waves or variable wind forcing at the open boundaries. Although there were limitations in all the OBCs that we tested the best overall performances were for the conditions proposed by Flather [1976], combined with a local solution approach proposed by Roed and Smesdtad [1984], and the flow relaxation scheme developed by Martinsen and Engedahl [1987].

Seasonal variability in the southwestern Atlantic

The circulation of the southwestern Atlantic Ocean is dominated by the Subtropical Gyre and the confluence of the Brazil and Malvinas currents. Observations indicate that the latitude of this confluence changes seasonally, lying farther north during the austral winter than during the summer. This phenomenon has important consequences for the local climate and marine population, as the latitude of the confluence also marks the boundary between the warm waters of the subtropical gyre and the cold waters of the Antarctic Circumpolar Current. We present evidence that these seasonal migrations may be related to changes in the transport of both the Brazil and Malvinas currents. A numerical model forced by climatological wind stress indicates that the transport of the Brazil Current decreases during winter months and increases during summer months. Geosat altimeter data corroborate the model results and also indicate that the transport of the Malvinas Current undergoes a seasonal cycle with phase opposite to that of the Brazil Current. Our hypothesis is that during the austral summer, a southward displacement of the latitude of the confluence is coincident with an acceleration of the flow in the subtropical gyre and a weakening of the transport of the Malvinas Current. This situation reverses during the winter when the Malvinas Current grows stronger, the Brazil Current transport decreases, and the latitude of the confluence of these two currents moves northward.

On the Upwelling of Downwelling Currents

Abstract The term “downwelling currents” refers to currents with a downslope mass flux in the bottom boundary layer. Examples are the Malvinas and Southland Currents in the Southern Hemisphere and the Oyashio in the Northern Hemisphere. Although many of these currents generate the same type of highly productive ecosystems that is associated with upwelling regimes, the mechanism that may drive such upwelling remains unclear. In this article, it is postulated that the interaction between a downwelling current and the continental slope generates shelfbreak upwelling. The proposed mechanism is relatively simple. As a downwelling current flows along the continental slope, bottom friction and lateral diffusion spread it onto the neighboring shelf, thus generating along-shelf pressure gradients and a cross-shelf circulation pattern that leads to shelfbreak upwelling. At difference with previous studies of shelfbreak dynamics (e.g., Gawarkiewicz and Chapman, Chapman and Lentz, and Pickart), the shelfbreak upwelli...

On the Separation of the Brazil Current from the Coast

Abstract A series of numerical experiments, using analytical and numerical models, leads to the conclusion that the separation of the Brazil Current from the coast can be related to the northward momentum of the Malvinas Current. Experiments in which the Maivinas Current has a low transport show the Brazil Current separating where the curl of the wind stress vanishes, seven degrees south of the observed separation latitude of 38°S. If, however, the flow distribution at the Drake Passage is adjusted so that the transport of the Malvinas Current is increased, then the model predicts that the latitude where the Brazil Current separates from the coast is near its observed value.

On the implementation of open boundary conditions for a general circulation model: The three‐dimensional case

This article discusses the results of an experimental survey on the performance of a select group of open boundary conditions (OBCs) for three- dimensional, primitive equation models. The focus is on passive conditions, where the model response at the open boundaries is dictated by the interior dynamics. The performance of the OBCs is tested in a series of numerical experiments conducted in a rotating channel which includes variable bottom topography and density stratification. The experiments are sele. cted to enhance nonlinear phenomena that comprises advection and the propagation of dispersive waves. The first two experiments study coastal upwelling with constant and time-varying wind forcing. The third experiment analyses the barotropic and baroclinic response of a coastal ocean due to the passage of a traveling storm. The open boundary condition with the best overall performance is the flow relaxation scheme on sea surface elevation and transport, a radiation condition for internal velocities, and a combined scheme of flow advection and relaxation for the temperature field. A modified gravity wave radiation scheme provides reasonable responses when combined with radiation conditions for internal velocities and an advection equation for temperature if the forcing is not changing direction rapidly at the open boundary. Schemes using the method of characteristics or traditional wave advection schemes fail when combined with radiation conditions for the baroclinic mode.

A kinematic analysis of the Indian/Atlantic interocean exchange

We analyze the Indian/Atlantic interocean exchanges in an eddy-permitting numerical simulation focusing on the Cape Basin, a region of the southeastern Atlantic bounded on the northwest by the Walvis Ridge and on the south by the Agulhas Ridge (near 40°S). To quantify the relative importance of the different dynamical mechanisms involved in the interocean exchange, we separated the climatological mean circulation from the transients. The analysis indicates that Agulhas eddies influence not only the transient but also fluxes associated with the mean circulation (eddy fluxes, for example, supply most of the energy of the Benguela Current). A distinctive characteristic of the eddy variability within the Cape Basin is the co-existence of cyclonic and anticyclonic vortices in dipole structures that resemble the Heton model of Hogg and Stommel (Proc. R. Soc. London. A 397 (1985) 1). Anticyclones are surface-intensified vortices that, in spite of their baroclinic structure, reach to deep layers. Cyclones are bottom-intensified vortices with dominant barotropic structure that projects into the upper layer. The propagation of both cyclones and anticyclones is strongly affected by the bottom topography. Our analysis shows that the Walvis Ridge and the Vema Seamount block the passage of bottom-intensified cyclones and changes the trajectories of the upper-intensified anticyclones. Although most anticyclones are able to escape the basin, the deep compensation generated by the ridge generates an energy loss of approximately 30%, and a change of the eddy trajectory to a more westward direction.

Dynamical analysis of the upwelling circulation off central Chile

[1] In this article we analyze the momentum and vorticity balances of a numerical simulation of the upwelling circulation off central Chile (34� –40� S) and its response to interannual local wind changes. Our analysis indicates that the path of the upwelling jet is strongly controlled by the bottom topography. This topographic steering causes the jet to separate from the coast at the Punta Lavapie cape (� 37� S). Although the zeroth-order momentum balance is dominated by the geostrophic terms, the circulation is also affected by nonlinear processes, which lead to the formation of large meanders and the shedding of cyclonic eddies north of Punta Lavapie during periods of wind relaxation. The relative contributions of the zeroth-order vorticity balance and the advective terms are also strongly controlled by changes in the coastline geometry and the bottom topography. Vorticity is created along the current axes and transported toward the coast and the Peru-Chile Trench, where it dissipates. South of Punta Lavapie the across-shelf transports are weaker with equatorward flows that are more stable than in the north. Additional numerical simulations

Large-Scale Forcing of the Agulhas Variability: The Seasonal Cycle

In this article the authors examine the kinematics and dynamics of the seasonal cycle in the western Indian Ocean in an eddy-permitting global simulation [Parallel Ocean Circulation Model, model run 4C (POCM-4C)]. Seasonal changes of the transport of the Agulhas Current are linked to the large-scale circulation in the tropical region. According to the model, the Agulhas Current transport has a seasonal variation with a maximum at the transition between the austral winter and the austral spring and a minimum between the austral summer and the austral autumn. Regional and basin-scale mass balances indicate that although the mean flow of the Agulhas Current has a substantial contribution from the Indonesian Throughflow, there appears to be no dynamical linkage between the seasonal oscillations of these two currents. Instead, evidence was found that the seasonal cycle of the western Indian Ocean is the result of the oscillation of barotropic modes forced directly by the wind.

A two-way nested simulation of the oceanic circulation in the Southwestern Atlantic

This article presents the results of a high-resolution (1/12°), two-way nested simulation of the oceanic circulation in the southwestern Atlantic region. A comparison between the model results and extant observations indicates that the nested model has skill in reproducing the best-known aspects of the regional circulation, e.g., the volume transport of the ACC, the latitudinal position of the BMC, the shelf break upwelling of Patagonia, and the Zapiola Anticyclone. Sensitivity experiments indicate that the bottom stress parameterization significantly impacts the mean location of the Brazil/Malvinas Confluence and the transport of the Zapiola Anticyclone. The transport of the Brazil Current strengthens during the austral summer and weakens during the austral winter. These variations are driven by the wind stress curl over the southwestern Atlantic. The variations of the transport of the Malvinas Current are out of phase with those of the Brazil Current. Most of the seasonal variability of this current is concentrated in the offshore portion of the jet, the inshore portion has a weak seasonality that modulates the magnitude of the Patagonian shelf break upwelling. Using passive tracers we show that most of the entrainment of deep waters into the shelf occurs in the southernmost portion of the Patagonian shelf and along the inshore boundary of the Brazil Current. Shelf waters are preferentially detrained near the Brazil/Malvinas Confluence. Consistent with previous studies, our simulation also shows that south of ∼42°S the Malvinas Current is composed of two jets, which merge near 42°S to form a single jet farther north.