The Burdwood Bank Circulation

A suite of high-resolution numerical simulations characterizes the oceanic circulation in the Burdwood Bank, a shallow seamount located in the northeastern end of the Drake Passage. Model analysis shows energetic upwelling and mixing uplifting deep and benthic waters into the photic layer. Tides and the Antarctic Circumpolar Current are the primary drivers of the bank's circulation. Tidal forcing is the main driver for the entrainment of deep waters into the upper layers of the bank and local wind forcing for the detrainment of these waters into the deep ocean. Passive tracer diagnostics suggest that the dynamical processes triggered by the Burdwood Bank could have a significant impact on local ecosystems and the biogeochemical balance of the southwestern Atlantic region, which is one of the most fertile portions of the Southern Ocean. Model results are robust—they are reproduced in a wide array of model configurations—but there is insufficient observational evidence to corroborate them. Satellite color imagery does not show substantial chlorophyll blooms in this region but it shows strong phytoplankton plumes emanating from the bank. There are several potential explanations for the chlorophyll deficit, including lack of light due to persistent cloud cover, deep mixing layers, fast ocean currents, and the likelihood that blooms, while extant, might not develop on the surface. None of these possibilities can be confirmed at this stage.Fil: Matano, Ricardo P.. State University of Oregon; Estados UnidosFil: Palma, Elbio Daniel. Universidad Nacional del Sur. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Combes, Vincent. State University of Oregon; Estados Unido

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

The Low-Frequency Variability of the Southern Ocean Circulation

Abstract: Long time series of sea surface height (SSH), sea surface temperature, and wind stress curl are used to determine the main modes of low-frequency variability of the Southern Ocean (SO) circulation. The dominant mode is a trend of increasing SSH at an average rate of 3.3 mm yr(-1). Similar trends have been reported in previous studies and the analysis indicates that the tendency of sea level increase over the SO has become more spatially homogeneous during the last decade, with changes in the increasing rate in 2000 and 2006. The other modes consist of stationary, basin-type modes, and an eastward-propagating wave. The stationary modes are particularly dominant in the Indian and Atlantic Ocean basins, where their spatial structure appears to be shaped by the basin geometry and the bottom topography. The wavelike patterns travel eastward with a period of approximately 10 years. Two waves were identified in the analysis: a complete cycle between 1997 and 2007 and a second cycle starting in 2000. Such waves have rarely been mentioned or identified in studies using recent satellite-derived SSH products.Keywords: In-situ, Signals, Expansion, Temperature, Sea level change, Wind, Antarctic circumpolar wave, Hemisphere annular mode, Scales, satellit

Formation and pathways of the intermediate water in the Parallel Ocean Circulation Model's Southern Ocean

The formation mechanisms and pathways of intermediate water in the Southern Ocean are analyzed from output of a high-resolution ocean general circulation model. Deep winter mixed layer formation in the Southern Ocean is diagnosed from the model results and is found to be mostly consistent with observations. Diapycnal water mass transformations by air-sea fluxes and internal mixing are quantified and split into mean and eddy components. The diapycnal formation of the water masses that constitute the Antarctic intermediate water layer in the southeast Pacific is found to occur mainly in the western Pacific Ocean in this model. In winter, convection up to 900 m is found to set the potential vorticity characteristics of this layer. Eddy fluxes of heat and buoyancy play an important role in the formation of the intermediate waters by transferring water from the southern parts of the subtropical gyres into the Antarctic Circumpolar Current (ACC) and vice versa. The effects of eddy fluxes are found to vary significantly along the path of the ACC. They are strongly concentrated in the regions near the Agulhas Return Current in the Indian Ocean and the Brazil-Malvinas Confluence in the Atlantic

Heat and mass balances of the South Atlantic Ocean calculated from a numerical model

The general circulation model of Bryan (1969), modified by the introduction of open boundary conditions at the Drake Passage and between Africa and Antarctica, has been used to study the mass and heat budgets of the South Atlantic Ocean. The model was initialized with the climatological annual mean values of temperature and salinity of Levitus (1982) and forced at its surface with the climatological wind stress data of Hellernian and Rosenstein (1983). After 3 years of integration the model reached a quasi-stationary state. A heat balance shows that the model transports 0.19 PW of heat toward the north across 30°S. While a large part of this heat is supplied by the atmosphere and involves the conversion of intermediate waters into surface waters, a comparison with climatological data of atmospheric heat fluxes suggests that an extra source of heat is necessary to maintain the northward heat flux.Copyrighted by American Geophysical Union

Climate Change Impacts on the Patagonian Shelf Break Front

We characterize long-term trends of sea surface temperature (SST), absolute dynamic topography, and chlorophyll-a (CHL) in the Patagonian shelf break front (SBF) using 27 years (1993–2019) of satellite data. Warming of the Argentinean shelf waters and the southwestward displacement of the Brazil-Malvinas Confluence (BMC) impact the northernmost extension of the SBF. Cooling of the Malvinas Current (MC) and the concurrent warming of the adjacent shelf waters lead to a significant increase of SST gradients along the outer shelf. The southwestward displacement of the BMC implies a similar shift of the SBF. An increase in CHL trend appears to be associated with southerly wind anomalies along the shelf break. We estimate a southward shift of the northernmost penetration of the MC of −0.11 ± 0.076°/decade.Fil: Franco, Barbara Cristie. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; ArgentinaFil: Ruiz Etcheverry, Laura Agustina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; ArgentinaFil: Marrari, Marina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Piola, Alberto Ricardo. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografía Naval. Departamento Oceanografía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; ArgentinaFil: Matano, R. P.. State University of Oregon; Estados Unido

Dynamical analysis of the upwelling circulation off central Chile

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 indicate that during periods with El Niño conditions, the area is affected with a general weakening of the currents and upwelling activity, although the northern region still shows the formation of eddies. During years with relatively stronger winds, in contrast, the upwelling activity and across-shelf transport processes are significantly increased. The results show that the Punta Lavapie cape has a large effect on the spatial and temporal variability of the coastal currents in the region off central Chile

A numerical study of the Southwestern Atlantic Sheld circulation : barotropic response to tidal and wind forcing

This article analyzes the barotropic circulation in the Southwestern Atlantic Shelf using a three-dimensional numerical model forced with winds and tides. South of 40°S, the shelf circulation is dominated by the propagation of the semidiurnal tides. In this region the diurnal tides are generally weak, except at the shelf edge where they resonate with northward propagating, continental shelf waves. North of 40°S, the tidal circulation is relatively weak, and the circulation is mainly driven by the winds. The wind-driven annual mean circulation is characterized by a broad northeastward flow south of approximately 40°S and is characterized by a southwestward flow farther north. The intense mixing associated with the Patagonian tides enhances the bottom friction that balances the energy input from the wind stress forcing. In contrast with previous results our simulation shows a detrainment of the northward volume transport with latitude due to an offshore flow along the edge of the Patagonian shelf break. The largest seasonal variations of the shelf circulation are observed in the region between 45°S and 25°S where, during the fall, there is a development of a clockwise gyre and a northeastward flow north of 40°S. The gyre weakens toward the winter, and the northeastward flow reverses directions

Shelfbreak upwelling induced by alongshore currents: analytical and numerical results

Alongshore flow in the direction of propagation of coastal trapped waves can result in upwelling at the shelfbreak. The intensity of this upwelling can be comparable in magnitude to wind-driven coastal upwelling, with its associated ecological features. Recent numerical experiments by Matano & Palma indicate that this upwelling results from convergence of Ekman transport at the shelfbreak. The mechanism for this phenomenon can be understood in terms of steady solutions to the shallow water equations in the presence of Coriolis force and bottom drag. Matano & Palma interpreted their numerical results in terms of the arrested topographic wave, but did not present direct comparisons. Here we present a family of analytical solutions to the equations of the arrested topographic wave that shows striking quantitative agreement with earlier numerical results.Keywords: Topographic effects, Ocean processes, Shallow water flow

Shelfbreak upwelling induced by alongshore currents: analytical and numerical results

Alongshore flow in the direction of propagation of coastal trapped waves can result in upwelling at the shelfbreak. The intensity of this upwelling can be comparable in magnitude to wind-driven coastal upwelling, with its associated ecological features. Recent numerical experiments by Matano & Palma indicate that this upwelling results from convergence of Ekman transport at the shelfbreak. The mechanism for this phenomenon can be understood in terms of steady solutions to the shallow water equations in the presence of Coriolis force and bottom drag. Matano & Palma interpreted their numerical results in terms of the arrested topographic wave, but did not present direct comparisons. Here we present a family of analytical solutions to the equations of the arrested topographic wave that shows striking quantitative agreement with earlier numerical resultsThis is the publisher's version of record. The original submission is copyrighted by Cambridge University Press and can be found here: http://www.tos.org/Keywords: shallow water flows, ocean processes, topographic effectsKeywords: shallow water flows, ocean processes, topographic effect