Variability of Antarctic bottom water flow into the North Atlantic (original) (raw)
Related papers
General Circulation of Antarctic Intermediate Water in the subtropical South Atlantic
2000
This study combines float data from different projects collected between 1991 and 2003 in the South Atlantic to describe the flow of Antarctic Intermediate Water (AAIW). Velocity space-time averages are calculated for various grid resolutions and with cells deformed to match the bathymetry, f/H or f/h (with H being the water depth and h being the thickness of the AAIW 5 layer). When judged by the degree of alignment between respective isolines and the resulting average velocity fields, the best grid is based on a nominal cell size of 3º (latitude) by 4º (longitude) with cell shapes deformed according to f/h. Using this grid, objectively estimated mean currents (and their associated errors), as well as meridional and zonal volume transports are estimated. Results show an anticyclonic Subtropical Gyre centred near 36ºS and spanning from 10 23º±1°S to 46° ± 1ºS. The South Atlantic Current meanders from 33ºS to 46ºS and shows a mean speed of 9.6 ± 7.8 cm s -1 (13 Sv ± 3 Sv). The northern branch of the Subtropical Gyre is located between 22ºS and 32ºS and flows westward with a mean speed of 4.7 ± 3.3 cm s -1 (12 Sv ± 2 Sv).
Journal of Geophysical Research, 2003
1] Interhemispheric signal transmission in the Atlantic Ocean connects the deep water production regions of both hemispheres. The nature of these interactions and large-scale responses to perturbations on timescales of years to millennia have been investigated using a global three-dimensional ocean general circulation model coupled to a dynamicthermodynamic sea ice model. The coupled model reproduces many aspects of today's oceanic circulation. A set of experiments shows the sensitivity to changes in different surface boundary conditions. Buoyancy changes in the Weddell and Labrador Seas exert a direct effect on the overturning cells of the respective hemisphere. They influence the density structure of the deep ocean and thereby lead to alterations in the strength of the ACC. Changing the wind stress south of 30°S influences the magnitude of the deep water production of both hemispheres. The interhemispheric effect in these experiments cannot be explained solely by advective mechanisms. Switching off the wind stress over the latitude band of the Drake Passage leads to a slow gradual decrease of the water mass transport in the ACC, resulting in an almost complete cessation. The model results prove the necessity to continue integrations over thousands of years until new equilibria are established.
Lagrangian circulation of Antarctic Intermediate Water in the subtropical South Atlantic
Deep Sea Research Part II: Topical Studies in Oceanography, 2005
This study combines float data from different projects collected between 1991 and 2003 in the South Atlantic to describe the flow of Antarctic Intermediate Water (AAIW). Velocity spacetime averages are calculated for various grid resolutions and with cells deformed to match the bathymetry, f/H or f/h (with H being the water depth and h being the thickness of the AAIW 5 layer). When judged by the degree of alignment between respective isolines and the resulting average velocity fields, the best grid is based on a nominal cell size of 3º (latitude) by 4º (longitude) with cell shapes deformed according to f/h. Using this grid, objectively estimated mean currents (and their associated errors), as well as meridional and zonal volume transports are estimated. Results show an anticyclonic Subtropical Gyre centred near 36ºS and spanning 10 from 23º±1°S to 46° ± 1ºS. The South Atlantic Current meanders from 33ºS to 46ºS and shows a mean speed of 9.6 ± 7.8 cm s -1 (8.5 Sv ± 3.5 Sv; 1 Sv = 1×10 6 m 3 s -1 ). The northern branch of the Subtropical Gyre is located between 22ºS and 32ºS and flows westward with a mean speed of 4.7 ± 3.3 cm s -1 (9.3 Sv ± 3.4 Sv). Evidence of a cyclonic Tropical Gyre divided in two sub-cells is visible on the stream function. 15
Shallow water modeling of Antarctic Bottom Water crossing the equator
Journal of Geophysical Research, 2004
1] The dynamics of abyssal equator-crossing flows are examined by studying simplified models of the flow in the equatorial region in the context of reduced-gravity shallow water theory. A simple ''frictional geostrophic'' model for one-layer cross-equatorial flow is described, in which geostrophy is replaced at the equator by frictional flow down the pressure gradient. This model is compared via numerical simulations to the one-layer reduced-gravity shallow water model for flow over realistic equatorial Atlantic Ocean bottom topography. It is argued that nonlinear advection is important at key locations where it permits the current to flow against a pressure gradient, a mechanism absent in the frictional geostrophic model and one of the reasons this model predicts less cross-equatorial flow than the shallow water model under similar conditions. Simulations of the shallow water model with an annually varying mass source reproduce the correct amplitude of observed time variability of cross-equatorial flow. The time evolution of volume transport across specific locations suggests that mass is stored in an equatorial basin, which can reduce the amplitude of time dependence of fluid actually proceeding into the Northern Hemisphere as compared to the amount entering the equatorial basin. Observed time series of temperature data at the equator are shown to be consistent with this hypothesis.
Nonuniform Upwelling in a Shallow-Water Model of the Antarctic Bottom Water in the Brazil Basin*
Journal of Physical Oceanography, 2004
A numerical model based on the shallow-water equations is developed to represent the flow of Antarctic Bottom Water (AABW) in the Brazil Basin (southwest Atlantic Ocean). The aim is twofold. First, an attempt is made to identify in a model that includes both simplified dynamics and realistic bathymetry (at 1/6° resolution) the impacts of the elevated diapycnal mixing rates near the Mid-Atlantic Ridge (MAR) documented by dissipation data of the Deep Basin Experiment (DBE). To this end, different assumptions regarding the distribution of the velocity across the AABW layer interface (w) are considered. Second, the extent to which the shallow-water model can replicate observations relative to AABW circulation in the basin, in particular the trajectory and velocity of neutrally buoyant floats released in the AABW during the DBE, is examined. The model flows are characterized by small Rossby numbers, except in the northward-flowing western boundary current where kinetic energy is largely ...
Journal of Marine Research, 2000
Data from a hydrographic section carried out in January-March 1994 offshore from the eastern coast of South America from 50S to 10N, are used to quantify the full-depth exchanges of water between the western boundary currents and the ocean interior. In the upper and intermediate layers, the westward transport associated with the southern branch of the South Equatorial Current was 49 Sv at the time of the cruise. The transports of the central and northern branches in the upper 200 m were 17 Sv and 12 Sv, respectively.After subtraction of the parts that recirculate in the subtropical, subequatorial, and equatorial domains, the fraction of the South Equatorial Current that effectively contributes to the warm water export to the North Atlantic is estimated at 18 Sv. The poleward boundary of the current southern branch is at 31S through the whole thickness of the subtropicalgyre, but the latitude of the northern boundary varies from 7°308S at the surface to 27S at 1400 m depth. The estimated latitude of its bifurcation into the Brazil Current and North Brazil Undercurrent also varies downward from about 14S at the surface to 28S at a depth of 600 m.
On the intermediate and deep water flows in the South Atlantic Ocean
Journal of Geophysical Research, 1997
A multiparameter analysis is applied on zonal and meridional hydrographic sections obtained for the South Atlantic Ventilation Experiment (SAVE) to determine the spreading and mixing of water masses in the South Atlantic Ocean, focusing our interest on the large-scale flow of intermediate and deep waters. The method utilizes all information from the hydrographic data set including temperature, salinity, dissolved oxygen, and nutrient fields. Mixing proportions are quantified and plotted along the eight sections considered. Results show no evidence of a primats' route of Antarctic Intermediate Water along the western boundary of the South Atlantic. In the eastern basin the eastward extension of the Upper North Atlantic Deep Water (NADW) in the Guinea Basin following the cyclonic subequatorial gyre is confirmed. In the Angola Basin a weak but thick NADW core layer is observed in conjunction with very little presence of Lower Circumpolar Deep Water (LCDW). High LCDW concentrations in Cape Basin are indicative of the communication of this basin to cold water sources in the south. The method is sensitive enough to detect for instance the presence of the Congo River Plume in the Angola Basin or the influence of the Weddell Sea Deep Water in the vicinity of the Romanche and Chain Fracture Zones in the equatorial region. In conjunction with the multiparameter analyses along SAVE sections, an analysis of components of the residual vector R indicates a middepth minimum in the RN/P utilization ratio. Both a suitable explanation for the minimum and the potential consequences for the multiparameter analyses of South Atlantic water mass circulation are still to be found.