Roman Y. Tarakanov

Abyssal Channels in the Atlantic Ocean: Water Structure and Flows

Foreword. Preface. Acknowledgements. 1. Geological and geophysical characteristics of the transform fault zones. 1.1 General description. 1.2 Charlie Gibbs Fracture Zone. 1.3 Vema Fracture Zone. 1.4 Romanche Fracture Zone. 1.5 Chain Fracture Zone. 1.6 Vema Channel. 2. Deep water masses of the South and North Atlantic. 2.1 General description. 2.2 Antarctic Intermediate Water. 2.3 Upper Circumpolar Water and Upper Circumpolar Deep Water. 2.4 North Atlantic Deep Water. 2.5 Lower Circumpolar Water and Lower Circumpolar Deep Water, Circumpolar Bottom Water, Southeast Pacific Deep Water, and Warm Deep Water. 2.6 Antarctic Bottom Water. 3. Source Regions, Abyssal Pathways, and Bottom Flow Channels (for waters of the Antarctic origin). 3.1 General description. 3.2 Weddell Sea and Weddell Gyre. 3.3 Agulhas and Cape basins. 3.4 Drake Passage, Scotia Sea, and Georgia Basin. 3.5 Antarctic Bottom Water in the Argentine Basin. 4. Exchange between the Argentine and Brazil basins Abyssal pathways and bottom flow channels (for waters of the Antarctic origin). 4.1 General description. 4.2 Vema Channel. 4.3 Santos Plateau. 4.4 Hunter Channel. 5. Further propagation of Antarctic Bottom Water from the Brazil Basin. 5.1 Brazil Basin. 5.2 Flow in the Guiana Basin and westward equatorial channels. 5.3 North American Basin. 5.4 Eastward Equatorial Channels. The Romanche and Chain Fracture Zones. 5.5 Vema Fracture Zone. 5.6 Eastern Basin Pathways and further propagation of Antarctic Bottom Water in the East Atlantic. 5.7 Kane Gap. 5.8 Angola Basin. 6. Flows through the Mid-Atlantic Ridge in the northern channels. Charlie Gibbs Fracture Zone and other fracture zones. Integrated conclusions. References. Index.

Convective and shear-induced turbulence in the deep Kane Gap

[1] The boundary layer above a 4569 m deep slope in the near-equatorial N-Atlantic Ocean Kane Gap, a throughflow for Antarctic Bottom Water (AABW), is characterized by two distinct turbulent regimes that differ by an order of magnitude in intensity depending on the direction of throughflow. During southward and downward flow, vertical mixing is vigorous. This is inferred from high-resolution temperature observations between 6 and 132 m above the bottom. For a representative case study of 2 days, average values are found for dissipation rate of e?=?2.1?±?1 × 10-9 W kg-1 and eddy diffusivity of Kz?=?7?±?4 × 10-4 m2 s-1. The mixing is across relatively large vertical overturns. During northward and upward flow, smaller overturns are more horizontal as in stratified shear flow (with representative 2 day mean e?=?6?±?3 × 10-11 W kg-1, Kz?=?4?±?2 × 10-5 m2 s-1). Stratification is approximately the same during both flow directions. Although the different turbulence regimes are partially associated with frictional boundary layers of large-scale flows above sloping topography, but not with those over flat bottoms, and partially with flow across a hill-promontory, internal waves are a dominant process in promoting turbulence. In addition, internal waves are observed to push stratification toward the bottom thereby importantly contributing to the mixing of AABW.

Deep Water Masses of the South and North Atlantic

The Southern Ocean and Antarctic Circumpolar Current isolate the Antarctic continent from other regions of the Earth. Thus, conditions in the study region provide the formation of a special water structure around Antarctica, whereas water structure in the northern regions is determined by interactions between waters of the North Atlantic and those of Arctic origin.

Internal Solitary Waves in a Layered Weakly Stratified Flow

The problem on internal waves in a weakly stratified two-layered flow is studied semi-analytically. The long-wave model describing travelling waves is constructed by means of scaling procedure with a small Boussinesq parameter. It is demonstrated that solitary wave regimes can be affected by the Kelvin–Helmholtz instability arising due to interfacial velocity shear in the upstream flow.

Internal waves in marginally stable abyssal stratified flows

The problem on internal waves in a weakly stratified two-layer fluid is studied semi-analytically. We discuss the 2.5-layer fluid flows with exponential stratification of both layers. The long-wave model describing travelling waves is constructed by means of a scaling procedure with a small Boussinesq parameter. It is demonstrated that solitarywave regimes can be affected by the Kelvin–Helmholtz instability arising due to interfacial velocity shear in upstream flow.

Bottom Water Flows in the Vema Channel and over the Santos Plateau Based on the Field and Numerical Experiments

The properties of Antarctic Bottom Water flows in the Southwest Atlantic were studied on the basis of hydrographic measurements and numerical modeling of the oceanic circulation. The CTD and LADCP profiles in the region of the Vema Channel and Santos Plateau were measured onboard the R/V “Akademik Sergey Vavilov”. Hydrographic observations at several locations over the Santos Plateau were carried out for the first time. The numerical simulation was performed using the Institute of Numerical Mathematics Ocean Model (INMOM). The observations of velocities were used for verification of the numerical model. The simulated three-dimensional velocity fields with high spatial resolution in the lower layer allow us to study the bottom currents over the entire length of the Vema Channel.

Spreading of Antarctic Bottom Water in the Atlantic Ocean

This paper describes the transport of bottom water from its source region in the Weddell Sea through the abyssal channels of the Atlantic Ocean. The research brings together the recent observations and historical data. A strong flow of Antarctic Bottom Water through the Vema Channel is analyzed. The mean speed of the flow is 30 cm/s. A temperature increase was found in the deep Vema Channel, which has been observed for 30 years already. The flow of bottom water in the northern part of the Brazil Basin splits. Part of the water flows through the Romanche and Chain fracture zones. The other part flows to the North American Basin. Part of the latter flow propagates through the Vema Fracture Zone into the Northeast Atlantic. The properties of bottom water in the Kane Gap and Discovery Gap are also analyzed.

Exchange Between the Argentine and Brazil Basins; Abyssal Pathways and Bottom Flow Channels (for Waters of the Antarctic Origin)

The zonally aligned Rio Grande Rise separates the Argentine Basin in the south from the Brazil Basin in the north. It is a high topographic obstacle for bottom water propagation to the north. Two meridional gaps intersect the Rise at ~39° W and ~28° W (Vema and Hunter channels, respectively). The depth in the Vema Channel exceeds 4,600 m as compared to the background depths of 4,200 m. The Hunter Channel is much shallower and the greatest depth does not exceed 4,000 m.

Further Propagation of Antarctic Bottom Water from the Brazil Basin

In this chapter we assume Wust’s (1936) sense of Antarctic Bottom Water propagation. In other words, this is the bottom water of the Antarctic rather than North Atlantic origin. However there is no generally accepted isotherm or value of any characteristics in literature for the upper boundary of Antarctic Bottom Water in the Equatorial and North Atlantic. Therefore, it is essential that quantitative estimates of Antarctic Bottom Water properties and transport depend on the choice of this boundary.

Flows through the Mid-Atlantic Ridge in the Northern Channels. Charlie Gibbs Fracture Zone and Other Fracture Zones

In the winter period, cold dense water is formed in the Norwegian and Greenland seas due to severe cooling and intense heat release to the atmosphere. The water mass from the Greenland Sea overflows the shallow threshold between Greenland and Iceland and flows into the Irminger Basin. The water mass from the Norwegian Sea overflows the threshold between Iceland, Faeroe, and Shetland Islands and flows into the Iceland Basin. The water mass, which is formed during the overflow over the latter thresholds, is called Iceland Scotland Overflow Water (ISOW). In the Iceland Basin this water is mixed with the significantly warmer and more saline waters in the Northeastern Atlantic, which are subject to the influence of Mediterranean waters. As a result, Iceland Scotland Overflow Water appears more saline and warmer than the deep waters propagating into the Atlantic through the Denmark Strait between Greenland and Iceland (Denmark Strait Overflow Water, DSOW). Iceland Scotland Overflow Water flows first to the west with the Deep Northern Boundary Current and then to the south along the eastern slope of the Reykjanes Ridge. The circulation scheme of different layers of the North Atlantic is shown in Fig. 2.6.