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.


INTRODUCTION
Antarctic Bottom Water (AABW) is formed over the Antarctic slope as a result of mixing of the cold and heavy Antarctic Shelf Water with the lighter, warmer, and more saline Circumpolar Deep Water [Orsi et al., 1999]. In the region of origin, Antarctic Shelf Water is formed in the autumn-winter season over the Antarctic shelf due to cooling of the relatively fresh Antarctic Surface Water to nearly freezing point temperature and increased salinity caused by ice formation. The resulting water mass with increased density descends and reaches the ocean floor. In the Atlantic Ocean the regions of dominating Antarctic Bottom Water formation are in the southern and western parts of the Weddell Sea.
Antarctic Bottom Water represents the coldest and deepest layer of the South Atlantic. A commonly accepted definition describes AABW as water with potential temperature cooler than 2°C [Wüst, 1936]. This layer can occupy a layer 1000 m thick and even more at the bottom of the Atlantic Ocean. The thickness decreases in the northern direction up to complete wedgingout at the bottom in the North Atlantic.
Generally, propagation of Antarctic waters in the bottom layer of the Atlantic Ocean is confined to depressions in the bottom topography. The pathways of AABW in the Atlantic Ocean are shown in Fig. 1. The general flow of these waters can be presented as follows [Morozov et al., 2010]. The bottom topography around the Vema Channel is shown in Fig. 2. The Vema Channel is the deepest one among the passages existing for Antarctic Bottom Water. Therefore, the coldest water (Weddell Sea Deep Water) can exit the Argentine Basin in the equatorward direction only through this channel [Zenk et al., 1993].
According to the moored measurements (two moorings), the mean transport of Antarctic Bottom Water (layer below 2°C isotherm) through the Vema Channel is estimated at 3.5 Sv. The mean velocities are 30 cm s -1 and the highest reach 60 cm s -1 . (Fig. 3).
However, the instantaneous transport measured by LADCP instruments (five sections across the channel in the middle part of the channel) appears lower and fluctuates between 2.5 and 3.5 Sv. Usually, the jet core is vertically mixed in a layer approximately 150 m thick. Owing to the Ekman friction the coldest core of the flow in the Vema Channel is usually displaced to the eastern slope of the channel.
In 2010, we carried out the measurements of currents in the region where Antarctic Bottom Water outflows from the Vema Channel to the Brazil Basin at latitude of 26°40' S. Let us compare the sections at the standard section (31°12' S) and in the northern part of the channel. The sections are presented in Fig. 4. The red line shows the location of the zero isotherm of potential temperature.
In the narrow passage in the northern part of the channel (the Vema Extension), where the channel becomes deeper and narrower, the isotherms of the potential temperature greater than 0°C do not reach the slopes of the channel as in the south. This means that the flow with a temperature of θ = 2°С and even with a temperature of θ = 0.2°С becomes wider. Using the available data we can compare only the Note the high speed core of AABW in the lowest 250 m and some rare current reversals caused by highly energetic eddies water flows with potential temperatures θ m 0°C. The mean velocities of the flow with such temperatures at the standard section are 23 cm/s, while at the northern section they decrease to 11 cm/s. Antarctic Bottom Water with higher temperatures flows above the western slope of the channel with velocities exceeding 20 cm/s. Unfortunately we could not extend the section farther to the west in the expedition in 2010 for a more precise calculation of the transport.
The square of the standard section for the water below 0°C is 6  10 6 m 2 , while in the north the similar square is almost four times smaller (1.4  10 6 m 2 ). The transport of this water across the standard section is 1.4 Sv, while the transport across the northern section is 0.16 Sv, which is almost 10 times smaller. Despite the fact that these sections were made not simultaneously, we relate this variability to the spatial variation of the flow. Thus a large amount of the coldest water does not reach the northern section remaining beyond the topographic obstacles and mixes with the overlying waters.    Such localization seems surprising because Antarctic Bottom Water transports through the Romanche and Chain fracture zones are of the order of 1 Sv, which is almost the same as the water transport through the Vema Fracture Zone. We believe that this phenomenon may be explained by stronger mixing in the Romanche and Chain Fracture Zones compared to the Vema Fracture Zone caused by internal tidal waves.

KANE GAP
The Kane Gap is located between the Grimaldi Mountains, which are a part of the Sierra Leone Rise and the Guinea Plateau near the African Continent (Fig. 6). The gap connects Gambia Abyssal Plain (Cape Verde Basin) and Sierra Leone Basin. The sill depth in the gap is 4502 m.  The temperature stratification of the flow is similar to the flow in the Vema Channel. The coolest and densest water of the flow is displaced to the western wall of the gap due to the Ekman friction. Lower salinities are also recorded here at the foot of the western slope. Since the Kane Gap is located in the Northern Hemisphere, Ekman friction displaces the densest water to the left wall of the channel (southwestern slope in our case) (Fig. 7).
The total transport below 1.9°C potential temperature isotherm based on LADCP measurements fluctuates between zero and 0.2 Sv based on our measurements in different years. Thus, the bottom water from the Vema Fracture Zone influences at least the northern part of the Sierra Leone Basin, while the bottom water from the Romanche Fracture Zone can spread to the north through the Kane Gap and influence the adjacent southern region of Cape Verde Basin. However, the bottom water transport does not exceed 0.2 Sv and can be influenced by tides.

DISCOVERY GAP
The northward propagation of bottom waters from the Canary Basin to the northeastern Atlantic occurs through the Discovery Gap. This region is the boundary for the further northward transport of bottom water with potential temperatures below 2°C. This passage is considered the terminal point of AABW spreading to the north in the sense that this is the water with a potential temperature less than 2°C. This is a narrow passage in the East Azores Fracture Zone at 37°N between the Madeira and Iberian abyssal basins [Saunders, 1987]. The passage is 150 km long. Its narrowest place is located at 37°20' N, 15°40' W. The width of the narrowest gap is 10 km and the depth of the sill is 4800 m. The measured mean velocities were 5 cm s -1 . The flux of bottom water colder than potential temperature θ = 2.05°C was estimated at 0.  One core displaced to the eastern slope was directed to the northeast with velocities of approximately 5 cm/s and the second core with slightly greater velocities was directed to the southwest and displaced to the western slope.

CONCLUSIONS
We summarized the characteristics of the transport of bottom water from its source region in the Weddell Sea through the main abyssal channels of the Atlantic Ocean. The analysis is based on the recent CTD-sections and moored current meters as well as on the historical data. The main properties of the bottom flow in the Vema Channel include a flow with a mean speed of 30 cm/s. The measurements in the Vema Channel that have been continuing for 30 years already revealed a temperature increase and recent fluctuations in the temperature of the coldest water. After the flow of Antarctic Bottom Water passes the Brazil Basin it splits into two flows. Part of the water flows through the Romanche and Chain fracture zones to the east. The other part flows to the northwest to the North American Basin. Part of the latter flow propagates through the Vema Fracture Zone into the Northeast Atlantic. We analyze an unsteady flow of bottom water in the Kane Gap. The terminal point for the Antarctic Bottom Water flow is the Discovery Gap.