BOTTOM TOPOGRAPHY AND DATA
The bottom topography based on satellite altimetry (GEBCO 2019) was used as the basis of the topography chart. We corrected the depths over the routes of our vessels using echo sounders.
Our analysis is based on CTD/LADCP measurements (up to 5 m above the bottom) in 2003, 2009, 2010, 2012, 2013, 2018, 2019, and 2020 conducted in the region of the outflow of AABW from the Vema Channel and also on the data of the WOCE A17 (WHP, 2002) section and quasi-zonal sections A09 in 2009 and 2018 along 24°S (http://cchdo.ucsd.edu). We also used the data of several stations from the WOD18 database. The locations of our and historical stations are shown in Fig. 2. The barotropic tide was removed from the resulting velocity profiles using the TPXO 9 model (Egbert and Erofeeva 2002).
BOTTOM CURRENTS AND THERMOHALINE PROPERTIES OF BOTTOM WATER
The data of CTD/LADCP sections in 2020 across the Vema Channel in the northern part of its extension demonstrate (Fig. 3) the existence of two coldest AABW flows (q<0.2°C). Repeated data since 2006 over the section across the Vema Channel at 31°S confirm this. One jet of AABW follows the deep channel bed of the Vema Channel. It is seen in Fig. 3 that the deepest core of the current is located at depths of 4700-400 m. The minimum potential temperature of this type of AABW in the northern region of the channel (26º43.2’S, 34º12.1’W) was recorded in 2003 (q=-0.094°C). It is noteworthy that there is a positive trend in the bottom potential temperature with time (Zenk and Morozov 2007). Potential temperature at the bottom at the same point in 2020 was q=-0.067ºC. Another jet of AABW was found over the gentle northwestern slope of the channel (Fig. 3). The core of the current with highest velocities was located at a depth of 4100-4200 m (150 m above the bottom) and approximately 600 m shallower than the core of the deeper jet of AABW.
Both cores are seen in Fig. 3 over sections 2 and 3 (26°30’- 27°00 S) in 2020. The western section 2 does not fully show the upper jet because we had to stop the casts in stormy weather. Section 2 was made along 26°25.0’S on April 3, 2020. The maximum velocity was recorded at the eastern station 6795 at 34°41.9’W (12 cm/s). On December 31, 2019 we made two stations along 26°20.0’S at 34°30.0’W and 34°40.0’W. The velocity in the bottom layer at the western station was directed to the northeast with a maximum of 9 cm/s at 130 m above the bottom. Velocity at the bottom at the eastern station was lower, 6-7 cm/s. Thus, the maximum velocities are close to 34°32’W longitude.
Let us consider the deeper currents near the outflow of AABW from the Vema Channel based on our CTD/LADCP casts in 2010-2020. The locations of the casts are in the region 26°30-26°50 S, 33°30-34°20 W (Fig. 9). The measurements showed that the continuation of the coldest AABW flow splits into two bottom jets. One of the jets (q=-0.087°C in 2012) was directed initially to the east with a slight deviation to the northeast. The second jet with warmer water (q=-0.063°C in 2012) was directed almost to the north. The flow is confined to the feet of higher mountains, while on the outer side of the flow there is a flatter elevation, and the channel is ~200 m deeper than the elevation; approximate width is ~10 km.
We searched for the continuation of the coldest AABW flow to the northeast of the channel. The stations were made in a quasi-zonal valley nearly at 26°30’S east of 33°30’W, which seemed to be a topographic continuation of the Vema Channel (Fig. 3). We did not find potential temperatures below zero at the bottom there, despite the fact that the sill depth at 33°30’W is slightly shallower than 4500 m. The vertical temperature gradient at the bottom (0.46°/50 m) west of this sill is very high so that potential temperature at station 2527 (26°37.2'S, 33°39.6'W) at a distance of 50 m above the bottom is q~0.4°C (Fig. 4) unlike q=-0.035°C at the bottom (4554 m). The flow with potential temperatures below 0.2ºC is confined to a bottom layer 40 m thick. The coldest continuation of the flow turns to the north because of the existence of this barrier.
Along with the eastward continuation of the AABW flow in the Vema Channel there is also a branch of this stream directed to the north (approximately along 34ºW). The northern channel is about 70 m deeper than the relatively flat plain in the west. Thermohaline properties of water in both smaller channels are similar to those in the Vema Channel.
The CTD/LADCP measurements in this channel revealed a northerly current with a velocity of approximately 5 cm s-1. The water in this current is characterized by the thermohaline properties similar to the water in the Vema Channel although it was slightly warmer (-0.063°C in 2012) compared to the potential temperature (-0.087°C in 2012) measured in the eastward flow.
A zonal section of four stations was occupied in 2018 along 33°30’W nearly at 25°34’S where a small channel was found based on the digital database (GEBCO 2019). The channel is approximately 150-200 m deep relative to the surrounding plateau (4700 m) (Fig. 5). This channel is apparently the continuation of the bottom flow that turned to the north at 33º30’W. The width of the channel here is about 7 km at a level of the 4800 m isobath, the maximum depth is 4950 m. Echo-sounder measurements confirmed the existence of this small channel. The measurements of the velocity component along this channel show the presence of a high-speed (more than 35 cm s-1) northward velocity core of the bottom flow approximately 100-150 m thick (Figs. 4, 5 and Fig. 4 in (Tarakanov et al., 2020)). Potential temperature of the bottom water in this channel was higher than zero. This layer with high velocities is located approximately below the θ=0.62°C isotherm. We estimated the transport in the bottom layer below θ<0.62°C as 0.150±0.007 Sv. This transport of bottom water is more than one order of magnitude smaller than the estimates of the total AABW flow across the section at 31°S. As usual in the Vema Channel, the core of cold fresher water is displaced to the eastern slope (i.e. to the right of the flow direction) with the formation of the sharpest deep thermocline over it (Jungclaus and Vanicek, 1999). This displacement is also seen in Fig. 3 over the section in the northern part of the Vema Channel.
The difference in the temperature profiles along the meridional channel (Fig. 4) shows gradual deepening of the isotherms (up to ~400 m) in the bottom layer from the Vema Channel Extension (station 2496) to the section (station 2710 at 25°34’S) especially notable in the layer colder than 0.60-0.65°C. This small channel is a continuation of the deepest bed of the Vema Channel. As the bottom flow propagates to the north its temperature increases and the thickness of the layer decreases, which leads to a decrease in the transport of coldest water. It is likely that this flow eventually disappears. Potential temperature of the flow at 25°30 S becomes higher than zero.
The pathway of the upper cold AABW flow is located over the western slope of the Vema Channel. Such an isolated core over the western terrace of the Vema Channel found on the repeated section (along 31°12’S) was observed in the potential temperature distributions in 1984 (-0.15°C), 1991 (-0.16°C), 1992 (-0.13°C), 2006 (-0.10°C), 2009 (-0.10°C), 2017 (-0.06°C), 2018 (-0.08°C), and 2020 (-0.06ºC). Velocity measurements performed since 2006 confirm the existence of this core; they range within 30-36 cm s-1 (Morozov et al., 2010).
In 2009, a CTD-section along 24°S was occupied by British scientists. The section was repeated in 2018 (https://cchdo.ucsd.edu). In 2009, two individual cores of low bottom temperatures were found on this section with potential temperatures in the first core ranging between -0.04°C and -0.05°C at 33°50'W (approximately at a depth of 4650 m) and in the second core at 31°25'W (in the depth range 5000-5200 m) the potential temperatures were between -0.03°C and -004°C. These cores slightly displaced in 2018. Potential temperatures in the cores in 2018 were approximately -0.03°C. The potential temperature section along this line in 2018 (24º S) is shown in Fig. 6.
It is clear that the two deeper jets considered above could not have such a cold continuation at 24°S because the upper core at 24°S was located at shallower depths (~4600 m) than the two deeper continuation flows (~4700 m) from the deep Vema Channel bed. The potential temperature of the cold water found at 24°00’S, 33°42’W at a depth of 4600 m was -0.049°C in 2009. In 2018 the potential temperature increased to -0.029°C. The lower core at 24°S was much colder (-0.03°C) than the two deeper jets of the continuation of AABW flow from the deep channel bed. Potential temperature at 25°30 S ranged within 0.069°-0.145°C above zero (Fig. 5).
It is likely that these two regions of relatively cold bottom temperature (at 24°S) could be associated with the continuation of the upper current of AABW from the Vema Channel that was initially spreading above the western wall of the channel. There is no other source of such cold water because the potential temperature of bottom water in the two channels continuing the Vema Channel bed is not as cold as the one recorded on the 24°S section. The measurements in 2009 and 2018 at 24°S were made at a distance of 300 km north of the channel. The inclination of the slope in this region is rather small: approximately 1:100. A jet of cold water in motion can maintain its position above such a slope. Shallow depressions on the continental slope (~50-100 m) were found exactly in the region of the location of cold-water cores, which could be eroded by the bottom water flow. We emphasize that there are no sources of such cold water other than the Vema Channel. Hence, this water arrives here with the upper jet of AABW. In 2018, 2019, and 2020 we made stations to locate the upper jet north of the Vema Channel based on the existence of cold temperatures and direction of the flow with a northerly component. We confirmed its location at a latitude of 25°34’S and on sections 4 and 5 around 25°S (Figs. 1, 7, 8). We also found this jet in the southern region closer to the Vema Channel at 26°20’S, 34°30’W.
This jet was traced by the low bottom potential temperatures and by the vectors of the bottom flow shown in Fig. 9. In 2020, we made two CTD/LADCP sections (Figs. 7, 8) to show that part of the bottom flow is directed into the canyon, which is seen in Figs. 2 and 9. A zonal canyon is located at 24°45’S; the canyon becomes wider approximately at 33°00'W. As seen from the distribution of temperature at the bottom from the historical data set (Fig. 2), this canyon diverts part of the flow of the coldest AABW down from the upper, quasi-geostrophic core of the AABW flow because the gravity force component directed normal to the flow increases over the steep slope of the canyon.
We conclude that the coldest type of AABW (q<0.0°C) flows north from the Vema Channel as two jets, one of which is located in the deep channel bed and the other approximately 200 m above the western slope of the channel. The deep channel bed of the Vema Channel does not end in the region 26°40’S, 34°00’W but continues as a widening to the north in the Brazil Basin. This flow generally diverges into two bottom flows along smaller channels. The potential temperature of the flow increases in the course of water spreading. Approximately at 25°30’S, the potential temperature in these flows becomes warmer than 0°C.
The potential temperature of the upper jet remains below zero even at 24°S. Measurements north of this latitude also revealed negative potential temperatures of the flow. This can be a continuation of the AABW jet described in (Sandoval and Weatherly 1999). It is likely that a third jet exists at shallower depths. We could trace it based on the CTD casts in 1994 and 2003, which found potential temperatures below zero; however, there were no LADCP measurements. The pathway of the jet slightly displaces in time based on the temperature measurements. Thus, we can construct a scheme for the continuation of the AABW flow from the Vema Channel based on our and historical measurements (Fig. 9).