E. G. Morozov

Circulation in the southwestern part of the Kara Sea in September 2007

During cruise 54 of the R/V Akademik Mstislav Keldysh to the southwestern Kara Sea (September 6 to October 7, 2007), a large amount of hydrophysical data with unique spatial resolution was obtained on the basis of measurements using different instruments. The analysis of the data gave us the possibility to study the dynamics and hydrological structure of the southwestern Kara Sea basin. The main elements of the general circulation are the following: the Yamal Current, the Eastern Novaya Zemlya Current, and the St. Anna Trough Current. All these currents are topographically controlled; they flow over the bottom slopes along the isobaths. The Yamal Current begins at the Kara Gates Strait and turns to the east as part of the cyclonic circulation. Then, it turns to the north and propagates along the Yamal coast over the 100-m isobath. The Eastern Novaya Zemlya Current (its core is located over the eastern slope of the Novaya Zemlya Trough) flows to the northeast. Near the northern edge of Novaya Zemlya, it encounters the St. Anna Trough Current, separates from the coast, and flows practically to the east merging with the continuation of the Yamal Current. A strong frontal zone is formed in the region where the two currents merge above the threshold that separates the St. Anna Trough from the Novaya Zemlya Trough and divides the warm and saline Arctic waters from the cooler and fresher waters of the southwestern part of the Kara Sea. This threshold, whose depth does not exceed 100–150 m, is a barrier that prevents the spreading of the Barents Sea and Arctic waters to the southwestern part of the Kara Sea basin through the St. Anna Trough.

Internal Tides. Global Field of Internal Tides and Mixing Caused by Internal Tides

Different approaches to the study of internal waves in the ocean are analyzed. Generation of internal tides over submarine ridges is considered on the basis of numerical models and measurements in the ocean. Energy fluxes from submarine ridges exceed many times the fluxes from continental slopes because the dominating part of the tidal flow is directed parallel to the coastline. Submarine ridges if normal to the tidal flow form an obstacle that can cause generation of large internal waves. Internal tides are extreme when the depth of the ridge crest is comparatively small with respect to the surrounding depths. Energy fluxes from most submarine ridges were estimated. They account for approximately one fourth of the total energy loss from the barotropic tides. Model estimates were compared with the measurements on moorings at 30 study regions in the oceans. Combined calculations and measurements result in a map of global distribution of internal tide amplitudes. The study is extended to the Arctic region. Extreme internal tides were recorded near the Mascarene Ridge in the Indian Ocean, Mid-Atlantic Ridge in the South Atlantic, Great Meteor bank and the Strait of Gibraltar.

Transport of antarctic waters in the deep channels of the Atlantic Ocean

Starting from 2002, the Shirshov Institute of Oceanology has been conducting investigations of Antarctic water transport in deep channels of the Atlantic Ocean, namely, the Vema Channel (31 ° S), Romanche Fracture Zone (0 ° N), and Vema Fracture Zone (11 ° N). The flow of bottom water in the Vema Channel is estimated as 4 Sv (1 Sv = 10 6 m 3 /s). Velocities in the channel reach 60 cm/s. A strong flow with velocities up to 30 cm/s was recorded in the Vema Fracture Zone. This flow includes the upper part of bottom waters of Antarctic origin and the lower part of North Atlantic Deep waters. The easterly transport of Antarctic waters is 0.1–0.7 Sv. In the Romanche Fracture Zone, maximum velocities reach 10 cm/s, while the entire easterly water transport is estimated as 0.1–0.8 Sv. The dominating propagation of Antarctic waters into the deep basins of the Northeast Atlantic occurs through the Vema Fracture Zone but not through the Romanche Fracture Zone due to strong mixing of deep waters in the latter channel caused by internal tidal waves. Bottom waters are formed at polar latitudes of the World Ocean. In the classic work by Wust [15], all waters of Antarctic origin are called Antarctic Bottom Waters (AABW). They propagate near the bottom in the Atlantic, being formed mainly in the Weddell Sea near the Antarctic slope as a result of mixing of cold and heavy Antarctic Shelf Water with lighter and warmer more saline Circumpolar Deep Waters. The pathways of Antarctic water propagation between the basins of the Atlantic are confined to depressions in the bottom topography. Antarctic Bottom Water from the Weddell Sea propagates through four passages in the South Scotia Ridge and through the South Sandwich Trench. The further propagation of AABW to the north into the Argentine Basin occurs through the Falkland Gap in the Falkland Ridge [14]. Part of this flow propagates along the southern and western margins of the Argentine Basin. The other part is trapped by the Subantarctic Front and flows to the east in the field of the Antarctic Circumpolar Current [14]. The waters of Antarctic origin are later transported to the Brazil Basin along three pathways: in the Vema Channel, in the Hunter Channel, and over the Santos Transport of Antarctic Waters in the Deep Channels of the Atlantic Ocean

Internal waves in a high-latitude region

Internal waves are studied in two high-latitude regions of the Kara Sea. The measurements were carried out with a towed CTD profiler and a distributed temperature sensor on a mooring. The lack of internal waves with an M2 tidal period and the presence of high-frequency waves are demonstrated on the basis of the measurements and numerical modeling.

Flows of Antarctic bottom water through fractures in the southern part of the North Mid-Atlantic Ridge

We study the flows of bottom waters of the Antarctic origin in deep fracture zones of the southern part of the North Mid-Atlantic Ridge. In the autumn of 2014, an expedition onboard the RV Akademik Sergey Vavilov carried out measurements of current velocities and thermohaline properties of bottom water in several quasi-zonal fractures in the southern part of the Northern Mid-Atlantic Ridge, which connect the deep basins of the West and East Atlantic, the Vema Fracture Zone (FZ) (10°50′ N) and a group of sub-equatorial fractures: Doldrums (8°15′ N), Vernadsky (7°40′ N), and a nameless fracture at 7°30′ N. The estimates of bottom water (θ < 2.0°C) transport through this group based on measurements from 2014 are approximately 0.28 Sv (1 Sv = 106 m3/s), which is close to 25% of the transport estimate through the Vema FZ (1.20 Sv) obtained in the same expedition. The coldest bottom water temperatures among the investigated fractures were recorded in the Vema FZ.

Flow of Antarctic Bottom Water at the output of the Vema Channel

The pathways of the coldest part of the Antarctic Bottom Water (AABW) with potential temperature θ < 0.0°C in the Vema Channel and the pathways of this water flowing out of the channel to the Brazil Basin are studied on the basis of the data collected during the Russian expeditions in 2003, 2009–2012, and historical CTD data. It is shown that the AABW flows to the north in the Vema Channel as two streams, one of which is located in the deep channel and the other approximately 300 m higher over the western slope of the channel. It was found that the northern end of the deep channel is not located near 26°40′ S, 34°00′ W as a widening in the northern direction to the Brazil Basin as follows from the dataset of digital topography (Smith and Sandwell, 1997) but continues in the eastern direction. A weaker northerly flow of the cold AABW was also found from the results of the measurements in 2012, which is confined to a branch of the Vema Channel continuation.

Antarctic bottom water flow in the western part of the Romanche Fracture Zone based on the measurements in October of 2011

The properties of the Antarctic Bottom Water flow in the region of its inflow to the channel of the Romanche Fracture Zone at 22°10′–22°30′ W are studied on the basis of CTD and LADCP profiling in the western part of the equatorial fracture zone. A deep water cataract was found at the sill over the southern wall of the fracture with a depth of approximately 4600 m, which is associated with the abyssal flow, whose potential temperature is lower than 1°C. The inflow of water into the channel of the fracture in this temperature range is fully localized over this sill. The minimum potential temperature θ recorded in 2011 near the bottom was equal to 0.51°C, which is lower approximately by 0.12°C than the minimum temperatures ever measured in the western part of the fracture. The water transport in the cataract was estimated at 0.2 Sv (1 Sv = 106 m3/s), which is approximately 30% of the known estimates of the total transport of Antarctic Bottom Water (θ < 1.9°C) through the fracture. The extremely high intensity of the cross isothermal mixing in the cataract region was found. The analysis of the bottom topography data, including the historical WOD09 dataset, shows that the inflow of water with 1.00° < θ < 1.70°C into the channel of the fracture is most likely fully localized in a few passages in the region of the survey in 2011, while the water exchange with the abyssal waters with θ > 1.70°C through the Romanche Fracture Zone between the West and East Atlantic can also occur through the depressions in the southern and northern walls of the fracture in the region of the Vema Deep.

Abyssal cataracts in the Romanche and Chain fracture zones

1211 The Romanche Fracture Zone is a deep (up to 7850 m in the Vema Deep at 18°30′ W), long passage in the equatorial zone of the Mid Atlantic Ridge in the Atlantic. This fracture along with the Chain Fracture Zone located a few degrees to the south (Fig. 1) pro vides the inflow of cold Antarctic waters from the western part of the Atlantic to the eastern basins of the equatorial zone. Then this water spreads to the south into the Angola Basin and, possibly, to the north through the Kane Gap. In the latter case, this water merges with the water transported from the West Atlantic through the Vema Fracture Zone at 11° N and fills the abyssal of the northeastern Atlantic basins. Thus, the Romanche and Chain fracture zones are very important elements in the water exchange of bot tom waters between the West and East Atlantic.

New model and field data on estimates of Antarctic Bottom Water flow through the deep Vema Channel

We used a numerical model of the ocean circulation with a high spatial resolution to obtain estimates of the kinematic characteristics of Antarctic Bottom Water flow through the abyssal Vema Channel in the southwestern part of the Atlantic Ocean. The results of simulations correspond to the data of direct velocity measurements made at several locations in the channel. The high horizontal and vertical resolution of the model in the bottom layer allowed us to study in detail the hydrodynamics of this flow over its entire length.

Measurements of bottom currents in underwater channels of the atlantic during cruise 36 of the R/V Akademik Sergey Vavilov

762 The investigation of the pathways of the Antarctic Bottom Water’s propagation in the abyssal channels of the Atlantic were continued during cruise 36 of the R/V Akademik Sergey Vavilov (October–November, 2012). We carried out measurements of the tempera ture, salinity, and current velocity in the Kane Gap, in the western part of the Romanche Fracture Zone, and in the northern part of the Vema Channel. The Con ductivity–Temperature–Depth profiling (CTD pro filing) was performed using an SBE 19 profiler. The current’s velocity was measured by a lowered acoustic doppler current profiler (LADCP RDI WHS 300 kHz). The cruise started in Rotterdam and ended in Ushuaia. The route of the cruise and the studied regions are shown in the figure. The Vema Channel, Romanche Fracture Zone, and Kane Gap are related to the key abyssal channels through which the Antarctic Bottom Water (AABW) with a potential temperature of θ < 2.0°C is trans ported from the South Atlantic to the northern basins of the Atlantic. The Vema Channel is a long (more than 700 km) and the deepest channel connecting the abyssal depths of the Argentine and Brazil basins. It is a pathway for the coldest part of the AABW (θ < 0.0°C) [2]. The equatorial Romanche Fracture Zone in the Mid Atlantic Ridge (figure) along with the Chain Fracture Zone located two degrees to the south are the main sources of cold bottom waters in the equatorial basins of the East Atlantic and in the Angola Basin [2]. The Vema Fracture Zone, through which the AABW also spreads to the East Atlantic and then flows to the north reaching the West European Basin, is located at 11° N. The flows of the AABW that propagated from the West Atlantic through the Vema and Romanche fracture zones merge in the region of the Kane Gap, which is the deepest bottom depression (approxi mately 4560 m) connecting the Sierra Leone Basin in the south and the Cabo Verde Basin in the north [2]. Unlike the measurements in the southern part of the Vema Channel, the measurements in its northern wide part were not numerous. In the 1990s, German scientists started the works in this region. Russian sci entists from the Shirshov Institute of Oceanology con tinued these investigations in 2003, 2004, 2009, and 2010 [1]. The main goals of the works in 2012 were the measurements of the thermohaline parameters and the assessment of the flow intensity of the coldest part of the AABW, which, as was supposed based on the digital data on the bottom topography [3], should flow out of the deep channel in this region (26°40′ S, 34°00′ W; figure). An echo sounder survey was an important component of the research in 2012. The results of the survey determined the selection of the locations of five CTD/LADCP profiling stations. The measurements revealed significant differences from database [3], which were manifested, first of all, in the principally different orographic forms of the bottom topography. In particular, it was found that the deep channel is directed here to the east northeast. It is not finished with a widening to the Brazil Basin but continues in the quasi zonal direction. The joint analysis of the data measured in 2010 and 2012 showed that the AABW flowing in the deep channel splits in the studied region into two branches. The most intense flow of cold water continues its motion along the channel to the east northeast while the second less intense jet of slightly warmer water flows to the north along a branch of the channel approximately 70 m deep. Both flows displace to the right to submarine elevations. We performed an echo sounder survey in the mid dle part of the Vema Channel near to a threshold (sill) revealed from database [3] with a depth of 4420 m (28°55′ S, 38°20′ W) (figure). These measurements revealed a sill, but its depth was estimated at 4620– 4670 m. The Romanche Fracture Zone (figure) is a deep passage in the Mid Atlantic Ridge in the equatorial Atlantic. Its length is approximately 800 km and its width ranges from 10 to 40 km. Its depth in the western part (0°50′ S, 22°25′ W; 1°10′ S, 22°35′ W) where the Measurements of Bottom Currents in Underwater Channels of the Atlantic during Cruise 36 of the R/V Akademik Sergey Vavilov