ISSN 1068-3739, Russian Meteorology and Hydrology, 2022, Vol. 47, No. 11, pp. 882–895. Ó Allerton Press, Inc., 2022.
Russian Text Ó The Author(s), 2022, published in Meteorologiya i Gidrologiya, 2022, No. 11, pp. 91–110.
Spatial Patterns of Dense Water Runoff
on the Antarctic Shelf and Continental Slope
P. N. Golovina*, N. N. Antipova, A. V. Klepikova, M. S. Molchanova,
S. V. Kashina, and I. A. Chistyakova
a
Arctic and Antarctic Research Institute, ul. Beringa 38, St. Petersburg, 199397 Russia
* e-mail: golovin@aari.ru
Received March 23, 2022
Revised May 26, 2022
Accepted June 24, 2022
Abstract—Observational data from the submesoscale hydrological polygon confirm the presence of
summer runoff of dense Antarctic shelf water (ASW) on the continental slope in the Commonwealth
Sea. It is provided by a huge amount of ASW accumulated in shelf depressions during winter. The ASW
overflows the shelf edge in the form of discrete meanders, which are formed as a result of baroclinic instability of a deep (~ 150–250 m) ASW front. The estimated instability scale along the dense water front
RdL » 1.5–3.2 km coincides with the observed spatial discreteness of ASW. At the same time, the distribution of near-bottom density currents affected by the bottom irregularities is noted on the shelf. The
Antarctic Slope Front (ASF) is formed on the slope due to the interaction of ASW with warm Circumpolar Deep Water (CDW). Due to the continuity of motion in the near-slope area, 3D cascading is accompanied by compensatory upwelling of CDW, which complements its large-scale advection. These
processes form the intrusive structure of water on the edge of the shelf and on the shelf. Depending on
the bathymetric characteristics of the slope in different parts of the polygon, the cascading of ASW has a
different regime.
DOI: 10.3103/S1068373922110085
Keywords: Antarctic shelf, continental slope, dense water runoff (cascading), submesoscale hydrological polygon, thermohaline and density water structure, density currents, baroclinic instability, intrusions, eddies, the Prydz Bay
1. INTRODUCTION
The formation of dense Antarctic shelf water (ASW) on the Antarctic shelfs occurs in winter [1, 11–15,
17, 18]. The most intensive formation of ASW takes place in the areas of breakup of near-barrier, flaw, and
coastal polynyas [10, 16, 20–22]. Filling shelf depressions, ASW overflows the shelf edge and flows down
the continental slope, like, for example, in the Commonwealth Sea [2–4, 8, 9, 20, 23]. The strongest source
of the ASW flow down the slope, the cascading, takes place in winter, during the ASW formation. However, a huge mass of ASW accumulated during the cold season in shelf depressions of the Antarctic seas
[10] also continue to flow down in some areas of the continental slope in summer. Such conclusion based
on analyzing summer oceanographic observations was made for the area of the continental slope in the
Commonwealth Sea [3, 8, 9].
All modern Russian field observations (2004–2016) in the Commonwealth Sea focused on studying the
ASW cascading were performed in the framework of seasonal summer activities of the Russian Antarctic
Expedition in the form of hydrological sections across the shelf and the slope. Most often only one section
was provided. When several sections were made, they were located at a significance distance from each
other: ~24–25 km [3, 8, 9]. At the same time, the distance between the measurement stations on the sections
was small: in the area of the shelf edge and the slope, it varied from 5.1 to 1.8 km and has not exceeded
1.8 km (sometimes 1 km) in the recent years (2011–2016). Thus, the submesoscale (~2–5 km) spatial resolution or even the eddy resolution (~0.5–2 km) was achieved in the section plane, which allowed analyzing
the stability of the Antarctic Slope Front (ASF) and investigating the formation of intrusions and eddy
lenses on the shelf edge and the slope [5, 6, 8].
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Such hydrological sections for the shelf edge and the slope contain a lot of information about the dense
water runoff patterns. Nevertheless, some features of the cascading can only be discussed in a hypothetical
form. The consideration of a single section or several sections located at a significant distance from each
other limits the possibilities of their interpretation. This is caused by the fact that all hydrophysical processes in the slope area are three-dimensional (3D), and the detailed but two-dimensional pattern can be retrieved in the section plane across the slope. Based on analyzing such field observations, it is just possible
to make assumptions about the form and scale of density currents on the shelf, about the patterns of the
dense water overflow, about the possibility of the instability occurrence during the ASW cascading, about
the effects of local bottom inhomogeneities on the slope on the cascading [4–9].
More correct (3D) description of the dense water runoff requires setting the target hydrological polygon
with the distances between the measurement stations not more than the local baroclinic Rossby deformation
radius, whose value in the ASW area was ~1.5–3 km [6]. In 2016, such submesoscale hydrological polygon
was performed on the shelf and the continental slope of the Commonwealth Sea. A part of the sections from
this polygon were united into a small-scale polygon, in which the distances between the sections and between the measurement stations on the sections are close in the value and are equal to »1.8–2 km.
The setting of the hydrological polygon is described, the measurement data are analyzed, the refined
bottom topography in the polygon area is investigated, the spatial (3D) patterns of dense water runoff and
the effects of submesoscale and local bottom inhomogeneities on the cascading are described, the cascading regime in different parts of the polygon is analyzed. The analysis of unique data of polygon field measurements in the shelf–slope area in the present paper allows studying the thermohaline and density structure of water in this area at a new qualitative level, which most reliably reflects ongoing hydrophysical processes. The results are important for verifying model studies of these processes.
2. DESCRIPTION OF FIELD OBSERVATION DATA
The submesoscale hydrological polygon between the meridians of 70° and 71° E consisting of nine
meridional sections across the slope was performed in summer (January 2016) in the area of the shelf and
the continental slope in the Commonwealth Sea (Fig. 1a). Going round the polygon was carried out by sections. Oceanographic studies were performed in the framework of seasonal activities of the 61th Russian
Antarctic Expedition on board the Akademik Fedorov research vessel. Figure 1b presents a more detailed
view of the polygon against a background of refined bottom terrain. In the eastern part of the polygon, the
distance between four sections along the circle of latitude is »10¢ (»4 km). In the western part of the polygon, the distance between five sections along the circle of latitude is »5¢ (»2 km). The distance between the
measurement stations on all meridional sections on the shelf edge and the slope is DX » 1.8 km. Therefore,
some sections and some stations on the sections can be united into the small-scale polygon, at which the
distance between the stations on the section and between the sections is close and equal to ~1.8–2 km
(Fig. 1b). The entire polygon (105 stations, 9 sections) was completed in very short time (»5 days). The
time of performing observations in the area of the shelf edge and the slope on each section was very small
and did not exceed 6–10 hours. Measurements of temperature and conductivity (salinity) at the stations were
carried out with the SeaBird SBE 911+ standard CTD with a vertical resolution not more than 1 m. Such
vertical discreteness of measurements allows studying a fine thermohaline structure, in particular, in the
bottom Ekman boundary layer (~20–30 m).
The construction of the refined bottom terrain on the shelf and the slope (Fig. 1b) is based on the detailed measurements of the ocean depth with a shipboard sounder when performing the sections on the
polygon. Up to 1000 sounder measurements bound to the exact coordinates were used. This allowed refining some details of bottom terrain on the shelf (Fig. 1b), although it is certainly difficult to reliably judge
about the small-scale bottom topography details between the sections.
The shelf in the polygon area is much vaster (Fig. 1) and deeper (~400–500 m) than the continental
slope, which is a distinctive feature of the bottom topography of the Antarctic seas [10]. When considering
the refined terrain at the polygon, larger (mesoscale) details of the bottom topography on the shelf attract attention. For example, in the western part of the polygon, there are quite extensive elevations: the banks with
depths below 340 m and even below 300 m (Fig. 1b). In view of this, the bottom slope on the shelf in this
part of the polygon reached significant values for shelf areas: s » Dh /Dl » 0.007–0.009. Here, Dh is the
depth difference between isobaths, Dl is the horizontal distance between isobaths along the normal. In the
eastern part of the polygon (closer to the Prydz Bay), the shelf is relatively smooth (table-like) with gentle
shallow depressions, its depth exceeds 460 m (Fig. 1b). Significant differences in bottom topography in different parts of the polygon lead to the fact that the depth difference along the shelf reaches 130–180 m in
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Fig. 1. (a) The scheme of the submesoscale hydrological polygon performed in January 2016 and (b) its zoomed variant
against a background of refined bottom terrain: the thin black lines are isobaths (m); the black rectangle is the small-scale
hydrological polygon distinguished in the submesoscale polygon with identical discreteness of measurements on the sections
and the spatial distance between the sections of ~2 km; the red lines are the sections along the shelf (I), shelf edge (II), upper
(III), middle (IV), and deep (V) parts of the continental slope.
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the polygon area. This means that the significant bottom slope s » 0.007–0.010 is formed in the eastern direction, along the shelf, which is comparable with the bottom slope across the shelf in the western part of
the polygon. In the western direction, the local bottom slope along the shelf sometimes reaches s » 0.06,
which is closer to the bottom slope on the continental slope (see below).
The bottom slope on the continental slope in the western part of the polygon also significantly differs
from its eastern part. In the east of the polygon, the slope is steep: the average value of s » 0.12–0.15. In the
east of the polygon, the slope is much gentler: the average s » 0.08–0.09. In some local areas in the steepest
part of the slope in the west of the polygon, the slope reaches very high values: s » 0.16–0.20 (Fig. 1b). An
important distinction is that the slope is smoother in the eastern part of the polygon, while it is rugged in the
western part. On the refined map in the western part of the polygon, there are small-scale bottom terrain details on the slope: a trough and a ridge (Fig. 1b). The configuration of the local elevation is refined, which
was detected before at the depths more than 1400 m on the section along 70° E [8]. Now, it is possible to say
that this is not a single elevation, but the ridge of rocks with a height up to 100 m, in front of which the
trough with a width (across the slope) of ~2 km directed to the east for a distance of at least 2 km (from the
section along 70°00¢ E to the section along 70°05¢ E) is situated (Fig. 2a). Then this trough sinks to the depth
of ~1500 m and merges with the trough directed along the slope along 70°10¢ E (Fig. 1b). Such sharp local
bottom terrain inhomogeneities are located in the zone where the slope turns into an abyssal. The refined
mesoscale and local (small-scale) terrain details play an important, sometimes key role in the propagation
and stability of density currents on the shelf and the continental slope of the Commonwealth Sea.
3. ANALYSIS OF DENSE WATER RUNOFF ON THE SHELF
AND THE SLOPE
3.1. The Shelf
For all sections of potential temperature q in the area of the submesoscale hydrological polygon on the
shelf and the continental slope, there are various forms of runoff or traces of runoff of cold ASW (Fig. 2),
although the polygon was executed in summer, when there is no mechanism of the ASW formation (the
convection induced by the ice formation). On the same sections of q for the correct analysis of processes in
the bottom layer, individual isopycnics are presented for the field of potential density relative to the deep part
of the slope: s 1500 = r 1500 (T = q 1500 ; S ; p = p1500) – 1000, which is the reference pressure level p = 1500 dbar
(Fig. 2). Figure 2 presents the example of the composite sections of q and s 1500 along 70°00¢ E and 70°15¢ E,
the full figure is provided in the supplementary materials at the website http:/link.springer.com.
The distributions of q and s 1500 in the bottom horizon were mapped for the reliable determination of the
ASW movement direction along the shelf and the slope (Fig. 3). These maps can also be used to analyze the
scales of the spatial variability of the shelf and slope cascading. The fully 3D representation of the features
and characteristics of the cascading in different parts of the shelf and the slope is complemented with longitudinal sections of q constructed along the shelf, the shelf edge, the upper, middle, and deep parts of the
slope (Fig. 4). Longitudinal sections are shown with lines I–V on the refined topographic map of sea bottom on the polygon (Fig. 1b). The description of propagation and the reliable analysis of the stability of
density currents in the bottom layer on the shelf and the slope are possible only in case of the joint consideration of longitudinal sections of q and the similar sections for conventional local potential density sz. They
correspond to the range of depths of each section of q. For example, this is s400 (relative to the reference
pressure level p = 400 dbar) for the shelf and the shelf edge, s650 for the upper part of the slope, s1000 for the
middle part of the slope, s1500 for the deep part of the slope (Fig. 5). The numerical characteristics of bottom
density currents on all sections along the slope and the shelf, namely, the values of q and s1500 in the bottom
horizon, the thickness of the bottom layer of cold and dense water (with q < –0.8°C) are presented in Fig. 6.
The analysis of the transversal composite sections of q and s1500 (Fig. 2) and the longitudinal individual
sections of q and sz (Figs. 4 and 5) allow stating that the ASW cascading as a process also occurs on the
shelf and the slope in summer. The feasibility of the summer cascading is provided by the presence of huge
masses of dense water accumulated on the shelf during the winter that recharge it, especially in the eastern
part of the polygon (Figs. 2f, 2g, 2h, and 2i in the supplementary materials, Figs. 3, 4a, and 5a). In this part
of the polygon, the shelf is the deepest (400–460 m), table-like one, or the one with the presence of
depressions (Figs. 1b and 3a). Here, the coldest (q < –1.8°C) and most dense (s1500 > 34.96) ASW is
observed (Fig. 3), its thickness reaches 150–250 m (along the isotherm of q » –0.8°C) (Figs. 2f, 2g, 2h, and
2i in the supplementary materials, Figs. 4a, 5a, and 6b). The most intense formation of dense ASW on the
shelf in winter (when the convection reaches the bottom) takes place in the areas of the regular breakup of
the Mackenzie near-barrier polynya near the Amery Ice Shelf in the Prydz Bay, as well as in the areas of the
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Fig. 2. The composite sections of potential temperature q (for p = pa) and conventional potential density s1500 (relative to the
reference level p = 1500 dbar) directed across the shelf and the slope: (a) along 70°00¢ E; (b) along 70°15¢ E. On the sections of
s1500, isopycnics are drawn only in the bottom layer starting from s1500 = 34.86. The black dash line is the refined bottom terrain based on the detailed bathymetric survey along the sections (up to 1000 measurements per section with a shipboard
sounder with precise coordinate referencing). The inset presents the local profiles of potential temperature q(z)5, salinity S(z)5
(smoothed by moving averaging over five values), and local conventional potential density sz (z)9 (smoothed by moving averaging over nine values) relative to the reference level p = 650 dbar for the upper part of the slope and relative to p = 1000 dbar
for its middle part. Here and below in Figs. 4 and 5, the station numbers are given above on the horizontal axis.
breakup of the Darnley coastal polynya (near Cape Darnley) (Fig. 1a) [2, 3, 9, 20, 23]. Moreover, the eastern
part of the polygon is closer to the Amery Ice Shelf (Fig. 1a), which is one of the possible ways (sources)
for the ASW cascading recharge on the shelf in the polygon area not only in winter but also in summer
[23].
Thus, the deep ASW front is formed in the eastern and central parts of the polygon on the shelf in winter
and is also preserved (observed) in summer. Moving westward along the shelf (toward the mean motion in
the Southern Hemisphere), ASW fills in shelf depressions and moves on the shelf elevation (Figs. 3, 4a, and
5a), which evidently impedes this motion (since the thickness of cold ASW in front of the elevation dramatically increases) (Figs. 4a and 5a) and directs the bottom density current to the shelf edge (Fig. 3). This
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Fig. 3. The distribution of (a) potential temperature q and (b) conventional potential density s1500 (relative to the reference
level p = 1500 dbar) in the bottom horizon of the submesoscale (and small-scale) polygon on the shelf and the continental
slope.
means that the shelf bottom topography features do not influence the propagation of density flows on the
shelf. This influence becomes the most complete when the thickness of the bottom density current is
smaller than the height of mesoscale topographic bottom inhomogeneities or is close to it.
In the shallower (~300–350 m) and steeper zone of the shelf in the west of the polygon (Fig. 1b), the
shelf water is less dense (s1500 » 34.90–34.93) (Fig. 3b), its thickness (along the isotherm of q » –1.7°C) is
rather small (»70–80 m) (Fig. 2). However, in some areas of the shelf (Fig. 2a) and in local shelf depressions (Fig. 2b) in the west of the polygon, the ASW thickness reaches 150 m.
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Fig. 4. The longitudinal sections of potential
temperature q in the bottom layer in the area of
the submesoscale polygon: (a, I) along the shelf;
(b, II) along the shelf edge; (c, III) along the upper part of the slope; (d, IV) along the middle part
of the slope; (e, V) along the deep part of the
slope. The scheme of sections I–V is presented in
Fig. 1b.
Fig. 5. The longitudinal sections of local conventional potential density sz in the bottom layer in
the area of the submesoscale polygon corresponding to the depth range of each section of potential temperature q (see Fig. 4): (a, I) along the
shelf, s400; (b, II) along the shelf edge, s400; (c,
III) along the upper part of the slope, s650; (d,
IV) along the middle part of the slope, s1000; (e,
V) along the deep part of the slope, s1500. The
scheme of sections I–V is presented in Fig. 1b.
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Fig. 6. The characteristics of bottom density currents on all sections along the shelf (I), shelf edge (II), upper (III), middle
(IV), and deep (V) parts of the continental slope in the area of the submesoscale poly gon: (a) potential temperature q in the
bottom horizon; (b) the thickness of the bottom layer of cold and dense water; (c) conventional potential density in the bottom
horizon s1500 (relative to the reference level p = 1500 dbar).
3.2. The Edge of the Shelf
The ASW overflows the edge of the shelf in summer not on all sections of the polygon (Fig. 2 in the
supplementary materials, Figs. 4b, 5b, and 6b), and the overflow intensity (the shelf cascading intensity),
namely, the thickness of the layer of dense and cold (q < –0.8°C) ASW on the shelf edge is different (Figs. 4b,
5b, and 6). The analysis of the sections of q and s z along the shelf and the edge of the shelf (Figs. 4a, 4b,
5a, and 5b) revealed that originally the continuous ASW front on the shelf approaches the shelf edge
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already as discrete frontal meanders. Probably, this is associated with the baroclinic instability of the ASW
front on the shelf, which confirms the previous assumptions [4, 9].
The spatial scale of instability of the baroclinic and thermocline ASW front (along the front), namely,
the local baroclinic Rossby deformation radius was evaluated: Rd L » ( g ¢x H sw ) 0. 5 / f . Here, Hsw » 100–
200 m is the thickness of the continuous front of dense ASW on the shelf (see Figs. 4a, 5a, and 6b);
g ¢x = gDr x /r is the buoyancy scale, and Dr x » 0.06–0.1 kg/m3 is the observed local horizontal density
difference on the shelf edge (Figs. 5b and 6c); f » 1.4 ´ 10–4 s–1 is the Coriolis parameter; g » 9.8 m/s2 is the
acceleration of gravity. The estimate of the value along the frontal instability scale is RdL » 1.5–3.2 km,
which coincides with the observed spatial discreteness of dense water runoff on the shelf edge (Figs. 4b, 5b,
6a, and 6b). It may be stated that as a result of the baroclinic instability of the ASW front, ASW overflows
the shelf edge in the form of discrete frontal meanders, the distance between which is ~ RdL. Thus, on the
one hand, the mean motion (the steady-state propagation of dense water in the bottom layer on the shelf
during the winter) is adapted to the mesoscale bottom inhomogeneities on the shelf that can favor the ASW
overflow in the certain areas of the shelf edge. On the other hand, if the bottom layer (front) of dense ASW
is very thick, which is actually observed even in summer (Figs. 4a and 5a), it is obvious that the baroclinic
instability formed on the front splits it into discrete frontal meanders (Figs. 4 and 5).
The flowing of cold ASW over the shelf edge and down the slope in interaction with the warm (q > 0°C)
and salter circumpolar deep water (CDW) forms the Antarctic Slope Front (ASF) (the difference in q on the
front reaches 1.2–2.2°C) [5]. This means that the cascading is among the major processes that initiate an
intense circulation in the area of the shelf edge and the continental slope [4, 9]. Due to continuity of the
motion in the near-slope region, the cascading is accompanied by the compensatory upwelling of CDW
along the inclined ASF isopycnics (Fig. 2 in the supplementary materials). Thus, the ASW cascading and
CDW upwelling are more often submesoscale (~3–5 km) (or even small-scale (~2 km) due to the observed
spatial discreteness of runoff) structure-forming processes that form the intrusive structure of water on the
shelf edge and the shelf. Then CDW penetrates to the shelf already in a modified state (MCDW) in the form
of relatively large warm intrusions or intrusion eddies [5, 9].
It should be explained here that the main reason for the appearance of warm water far in the southern
Antarctic in the area of the continental slope is still the large-scale oceanic advection of warm CDW to
these areas. However, the observed spatial intrusive structure of ASW (Figs. 4a and 5a) is formed under the
influence of the compensatory upwelling of MCDW induced by the discrete cascading. Its scales are much
smaller than the large-scale advective CDW flows. Thus, the compensatory upwelling being an analog of
horizontal advection complements the large-scale CDW advection, locally intensifying the MCDW
transport to the shelf. All sections of q and s1500 across the shelf and the slope (Fig. 2 in the supplementary
materials) and the longitudinal sections of q and s z on the shelf in the polygon area (Figs. 4a and 5a)
confirm this fact. The thickness of warm intrusions of MCDW on the shelf sometimes reaches 100–150 m
(Fig. 2 in the supplementary materials, Figs. 4a and 5a). However, the distance of penetration of MCDW to
the shelf and its intensity (the value of inversions of q in warm intrusions) on different sections and in
different parts of the polygon differ and depend on local cascading conditions.
3.3. The Continental Slope
In the eastern part of the polygon, the ASW slope cascading is weak. For example, the observed
thickness of the layer of dense water flowing over the shelf edge (with the temperature q < –0.8°C) does
not exceed 30–40 m, although the layer (front) of cold and dense ASW on the shelf sometimes reaches a
thickness of >200 m (Figs. 2g, 2h, and 2i in the supplementary materials, Figs. 4a, 4b, 5a, 5b, and 6b). One
of the main reasons for such weak summer cascading is the small bottom slope on the shelf and the presence
of depressions (Figs. 1b and 2), which hampers the runoff of dense water. In the upper and middle parts of
the slope, ASW propagates in the form of meanders or plumes (Figs. 2g, 2h, and 2i in the supplementary
materials), whose width is close to the value of RdL (~2–4 km) (Fig. 3). Their thickness is small and equal to
~20–40 m, which coincides with the thickness of the bottom Ekman boundary layer (Figs. 4c, 4d, 5c, 5d,
and 6b). In the lower horizons (in the deep part of the slope), there are thin discrete plumes of cold and
dense but transformed water of shelf origin (q > –0.8°C) (Figs. 2g, 2h, and 2i in the supplementary
materials, Figs. 4e, 5e, and 6a). Thus, it may be stated that there is the colder and denser water on the slope.
However, the question arises how it moves along the slope.
When baroclinity is absent on the slope, but there is an undisturbed layer of cold water within the bottom
Ekman layer, it may be supposed that not the cascading itself (the process of dense water runoff) but its
result, namely, the “traces” of the previous cascading is observed. There may be no runoff since the density
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of the cold water on the slope coincides with the one of surrounding water. Such situation is observed in
summer on the slopes of many Antarctic seas, where the winter cascading is registered [10]. In the present
case, on the eastern sections of the polygon, there are discrete meanders and plumes of cold but slightly
denser (heavier) water than surrounding slope water (Figs. 2e, 2f, 2g, 2h, and 2i in the supplementary
materials). Therefore, the authors assume that the summer cascading in the eastern part of the polygon exists
in an undisturbed (quasigeostrophic) mode. The stable (in the absence of inversions of local potential
density on the profiles of s z in the bottom layer) propagation of ASW along a rather smooth slope (Fig. 1b)
occurs mainly along isobaths in the western direction (in the direction of the mean motion) (Fig. 3) within
the bottom Ekman boundary layer (Fig. 2 in the supplementary materials). The longitudinal sections of q
and s z also clearly demonstrate the westward displacement of the ASW plumes along the slope for a
distance above 10 km in the process of runoff from the shelf edge to the deep part of the slope (Figs. 4c, 4d,
4e, 5c, 5d, and 5e). This means that the distances passed by the plume along the slope are 3–5 times larger
than the distance down the slope.
In the western part of the polygon, the summer ASW cascading is more noticeable (significant). The
thickness of ASW flowing down in the area of the shelf edge and in the upper part of the slope can reach
>80–100 m, for example, on the section along 70° E (Figs. 2a, 4b, 4c, 5b, 5c, and 6b). Here, runoff occurs in
the form of a meander that is not observed further down the slope in the section plane, since it moves
mainly in the western direction beyond the polygon (Fig. 2a). A comparatively high intensity of the summer cascading in the west of the polygon as compared to the east is evidently associated with a significant
shelf slope s » 0.007–0.009 (Fig. 1b), which facilitates the dense water runoff along the shelf and then along
the slope.
On the nearest section along 70°05¢ E (»2 km to the east), the ASW overflow is less pronounced (Fig. 2b
in the supplementary materials, Figs. 4b, 5b, and 6b). On the next section along 70°10¢ E, the ASW runoff in
the shelf edge area is almost not identified based on temperature and density (q > –0.3°C) (Figs. 2c, 4b,
5b, and 6), and dense and cold water is observed as discrete plumes in the upper and middle parts of the
slope (Fig. 2c in the supplementary materials). The thickness of the upper plume is small and does not
exceed 50 m, and the deep cold plume (~900–1100 m) is very thick, its thickness reaches ~200 m (along the
isotherm q » –0.8°C). During its propagation along the slope, the instability features are observed. They
are expressed as thermohaline intrusions on the vertical profiles q(z)5 and S(z)5 smoothed by moving
averaging over five values. They correspond to the inversions of local potential density on the profiles
s1000(z)9 (the reference pressure level p = 1000 dbar coincides with the mean depth of the detected plume)
smoothed by moving averaging over nine values.
The instability is found both in the bottom horizons and at the top of the plume (at the height of >200 m
from the bottom) as well as at the neighboring, deeper station of the section (at a distance of ~1.8 km) at a
significant height (~200–300 m from the bottom). Here, the thick cold intrusion with a thickness of ~100 m
is observed, which is accompanied by density inversions. Its occurrence may be explained only assuming
that the cold water moves from the area of the shallower slope resulting from the instability of the thick
ASW plume or meander (with a thickness of ~200 m) flowing along the slope. The observed intrusive
layering may result from the local baroclinic instability on the boundary of the density current (plume or
meander) accompanied by the hydrostatic instability (the inversions on the profiles s1000(z)9), which is
formed as a result of the local mixing of water with different thermohaline and density characteristics [5, 7].
The horizontal size of the cold intrusion (the displacement off the slope) or the transfrontal scale of the ASF
instability is »1.5–2 km (Fig. 2c in the supplementary materials), which coincides with the previously
estimated RdL for the ASF [6, 7]. The density inversions indicating the local hydrostatic instability very
likely exist, since their thickness reaches ~10–40 m. This means that their identification is provided by a
great number of measurements, when inversions do not disappear on the profiles s1000(z)9 even in case of
smoothing over nine values.
It should be noted that warmer water is situated further up the slope from the thick deep cold plume (in
the plane of the section along 70°10¢ E). Therefore, its occurrence at such depths may be explained only assuming that the intense runoff of cold and dense water occurs nearby, since it sank to a large depth.
Indeed, »2 km to the east, on the section along 70°15¢ E, there is an intense shelf and slope cascading of
ASW (Fig. 2b). On the shelf, this section crosses the elevation behind which a local depression with a depth
of ~50 m filled with cold and dense ASW is located closer to the shore (Figs. 1b and 2b). As noted above,
such features of bottom topography on the shelf (elevation) favor the ASW runoff exactly in the area of this
section, when its density current flows to the shelf edge (Figs. 3, 4a, 4b, 5a, and 5b). At the same time, very
complex bottom topography, namely, an increase in the shelf slope in the elevation area and a local increase
in the ASW front thickness in front of the elevation may facilitate the occurrence of local baroclinic instaRUSSIAN METEOROLOGY AND HYDROLOGY
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bility of the ASW front already on the shelf. The similar pattern has been repeatedly observed on the shelf
edge with a significant increase in the bottom slope [5, 6, 9].
On this section, ASW overflows the shelf edge in the form of the thick layer with a thickness of ~70 m
and flows down the slope as a meander. During the sinking, a dramatic increase in the ASW layer thickness
is periodically observed: >250 m in the upper part of the slope and >160 m in the middle part (Figs. 4c, 4d,
5c, 5d, and 6b). Probably, a reason for the occurrence of such local thickening of the bottom density current
is an increase in the resistance and, hence, in the turbulent entrainment at its top with the accelerated movement along the steep slope after the overflow. At the same time, there is a local increase in the horizontal
density gradient, which leads to an increase in the velocity shear at the density interface. The local
baroclinic instability occurs in the ASF area, it is accompanied by the local hydrostatic instability and the
subsequent intrusive layering (Fig. 2b) [5, 7, 9].
The generated cold intrusions propagate beyond the bottom density current (beyond the ASF) and are
registered at a distance of ~2–3 km from the slope in deeper parts of the slope in the horizons located at
significant heights from the bottom (up to 300–400 m) (Figs. 2b, 4d, and 5d). This horizontal scale of intrusions perpendicular to the slope characterized the transfrontal scale of the ASF instability. In fact, the
moment of separation of the cold intrusion from the ASF, the density of which is still greater than that of
surrounding water, is recorded on the section along 70°15¢ E. It may be characterized as the moment of realization of deep water ventilation as a result of local baroclinic instability of the bottom density current
(Figs. 4d and 5d). The similar cases of instability and intrusive layering in the ASF area that are among the
main mechanisms of the deep water ventilation, have been studied before not only on the continental slope
but also on the shelf edge [5–7, 9].
The above ASW meander runoff in the plane of the section along 70°15¢ E is mostly directed down the
slope (Figs. 2b, 3, 4b, 4c, 4d, 5b, 5c, 5d, 6a, and 6b). Only some elements of this meander are observed on
the neighboring section along 70°10¢ E in the direction of the mean flow (Figs. 2 and 3), which is also a
feature of its variability. On these sections, the bottom density current is ageostrophic (Fig. 2 in the
supplementary materials). The horizontal instability scale corresponding to the spatial scale of variability
of q and s1500 in the bottom horizon of the small-scale polygon can be estimated at ~2 km (Figs. 7a and 7c).
Its value is consistent with the estimated RdL for the ASF and the estimated horizontal scale of intrusive
layering in the bottom layer on the shelf edge and the slope obtained before [5, 6, 9]. Then, the directions of
currents that can be determined proceeding from the analysis of q and s1500 fields in the bottom horizon and
in the horizon situated 20 m over it, are opposite (Fig. 7), which confirms once more the fact of instability
of the density current (meander) flowing down the slope. The height of 20 m between the horizons
determines the vertical scale of baroclinic instability, which is close to the previously obtained
characteristic vertical scale of intrusive layering in the bottom layer equal to ~20–40 m [5].
The observed predominant propagation of ASW down the slope in the form of the meander (Fig. 2b) is
associated not only with the intense cascading, which is the intensive flow of dense water over the shelf
edge, but also with the small-scale topographic inhomogeneity of the slope, namely, with the trough that is
observed on the refined bottom terrain map (Fig. 1b). It seems to favor the “organization” of the ASW runoff, partly “capturing” and directing the density current down the slope, especially when its thickness is
comparable with the depth of the trough or is smaller (Figs. 2b and 3). At the same time, on a relatively
smooth slope in the east of the polygon, there are almost no obstacles for the predominant movement of
dense water along isobaths. Such analysis of the effect of local topographic bottom inhomogeneities on the
slope on the regime of the bottom density current propagation has also been performed before [7, 8, 10].
Already at a distance of »2 km east of the flowing thick ASW meander (on the section along 70°20¢ E),
the cascading is weak (Fig. 2 in the supplementary materials, Figs. 4b, 5b, and 6b). On the section along
70°30¢ E, there is no ASW runoff on the shelf edge and in the upper part of the slope (Figs. 4b, 5b, and 6b).
Here, the reverse (compensatory) inflow (upwelling) of warm CDW (q > 0°C) is observed (Figs. 4b, 4c, 5b,
5c, 6a, and 6c). It may be supposed that CDW perhaps reaches the upper part of the slope, if considering the
refined bottom terrain on the section, and propagates (in the form of MCDW) far to the shelf over the cold
and dense ASW (Fig. 2). There is an obvious 3D pattern of the discrete cascading of ASW and the
compensatory upwelling of CDW generated by it on the slope (MCDW on the shelf). The latter is formed
not only higher than the denser and colder ASW meander (Fig. 2b), but also nearby on the slope and the
shelf edge (Figs. 2, 4b, 4c, 5b, 5c, 6a, and 6c). In the middle and deep parts of the slope on the sections
along 70°20¢ E and 70°30¢ E, there are the thin plumes of cold and dense water (Fig. 2 in the supplementary
materials) flowing from the eastern areas of the slope (Figs. 3, 4d, 4e, 5d, and 5e). Propagating to the west
along the slope, these plumes merge in its deep part with the meander sinking in the plane of the section
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Fig. 7. The distribution of (a, b) potential temperature q and (c, d) conventional potential density s1500 (relative to the reference level p = 1500 dbar) in (a, c) the bottom horizon, as well as (b, d) in the horizon 20 m above the bottom horizon in the area
of the small-scale polygon (included into the submesoscale polygon, see Fig. 1b). The arrows show the directions of possible
local horizontal currents.
along 70°15¢ E, which, at the depth of ~1500 m, wedges out to a thickness of ~20–30 m, which is close to
the thickness of the Ekman boundary layer (Figs. 2b, 3, 4e, 5e, and 6).
In the west of the polygon on the sections along 70°00¢ E and 70°05¢ E, there is the high small-scale
ruggedness of the bottom in the deep part of the slope (below 1400 m) (Fig. 1b), which affects the regime of
dense water runoff. If the thickness of the bottom density current is small, it can be completely or partly
captured by the trough directed along the slope (Figs. 1b and 2). If the thickness of the density current exceeds the height of underwater rocks limiting the trough, the local hydrodynamic instability of this current
may occur over them. As a result of the intrusive layering, eddy lenses can be generated over such local
bottom inhomogeneities on the slope (Fig. 2a), which have been analyzed in the earlier studies [8]. They
are independent elements in the system of deep water ventilation on the continental slopes of the Antarctic
seas [7].
4. DISCUSSION OF RESULTS
The analysis of field measurements on the submesoscale and small-scale polygons confirmed the presence of the summer ASW runoff in some areas of the shelf and the continental slope of the Commonwealth
Sea. The continuation of winter cascading in summer is provided by a huge amount of ASW accumulated
in shelf depressions during the winter. The ASW overflows the shelf edge as discrete meanders, which are
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formed as a result of baroclinic instability of the thick (~150–250 m) ASW front on the shelf. The estimates
obtained along the frontal instability scale (the local baroclinic Rossby deformation radius) are RdL »
1.5–3.2 km, which coincides with the observed spatial discreteness of the cascading.
Simultaneously, there is the effect of bottom topography features on the propagation of bottom density
currents. It becomes the most complete when the thickness of the current is smaller than the height of
mesoscale topographic inhomogeneities of the bottom or is close to it.
The ASF is formed as a result of the slope cascading of cold ASW in interaction with warm CDW. The
intensity of the near-slope circulation increases. There is the 3D pattern, when due to the continuity of motion in the near-slope area, the cascading is accompanied by the compensatory CDW upwelling, which is an
analog of the large-scale horizontal advection of CDW. The ASW cascading and CDW upwelling form the
intrusive structure of water on the shelf and the slope.
Different forms and regimes of the cascading are observed in different parts of the polygon depending
on the bathymetric structure of the slope. In the eastern part of the polygon on a relatively smooth slope,
there is an undisturbed (quasigeostrophic) regime of ASW runoff in the form of discrete meanders or
plumes with a width close to the value of RdL (~2–4 km), and the thickness is close to the value of the bottom Ekman boundary layer (~20–40 m). The runoff occurs mainly along isobaths in the direction of the
mean motion.
In the western part of the polygon, there is a much more intense ASW overflow, which is favored by the
great shelf slope, shelf topography features, and ASW front instability already on the shelf. Therefore,
runoff more often occurs in the form of discrete meanders propagating mainly down the slope, which is facilitated by the steep slope. Here, the cascading is ageostrophic (unstable). At the top of the bottom density
current, the intrusive layering of water with different thermohaline and density characteristics results from
the baroclinic instability accompanied by the hydrostatic instability. The horizontal size of cold intrusions
(in the direction from the slope) equal to 1.5–2 km corresponds to the spatial scale of variability of q and
s1500 in the bottom horizon (~2 km). It also matches with the estimated transfrontal scale of the ASF instability and with the estimated RdL (along the frontal instability scale) for the ASF from the earlier studies.
The vertical scale of baroclinic instability is ~20–40 m, it coincides with the estimated characteristic vertical scale of intrusive layering. Some sections directly presented the moment of separation of cold intrusions
from the slope, which may be characterized as the moment of realization of the deep water ventilation
mechanism resulting from the local baroclinic instability of the ASF.
One of the reasons for the predominant propagation of meanders down the slope is the effect of smallscale bottom inhomogeneities, which “organize” runoff, partly or completely capture and direct the density
current down the slope.
In the middle and deep parts of the slope, bottom inhomogeneities can completely change the runoff
regime depending on the ratio of their vertical size and the thickness of the bottom density current. Hydraulic jumps leading to the intense turbulent mixing, which is one of the main mechanisms for the deep and
bottom water ventilation, and lee waves can be generated. In case of their instability (kinematic one), the
local turbulent mixing also occurs. Eddy lenses, which are an independent element of the deep and bottom
water ventilation, can be formed over local elevations as a result of the hydrodynamic instability of the
current.
FUNDING
The research was supported by the Russian Science Foundation (grant 22-27-00013).
SUPPLEMENTARY MATERIALS
Supplementaty materials are available for this article at DOI: 10.3103/S1068373922110085 and are
accessible for authorised users.
REFERENCES
1. N. N. Antipov, A. I. Danilov, and A. V. Klepikov, “Circulation and Structure of Water in the Western Weddell
Sea from Weddell-1 Expedition Data,” in The Results of Antarctic Studies (Gidrometeoizdat, St. Petersburg,
1995) [in Russian].
2. N. N. Antipov and A. V. Klepikov, “Interannual Variability of Water Masses in the Area of Bottom Water Formation in the Prydz Bay,” Problemy Arktiki i Antarktiki, No. 3 (2017) [in Russian].
RUSSIAN METEOROLOGY AND HYDROLOGY
Vol. 47
No. 11 2022
SPATIAL PATTERNS OF DENSE WATER RUNOFF
895
3. N. N. Antipov and A. V. Klepikov, “Oceanographic Conditions in the Prydz Bay from the 1997–2007 AARI
Expedition Data,” Problemy Arktiki i Antarktiki, No. 2, 48 (2007) [in Russian].
4. P. N. Golovin, “Conditions of the Formation and Runoff of Shelf Waters in Different Water Areas of the Arctic
and Antarctic,” Meteorol. Gidrol., No. 12 (2012) [Russ. Meteorol. Hydrol., No. 11–12, 37 (2012)].
5. P. N. Golovin, N. N. Antipov, and A. V. Klepikov, “Intrusive Layering of the Antarctic Slope Front,”
Okeanologiya, No. 4, 56 (2016) [Oceanology, No. 4, 56 (2016)].
6. P. N. Golovin, N. N. Antipov, and A. V. Klepikov, “Studying the Stability of the Antarctic Slope Front in the
Commonwealth Sea,” Meteorol. Gidrol., No. 11 (2013) [Russ. Meteorol. Hydrol., No. 11, 38 (2013)].
7. P. N. Golovin, N. N. Antipov, and A. V. Klepikov, “Mechanisms of Deep and Bottom Water Ventilation over the
Antarctic Continental Slope,” Meteorol. Gidrol., No. 6 (2019) [Russ. Meteorol. Hydrol., No. 6, 44 (2019)].
8. P. N. Golovin, N. N. Antipov, and A. V. Klepikov, “Peculiarities of Dense Water Cascading off the Deep Part of
Antarctic Continental Slope (A Case Study for the Commonwealth Sea),” Meteorol. Gidrol., No. 12 (2018) [Russ.
Meteorol. Hydrol., No. 12, 43 (2018)].
9. P. N. Golovin, N. N. Antipov, and A. V. Klepikov, “Downflow of the Antarctic Shelf Water at the Shelf and Continental Slope of the Commonwealth Sea in the Summer Season and Its Effect on the Bottom Water Formation in
the Southern Ocean,” Okeanologiya, No. 3, 51 (2011) [Oceanology, No. 3, 51 (2011)].
10. P. G. Baines and S. A. Condie, “Observations and Modelling of Antarctic Downslope Flows: A Review,” in
Ocean, Ice, and Atmosphere: Interactions at the Antarctic Continental Margin, Antarctic Res. Ser., 75 (1998).
11. E. Fahrbach, G. Rohardt, N. Scheele, M. Schroder, V. Strass, and A. Wisotzki, “Formation and Discharge of Deep
and Bottom Water in the Northwestern Weddell Sea,” J. Mar. Res., 53 (1995).
12. A. Foldvik, T. Gammelsrod, S. Osterhus, E. Fahrbach, G. Rohardt, M. Schroder, K. W. Nicholls, L. Padman, and
R. A. Woodgate, “Ice Shelf Water Overflow and Bottom Water Formation in the Southern Weddell Sea,” J.
Geophys. Res., 109 (2004).
13. A. Foldvik, T. Kvinge, and T. Torresen, “Bottom Currents near the Continental Shelf Break in the Weddell Sea,”
in Oceanology of the Antarctic Continental Shelf, Antarctic Res. Ser., 43 (1985).
14. T. D. Foster and E. C. Carmack, “Frontal Zone Mixing and Antarctic Bottom Water Formation in the Southern
Weddell Sea,” Deep Sea Res., 23 (1976).
15. T. D. Foster, A. Foldvik, and J. H. Middleton, “Mixing and Bottom Water Formation in the Shelf Break Region of
the Southern Weddell Sea,” Deep Sea Res., No. 11, 34 (1987).
16. G. Fuseo, G. Budillon, and G. Sperie, “Surface Heat Fluxes and Thermohaline Variability in the Ross Sea and
Terra Nova Bay Polynya,” Cont. Shelf Res., 29 (2009).
17. M. Hoppema, O. Klatt, W. Roether, E. Fahrbach, K. Bulsiewisz, C. Rodehacke, and G. Rohardt, “Prominent
Renewal of Weddell Sea Deep Water from a Remote Source,” J. Mar. Res., 59 (2001).
18. J. H. Middleton, T. D. Foster, and A. Foldvik, “Low-frequency Currents and Continental Shelf Waves in the
Southern Weddell Sea,” J. Phys. Oceanogr., 12 (1982).
19. R. D. Muench and A. L. Gordon, “Circulation and Transport of Water along the Western Weddell Sea Margin,” J.
Geophys. Res., 100 (1995).
20. K. Ohshima, Y. Fukamachi, G. Williams, S. Nihashi, F. Roquet, Y. Kitade, T. Tamura, D. Hirano, L. HerraizBorreguero, I. Field, M. Hindell, S. Aoki, and M. Wakatsuchi, “Antarctic Bottom Water Production by Intense
Sea-ice Formation in the Cape Darnley Polynya,” Nature Geosci., No. 3, 6 (2013).
21. K. Snow, A. M. Hogg, B. M. Sloyan, and S. M. Downes, “Sensitivity of Antarctic Bottom Water to Changes in
Surface Buoyance Fluxes,” J. Climate, 29 (2016).
22. K. Snow, B. M. Sloyan, S. R. Rintoul, A. M. Hogg, and S. M. Downes, “Controls on Circulation, Cross-shelf
Exchange, and Dense Water Formation in an Antarctic Polynya,” Geophys Res. Lett., 43 (2016).
23. G. P. Williams, L. Herraiz-Borreguero, F. Roquet, T. Tamura, K. I. Ohshima, Y. Fukamachi, A. D. Fraser, L. Gao,
H. Chen, C. R. McMahon, R. Harcourt, and M. Hindell, “The Suppression of Antarctic Bottom Water Formation
by Melting Ice Shelves in Prydz Bay,” Nat. Commun., 7 (2016).
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