Introduction
Antarctic Bottom Water (AABW) covers ca. 58 % of the ocean's abyssal layer
and fills and ventilates the deepest basins of the global ocean (Johnson,
2008). Dense bottom water formation and spread, which occur mainly in deep
western boundary currents, are important physical processes that contribute
to the variability of the global overturning circulation deep cell (Talley,
2013). The meridional overturning circulation cells are the main mechanisms
that are responsible for the interbasin exchange of mass, heat, salt,
carbon, and nutrients; therefore, these cells contribute substantially to
global climate regulation (e.g., Rahmstorf, 2006; Lumpkin and Speer, 2007).
The AABW formation occurs regionally around the Antarctic margin through the
mixing of intermediate waters with near-freezing point shelf waters (e.g.,
Carmack and Foster, 1975; Foldvik et al., 1985; Nicholls et al., 2009). The
latter results from shelf waters that are modified by atmospheric
interaction and brine rejection during sea ice production and also by
cooling through basal contact with floating ice shelves. The Weddell Sea is
considered the main contributor to the AABW formation (Rintoul, 1998; Orsi
et al., 1999). The Weddell Sea Bottom Water (WSBW) is the densest local AABW
variety within the Weddell Sea and is mostly confined to the Weddell Basin
(Orsi et al., 1993). The Weddell Sea Deep Water (WSDW) overlies the WSBW and
is light enough to spread into the global ocean through the narrow passages
of the South Scotia Ridge (Muench and Hellmer, 2002; Franco et al., 2007).
The WSDW can be formed either directly by entrainment during the downslope
flow of dense plumes from the shelf or by mixing of the WSBW with Warm Deep
Water (WDW; Orsi et al., 1993, 1999; Meredith et al., 2000).
Occasionally, bottom water formation can occur in the open ocean region
through deep convection, such as occurred with the Weddell Polynya events
observed during the successive winters of 1974–1976 (Gordon, 1977). During
this process, there is an intense heat loss to the atmosphere, which
decreases vertical stability and allows open ocean deep convection to depths
much greater than usual (Killworth, 1983).
The dominant characteristics of regional AABW varieties depend on the local
type of shelf water and on complex local physical processes that are coupled
and related to sea ice formation, such as the opening of coastal polynyas,
ice shelf basal melting, and mixing with overlying waters (Gill, 1973;
Carmack and Foster, 1975; Foldvik et al., 1985; Orsi et al., 1999; Nicholls
et al., 2009). The Antarctic sea ice area undergoes a large seasonal cycle,
varying in extent by approximately 83 % (Parkinson and Cavalieri, 2012).
In the summer, freshwater from ice melt plays a key role in the modification
of Antarctic Surface Waters (AASW), whereas brine rejection in winter
resulting from sea ice formation enhances shelf water salinity and
contributes to dense water formation.
Several studies have reported changes in the hydrographic properties of the
AABW source waters during recent decades. These changes include freshening
of dense waters in the Weddell and Ross Seas shelf regions (e.g., Jacobs et
al., 2002; Jacobs and Giulivi, 2010; Jacobs, 2004; Hellmer et al., 2011;
Azaneu et al., 2013). In addition, long-term warming was observed for the
intermediate waters of the Antarctic Circumpolar Current (ACC; during the 1990s;
Gille, 2002) and WDW within the Weddell Gyre (1970s–1990s, Robertson et
al., 2002). In turn, properties of the AABW recently formed have also
changed over the years. The WSBW in the inner Weddell Sea and the Prime
Meridian experienced warming during the second half of the 1990s
(0.01 and 0.003 ∘C yr-1, respectively;
Fahrbach et al., 2004). A more extended study reported a WSDW and WSBW
temperature and salinity increase (1998–2008) after a decreasing period
in 1984–1992 (Fahrbach et al., 2011). Abyssal waters in the
Australian–Antarctic Basin and Princess Elizabeth Trough experienced
warming (0.1 and 0.05 ∘C) and freshening (-0.005 and
-0.01) between 1994/1995 and 2007 (Johnson et al., 2008). Moreover,
freshening has been observed in the AABW within the Indian and Pacific
sectors of the Southern Ocean (1995–2005; Rintoul, 2007). Freshening of the
deep Amundsen–Bellingshausen, Australian–Antarctic and Weddell Basins
between 1991 and 2008 was also identified in World Ocean Circulation Experiment (WOCE) data; the latter basin
exhibited the smallest trend (Purkey and Johnson, 2013). The contraction of
the AABW was reported by Purkey and Johnson (2010) during the 1990s–2000s.
Azaneu et al. (2013) also observed a reduction in the volume, as well as
warming and a density decrease, in the AABW during the past 50 years
(1958–2011). Consistent with these findings, a decrease in the WSBW
contributions (∼ 20 %) to the total water mass mixture in
the Weddell Basin was identified during the 1980s–1990s (Kerr et al.,
2009). Reduced AABW volume can result from changes in the properties of the
produced dense water or from a decrease in its formation rate. The latter
can be one of the factors linked to the aging and reduced ventilation rate
of WSBW and WSDW at the Prime Meridian between 1984 and 2011 (15 and 21 %
reductions in the ventilation rates, respectively); larger reductions
in the ventilation rates of WSBW near the bottom water sources (26–30 %)
occurred during the period 1992–2011 (Huhn et al., 2013).
Despite the possible global implications of these long-term changes, the
limited and summer-biased sampling in the Southern Ocean precludes a better
understanding of the connections between these processes and the embedded
physical mechanisms. Assimilation of data into ocean models offers a
potentially useful tool to fill the information gaps of observational data
in terms of spatial and temporal resolution by providing a more synoptic and
holistic view of the main processes acting on observed changes. However, the
capability and consistency of these products still need to be evaluated and
investigated (Dotto et al., 2014), particularly for the deep Southern Ocean,
where the available in situ data set is still sparse in both time and space (e.g.,
Rintoul et al., 2012).
Our study aims to perform a detailed evaluation of the representation of
hydrographic properties and the variability of the AABW in the Southern
Ocean from the Estimating the Circulation and Climate of the Ocean – Phase II (ECCO2) reanalysis output. Over the past decade, several
global ocean data assimilation products have been developed based on the
synthesis of observations through the physics described by global ocean
general circulation models (Lee et al., 2010). The original ECCO project was
established in 1998 as part of the WOCE, with the intent of generating a
quantitative reproduction of the time-evolution of ocean states (Menemenlis
et al., 2008); this project has focused on decadal and long-term climate
changes (Wunsch et al., 2009). For example, Wunsch and Hemibach (2014) used
an updated ECCO product to describe temperature and heat content changes
primarily in the abyssal ocean. The authors found a linearly decreasing
trend in the integrated global heat content below 2000 m. Moreover, they
also found an important regional heterogeneity that was most evident with
respect to a warming in the western Atlantic and sectors of the deep
Southern Ocean. However, previous ECCO solutions were limited by the coarse
resolution and absence of sea ice representation. Thus, the ECCO2 was
created to improve these deficiencies by producing a global eddy-permitting
solution that includes sea ice. ECCO2 presents dynamically consistent
simulations because it is generated by a model that is forward run using
optimized values of control parameters (Menemenlis et al., 2008). Therefore,
the reanalysis product ECCO2 presents itself as a potentially valuable tool
for ocean variability and long-term change studies, emphasizing that the
assessment of the ECCO2 reanalysis products based on their reproduction of
the properties and variability of such climate-impacting water masses as the
AABW is fundamental for ensuring the future use of these data with respect
to the Southern Ocean hydrography and decadal variability. In Sect. 2, we
describe ECCO2 in greater detail and present the observational data used for
comparison. We also describe in detail how we compared the distinct
data sets, aiming to investigate the representation of AABW hydrographic
properties, ocean current velocity and transport, and dense water
variability. The comparison results are presented in Sect. 3, in which we
first evaluate the climatological representation of hydrographic data over
ocean layers (Sect. 3.1) and along hydrographic sections (Sect. 3.2).
Velocity and transport are evaluated in Sect. 3.3 based on four case
studies. In Sect. 3.4, the long-term variability of AABW
properties is determined Finally, a general discussion, comparison with other studies and
final considerations are presented in Sect. 4.
ECCO2 Southern Ocean representation
Climatological representation of the Southern<?xmltex \hack{\newline}?> Ocean water masses
The general distribution of the hydrographic properties represented by the
reanalysis in the θ–S space agrees with the observations (Fig. 2).
Nonetheless, the reanalysis maximum salinity values are displaced toward
lower absolute values for all sectors. Such a displacement is represented by
relatively fresh intermediate waters in the reanalysis output and ultimately
indicates an underrepresented penetration of the North Atlantic Deep Water
(NADW) into the study area. The AABW is present in all sectors; however, the
colder and saltier bottom water variety from each sector is not represented.
One exception is the densest bottom water layer from the Weddell Sea (WSBW;
γn≥ 28.4 kg m3).
Although the reanalysis output can reproduce the average shape of the
θ–S diagrams from observational data, additional results show that
the water masses in most of the Southern Ocean are poorly represented by the
ECCO2 product during the last 8 years evaluated in this study
(2004–2011). The spatial distribution of the hydrographic properties over
the last 8 years of the data set (2004–2011; Fig. 3) shows the
appearance of a dense plume in the SL during the period 2006–2007. In the
last two years, waters with γn≥ 28.4 kg m-3 not
only fill the deep basins of the Weddell Sea but also reach the surface and
spread within the region. Abrupt changes in water masses can be identified
since 2004 in the AABW area estimates (see Sect. 3.2), with dense waters
occupying most of the water column in some regions at the end of the studied
period. These hydrographic conditions are clearly not observed in the real
ocean and suggest that the representation of physical processes in the ECCO2
product is inadequate during this period.
Two-year ECCO2 average density fields in the (a) SL and (b) BL from 2004 to 2011. Blue denotes waters
lighter than γn= 28.27 kg m-3.
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(a) ECCO2 sea ice area (%) annual anomalies relative to the
long-term (1992–2011) average for the period 2004–2011. (b) Comparison of
monthly (gray) and annual (black) time series of sea ice area (%) from
the ECCO2 (solid line) and remote sensing (dashed line) data.
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For a better understanding of the unrealistic representation of hydrographic
conditions in the last few years of the ECCO2 output, we evaluated the sea
ice field reproduced by the reanalysis. The annual anomalies in the sea ice
concentration (with respect to the 1992–2011 average; Fig. 4a) show that
the decrease of this parameter culminated with the opening of an open ocean
polynya east of the Prime Meridian in 2005 and the subsequent retreat of sea
ice in the following years. The sea ice concentration time series (Fig. 4b)
exhibit two main temporal changes: a reduction in the maximum winter values
in 2001 and a more evident decreasing trend after 2004. Sea ice thickness
also exhibits a similar pattern; however, the maximum thickness decreased
after 1998 (not shown). The time series also show a strong seasonality. In
winter, the reanalysis output overestimates the satellite data by more than
20 %. However, in summer, the difference between the reanalysis and
satellite data is less than 5 %. In the final years of the studied period,
the sea ice concentration time series produced by the reanalysis are more
consistent with the satellite observations. However, this reduction in the
sea ice coverage results primarily from the polynya surging and the
unrealistic sea ice retreat in the Weddell Sea and Indian Ocean sectors.
ECCO2 average fields (1992–2004) of θ, S, and γn for the SL (100–150 m). Black contours indicate
isopycnals (units are kg m-3). The bottom right panel shows the
difference between the reanalysis and observed density fields.
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Same as in Fig. 5 except for the IL
(409–634 m).
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The aforementioned sea ice features possibly contributed, and are highly
related, to the large amount of dense water reproduced by the ECCO2 product
in most of the Southern Ocean after 2004. In the coupled MITgcm, the surface
wind stress and both heat and freshwater fluxes are computed from
atmospheric states that are modified by a sea ice model at each time step
(Adcroft et al., 2012). Changes in the sea ice concentration directly affect
the formation of AABW by altering the heat flux, sea ice melting, and brine
rejection; moreover, changes in the sea ice concentration also affect the
formation of AABW due to the effect of these changes on the transference of
momentum from wind to ocean currents (Curry and Webster, 1999).
Sea ice thickness and compactness strongly moderate the occurrence of deep
convection in the Weddell Sea region (Martin et al., 2013). Several factors,
such as brine rejection from extensive sea ice formation (Heuzé et al.,
2013), can contribute to decreasing the stability of the water column and
initiating convective mixing. Cold surface waters mix with warmer and
saltier intermediate waters. The heat transported to the surface impedes the
formation of new ice, which helps maintain the polynya, while heat loss at
the surface leads to the cooling of deep waters (Morales Maqueda et al.,
2004). This mechanism leads to a high production of dense waters that are
directly injected into deeper levels (normally below 2000 m) in the open
ocean area of the Weddell Sea primarily after 2004. A further retreat in the
sea ice after the polynya formation can also contribute to the
intensification of ocean currents via the transference of momentum from wind
to ocean currents, which facilitates the spreading of the AABW dense plume.
As a result, the hydrographic properties of the Southern Ocean are
inadequately represented during most of the recent years in the ECCO2
reanalysis output (2004–2011); therefore, this period is treated with
caution in the subsequent analysis and is disregarded in time average
estimates.
The exclusion of the 2004–2011 anomalous period from the temporal average
does not induce substantial changes in the shape of the θ–S
diagrams (not shown). However, when only the period from 1992 to 2004 is
considered, the ratio between the volume of waters with densities between
28.1 and 28.4 kg m-3 relative to waters denser than 28.27 kg m-3
increased 7-fold in the Weddell Sea sector and 1.12-fold in the Indian
Ocean sector.
The major oceanographic features are spatially reproduced by the reanalysis
output in the averaged fields (1992–2004) of θ, S, and γn
for each defined layer (i.e., SL, IL, and BL) (Figs. 5, 6, and 7).
In the SL (Fig. 5), the coldest water mass is observed in the southern
Weddell Sea shelf (∼ -1.9 ∘C). Shelf waters with the
highest salinities (S≥ 34.55) are present in the Ross and Weddell Sea
continental shelves. The densest (γn≥ 28.4 kg m-3)
water masses are located in the south and southwestern portion of the
Weddell Sea continental shelf and western shelf of the Ross Sea. Relatively
less dense waters (γn ∼ 28.27 kg m-3) are
present in the northwestern Weddell Sea shelf. Waters with θ≤ -1.5 ∘C
are present in the Weddell Sea, Ross Sea, and Prydz Bay
continental shelves in the averaged IL field (Fig. 6). The IL contains the
core of the Circumpolar Deep Water (CDW). Thus, the open ocean regime of the
Weddell Sea exhibits θ values that are relatively lower
(0 ∘C ≤θ≤ 1 ∘C) than those in
the other sections. This occurs due to the temperature attenuation caused by
mixing of the Winter Water and CDW while it flows along the Weddell Gyre (WG) (Weppernig et
al., 1996), resulting in WDW formation. The Ross Sea presents the saltiest
water masses of all the shelves (S≥ 34.72) and the open ocean
(S≥ 34.7) in the IL. The densest waters (γn≥ 28.27 kg m-3)
of the IL fill the southern and western portions
of the Weddell Sea shelf region and west of the Ross Sea shelf. The general
features of the BL (Fig. 7) are consistent with the description by
Orsi et al. (1999) regarding the differences in bottom waters between basins based
on observational data. According to Orsi et al. (1999), the coldest and
freshest bottom water masses occur in the southwestern Weddell Sea sector,
the warmest and saltiest waters are found in the northwestern Ross Sea
sector, and intermediate characteristics are present in the Western Pacific
sector. The Weddell Sea and western region of the Indian sectors present the
densest bottom water masses (γn≥ 28.3 kg m-3). The
Ross Sea and B&A sectors encompass waters lighter than γn= 28.27 kg m-3
(the AABW threshold), which are limited to the Ross gyre region.
Despite the reproduction of the main oceanographic features by the ECCO2
reanalysis output, there are certain discrepancies regarding the absolute
θ–S values that lead to differences between density fields (Figs. 5, 6,
and 7). The θ and S difference fields are presented in the
Supplement (Fig. S1). The reanalysis representation of the SL in
the Southern Ocean is generally denser than the observations (Fig. 5), which
is primarily because the waters are saltier than expected. As a result of a
fresher intermediate layer, the IL density is underestimated in the B&A
and Ross Sea sectors (Fig. 6). The Weddell Sea is also represented by a
relatively low density; however, this finding is related to the temperature
overestimation and the close similarity of the salinity values to the
expected values (i.e., a difference of between -0.01 and 0.01). In the
Indian and Western Pacific sectors, the temperature underestimation leads to
waters that are denser than observations. In the BL (Fig. 7), the deep water
masses are predominantly lighter than suggested by the observations, which
is caused by waters that are warmer than expected and by the general absence
of colder bottom water variety of the AABW.
Same as in Fig. 5 except for the BL (3000 m to the
seabed).
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The relative accuracy of the ECCO2 reanalysis in representing the θ,
S, and γn fields at the defined layers during the reliable period
is summarized in a Taylor diagram. The hydrographic properties are not
equally well represented between the sectors. Most of the sectors and
properties in the SL (Fig. 8a) underestimate the spatial variability of the
observations (SD ≤ 1). The Indian density field, in
addition to the B&A salinity, comprises the poorest representation of the
observed fields. The Ross Sea sector density field reveals the most
consistency between reanalysis and observations in the SL layer. All other
sectors and parameters have correlation coefficients lower than 0.9. The
temperatures in the Ross Sea, Indian, and Western Pacific sectors and the
density in the B&A sector are the best-represented fields in the IL,
exhibiting correlation coefficients that exceed 0.9 (Fig. 8b). The salinity
fields are the most poorly represented fields for all sectors. Every field
and sector evaluated in the IL presents an SD less than 1,
which indicates smaller amplitudes in the variation pattern compared with
observational data. The BL field values exhibit large variations in the
SD values, with salinity having the lowest values (Fig. 8c).
The temperature and density fields in the Ross Sea and B&A sectors are
most similar to the observational fields. For all sectors and properties,
the correlation coefficients are less than 0.8, i.e., lower than in the
other layers. In general, the hydrographic properties are closer to the
reference values, i.e., the properties are better represented in the IL; the
poorest representation is in the BL.
Taylor diagram for the period 1992–2004 within each defined
layer: (a) SL (100–150 m), (b) IL
(409–634 m), and (c) BL (3000 m to the seabed); the
diagrams are separated according to the ocean sectors (Fig. 1) and are
indicated by numbers (Indian – 1, Western Pacific – 2, Ross Sea – 3,
Bellingshausen and Amundsen sectors – 4, and Weddell Sea – 5). Dark blue,
green, and light blue points indicate θ, S, and γn,
respectively. The gray curves, dashed curves, and dashed
radii refer to the normalized standard deviation, centered root-mean-square
error, and correlation, respectively. WOD refers to the observed data set
used as a reference for the comparison (see the text for details).
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Hydrographic properties of the repeat section WOCE SR3. Average
fields of θ, S, and γn during section occupations are
presented for the ECCO2 data. The bottom-right panel refers to the difference between the
reanalysis and observed density fields. Dotted lines indicate isolines of
θ= 0 ∘C, S= 34.64, and γn= 28.27 kg m-3
based on the ECCO2 reanalysis (black) and observations (white).
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The results based on the climatological ocean state indicate that the
hydrographic properties represented by the reanalysis have an average
distribution in the θ–S space that is similar to the observations.
Following the opening of the polynya in 2004, the Weddell Sea sector of the
Southern Ocean became flooded with dense waters, indicating that the
relevant physical processes are not correctly represented by the reanalysis
during this period. Before 2004, the main oceanographic surface water
features are reproduced by the reanalysis; however, these waters are
generally denser than suggested in the observations, while deep waters are
primarily less dense than expected. The intermediate layer is statistically
the closest to the observations, while the deep waters have the worst
representation. The misrepresentation of surface waters is possibly due to
the lack of accuracy in reproducing several complex processes that act on the
ocean surface in the reanalysis output. This limitation is compounded in
coastal waters by the absence of ice shelf water, an important predecessor of AABW
(Foldvik et al., 2004). This absence was expected because ice shelves are
not considered in the ECCO2 product. Less dense deep waters are related to
the lack of the coldest and densest AABW varieties, which possibly result
from the absence of newly formed dense waters spilling off the shelf in the
reanalysis. Moreover, the coarse vertical grid resolution at greater depths
implies that deep waters are represented by the average properties of
approximately 400 m of water column, which leads to a poor representation of
the approximately 100 m-thick bottom water layer.
Same as in Fig. 9, except for section WOCE SR4.
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The average differences between the reanalysis and observed fields are
comparable to the performance of the more accurate models among those
evaluated by Heuzé et al. (2013). The authors compared the AABW
hydrographic properties of 15 climate models with observed climatology.
The models considered to be most accurate by the authors exhibited
temperatures, salinities and densities within 0.5 ∘C, 0.1, and 0.05 kg m-3
of the observed climatology, respectively. Considering the BL,
the average differences between the ECCO2 estimates and the observations are
0.24 ∘C, 0.09 and -0.038 kg m-3 for temperature, salinity, and
density, respectively. These values are lower than those defined by
Heuzé et al. (2013), indicating that the ECCO2 discrepancies, in terms
of absolute values, are comparable to other circulation models. Moreover,
most of these models also use deep convection as an important mechanism for
dense water formation.
Water mass representation along repeat <?xmltex \hack{\newline}?> hydrographic sections
Following the previous time-averaged analysis, we considered only the period
from 1992 to 2004 of the ECCO2 data set to compare the averaged hydrographic
properties from observations and reanalysis output along the historical
sections (Figs. 9–11). Complementary figures for the θ and
S difference fields can be found in the Supplement (Fig. S2).
The 0 ∘C isotherm from the reanalysis average temperature profile
for section WOCE SR3 (Fig. 9), located within the Australian–Antarctic
Basin, exhibits a more restricted extension than suggested in the
observations at the surface; moreover, this isotherm is absent in deep
waters. This finding provides additional evidence of the overestimation of
surface water temperatures and that the reanalysis bottom waters do not
reach the minimum observational temperatures. Despite the coincidence of the
S= 34.64 isohaline, the salinity maximum of intermediate waters is lower
than the observed values. The subsurface density overestimation is
coincident with the temperature bias indicated by the θ= 0 ∘C isoline; however, this result is caused by the smaller
thickness of the surface fresh water layer that is reproduced in the
reanalysis compared with the observations. The change from a denser
(lighter) intermediate water to the south (north) of 63 ∘S
coincides with the southern limit of the eastward transport (“southern ACC
front”; Rintoul and Bullister, 1999), which was identified by Orsi et al. (1995) as a
front located south of the polar front that is usually marked by the
southernmost extent of maximum temperatures; this result is coincident with
the reanalysis temperature profile. Thus, the observed change in density
representation may be linked to a dynamical barrier. The temperature
differences exhibited in deep waters contribute to an AABW layer that is
lighter and thinner than expected. Thus, it is likely that the AABW layer in
this region is primarily supplied by the relatively warmer and saltier
bottom water from the Ross Sea because the colder and fresher Adelie Land
Bottom Water is absent.
Same as in Fig. 9, except for section WOCE SR2.
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The property fields of the sections within the Weddell Sea and along the
Prime Meridian are consistent between the reanalysis and observational data,
which is indicated by the highly coincidental 0 ∘C isotherm and
34.64 isohaline (Figs. 10 and 11). The major regional features are
represented by the reanalysis output, e.g., the characteristic doming of the
Weddell Sea cyclonic gyre (Fahrbach et al., 2004) and the signature of the
Antarctic slope front, which is a boundary between the shelf waters and the
CDW over the Antarctic continental slope (Jacobs, 1991). The warm core of
the WDW that is adjacent to the continental shelf is represented in the
reanalysis for both sections. Despite the coincident 0 ∘C
isotherm, the bottom waters do not reach the low temperatures of the WSBW
(-0.7 ∘C). Consequently, the bottom waters are lighter than
expected in both sections in the reanalysis product (Figs. 10 and 11, bottom-right panel); however,
there is good agreement in the AABW upper limit. Therefore, in the sections
within the Weddell Sea, the absence of the denser AABW variety (WSBW) is
compensated in terms of volume by the WSDW. However, for section WOCE SR3,
the 28.27 kg m-3 isopycnal is deeper and, consequently, the dense water
layer is thinner (Fig. 9). The density overestimation at the western end of
WOCE SR4 is not associated with a dense downward flow along the continental
slope. Instead, this overestimation is a result of the combination of waters
that are saltier (∼ 100 to 400 m) and colder (∼ 400 to 1500 m)
than suggested by the observational data. The downward flow
is not identifiable in any of the cross-slope sections.
Estimates based on the reanalysis output of the area occupied by the AABW
layer along WOCE sections (Fig. 12) slightly underestimate the AABW area and
density before 2004 (average area differences of 14.19, 2.36, and 5.26 %
for WOCE SR3, SR2, and SR4, respectively). The larger differences in the AABW
area found in WOCE SR3 were expected because the 28.27 kg m-3 isopycnal
was deeper than suggested by the observations in the averaged section (Fig. 9).
The WOCE SR3 time series (Fig. 12a) does not exhibit abrupt changes in
any of the estimates, which is consistent considering its distance from the
polynya region. However, section WOCE SR2 (Fig. 12b) exhibits an unrealistic
change in the AABW area (from ∼ 66 to ∼ 90 %) and density
after 2004. At the end of the time series, AABW occupies
most of the water column, which is composed of waters denser than 28.4 kg m-3
after 2008. The AABW area and density in section WOCE SR4 (Fig. 12c) begin to
increase concurrently with the stabilization of the AABW area
in section WOCE SR2 (approximately in 2006). Moreover, AABW fills nearly the
entire water column in the final 2 years of the data set (i.e.,
2010–2011). The mean differences between the estimates from the reanalysis
and observational data sets are ∼ 16 and ∼ 4
times higher in the most recent years for sections SR2 and SR4,
respectively. Clearly, observational data do not exhibit this pattern. As
discussed previously, these abrupt changes in the AABW area and the mean
density are the result of the high dense water production reproduced by the
reanalysis during the most recent years. The opening of the polynya and the
deep convection process began near the Prime Meridian; thereafter, the
produced dense water was advected along the Weddell Gyre, which explains the
delay between the WOCE SR2 and SR4 property changes. Finally, these results
show consistency between area estimates within the Weddell Sea from the
reanalysis and observational data sets until 2004 and confirm the inability
of the ECCO2 product to reproduce hydrographic properties and structures
after 2004.
The percentage of the section area occupied by the AABW (upper
panels) and AABW (bottom panels) layer average density for the WOCE repeat
sections (a) SR3, (b) SR2, and (c) SR4. Squares denote the observed
estimates. The reanalysis estimates restricted to available observations are
represented by circles. Open markers denote incomplete section occupation.
Black lines refer to estimates based on the complete spatial and temporal
ECCO2 data.
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The analysis of the hydrographic properties along the three WOCE sections
revealed that within the Weddell Sea (WOCE SR2 and SR4), the reanalysis is
capable of representing the major hydrographic features, e.g., the doming of
the Weddell Sea Gyre, WDW temperature and salinity maxima, and the Antarctic
slope front before the polynya opens. The AABW upper limit and area are
coincident with the observations; the average differences are less than
6 % compared with the observational data estimates before 2004. However,
in section SR3, the 28.27 kg m-3 isopycnal is deeper than expected,
which leads to larger differences in the estimated area (14 %). In all
sections, deep waters are warmer and less dense than expected. In the
Weddell Sea, the densest AABW variety (WSBW) is absent and replaced by the
warmer WSDW, while in SR3, the AABW layer is thinner. After 2004, the
Weddell Sea is filled with dense waters produced by intense deep convection
that occupy most of the water column at the end of the time series,
producing inconsistent and unreliable hydrographic conditions.
Velocity and volume transport assessment
The assessment of the AABW current velocity and volume transport variability
is performed in four case studies based on data previously reported in the literature
(see Table 1 for details), and each case study is highlighted below.
Case study I: Kerguelen Plateau/Indian sector
The Kerguelen Plateau deep western boundary current was recently identified
as a significant pathway of deep water transport from the Southern Ocean
toward lower latitudes. This region (Fig. 1) is characterized by a narrow
and intense northwestward flow close to the plateau and a southeastward flow
offshore (Fukamachi et al., 2010; hereafter referred to as F10; case study I). Based on
the 28.27 kg m-3γn isopycnal, the AABW layer exhibits a
thickness of approximately 1500 m in the ECCO2 reanalysis, which coincides
with F10 estimates (see the Supplement; Fig. S3). However, the
AABW layer is warmer in the reanalysis than in the observations presented by
F10. The ECCO2 deep western boundary current mean transport (3.7 ± 1.3 Sv;
Fig. 13a) during the observed period (February 2003 to January 2005) is
much lower than the 12.3 ± 5.6 Sv estimated by F10. After 2006, there
is a continuous increase in the AABW flow (Fig. 13b) that leads to an AABW
transport of 12 Sv at the end of the time series, which is close to the mean
flow reported by F10 (i.e., 12.3 Sv). However, the years with higher AABW
transport coincide with the period of active deep convection, during which
the ECCO2 reanalysis produces unreliable hydrographic conditions. The
reanalysis output does not represent the observational variability pattern
because the correlation between time series is low (r= -0.2). The rugged
bathymetry and poor model spatial resolution can contribute to the
difficulty of the reanalysis output reproducing the deep and bottom volume
transport variability. Another important factor that could have contributed
to a low correlation in this analysis is the short length of the available
time series.
(a) Monthly AABW volume transport (Sv) obtained from the ECCO2
data (black line) and Fukamachi et al. (2010) (gray line) for Section I (see Fig. 1).
The averaged ECCO2 reanalysis estimate is indicated by the black dotted
line. (b) Monthly AABW volume transport (Sv) from the ECCO2 data during the
entire data period. The gray box highlights the period presented in panel
(b).
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Case study II: Endurance Ridge/Weddell Sea <?xmltex \hack{\newline}?>sector
The deep and bottom water properties in the northern limb of the Weddell Sea
were evaluated by Gordon et al. (2010) and McKee et al. (2011) using data
from bottom moorings M2 (3096 m; 62∘38′ S; 043∘15′ W)
and M3 (4560 m; 63∘32′ S; 41∘47′ W) (Fig. 1; case study II). The
latter is associated with the eastward flow of the WSBW originating in the
southwestern Weddell Sea, whereas measurements from M2 are related to the
WSDW formed further north. The monthly velocity time series of the deepest
current meters from each mooring are well correlated (r= 0.58 for M2 and
r= 0.57 for M3) with velocities from the ECCO2 reanalysis at approximately
the same depths (Fig. 14a). Despite the statistical significance of these
correlations, current meter M2 has a short time series and an 18-month
gap during the 4-year sampling period; therefore, these correlation
results should be considered with caution. The amplitude of seasonal
variations is higher in the time series from observational data and more
evident at M3. The different time series patterns of variability might be
related to distinct water mass sources and forcing of the water flowing
through each mooring. In general, the average current velocities from the
reanalysis are lower than those from the observational data except during
the low-current velocity periods at M3 (December, January, and February);
during these periods, the velocities are very similar. During the polynya
period, there is an increase in the reanalysis velocity that leads to values
that are approximately the same as the observational average (∼ 12 cm s-1)
during July 2006 at M3; however, the reanalysis results
largely exceed the observational average at the end of the time series (Fig. 14b).
The velocity from M2 position decreases after 2007 and reaches minimum
values in 2010, suggesting a condition of slower coastal currents during the
polynya event.
(a) Comparison between monthly time series of current meter
(gray) and the ECCO2 (black) velocity (cm s-1) at ∼ 3100 m based
on the M2 position (upper panel) and at ∼ 4580 m based
on the M3 position (bottom panel; see mooring positions in Fig. 1).
(b) Monthly ECCO2 time series of current velocity (cm s-1) at the M2
position (black line) and the M3 position (gray line) for the period
1992–2011. Both time series are filtered using a 3-month moving
average. The boxes highlight the period shown in (a).
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Case study III: Prime Meridian/Weddell Sea<?xmltex \hack{\newline}?> sector
Also within the Weddell Sea sector is the presence of a mooring array along
the Prime Meridian that was evaluated by Klatt et al. (2005), which is
hereafter referred to as K05 (case study III). At the M233 location, which is immersed in
the Antarctic Coastal Current, the reanalysis absolute values are generally
lower than suggested by the observations; the data are only similar during
the weaker phases (December–January) of the observational data (Fig. 15a).
After 2007, the abrupt increase in the velocity from the ECCO2 output leads
to mean values of -12.7 ± 1.5 cm s-1. This velocity is much higher
than the previous average found in the ECCO2 time series (-2.6 ± 1.9 cm s-1)
and also from the observational mean values (-4.5 ± 2.4 cm s-1).
The variability pattern is well correlated at M233 (r= 0.68); a
clear annual cycle is found in both time series. Due to increases in area
(Fig. 12) and velocity (Fig. 15), the contribution of AABW transport south
of 60∘ S to the cross-section volume transport (Section IV; Fig. 1)
increases from approximately 38 % at the beginning of the time series to
approximately 80 % in the latter years. Two main changes occur in AABW
transport (Fig. 15b), i.e., an intensification in AABW transport in 2004 and
even higher values after 2008. The average AABW transport after 2008 (40 Sv ± 23) is much higher than the K05 estimates (28 Sv).
(a) Comparison between the monthly time series of the current
meter (gray) and ECCO2 (black) zonal velocity (cm s-1) at
∼ 2000 m based on the M233 position (see mooring positions in
Fig. 1); (b) monthly ECCO2 time series of volume transport (Sv) in Section IV
considering the entire water column (black line) and only the AABW layer
(gray line).
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Case study IV: western Weddell Sea/Weddell Sea <?xmltex \hack{\newline}?>sector
Fahrbach et al. (2001) (hereafter referred to as F01) evaluated the export
of newly formed dense bottom waters toward the northern limb of the Weddell
Gyre using a mooring array at the northern tip of the Antarctic Peninsula
(Section II; Fig. 1; case study IV). ECCO2-estimated AABW volume transport is also compared with
the Kerr et al. (2012) model estimates, which are hereafter referred to as K12.
The absolute transport values from the ECCO2 and observational data differ
because of the different dense layer definitions, whereas the time series
exhibit a good temporal correlation (r= 0.52 between the ECCO2 and F01;
Fig. 16a). An increase in the observed transport after 1994 (from an average
value of 3.5 ± 0.6 Sv prior to this year to 3.9 ± 0.5 afterwards)
is also observed in the ECCO2 time series (from an average of 2.3 ± 0.19 Sv
to 8.5 ± 3.4 Sv). The volume transport estimates based on the
OCCAM model evaluated by K12 also show this variability pattern; however,
the absolute values are higher than the reanalysis estimates despite the
same AABW layer definition. The AABW flow represented by the ECCO2 output
has a well-defined seasonal cycle in which weaker flows occur in January and
stronger flows occur in June (Fig. 16b). However, the months of maximum and
minimum ECCO2 transport lag those determined by Fahrbach et al. (2001) by
1 month (December and May). Before 2004, the average AABW flow reproduced
by the ECCO2 reanalysis at Section II (6.7 Sv) is very similar to the volume transport
determined by Naveira Garabato et al. (2002) (6.7 ± 1 Sv) and Franco et al. (2007) (∼ 9 Sv) for WSDW exported over the South Scotia
Ridge. The decrease in the reanalysis AABW volume transport after 2006 is
related to a change in the local circulation that persists until 2010. This
circulation change after the polynya event is caused by the surging of
currents flowing to the south along the shelf break, which dominates the
northern portion of the Antarctic Peninsula shelf region in 2010 (not
shown). For most of the studied period, only WSDW contributed to the AABW
volume transport in the ECCO2 reanalysis (Fig. 16b). However, close to the
opening of the polynya and primarily thereafter, there are episodic events
of the direct injection of WSBW into the bottom layer (December 1998 to
March 2000; April 2003 to September 2008; November 2010 to December 2011).
During the period 2003–2008, the ECCO2 average WSBW transport (1.86 Sv) is
in accordance with that reported by F01 (1.3 ± 0.4 Sv). However, with
the abrupt increase after 2010, the WSBW transport becomes responsible for
almost all of the AABW transport. Although Section II is located north of a bottom
water formation area, the origin of this dense bottom water is in southern
regions. Therefore, the variability of dense water transport reported herein
must be affected not only by newly formed bottom water but also by remote
thermohaline and wind forcings (Fahrbach et al., 2001), including the polynya
event and anomalous dense water production.
(a) Section II (Fig. 1) annual transport (Sv) based on the ECCO2 (black
line), observational (gray line; from Fahrbach et al., 2001) and OCCAM
(light gray line; from Kerr et al., 2012) data. The ECCO2 and OCCAM
estimates are based on the AABW; (b) ECCO2 Section II annual (black solid line) and
monthly (gray solid line) volume transport (Sv) for the AABW layer. Monthly
time series considering the WSBW layer are also presented (gray dashed
line).
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The ventilation of the AABW to the deep Southern Ocean was evaluated by the
cross-shelf break ECCO2 bottom layer transport, which was determined within
the northwestern Weddell Sea (east of the 1000 m isobath and from
72 to 65∘ S along the longitude 305∘;
Section III – Fig. 1). The AABW production occurs primarily in pulses (Schröder et
al., 2002) that can be seen in the ECCO2 time series during three episodic
periods (1994–1999, 2000–2004, and 2008–2011) (Fig. 17). The first two
events exhibit an average transport of 0.25 ± 0.25 Sv (positive values
to the east, i.e., offshore) and coincide with the AABW production periods
determined from the OCCAM simulation (K12). WSBW is also produced during
this period, although this water mass is not detectable in the open ocean.
The produced WSBW is possibly diluted via mixing because of the
underrepresented volume transport that is reproduced until 2004 (0.06 ± 0.12 Sv)
relative to the 1.1 ± 0.5 Sv bottom water (θ≤-0.7 ∘C)
formation rate estimated by Huhn et al. (2008)
in the same area. The intensification of AABW and WSBW production after 2008
leads to maximum values (3.6 and 1.9 Sv, respectively) that exceed the
values estimated by the observations.
Annual (thick lines) and monthly (thin lines) ECCO2 volume
transport in Section III of the AABW (black lines) and WSBW (gray lines).
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The case studies reveal that the temporal variability of the dense water volume
transport (case study IV) and current velocity (case studies II and III) determined from the reanalysis are
correlated at a statistically significant level with the observationally
based estimates in the regions that are most important for AABW export.
Moreover, the intermittent characteristic of AABW production (case study IV) is reproduced
by the reanalysis output. However, in Section I (located near the Kerguelen Plateau;
case study I), the rugged bathymetry and the relatively low model resolution are
possibly important factors that contribute to the difficulty in reproducing
the regional variability in AABW volume transport. Moreover, the relatively
short length of the observational time series must be taken into
consideration in the correlation analysis results. The absolute AABW current
velocity and volume transport in all of the analyzed cases are
underestimated by the ECCO2 reanalysis product before 2004. There is no
export of WSBW along the northern tip of the Antarctic Peninsula (Section II; case study IV) before
2004, while its transport through the meridional Section III is considerably low,
making it indistinguishable in the open ocean.
The reduced production and export of dense flow from the shelf is possibly
related to the spatial resolution of the model used to create the reanalysis
product, limiting the representation of coastal processes and coupled
atmosphere–ocean–ice interactions. Furthermore, the diffusion scheme
generally applied in z level global circulation models could also be a
factor. In this case, the dense water formed in the shelf region mixes
horizontally and vertically when flowing down the slope (Heuzé et al.,
2013); therefore, the dense signal can be lost along the downward flow as a
result of mixing with intermediate waters. In the open ocean, the
underestimation in the volume transport of AABW is affected by the
difficulty in reproducing the export of dense waters from the Antarctic
shelves (case study IV) and also by the low current velocities (case studies II and III).
During more recent years, with the effect of the persistent polynya and erroneous deep water
formation, AABW transport estimates greatly exceed the observationally based
estimates in all of the analyzed case studies.
Long-term AABW variability
At the beginning of the analyzed time period (1992–1995), the four-year
average AABW layer thickness reproduced by the ECCO2 reanalysis product
(Fig. 18) shows a geographical distribution that is similar to (albeit
slightly thinner than) the 20-year average observationally based dense layer
(Fig. 18, bottom-right panel). The absence in the reanalysis of the thin
dense layer seen in observations at the eastern part of the B&A sector
and the northern Ross Sea sector is related to the temperature
overestimation. A retreat of the dense deep layer in the B&A and Ross Sea
sectors with time can be clearly seen in the AABW thickness. By the period
2004–2007, the AABW from the Ross Sea and B&A sectors is restricted to
the core of the Ross Gyre. A contraction of the AABW layer from the 1980s to the
2000s was recently reported by Purkey and Johnson (2012), who noted that the
Australian–Antarctic and Amundsen-Bellingshausen basins present the highest
isopycnal deepening rates (-13.2 ± 6.7 m yr-1 and -11.4 ± 2.9 m yr-1,
respectively). A volume decrease of dense waters is also
reported by Azaneu et al. (2013), who showed a -8.06 ± 0.38 m yr-1
average AABW upper limit rate of change for the period 1958–2010.
Despite the substantial retreat of the AABW layer in the ECCO2 output during
the period 1992–2004, the average deepening rate of the AABW isopycnal from
the reanalysis output (-3.06 ± 1.81 m yr-1) is smaller than the
observational estimates determined by Purkey and Johnson (2012) and Azaneu
et al. (2013). The consequences of deep convection and dense bottom water
injection in the Weddell Sea during the polynya period are observed by the
overestimation of the AABW layer thickness in the 2004–2011 average.
Four-year averages of the AABW layer thickness (m) based on the
ECCO2 data for the period 1992–2010. The bottom-right panel shows the
20-year average AABW layer thickness based on observational data.
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The temporal trends based on the regional AABW annual time series exhibit
different rates of change for each Southern Ocean sector (Table 2). During
the period prior to the opening of the polynya (1992–2004), all sectors
experience warming, freshening, and density decreases except for the Weddell
Sea sector. This region warms until 2000 (0.0028 ∘C yr-1) and
then begins to become colder and fresher. Prior to a deep convection event
and opening of the polynya, the vertical transport of oceanic heat
vertically melts the sea ice and causes a decrease in the sea ice
concentration and thickness (Martin et al., 2013). Thus, the decrease in the
sea ice concentration and thickness observed ca. 2000 is potentially
caused by oceanic heat in the Weddell Sea, which leads to an early cooling
in the region.
Linear fits of the annual regional AABW time series based on the
ECCO2 outputa.
Southern Ocean hydrographic sector
Trend in yr-1
Hydrographic property
1992–2004
1992–2011
Bellingshausen and Amundsen
θ (∘C)
0.0041
–
S
-0.0001
–
γn (kg m-3)
-0.0006
–
Ross Sea
θ (∘C)
0.0029
-0.0002
S
0.0000
-0.0003
γn (kg m-3)
-0.0006
-0.0005
Western Pacific
θ (∘C)
0.0048
0.0016
S
-0.0001
-0.0002
γn (kg m-3)
-0.0010
-0.0006
Indian
θ (∘C)
0.0019
-0.0078
S
-0.0004
-0.0007
γn (kg m-3)
-0.0009
0.0006
Weddell Sea
θ (∘C)
-0.0020
-0.0283
S
-0.0004
-0.0010
γn (kg m-3)
-0.0004
0.0045
a Bold font indicates a statistically significant trend (P≤0.05).
The variability pattern of warming, freshening, and lightening of dense
waters reproduced in most of the ocean sectors during the period 1992–2004
agrees with several observational studies (e.g., Fahrbach et al., 2004;
Rintoul, 2007; Aoki et al., 2005; Purkey and Johnson, 2013). Azaneu et al. (2013)
estimated an AABW average temperature, salinity, and density trend of
0.0015 ∘C yr-1, -0.0002 yr-1, and -0.0005 kg m-3 yr-1
(1958–2011), although freshening is not a clear
pattern throughout the Southern Ocean. The analogous average rates for the
Southern Ocean sectors (i.e., 0.0023 ∘C yr-1, -0.0002 yr-1,
and -0.0007 kg m-3 yr-1 for temperature, salinity,
and density, respectively), as determined with the ECCO2 output (considering
the period 1992–2004), are consistent with those found by Azaneu et al. (2013). The ECCO2 product is based on the assimilation of a large number of
observational data throughout the model simulation. Therefore, in addition
to interannual variability, we expect that the reanalysis should be capable
of representing the observed low frequency variability. However, it is most
likely that in ECCO2, the trends found are a consequence of a “natural”
drift of the model. The representation of a reduced export of dense waters
from the shelf and the low current velocities contributed to a longer
residence period of AABW within the deep basins of the Southern Ocean. A
longer residence period can enhance the mixing with the warmer CDW, which
can produce the observed warming trend and the loss of the dense water
signal with time. A similar process is suggested to be occurring in the real
ocean due to the possible recent decrease in dense water production (Azaneu
et al., 2013). In the case of ECCO2, however, the process is a consequence
of its inability to properly reproduce the dense water production and
ocean–ice interactions rather than a representation of a real climate
trend. When the entire period is considered (i.e., 1992–2011), the
anomalous production of dense waters produces a cooling trend; this trend is
most evident in the Indian (-0.0078 ∘C yr-1) and Weddell Sea
(-0.0283 ∘C yr-1) sectors, which is directly influenced by
the polynya surging. Four of the models analyzed by Heuzé et al. (2013)
also exhibited bottom waters that were warmer than the real ocean and a
decreasing temperature trend; however, the rate of change (∼ -0.0015 ∘C yr-1)
found in their study is lower than those suggested by the ECCO2 reanalysis product.
Discussion and conclusions
The assessment of the ECCO2 output shows substantial changes in the
hydrographic conditions in the simulated years, such as a warming of dense
waters in the Southern Ocean (Table 2) and a retreating of the AABW layer in
the B&A and Ross Sea sectors (Fig. 18). However, the most striking
modification observed is the appearance of a dense water plume in the
Weddell Sea sector due to deep ocean convection. This feature is related to
the opening of an oceanic polynya that is clearly visible after 2005 and is
located near the center of the Weddell Gyre (Fig. 4a). This phenomenon
contributes to the unrealistic representation of the Southern Ocean
hydrographic properties primarily after 2004.
Before 2004, the ECCO2 reanalysis is able to grossly represent the main
Southern Ocean oceanographic features, including the dense waters over the
shelf (Figs. 5–7). The water column structure and the AABW upper
limit are consistent between the reanalysis and observations within the
Weddell Sea (Figs. 9–11 and 18). Moreover, estimates of the area
occupied by the dense layer based on the ECCO2 product are underestimated by
less than 6 % compared with the observational estimates (Figs. 12). The
current velocity of deep waters near the Endurance Ridge (case study II) and at the Prime
Meridian (case study III) and AABW transport east of the Antarctic Peninsula (case study IV) are well
correlated with the observations.
Despite these agreements, the ECCO2 output does not agree well with respect
to the AABW layer thickness and transport variability out of the Weddell Sea
(Figs. 8 and 12). Moreover, surface waters around the Southern Ocean are
generally depicted as being saltier and denser than suggested by the
observations. The bottom layer is warmer and less dense than expected; the
densest bottom water variety is lacking in the reanalysis data (Figs. 5, 7,
9–11). Furthermore, the estimated AABW volume transport and absolute
velocity values are underestimated in all cases (Figs. 13–17) until the polynya event occurs. In addition, there is no export of WSBW
through Section II at the northern tip of the Antarctic Peninsula before 2004
(case study IV). The misrepresentation of the surface layer is possibly the consequence of an
inadequate representation of the complex processes that occur at the
ocean–ice–atmosphere interface. The overestimation of the sea ice
concentration in winter requires intense sea ice formation and subsequent
brine rejection, which likely promotes high salinity surface waters. The
underestimation of AABW transport and the presence of warm and less dense
waters are possibly the consequence of the spatial resolution of the model,
which limits the representation of coastal processes, and the diffusion
scheme used in the model. Considering that the flow exported from the
continental shelf is already underrepresented, the dense and cold signal is
diluted during its downward flow due to vertical and horizontal mixing.
Consequently, AABW transport in the open ocean is affected by the limited
export of dense waters and also by the low current velocities.
An important feature noted in the reanalysis output is the reproduction of
WSBW transport through the meridional Section III (northwestern Weddell Sea;
case study IV) even before 2004. Although also considerably lower than suggested by the
observations, this flow represents the ventilation of the Weddell Sea bottom
waters with the newly formed dense waters. None of the 15 general
circulation models analyzed by Heuzé et al. (2013) are capable of
reproducing the spilling and sinking of dense waters from the shelf. Half of
the models use open ocean deep convection as the main mechanism to create
deep water. In the ECCO2 reanalysis, this mechanism surges only after 2004;
thereafter, this mechanism has the larger effect on the transport of AABW
and the hydrographic properties not only in the Weddell Sea but also in
remote areas, e.g., in Section I of the Indian sector.
Deep convection generally begins with a decrease in stability within the
water column, which can be caused by surface water buoyancy loss or an
increase in the buoyancy of deep waters. Several factors, such as increased
heat loss to the atmosphere, changes in surface fluxes, and brine
rejection from ice formation, can trigger the process and allow for the mixture of
cold surface waters with saltier intermediate waters (Martin et al., 2013).
In addition, some aspects can precondition this process, e.g., the strong
cyclonic gyre that is present in the Weddell Sea, which is naturally
associated with upwelling of the pycnocline, and also the interaction of the
circulation with the seabed topography near the Maud Rise (Gordon and Huber, 1990).
Martin et al. (2013) evaluated two experiments with the Kiel Climate Model
and found a quasi-periodic occurrence of deep convection in the Atlantic
sector of the Southern Ocean. According to the authors, the main trigger for
deep convection in these experiments was the heat accumulation at
intermediate depths. The inflow of warm NADW into the Atlantic-Indian Basin
is the primary heat source, which is trapped in the gyre circulation by the
westward return flow to the south of the Weddell Gyre. Heat then accumulates
at a depth of approximately 1500 m during periods without convection and
subsequently spreads in depth, acting to warm the entire water column below
100 m. Sea ice compactness regulates the occurrence of deep convection;
however, brine rejection is not essential for triggering the mixing process.
The ECCO2 output comprises a short period of time that includes only one
deep convection event, which limits the understanding of this process.
However, the heat accumulation at intermediate depths described by
Martin et al. (2013) is not evident in the ECCO2 output (see the Supplement, Fig. S4).
Although the heat accumulation is absent, the warming of
deep waters and cooling at intermediate depths are both present prior to the
polynya event. The cooling of WDW (-0.0374 ∘C yr-1) in
ECCO2 was also identified by Dotto et al. (2014) within the Weddell Sea,
which reinforces the idea that the AABW warming is the consequence of the
weak export and mixing of less dense waters.
We believe that the difficulty in reproducing the export of dense waters
along the continental slope in the reanalysis, which produces less dense
deep and bottom layers, results in a less stratified Southern Ocean, as
suggested by de Lavergne et al. (2014). The warming trend of AABW and the
too strong seasonality of sea ice are also important contributors to
decreases in the stability of the water column and create favorable
conditions for the initiation of deep convection. Thus, warmer waters in the
deep and bottom layers and saltier waters at the surface should be the
determinant factors for convective mixing and the homogenization of the
water column in the ECCO2 reanalysis. The heat transported to the surface
melts the sea ice and hinders the formation of new ice, leading to the
opening and the maintenance of the large oceanic polynya after 2004. Deep
waters become colder due to heat loss at the surface, leading to the
production of dense waters at deeper levels (typically below 2000 m) in the
Weddell Sea after 2004.
Deep convection affects not only the hydrographic properties of the water
masses but also the velocity and transport of AABW around most of the
Southern Ocean. Two major changes are observed in the transport of dense
waters after the opening of the polynya (Fig. 15). The first change occurs
ca. 2004 due to the large volume of dense water being produced; the second
change occurs primarily after 2007 as a consequence of an increase in ocean
current velocities. With the initiation of deep convection, it appears that
the reanalysis data more closely represent the real ocean, reproducing dense
water velocities, transport and export of WSBW that mimic the observations.
However, primarily after 2007, WSBW is widely produced by deep convection
and the current velocities increase, leading to estimates of AABW transport
that largely exceed the observations in the analyzed cases. Deep convection
can also contribute to increased transport estimates due to its effect on
oceanic pressure gradients. This mechanism causes a rise in the isopycnals
south of the ACC and an increase in the ACC strength. Consequently, the
Weddell Gyre (Martin et al., 2013) and the current velocities are also
intensified. Moreover, the sea ice retreat enables a more direct effect of
wind stress over the ocean, which may have also intensified the Weddell
Gyre's barotropic circulation depicted in the reanalysis data and
consequently contributed to convective mixing. Moreover, the transport by
ocean currents is enhanced, which facilitates the spreading of the dense
AABW plume.
The results reveal that the ECCO2 reanalysis product requires extensive
improvements regarding the representation of AABW in the Southern Ocean.
Based on the careful comparisons performed, reanalysis output from the
1992–2004 period can be suitable for studies focused on the AABW
climatological distribution and velocity/transport high-frequency
variability within the Weddell Sea. Low-frequency AABW variability is
directly influenced by model drift and should not be investigated using the
reanalysis output. During this period, the misrepresentation of the bottom
layer in the ECCO2 product is comparable with the performance of other
models (Heuzé et al., 2013), which demonstrates that dense water
representation is a limiting factor in hydrographic modeling. Most of these
models also use deep convection as an important mechanism for dense water
formation. However, in the ECCO2 reanalysis product, a combination of
factors leads to an unrealistic scenario in the more recent years, i.e., the
currents are much stronger than expected and the Weddell Sea is flooded with
very dense waters that occupy nearly the entire water column. Therefore, the
use of data from the latter period (2005–2011) is not advisable for any
purpose.
These findings reinforce the need for an accurate representation of sea ice
coverage and variability and of the processes occurring over both shelves
and slopes for conducting adequate simulations of deep Southern Ocean
hydrography. Improvements in the limitations related to model resolution,
ice shelf representation, and sub-grid physical parameterizations, e.g.,
vertical mixing, convective processes, downslope flow, and sea ice growth,
must be considered for optimal results. Despite the deficiencies of the
ECCO2 output evaluated in this study, it is important to highlight the
improved performance of this reanalysis in reproducing the AABW properties
(for the period 1992–2004) in comparison with other commonly used modeled
data, such as OCCAM and ORCA025 (Renner et al., 2009; K12).
The ECCO2 reanalysis data set is an ongoing project, and several efforts have
been made to improve key points, e.g., ice shelf representation. Recent
studies have applied regional ECCO2 configurations modified to include
explicit representations of ice shelves to investigate the process in the
ocean and ice interface (Heimbach and Losch, 2012; Schodlok et al., 2012; Xu et
al., 2012). Moreover, the centerpiece of the ECCO2 is the application of an
adjoint-method optimization of the global eddying configuration, which would
enable a much larger number of control variables than the Green's function
method. The adjoint-method optimization method has already been proven to be
valuable (e.g., Mazloff et al., 2010; Ito et al., 2010), and its application
both globally and for a longer time period than the actual SOSE (Southern
Ocean State Estimation) regional solution would produce a substantial
improvement.
Finally, long-term observational studies in important regions of the AABW
export must be conducted. The focus of these studies should be extended
beyond the Weddell and Ross Sea regions to less explored (but also important)
areas, such as the Kerguelen Plateau region, which has shown the poorest
representation of AABW variability. In addition, the lack of observations for
winter months affects the quality of reanalysis assimilation and limits the
assessment of these data sets and models. The use of new technologies for
sampling during wintertime under seasonal ice cover, such as Argo, Deep Argo
floats, sea gliders, and ice-tethered buoys, and the use of marine mammals as
platforms for data collection in shelf and shelf break regions should be
encouraged.