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Changes in subduction in the South Atlantic Ocean during the 21st century in the CCSM3


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The Community Climate System Model version 3 is used to analyse changes in water mass subduction rates in the South Atlantic Ocean over the 21st century. The model results are first compared to observations over 1950-2000, and shown to be rather good. The subduction rates do not change significantly over the 21st century, but the densities at which water masses form become significantly lighter. The strong westerly winds in this region do not change much, which suggests small changes to the rate at which the Atlantic sector of the Southern Ocean takes up heat and carbon dioxide over the 21st century.
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Changes in subduction in the South Atlantic Ocean during the
21st century in the CCSM3
Marlos Goes,
Ilana Wainer,
Peter R. Gent,
and Frank O. Bryan
Received 21 November 2007; revised 2 February 2008; accepted 14 February 2008; published 18 March 2008.
[1] The Community Climate System Model version 3 is
used to analyse changes in water mass subduction rates in the
South Atlantic Ocean over the 21st century. The model
results are first compared to observations over 19502000,
and shown to be rather good. The subduction rates do not
change significantly over the 21st century, but the densities at
which water masses form become significantly lighter. The
strong westerly winds in this region do not change much,
which suggests small changes to the rate at which the Atlantic
sector of the Southern Ocean takes up heat and carbon
dioxide over the 21st century.
Citation: Goes, M., I. Wainer,
P. R. Gent, and F. O. Bryan (2008), Changes in subduction in the
South Atlantic Ocean during the 21st century in the CCSM3,
Geophys. Res. Lett., 35, L06701, doi:10.1029/2007GL032762.
1. Introduction
[2] Water mass formation in the Atlantic sector is an
important component of Southern Ocean ventilation. Com-
ponents of both Subantarctic Mode Water (SAMW) and
Antarctic Intermediate Water (AAIW) are formed in this
basin, see Sloyan and Rintoul [2001] for example, and much
of the Antarctic Bottom Water is formed in the Weddell Sea.
This region is important because it takes up a lot of heat and
from the atmosphere. Therefore, an important question
is how water mass formation will c hange ov er the
21st century, especially if formation rates weaken, and the
South Atlantic can’t take up as much heat and CO
as it
does at present? There are conflicting published answers to
this question. Russell et al. [2006] analyze future changes in
the Southern Hemisphere from the latest GFDL cl imate
models. They suggest that the ocean may take up more CO
in the future, because of a small increase in the strong
westerlies over the high latitude Southern Ocean. Le Quere
et al. [2007] estimate from observations that the Southern
Ocean sink of CO
has weakened between 1981 and 2004 at
a faster rate than expected from just the reduced solubility
as CO
increases. They attribute this additional weakening
to the observed increase in the Southern Ocean westerlies.
3] Possible changes in South Atlantic water mass for-
mation will be analyzed using simulations from the Com-
munity Climate System Model version 3 (CCSM3). The
justification is that SAMW and AAIW are simulated much
better in the ocean components of climate models used in
the IPCC fourth assessment report than in previous reports,
see Sorensen et al. [2001, Figure 2], for example. Very
recently, Sloyan and Kamenkovich [2007] have made a
detailed study of the SAMW and AAIW simulation in eight
of the climate models used in the IPCC fourth assessment
report. The CCSM3 is one of the three best models in
simulating the density structure of the Antarctic Circumpo-
lar Current, the AAIW sa linity minimum in the Indian
Ocean, and the simulation of SAMW and AAIW in general.
Their study is only of present day simulations, and shows
results mostly from the southern Indian and Pacific Oceans.
In this paper, we show results just from the South Atlantic
Ocean, and include projections of future changes over the
21st century.
2. The CCSM3 Model and Integrations Analysed
[4] The CCSM3 is a general circulation climate model that
couples atmosphere, land, ocean, and sea ice components. An
overview and a description of its present day climate simu-
lation using T85 (1.4°) atmosphere and land resolution, and a
nominal 1° ocean and sea ice resolution are given by Collins
et al. [2006]. First, two long control integrations with 1870
values of greenhouse gases were run. Then a total of nine
20th century integrations, that branched from the control runs
at different times, wer e run for 130 years; 18701999. These
20th century integrations were all forced using observed
variations of solar input, volcanic and other aerosols, and
observed values of greenhouse gases such as CO
21st century integrations continued for 100 years from the
end of the 20th century runs, and spanned years 2000 2099.
The runs analyzed in this paper all used the A1B SRES
scenario for the projected future increase in CO
, which
increases from 370 ppm in 2000 to 690 ppm in 2100. There
were seven members in this ensemble of 21st century
integrations, and much more detail on all these runs is given
by Meehl et al. [2006, section 2].
3. The 20th Century South Atlantic Simulation
[5] This section will be very brief, and the reader is
referred to the recent Sloyan and Kamenkovich [2007] study
for a more thorough analysis of SAMW and AAIW in
present day simulations from eight climate models incl ud-
ing the CCSM3.
6] Figure 1a shows the zonal average salinity in color
and the potential density contour lines in white in the South
Atlantic down to 1.5 km from the Levitus et al. [1998] data.
It shows the salinity minimum that defines AAIW going
from the surface at 50° –55°S, reaching a maximum depth
of about 840 m near 35°S, and then rising to a depth near
680 m equatorward of 20°S. Figure 1b shows the same
fields, but for the ensemble mean of 20th century runs
between 19501999. It has a similar subsurface salinity
GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L06701, doi:10.1029/2007GL032762, 2008
Department of Physical Oceanography, University of Sao Paulo, Sao
Paulo, Brazil.
National Center for Atmospheric Research, Boulder, Colorado, USA.
Copyright 2008 by the American Geophysical Union.
L06701 1of5
distribution, with the salinity minimum at the surface near
45° –50°S, reaching a maximum depth of about 830 m near
40°S, and then rising to a depth of about 700 m equatorward
of 30°S. Thus, the salinity is about 0.2 psu fresher than
observations near the surface at 55°S, and 0.1 psu more
saline at depth equatorward of 30°S.
7] In observational studies, the location of the salinity
minimum is always taken as the s = 27.2 isopycnal. A very
nice feature of the CCSM3 simulation is that the salinity
minimum below 300 m is also characterized by the same
isopycnal. A quantitative measure of the model error is the
depth of the s = 27.2 isopycnal and the average salinity on
this isopycnal at 30° and 50°S. These values are 730 m and
34.45 psu at 30°S in the model, and 825 m and 34.37 psu in
the Levitus et al. [1998] data. At 50°S, the values are 150 m
and 33.92 psu and 165 m and 34.06 psu in the model and
observations, respectively. These errors imply that the
model temperature is >1° C too cold around 150 m at
50°S, and <1°C too warm around 730 m at 30°S. Overall,
we conclude that the simulation of water mass formati on in
the South Atlantic in CCSM3 is rather good. This gives
confidence that the model will show realistic changes in the
21st century integrations.
4. Water Mass Formation Changes During the
21st Century
[8] We first look at the changes in salinity and potential
density over the 21st century, and their variability across the
ensemble of 21st century runs. Figure 1c shows the zonal
average salinity and potential density in the South Atlantic
for the ensemble mean of 21st century runs between 2080
2099. It shows that AAIW becomes fresher, and the
associated isopycnals deepen considerably, over the 21st
century. Contrasting changes in the surface ocean occur
elsewhere: the Weddell Sea freshens significantly, but the
area around 25°S becomes more saline.
9] The solid line in Figure 2a shows the ensemble and
zonal average of the s = 27.2 isopycnal depth at 30°S
versus time between 18702100, and Figure 2b shows the
same for salinity on the 27.2 isopycnal. The shading shows
the standard deviation across the ensemble. The depth of the
27.2 isopycnal shows variabil ity on the order of 10 m
between 18701980, but then begins to deepen monoton-
ically. The deepening starts late in the 20th century, but then
continues strongly throughout the 21st century, so that the
depth reaches 810 m in 2100, compared to the average of
720 m for 18701980. Figure 2b shows similar changes
with time for salinity on the 27.2 isopycnal. The average
value, with small variability, is nearly 34.45 psu over 1870
1980, but then starts to get fresher and this continues over
the 21st century to a value of 34.41 psu at 2100. The
standard deviatio n sho ws tha t both the deepenin g and
freshening of the 27.2 isopycnal are highly significant
compared to the ensemble variability.
10] The upper 1 km of the ocean warms throughout the
South Atlantic region, with a near-surface temperature rise
of 12°C. However, Figure 2 shows that the 27.2 isopycnal
Figure 1. Plot of zonal average salinity and density contours in the South Atlantic: (a) Levitus et al. [1998] data, (b) CCSM3
ensemble average h19501999i, and (c) CCSM3 ensemble average h20802099i.
deepens sufficiently that the temperature on it actually goes
down a little.
11] The measure of water mass formation rates used is to
diagnose the subduction across the base of the ocean mixed
layer as a function of potential density, see Karstensen and
Quadfasel [2002]. The model mixed layer depth is the
depth where the local buoyancy gradient equals the max-
imum bulk buoyancy gradient calculate d between the
surface and any depth, see Large et al. [1997, p. 2427].
First, the annual maximum depth of the mixed layer is
averaged over a particular decade to calculate the depth, H.
Subduction is then calculated with respect to this depth
under the assump tion that most wate r mass formation
occurs in winter when the mixed layer depth is a maximum.
It is also assumed that using the decadal average value does
not alias the interannual variability. The total subduction is
calculated as,
Sub sðÞ¼
u rH þ w½dA; ð1Þ
where u and w are the decadal averaged horizontal and
vertical velocities at the depth H. The total subduction is an
integral of the local positive subduction over the area
defined by the adjace nt isopycnals s Ds/2 and s + Ds /2,
where Ds = 0.2.
12] Figure 3 shows the transport of subducted waters for
24.9 < s < 27.5 in each decade between 1870 2100 from
one of the 20th and 21st century runs. The blue line is the
average value for h18701999 i, and the red line the average
value for h20002099i. For 24.9 < s < 26.3, the subduction
transports are relatively small, and do not change much over
the 21st century. However, there are signif icant changes for
26.3 < s < 27.1, which is the density range at which SAMW
and AAIW are formed in the CCSM3. The largest changes
are a 3 Sverdr up (Sv) increase at s = 26.8, and a 4 Sv
decrease at s = 27.0, with much smaller increases of <1 Sv
at s = 26.4 and 26.6. The total subduction transport in this
density range is about 15 Sv, which is close to the
observational estimate of Karstensen and Quadfasel
[2002], and the inverse model estimate of Sloyan and
Rintoul [2001]. Total subduction in this density range does
not change significantly over the 21st century.
13] Figure 4 shows how the subduction is divided into
the lateral and vertical velocity components at the higher
densities in the range 26.5 < s < 27.3 versus time 1870
2100 from the same 20th and 21st century run as Figure 3.
The lateral component dominates the vertical component in
all these density classes due to the frontal characteristic of
this region, in agreement with the results of Karstensen and
Quadfasel [2002]. In fact, the vertical component is nega-
tive at s = 26.8 and 27.0. Figure 4 shows that the total
Figure 2. Plot of (a) depth of the s = 27.2 isopycnal, and
(b) salinity on 27.2 at 30° in the South Atlantic versus time
18702100. The line is the ensemble mean, and the shading
is the ensemble standard deviation.
Figure 3. Transport of subducted waters for each decade between 18702100 versus density 24.9 < s < 27.5 in one 20th
and 21st century run. The blue line shows the average h18701999i, and the red line the average h20002099i.
subduction rates increase at s = 26.6 and 26.8 over the
21st century, but decrease at s = 27.0 and 27.2. There is
very little subduction at s = 27.2 after about 2040 for this
A1B scenario run. This again shows that the total subduc-
tion rates of SAMW and AAIW do not change much over
the 21st century, but move to a significantly lower density
by about Ds = 0.2.
14] Figure 5a shows the decadal maximum mixed layer
depth H, and the density at this depth from the same
21st century run for h20002009i, and Figure 5b shows
the same fields for h20902099i h2000 2009i. There is
relatively little change in the maximum mixed layer depth
over the 21st century, although the location of the deepest
mixed layer near 40°S moves a little to the south. However,
there is a consistent change to lighter densities throughout
the South Atlantic. The largest change of Ds = 0.5 occurs
between 30° and 40°S where the h2000 2009i density is
around s = 26.0. South of 45°S, there is a consistent change
to a ligh ter density by Ds = 0.2 all across the South
Atlantic, which covers the formation region of SAMW
and AAIW. Figure 5b shows how the areas of different
density classes at the base of the maximum mixed layer
change over the 21st century. Additional calculations show
that the changes in subduction over the 21st century at the
higher densiti es shown in Figures 3 and 4 are mostly due to
the changing areas of the density intervals over which the
integral is taken, with the integrand in equation (1) staying
nearly constant.
5. Conclusions
[15] There are four main conclusions from this study:
16] a) The CCSM3 simulation in the South Atlantic, and
of AAIW properties in particular, is good enough to give
confidence in the model’s projected changes over the
21st century.
17] b) There is natural variability, but small trends, in
the ocean simulation up to 1980. Then changes and trends
appear, which continue throughout the 21st century. These
changes under the A1B scenario forcing are very signif-
icant with respect to both variability within each run, and
the variability across the ensemble of 21st century runs.
18] c) In the South Atlantic, the isopycnals become
deeper and the water on them becomes a little fresher and
colder over the 21st century. Isopycnals getting deeper and
salinity decreasing on isopycnals is exactly the interior
ocean signal that Bindoff and McDougall [1994] showed
would result from warming and freshening at the ocean
19] d) Analysis of the subduction across the maximum
mixed layer depth shows that the total SAMW and AAIW
formation rates do not change significantly over the
21st century. However, by 2100 these water masses are
Figure 4. Plot of the lateral and vertical velocity subduction rates for 26.5 < s < 27.3 in the South Atlantic versus time
18702100 in one 20th and 21st century run.
formed at significantly lighter densities by Ds = 0.2 than
in 2000. This pattern of change in subduction rates is
consistent across the ensemble of CCSM3 21st century
A1B integrations.
20] Thus, the CCSM3 suggests that the uptake of heat
and CO
in the South Atlantic over the 21st century will not
be affected by reduced rates of water mass formation. This
conclusion is supported by analyses of water mass trans-
formation rates at the ocean surface and passive tracers done
on the same model runs, which are not shown. Russell et al.
[2006] analyze future changes in the Southern Hemisphere
from the latest GFDL climate models. They suggest that the
ocean may take up more CO
in the future, because of a
small increase in the strong westerlies over the high latitude
Southern Ocean. These westerlies increase by only 23% in
the South Atlantic sector over the 21st century in the
CCSM3, which is not statistically significant, although the
maximum does move slight ly polewards. Thus, this mech-
anism for increased ocean uptake does not occur in the
CCSM3. Note again that Le Quere et al. [2007] suggest that
increased winds have led to reduced CO
uptake over
1981 2004. Finally, the question of whether the Southern
Ocean continues to take up the same fraction of CO
emissions in the future can only be calculated in a model
with a full carbon cycle. Fung et al. [2005] show results
from such a model embedded in an earlier version of this
climate model, CSM1.4. They show that, even if future
ocean circulation changes do not affect the uptake of CO
whether the ocean continues to take up the same fraction of
emissions depends upon the rate of atmospheric CO
increase and the reduced solubility due to rising sea surface
temperatures. Unfortunately, future projections with a car-
bon cycle component have not been performed with the
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F. O. Bryan and P. R. Gent, National Center for Atmospheric Research,
P.O. Box 3000, Boulder, CO 80307-3000, USA. (
M. Goes and I. Wainer, Department of Physical Oceanography,
University of Sao Paulo, 05508-900 Sao Paulo, Brazil.
Figure 5. Density, in color, at the annual maximum mixed
layer depth, black contours, in the 21st century run: (a)
h20002009i and (b) h20902099ih2000 2009i.
... . Results also show a statistically significant shoaling of the isopycnals within the circumpolar AAIW, accompanying a decrease in density, and an equatorward spreading of the salinity anomalies at the sub-surface [Durack and Wijffels, 2010;Schmidtko and Johnson, 2012]. A further decrease in the AAIW density is also projected for the 21st century in climate models [Goes et al., 2008]. ...
... We consider two locations in the central part of the basin, at 25 • W/30 • S and 25 • W/35 • S(Figure 4). At both latitudes, SODA (black line) shows an increase in salinity and temperature in the late 1980s/beginning of 1990s until the end of the series(Figure period (Figure 4e,f); a feature that agrees with climate projections of the AAIW[Goes et al., 2008]. The effect of the density decrease at the minimum salinity depth is more prominent at 35 • S than at 30 • S. ...
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In this study we investigate the sub-surface salinity changes on decadal timescales across the Subtropical South Atlantic Ocean using two ocean reanalysis products, the latest version of the Simple Ocean Data Assimilation and the Estimating the Circulation and Climate of the Ocean, Phase II, as well as with additional climate model experiments. Results show that there is a recent significant salinity increase at the core of the salinity minimum at intermediate levels. The main underlying mechanism for this sub-surface salinity increase is the lateral advective (gyre) changes due to the Southern Annular mode variability, which conditions an increased contribution from the Indian Ocean high salinity waters into the Atlantic. The global warming signal has a secondary but complementary contribution. Latitudinal differences at intermediate depth in response to large-scale forcing are in part caused by local variation of westward propagation features, and by compensating contributions of salinity and temperature to density changes.
... This is not the case in iPOP2-TRACE: AAIW northward penetration is not controlled by upstream AAIW production. We compare the AAIW subduction rate, which is the subduction across the base of the ocean mixed layer in the South Atlantic AAIW formation region (Goes et al., 2008). The AAIW subduction rate is 4.6 sverdrup (Sv, 10 6 m 3 /s) during LGM and 6.0 Sv during HS1 in iPOP2-TRACE, indicating the upstream AAIW production during HS1 is not lower but even higher. ...
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... In contrast to SAMW, AAIW fills the layer at depths of 800-1000 m in the subtropical and tropical oceans, with only minor changes of its properties (Georgi, 1979;Piola and Georgi, 1982;Wong, 2005;Iudicone et al., 2007). Previous studies have already proved that the formation and erosion of SAMW and AAIW are closely linked to interactions at the air-sea surface Rintoul, 2001, Banks et al., 2002;Goes et al., 2008;Saenko et al., 2011), suggesting that changes in atmospheric forcing, such as the strength of westerlies and associated heat flux, could strongly impact the production and characteristics of SAMW and AAIW in the South Pacific. ...
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Based on an eddy permitting ocean general circulation model, the response of water masses to two distinct climate scenarios in the South Pacific is assessed in this paper. Under annually repeating atmospheric forcing that is characterized by different westerlies and associated heat flux, the response of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) is quantitatively estimated. Both SAMW and AAIW are found to be warmer, saltier and denser under intensified westerlies and increased heat loss. The increase in the subduction volume of SAMW and AAIW is about 19.8 Sv (1 Sv = 10⁶ m³ s⁻¹). The lateral induction term plays a dominant role in the changes in the subduction volume due to the deepening of the mixed layer depth (MLD). Furthermore, analysis of the buoyancy budget is used to quantitatively diagnose the reason for the changes in the MLD. The deepening of the MLD is found to be primarily caused by the strengthening of heat loss from the ocean to the atmosphere in the formation region of SAMW and AAIW.
... The changes in surface temperature, and consequently the winter mixed layers, may account for the small changes (ECHO-G), or increases in (CM2.1), in the net subduction of SAMW in these models (Fig. 6d). Banks et al. (2002) (for the Indian Ocean) and Goes et al. (2008) (for the South Atlantic) concluded that changes in the SAMW density over time did not necessarily result in formation and subduction changes. We find a small change in the model mean subduction of SAMW in the Atlantic basin (1 Sv); however, there is a 14% decrease in the model mean in the Indian Ocean. ...
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... In this region, the heat and freshwater fluxes, rather than the Ekman transport, largely determine the subduction rate of fluid into the permanent thermocline (MNW). Karstensen and Quadfasel (2002) and Goes et al. (2008) diagnosed the annual mean subduction rate (for densities lighter than 27.3 kg m 23 ) at a similar magnitude in the Southern Hemisphere. We apply the methods of MNW to analyze long-term changes in water mass subduction rates on neutral density surfaces (g n ) and to determine the contribution of heat, freshwater fluxes, and Ekman transport on the changes in the net buoyancy flux driving subduction in the CSIRO Mk3.5 simulation. ...
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Changes in the temperature, salinity, and subduction of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) between the 1950s and 2090s are diagnosed using the CSIRO Mark version 3.5 (Mk3.5) climate system model Caps under a CO2 forcing that reaches 860 ppm by the year 2100. These Southern Ocean upper-limb water masses ventilate the ocean interior, and changes in their properties have been related to climate change in numerous studies. Over time, the authors follow the low potential vorticity and salinity minimum layers describing SAMW and AAIW and find that the water column in the 2090s shifts to lighter densities by approximately 0.2 kg m-3. The model projects a reduction in the SAMW and AAIW annual mean subduction rates as a result of a combination of a shallower mixed layer, increased potential vorticity at the base of the mixed layer, and a net buoyancy gain. There is little change in the projected total volume of SAMW transported into the ocean interior via the subduction process; however, the authors find a significant decrease in the subduction of AAIW. The authors find overall that increases in the air-sea surface heat and freshwater fluxes mainly control the reduction in the mean loss of the SAMW and AAIW surface buoyancy flux when compared with the effect of changes supplied by Ekman transport because of increased zonal wind stress. In the A2 scenario, there are cooling and freshening on neutral density surfaces less than 27.3 kg m-3 in response to the warming and freshening observed at the ocean's surface. The model projects deepening of density surfaces due to southward shifts in the outcrop regions and the downward displacement of these surfaces north of 45°S. The volume transport across 32°S is predicted to decrease in all three basins, with southward transport of SAMW and AAIW decreasing by up to 1.2 and 2.0 Sv (1 Sv ≡ 106 m3 s-1), respectively, in the Indian Ocean. These projected reductions in the subduction and transport of mode and intermediate water masses in the CSIRO Mk3.5 model could potentially decrease the absorption and storage of CO2 in the Southern Ocean.
Understanding the time-evolution of subduction systems on Earth, from their initiation into pro-gressive subduction to eventual termination, is lacking in important ways. Numerical models of subducting systems are central to developing an improved understanding, yet achieving Earth-like subduction (asymmetric and single-sided) within a convecting mantle remains a major challenge. A few of the more salient features of subduction zones, such as 1) partitioning of subduction velocity into the two modes of subduction (plate advance and slab rollback), 2) the accurate along-trench curvature of subduction zones, and 3) episodic character of tectonic stress regimes in the overriding plate are not features that emerge self-consistently. Some progress has been made using simplified models of free subduction that avoid prescribing the kinematics (convergence rate, plate speed) or geometry (trench location, dip angle) of the system, thereby allowing it to evolve naturally, driven only by its own negative buoyancy (Faccenna et al., 2001; Funiciello et al., 2003; Schellart, 2004; Morra et al., 2006; Royden and Husson, 2006; Stegman et al., 2006; Schellart et al, 2007; Goes et al, 2008). The strength of the subducting plate controls the particular style of subduction, thereby producing an associated upper mantle slab geometry, (Billen, 2008; Giuseppe et al., 2008; Stegman et al., 2010; Ribe, 2010). However, such models typically adopt simplified descriptions for the subducting plate (uniform viscosity, thickness, and density) and assume no heat transfer (i.e. the energy equation is not included). We investigate the development of subduction in fully dynamic system within a convecting mantle with initial conditions based on a prescribed thermal boundary layer that includes a single, mature oceanic plate surrounded by younger oceanic plates. We make use of the finite-volume multigrid code StagYY (Tackley, 2008) to run the models. Each model run uses a depth-and temperature-dependent rheology (Arrhenius law). Plastic yielding is used wherein the yield stress has both brittle and ductile components. By specifying a temperature at each grid point, the geometry and physical properties of plates and background mantle are specified. Because the viscosity is a function of temperature and stress, the system is extremely non-linear and the integrated strength of the lithosphere in critical regions (such as where bending occurs) is strongly time-dependent. Subduction is initiated by adding a small perturbation. The boundary conditions for the 2D models are the same across all model runs. The prescribed thermal boundary layer overlies an isothermal mantle with a bottom insulating (zero heat flux) thermal boundary condition. A zero-density low-viscosity "sticky-air" layer is defined at the top of the model. The sticky-air layer is necessary for producing realistic subduction (Schmeling et al. 2008). At the sidewalls a wrapping boundary condition is applied. We continue investigating the development of subduction within this model framework, moving towards more realistic schemes. Our future work looks toward modeling in three spatial dimensions and using the results of the simple subduction models to examine more complex interactions.
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The paleoclimate version of the National Center for Atmospheric Research Community Climate System Model version 3 (NCAR-CCSM3) is used to analyze changes in the water formation rates in the Atlantic, Pacific, and Indian Oceans for the Last Glacial Maximum (LGM), mid-Holocene (MH) and pre-industrial (PI) control climate. During the MH, CCSM3 exhibits a north-south asymmetric response of intermediate water subduction changes in the Atlantic Ocean, with a reduction of 2 Sv in the North Atlantic and an increase of 2 Sv in the South Atlantic relative to PI. During the LGM, there is increased formation of intermediate water and a more stagnant deep ocean in the North Pacific. The production of North Atlantic Deep Water (NADW) is significantly weakened. The NADW is replaced in large extent by enhanced Antarctic Intermediate Water (AAIW), Glacial North Atlantic Intermediate Water (GNAIW), and also by an intensified of Antarctic Bottom Water (AABW), with the latter being a response to the enhanced salinity and ice formation around Antarctica. Most of the LGM intermediate/mode water is formed at 27.4 <σθ < 29.0 kg/m3, while for the MH and PI most of the subduction transport occurs at 26.5 < σθ < 27.4 kg/m3. The simulated LGM Southern Hemisphere winds are more intense by 0.2-0.4 dyne/cm2. Consequently, increased Ekman transport drives the production of intermediate water (low salinity) at a larger rate and at higher densities when compared to the other climatic periods.
Understanding natural climate variability in the North Atlantic region is essential not only to assess the sensitivity of atmosphere–ocean climate signal exchange and propagation, but also to help distinguish between natural and anthropogenic climate change. The North Atlantic Oscillation is one of the controlling modes in recent variability of atmosphere–ocean linkages and ice/freshwater fluxes between the Polar and North Atlantic Ocean. Through these processes the NAO influences water mass formation and the strength of the Atlantic Meridional Overturning circulation and thereby variability in ocean heat transport. However, the impact of the NAO as well as other forcing mechanisms on multidecadal timescales such as total solar irradiance on Eastern North Atlantic Central Water production, central water circulation, and climate signal propagation from high to low latitudes in the eastern subpolar and subtropical basins remains uncertain. Here we use a 1200yr long benthic foraminiferal Mg/Ca based temperature and oxygen isotope record from a ~900m deep sediment core off northwest Africa to show that atmosphere–ocean interactions in the eastern subpolar gyre are transferred at central water depth into the eastern boundary of the subtropical gyre. Further we link the variability of the NAO (over the past 165yrs) and solar irradiance (Late Holocene) and their control on subpolar mode water formation to the multidecadal variability observed at mid-depth in the eastern subtropical gyre. Our results show that eastern North Atlantic central waters cooled by up to ~0.8±0.7°C and densities decreased by σθ=0.3±0.2 during positive NAO years and during minima in solar irradiance during the Late Holocene. The presented records demonstrate the sensitivity of central water formation to enhanced atmospheric forcing and ice/freshwater fluxes into the eastern subpolar gyre and the importance of central water circulation for cross-gyre climate signal propagation during the Late Holocene.
Subduction, water mass transformation, and transport rates in the Indo-Pacific Ocean are diagnosed in a recent version of the Canadian Centre for Climate Modelling and Analysis coupled model. It is found that the subduction across the base of the winter mixed layer is dominated by the lateral transfer, particularly within the relatively dense water classes corresponding to the densest mode and intermediate waters. However, within lighter densities, including those characterizing the lighter varieties of mode waters, the vertical transfer has a strong positive input to the net subduction. The upper-ocean volume transports across 30°N and 32°S are largest within the density classes that correspond to mode waters. In the North Pacific, the buoyancy flux converts the near-surface waters mostly to denser water classes, whereas in the Southern Ocean the surface waters are transformed both to lighter and denser water classes, depending on the density. In response to a doubling of CO2, the subduction, transformation, and transport of mode waters in both hemispheres shift to lighter densities but do not change significantly, whereas the subduction of intermediate waters decreases. The area of large winter mixed layer depths decreases, particularly in the Southern Hemisphere. In the low latitudes, the thermocline water flux that enters the tropical Pacific via the western boundary flows generally increases. However, its anomaly has a complex structure, so that integrated estimates can be sensitive to the isopycnal ranges. The upper part of the Equatorial Undercurrent (EUC) strengthens in the warmer climate, whereas its lower part weakens. The anomaly in the EUC closely follows the anomaly in stratification along the equator. The Indonesian Throughflow transport decreases with part of it being redirected eastward. This part joins with the intensified equatorward thermocline flows at the western boundaries and contributes to the EUC anomaly.
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The Antarctic intermediate and deep water masses present in the Atlantic sector of the Southern Ocean were quantified through the inverse method known as Optimum Multiparameter (OMP) analysis. The method was applied to the Simple Ocean Data Assimilation (SODA) product, which assimilates real ob-served ocean data into a hydrodynamic model. Results here show that the SODA dataset is able to capture reasonably well the intermediate and deep water mass (i.e. Warm Deep Water, Weddell Sea Deep Water, Weddell Sea Bottom Water, and Circumpolar Deep Water) regional distribution and contribution to the total mixture in the Weddell Sea and Weddell-Scotia Confluence. Those regions are, respectively, the main Antarctic Bottom Water source and export areas to the global ocean. We infer some aspects of the ocean circulation from the water mass distribution obtained. However, some efforts are still needed to better represent the deep salinity in these areas. The weak representation of this hydrographic parameter could be associated with the model's lack of important cryospheric processes directly involved with bottom water formation.
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Climate change scenario simulations with the Community Climate System Model version 3 (CCSM3), a global coupled climate model, show that if concentrations of all greenhouse gases (GHGs) could have been stabilized at the year 2000, the climate system would already be committed to 0.4°C more warming by the end of the twenty-first century. Committed sea level rise by 2100 is about an order of magnitude more, percentage-wise, compared to sea level rise simulated in the twentieth century. This increase in the model is produced only by thermal expansion of seawater, and does not take into account melt from ice sheets and glaciers, which could at least double that number. Several tenths of a degree of additional warming occurs in the model for the next 200 yr in the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) B1 and A1B scenarios after stabilization in the year 2100, but with twice as much sea level rise after 100 yr, and doubling yet again in the next 100 yr to 2300. At the end of the twenty-first century, the warming in the tropical Pacific for the A2, A1B, and B1 scenarios resembles an El Niño-like response, likely due to cloud feedbacks in the model as shown in an earlier version. Greatest warming occurs at high northern latitudes and over continents. The monsoon regimes intensify somewhat in the future warmer climate, with decreases of sea level pressure at high latitudes and increases in the subtropics and parts of the midlatitudes. There is a weak summer midlatitude soil moisture drying in this model as documented in previous models. Sea ice distributions in both hemispheres are somewhat over- extensive, but with about the right ice thickness at the end of the twentieth century. Future decreases in sea ice with global warming are proportional to the temperature response from the forcing scenarios, with the high forcing scenario, A2, producing an ice-free Arctic in summer by the year 2100.
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The ventilation of the permanent thermocline of the Southern Hemisphere gyres is quantified using clima- tological and synoptic observational data. Ventilation is estimated with three independent methods: the kinematic method provides subduction rates from the vertical and horizontal fluxes through the base of the mixed layer, the water age uses in situ age distribution of thermocline waters, and the annual-mean water mass formation through air-sea interaction is calculated. All three independent estimates agree within their error bars, which are admittedly large. The subduction rates are mainly controlled through their vertical and lateral components with only minor transient eddy contributions. The vertical transfer, derived from Ekman pumping, ventilates over most of the areas of the subtropical gyres, while lateral transfer occurs mainly along the Subtropical and Subantarctic Fronts, where it injects mode and intermediate waters. For the permanent thermocline the overall ventilation of the South Atlantic is about 21 Sv (Sv ( 106 m3 s 21). Of this, lateral transfer contributes 10 Sv, mainly in the Brazil-Malvinas confluence zone and to the northeast of Drake Passage. The effective vertical transfer at the bottom of the mixed layer is only two-thirds of the Ekman pumping due to strong northward forcing of the mixed layer itself. The Indian Ocean is ventilated at a rate of 35 Sv with equal lateral and vertical contributions. The South Pacific's overall ventilation is 44 Sv of which the lateral input contributes little more than half. West of 1308W, the South Pacific is ventilated through Ekman pumping and with only minor lateral transfer. In the east lateral transfer dominates between 10 8 and 208S and along the Subantarctic Front in a narrow density range. Combining overall transports with earlier estimates for the Northern Hemisphere gives a ventilation of the World Ocean's permanent thermocline of about 160 Sv. Analysis of atmospheric reanalysis air-sea flux data reveals an overall increase in the formation of thermocline waters for all three Southern Hemisphere oceans.
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Abstract Antarctic Intermediate Water is formed ,at the ,high mid-latitudes of the Southern Ocean. In many ,ocean ,general circulation model ,simulations with coarse resolution and a z-coordinate, a mid-depth salinity minimum characteristic of this intermediate water is reproduced. However, for the real ocean it remains unclear which are the dominant,processes in the formation of this water mass and which are the source regions for contributing surface waters. To elucidate ,such processes ,and ,quantify intermediate water formation rates, two experiments with an ocean general circulation model were conducted. In one experiment, the traditional parameterization of horizontal and vertical mixing was applied, while the second model included the Gent-McWilliams parameterization for an eddy-induced transport velocity. In the latter application, the production and meridional export of intermediate water was found to be larger than in the
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Changes in atmospheric forcing can affect the subsurface water column of the ocean by three different mechanisms. First, warmed mixed-layer water that is subducted into the ocean interior will cause subsurfce warming; second, the subducted surface water can be freshened through changes in evaporation and precipitation; and third, the properties at a given depth may be changed by the vertical displacement of isotherms and isohalines without changes of water masses. Linear relations are derived for the relative strength of each process in terms of the observed changes of potential temperature and salinity in two different coordinate frames: 1) constant density surfaces, and 2) isobaric surfaces. -from Authors
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The Southern Ocean's Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are two globally significant upper-ocean water masses that circulate in all Southern Hemisphere subtropical gyres and cross the equator to enter the North Pacific and North Atlantic Oceans. Simulations of SAMW and AAIW for the twentieth century in eight climate models [GFDL-CM2.1, CCSM3, CNRM-CM3, MIROC3.2(medres), MIROC3.2(hires), MRI-CGCM2.3.2, CSIRO-Mk3.0, and UKMO-HadCM3] that provided their output in support of the Intergovernmental Panel on Climate Change's Fourth Assessment Report (IPCC AR4) have been compared to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atlas of Regional Seas. The climate models, except for UKMO-HadCM3, CSIRO-Mk3.0, and MRI-CGCM2.3.2, provide a reasonable simulation of SAMW and AAIW isopycnal temperature and salinity in the Southern Ocean. Many models simulate the potential vorticity minimum layer and salinity minimum layer of SAMW and AAIW, respectively. However, the simulated SAMW layer is generally thinner and at lighter densities than observed. All climate models display a limited equatorward extension of SAMW and AAIW north of the Antarctic Circumpolar Current. Errors in the simulation of SAMW and AAIW property characteristics are likely to be due to a combination of many errors in the climate models, including simulation of wind and buoyancy forcing, inadequate representation of subgrid-scale mixing processes in the Southern Ocean, and midlatitude diapycnal mixing parameterizations.
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Nine hydrographic sections are combined in an inverse box model of the Southern Ocean south of ∼ 12°S. The inverse model has two novel features: The inclusion of independent diapycnal flux unknowns for each property and the explicit inclusion of air-sea fluxes (heat, freshwater, and momentum) and the water mass transformation they drive. Transformation of 34 × 106 m3 s-1 of Antarctic Surface Water by air-sea buoyancy fluxes, and cooling and freshening where Subantarctic Mode Water outcrops, renews cold, fresh Antarctic Intermediate Water of the southeast Pacific and southwest Atlantic. Relatively cold, fresh mode and intermediate water enter the subtropical gyres, are modified by air-sea fluxes and interior mixing, and return poleward as warmer, saltier mode and intermediate water. While the zonally integrated meridional transport in these layers is small, the gross exchange is approximately 80 × 106 m3 s-1. The air-sea transformation of Antarctic surface water to intermediate water is compensated in the Southern Ocean by an interior diapycnal flux of 32 × 106 m3 s-1 of intermediate water to upper deep water. The small property differences between slightly warmer, saltier intermediate water and cold, fresh Antarctic Surface Water results in a poleward transfer of heat and salt across the Polar Front zone. Mode and intermediate water are crucial participants in the North Atlantic Deep Water overturning and Indonesian Throughflow circulation cells. The North Atlantic Deep Water overturning is closed by cold, fresh intermediate water that is modified to warm, salty varieties by air-sea fluxes and interior mixing in the Atlantic and southwest Indian Oceans. The Indonesian Throughflow is part of a circum-Australia circulation. In the Indian Ocean, surface water is converted to denser thermocline and mode water by air-sea fluxes and interior mixing, excess mode water flows eastward south of Australia, and air-sea fluxes convert mode water to thermocline water in the Pacific.
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A coupled climate model with poleward-intensified westerly winds simulates significantly higher storage of heat and anthropogenic carbon dioxide by the Southern Ocean in the future when compared with the storage in a model with initially weaker, equatorward-biased westerlies. This difference results from the larger outcrop area of the dense waters around Antarctica and more vigorous divergence, which remains robust even as rising atmospheric greenhouse gas levels induce warming that reduces the density of surface waters in the Southern Ocean. These results imply that the impact of warming on the stratification of the global ocean may be reduced by the poleward intensification of the westerlies, allowing the ocean to remove additional heat and anthropogenic carbon dioxide from the atmosphere.
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The Community Climate System Model version 3 (CCSM3) has recently been developed and released to the climate community. CCSM3 is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler. CCSM3 is designed to produce realistic simulations over a wide range of spatial resolutions, enabling inexpensive simulations lasting several millennia or detailed studies of continental-scale dynamics, variability, and climate change. This paper will show results from the configuration used for climate-change simulations with a T85 grid for the atmosphere and land and a grid with approximately 1 degrees resolution for the ocean and sea ice. The new system incorporates several significant improvements in the physical parameterizations. The enhancements in the model physics are designed to reduce or eliminate several systematic biases in the mean climate produced by previous editions of CCSM. These include new treatments of cloud processes, aerosol radiative forcing, land-atmosphere fluxes, ocean mixed layer processes, and sea ice dynamics. There are significant improvements in the sea ice thickness, polar radiation budgets, tropical sea surface temperatures, and cloud radiative effects. CCSM3 can produce stable climate simulations of millennial duration without ad hoc adjustments to the fluxes exchanged among the component models. Nonetheless, there are still systematic biases in the ocean-atmosphere fluxes in coastal regions west of continents, the spectrum of ENSO variability, the spatial distribution of precipitation in the tropical oceans, and continental precipitation and surface air temperatures. Work is under way to extend CCSM to a more accurate and comprehensive model of the earth's climate system.
The effects of more realistic bulk forcing boundary conditions, a more physical subgrid-scale vertical mixing parameterization, and more accurate bottom topography are investigated in a coarse-resolution, global oceanic general circulation model. In contrast to forcing with prescribed fluxes, the bulk forcing utilizes the evolving model sea surface temperatures and monthly atmospheric fields based on reanalyses by the National Centers for Environmental Prediction and on satellite data products. The vertical mixing in the oceanic boundary layer is governed by a nonlocal K-profile parameterization (KPP) and is matched to parameterizations of mixing in the interior. The KPP scheme is designed to represent well both convective and wind-driven entrainment. The near-equilibrium solutions are compared to a baseline experiment in which the surface tracers are strongly restored everywhere to climatology and the vertical mixing is conventional with constant coefficients, except where there is either convective or near-surface enhancement. The most profound effects are due to the bulk forcing boundary conditions, while KPP mixing has little effect on the annual-mean state of the ocean model below the upper few hundred meters. Compared to restoring boundary conditions, bulk forcing produces poleward heat and salt transports in better agreement with most oceanographic estimates and maintains the abyssal salinity and temperature closer to observations. The KPP scheme produces mixed layers and boundary layers with realistically large temporal and spatial variability. In addition, it allows for more near-surface vertical shear, particularly in the equatorial regions, and results in enhanced large-scale surface divergence and convergence. Generally, topographic effects are confined locally, with some important consequences. For example, realistic ocean bottom topography between Greenland and Europe locks the position of the sinking branch of the Atlantic thermohaline circulation to the Icelandic Ridge. The model solutions are especially sensitive to the under-ice boundary conditions where model tracers are strongly restored to climatology in all cases. In particular, a factor of 4 reduction in the strength of under-ice restoring diminishes the abyssal salinity improvements by about 30%.