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Southern Ocean Mixed
Layer Depths – Assessment in CMIP5 Models: Historical Bias and Response
to Forcing In this paper Sallée et
al. report the results of
their assessment of the development of the deep water
Southern Ocean
winter mixed layer in the climate models that participated in the 5th
Coupled Model Intercomparison Project (CMIP5). A key to the ventilation
of the ocean interior are the deep winter convection regions, and
property changes of these regions have been related to changes of
climate in many studies. When compared to observations the simulation of
these models is consistently too shallow, too light and shifted towards
the equator. The shallow bias is associated mostly with an excess of
annual-mean freshwater input at the surface of the ocean that over
stratifies the surface layer thereby preventing the development of deep
water in winter. Contrasting with this, future changes that have been
modelled are mostly associated with reduced loss of heat during winter
that leads to winter mixed layers that are even shallower. In the
Pacific Basin the mixed layers shallow most strongly under future
scenarios, and this is associated with a reduction of the volume of
ventilated water in the interior. Sallée et
al. found that there was a
strong state dependency on change of depth of the mixed layer in the
future, with models with larger historical mixed layer depths. It was
expected that, given the biased shallow in most models, most CMIP5
climate models might underestimate winter mixed-layer shallowing, with
implications that are important for the sequestration of heat, as well
as gases such as carbon dioxide, and therefore for climate. At the ocean surface the mixed layer is the
gateway for all exchanges between the atmosphere and the ocean. In the
Southern Ocean, where intense winds and buoyancy flux extremes lead to
the formation of the thickest mixed layers on Earth (de Boyer et
al., 2004) this gateway
function is especially important. These deep mixed layers provide a
conduit a conduit for the sequestration of heat and gases (including
carbon dioxide), from the atmosphere into the interior of the ocean
(Sabine et al., 2004; Ito et
al., 2010; Sallée et
al., 2012). Therefore, the
assessment of how well the mixed layer of the Southern Ocean is
represented in climate models as it can affect the accuracy of future
projections. In the Southern Ocean the deepest mixed layers
form in winter directly north of the Antarctic Circumpolar Current (ACC)
(e.g., McCartney, 1977; Sallée et
al., 2006; Dong et al.,
2008). Mode and intermediate waters acquire their physical and
biogeochemical properties in the circumpolar band of thick mixed layer
prior to being subducted into the ocean interior. The thermocline of the
Southern Hemisphere subtropical gyres are then ventilated by these
waters (Sallée et al.,
2010a). It has long been recognised that mode and intermediate waters
are key water masses in the determination of the global distribution and
budgets of heat, carbon and nutrients (e.g., Sarmiento et
al., 2004; Sabine et
al., 2004; Ito at
al., 2010; Sallée et
al., 2012), as a result of
their large thickness and their surface formation. Estimates suggest, in
particular, that more than 40 % of the total oceanic anthropogenic
carbon has entered the ocean south of 40oS. There are also
indications from palaeoclimatic records that in the Southern Ocean a
breakdown in stratification contributed to the rise of atmospheric
carbon dioxide at the end of the Last Glacial Maximum (LGM) (Toggweiler
& Russel, 2008; Anderson et al.,
2009). This emphasises the importance of representing accurately the
mixed layer of the Southern Ocean so that past present and future
climate can be modelled accurately. A wide variation in the ability to represent deep
water mixed layer in the Southern Ocean is exhibited by climate models
of the 3rd Coupled Model Intercomparison Project (CMIP3)
(Downes et al., 2009, 2010).
Various improvements to the parameterisation of mixed layer dynamics
have been suggested since then, and some of these have been implemented
in the models that contribute to Coupled Model Intercomparison Project
Phase 5 (CMIP5). The presence of surface waters is possibly the
most significant characteristic of the mixed layer of the ocean, which
is contrasts with the atmospheric boundary layer, which results in both
wave breaking and Langmuir circulation at the surface (Noh & Min, 2004).
There are a number of turbulent kinetic closure schemes that have been
developed with the aim of parameterising this complex physics, which is
associated with convection and restratification of the mixed layer. In
some models new generation turbulence closure schemes have been
implemented (e.g., IPSL group, J. L. Dufresne et
al.), climate change
projections using IPSL CM5 Earth System Model: from CMIP3 to CMIP5,
submitted to Climate Dynamics,
2012, with representation of double diffusion processes (Merryfield et
al., 1999), Langmeyer cells
(Axell, 2002) and surface wave breaking (Mellor & Blumberg, 2004;
Burchard & 2008). Also, the
restratification effects of the finite-amplitude, sub-mesoscale mixed
layer eddies have been included in some models (e.g., CCSM4 group,
Danabasoglu et al., 2012)
using the mixed layer parameterisation of Fox-Kemper et
al. (2008) as was implemented
by Fox-Kemper et al. (2011).
It might also be expected that other model developments would improve
the representation of the mixed layer. It has been shown (Lee et
al., 2011) that increased
resolution of a model can improve the representation of the ocean
advection of buoyancy and the stratification of the Southern Ocean,
which translates into a mixed layer representation that is much more
realistic.
Sallée
et al. suggest it might be
expected that if improvements have been made to the representation of
fluxes of heat, freshwater and momentum at the air surface interface,
through improvements to the atmospheric models or to atmospheric-ocean
coupling, to benefit the representation of the ocean mixed layer.
Surface winds in the Southern Hemisphere also have a strong impact on
the mixed layer in the Southern Ocean (e.g. Sallée et
al., 2010b) and it has been
shown that they are influenced by the representation of the recovery of
ozone over the first half of the 21st century (Son et
al., 2008, 2010). All CMIP5
models differ from CMIP3 in that they include a representation of
changes in the stratospheric ozone (see information about ozone forcing
in Bracegirdle et al., 2013);
Sallée et al. note that all
models use the same ozone forcing, and therefore, in different models
the effect may be of different intensity. In this paper Sallée et
al. assess the present-day
skill and projected changes that are simulated by the CMIP5 models, with
a focus on the mixed layer of the Southern Ocean. No study to date has
consistently analysed the causes of the present-day Southern Ocean mixed
layer bias in climate models, in spite of the result of 10 years of Argo
profiles in the Southern Ocean that are now available allow a robust
understanding of the structure, characteristics and construction of the
real Southern Ocean winter mixed layer (Dong et
al., 2008; Sallée et
al., 2010b). Sallée et
al. present here such a
consistent analysis. Sallée et al.
attempt to identify the most important forcings that lead to biased
information, though a detailed study of the influence of particular
parametrisation schemes is beyond the scope of this paper. For
projections in the future Sallée et
al. provide a summary of
multi-model projections and examine if there is a state dependence in
the model response. The analysis is focused on the assessment of the
deep mixed-layer band that develops in winter and leads to the formation
of mode and intermediate water. The implications of representation of
mixed layer for modelled mode and intermediate water masses are added to
the end of this paper, and are discussed further in the framework of the
Southern Ocean overturning circulation in a companion paper (Sallée et
al., 2013).
Southern Ocean Mixed Layer Representation In the Southern Ocean there is a strong seasonal
cycle in the Mixed Layer Depth (MLD) that exceeds more than 400 m in
some locations to the north of the
Antarctic Circumpolar Current (ACC) (Sallée et
al., 2010b). The water column
is destabilised by the winter cooling and the MLD is increased to the
extent that the maximum MLD are found in the late austral winter
(September) before the shallow summer mixed layer is rapidly
re-established by warming in spring and early summer. In this paper, as
introduced above, Sallée et al.
focused mostly on the deep winter mixed layer convection that develops
on the equatorwards, northern, edge of the ACC. Sallée et
al. found it useful for
documenting the ability of models to represent summer mixed layer and
the amplitude of the seasonal cycle before they tackled the analysis of
winter Mixed Layer depth representation. The characteristics of water
subducted in winter are set all year round; while the winter mixed layer
depth is crucial for the ventilation of the Southern Ocean (McCartney,
1977; Hanawa & Talley, 2001; Sallée et
al., 2010a). Also, the depth
in summer is critical for the chemistry of the ocean surface and
biological activity (Lovenduski & Gruber, 2005; Sallée et
al., 2010a), which are
processes that are implemented in Earth System Models that participated
in CMIP5. In the Southern Ocean the structure of the mixed
layer is characterised by a circumpolar band of deep mixed layers that
reach 60-90 m in summer (February), in the latitude band 50oS-60oS.
Mixed layers are shallower, at around 50 m, outside of this band. In
winter the deep circumpolar band is strongly destabilised to reach
depths of up to 400-700 m. In winter the band is narrower and
concentrated only on the equatorwards edge of the ACC. Mode and
intermediate waters are formed at this location (McCartney, 1977; Hanawa
& Talley, 2001; Sallée et al.,
2006, 2008a). The models tend to be biased shallow, on average, compared
with observations, in summer and in winter, in the band of deepest mixed
layers. In winter as well as summer the multi-model average of bias is
significant. The multi-model average of bias reaches 50-70 m in summer,
while it reaches 100-200 m in winter. Sallée et
al. also found that models
simulate a band of deep mixed layer that is too wide, which extends too
far towards the equator, as was revealed by deep bias on the northern
edge of the deepest mixed layer band (average deep bias of 100-200 m). Compared with observations the summer and winter
mixed layer depth biases translate into a misrepresentation of the mixed
layer seasonal cycle. According to Sallée et
al. the MLD seasonal cycle
amplitude is too small by as much as 200 m in regions of deep mixed
layer convection in the Eastern Indian Ocean, the mid-Pacific and
eastern Pacific basins. In subtropical regions immediately to the north
of the maximum mixed layer depth sector, in contrast, the seasonal cycle
amplitude is too large when compared with observations by 100-200 m on
average. There are important implications for the formation of mode and
intermediate water of these significant biases. (Sallée et
al., 2010a) have shown the
importance of seasonal cycle and regional structure of the deepest mixed
layer depth of the Southern Ocean in the subduction of water masses. In
winter mixed layer depth and seasonal cycle amplitude the significant
deep bias in the subtropical regions, western
Indian Ocean and
western Pacific Ocean,
suggest that the amount of subtropical mode and intermediate water
subducted in climate models is too large. According to Sallée et
al. they have shown,
consistent with this, in a companion paper that the density of mode
water is biased light as the result of unrealistically too large
formation of subtropical mode water in the western Indian and western
Pacific sectors, and the formation of subantarctic mode water in the
eastern Indian and Pacific sectors is too weak (Sallée et
al., 2013).
Understanding the bias in winter mixed layer depth As described above, the multimodal average of MLD
bias can be used to understand the general shortcomings of the ensemble
of models, though it hides a range of structures that are very distinct
across the models. In this section the spread of MLD patterns in each
model is detailed, and the forcings are analysed to understand better
what primarily leads to the distinct MLD representations across the
models. The focus was on the circumpolar band of very deep mixed layer
(MLDmax) that develops on the northern edge of the Antarctic
Circumpolar Current (ACC) in winter, where the mode and intermediate
waters form (McCartney, 1977; Hanawa & Talley, 2001; Sallée et
al., 2006).
Conclusions and discussion The representation of the winter mixed layer in
the Southern Ocean has been evaluated in 21 climate models that
participated in the CMIP5 exercise. In the climate models that were
analysed the region MLDmax, where mode and intermediate
waters form and are preconditioned, is shallower, lighter and more
towards the equator than it has been observed. There are important
implications for characteristics of mode and intermediate waters and the
rate at which they enter the interior of the ocean. For the dissolution
and sequestration of carbon dioxide in the interior of the ocean this is
of primary importance (Séférian et
al., 2012; Sallée et
al., 2012). This paper is the first to have unravelled the
primary drivers of the bias that need to be looked at if the
representation of deep MLD in the Southern Ocean in climate models,
while MLD bias in the Southern Ocean in climate models has previously
been documented (e.g. Downes et
al., 2009). Sallée et al.
identified that fluxes of freshwater increase artificially the
stratification of MLDmax, and this biases the depth and
density of the surface layer by preventing convection in the deep mixed
layer. According to Sallée et al.
they are not arguing for the thermal stratification or winter buoyancy
flux not having an impact on MLDmax, but they identified the
annual mean freshwater flux as the primary source of error. One of the
largest shortcomings of knowledge in the Southern Ocean has long been
identified as observational uncertainty and technical difficulties in
obtaining good estimates of annual mean buoyancy flux in the Southern
Ocean (e.g. Liu et al.,
2011). The analyses of Sallée, however, offers a fresh perspective for
modelling teams to adjust their Southern Ocean fluxes to best represent
the amount of haline stratification at the base of the mixed layer that
has been well observed. CMIP5 models simulate shallowing, a lightening
and a meridional shift of MLDmax under increased radiative
forcing scenarios. The meridional shift is towards the equator in the
Pacific sector and towards the pole in the Indian sector, and is
associated with shift in ACC position (Meijers et
al., 2012). The shallowing is
linked strongly to increased fluxes of heat in winter and occurs mostly
in the Pacific region. Sallée et
al. found a strong state of dependency between historical and future
change in MLD: those models that have the strongest historical bias in
MLD indicate there will be little change in the future, whereas those
that have a present-day MLD that is closer to observations indicate
significant shallowing of the MLD under future forcing scenarios.
Importantly, this suggests that changes in the future in MLD might be
larger than in indicated by most models, given that most models are
biased shallow. Mixed-layer properties are linked tightly to the volume
and properties of ventilated layers in the interior of the ocean, in
historical runs, as well as for future changes. The state dependency in
mixed layer could therefore potentially indicate that most models
simulate a reduction that is too weak in the volume of the ventilated
layer. This would have large implications for sequestration of heat,
freshwater and gases such as oxygen and carbon, and could indicate that
this potential climate change feedback may be underestimated by the
current generation of models. Sallée, J. B., et al. (2013). "Assessment of
Southern Ocean mixed-layer depths in CMIP5 models: Historical bias and
forcing response." Journal of Geophysical Research: Oceans 118(4):
1845-1862.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |