Climate Implications of Subantarctic Mode Water and Antarctic Intermediate

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Climate Implications of Subantarctic Mode Water
and Antarctic Intermediate
Bernadette Sloyan CSIRO Marine and Atmospheric
Research
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SAMW Formation Regions Southern Eastern Indian
and Pacific Oceans
Proxy for winter mixed layer depth is 95 Oxygen
saturation. Deep mixed layers identify sites of
SAMW formation.
From Talley 1999
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Distribution SAMW and AAIW globally significant
water masses
Global distribution of low-salinity intermediate
water. Formation region marked with X, and
strong mixing with
LSW
NPIW
AAIW
From Talley 1999
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Progression of SAMW and AAIW salinity from WOCE
data
5o salinity bins north of Polar Frontal Zone
SAMW 26.7-27.0
AAIW 27.0-27.2
AAIW 27.2-27.4
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Progression of SAMW and AAIW Potential
Temperature from WOCE data
5o temperature bins north of Polar Frontal Zone
SAMW 26.7-27.0
AAIW 27.0-27.2
AAIW 27.2-27.4
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Climate model simulation Comparison with CSIRO
Atlas of Regional Seas (CARS2006)
CARS2006 depth, temperature and salinity for
Austral Spring (Sept, Oct, Nov) on isopycnal
surfaces that define SAMW and AAIW.
Subantarctic Front
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Climate Models IPCC AR4
Define model mean climate of 20th century as
20-year average (1981-2000)
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Climate Model Simulation of SAMW and AAIW
Salinity
RMS ensemble isopycnal salinity error (psu)
relative to CARS2006 for Austral Spring. Ensemble
is for 5 (26.7, 26.9 27.0 27.2 and 27.4) observed
isopycnal surfaces that define SAMW and AAIW.
Subantarctic Front
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Climate Model Simulation of SAMW and AAIW
Isopycnal Potential Temperature
RMS ensemble isopycnal potential temperature
error (oC) relative to CARS2006 for Austral
Spring. Ensemble is for 5 (26.7, 26.9 27.0 27.2
and 27.4) observed isopycnal surfaces that define
SAMW and AAIW.
Subantarctic Front
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Climate Model Simulation of SAMW and AAIW
Isopycnal Depth
RMS ensemble isopycnal depth error (m) relative
to CARS2006 for Austral Spring. Ensemble is for 5
(26.7, 26.9 27.0 27.2 and 27.4) observed
isopycnal surfaces that define SAMW and AAIW.
Subantarctic Front
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Observed and Climate Model Simulated SAMW
Potential Vorticity in Eastern Pacific Ocean
(90oW)
PV (10-14 (cm s)-1). Observed depth and latitude
extent of SAMW low potential vorticity layer
overlain on model simulated distribution.
Overlain on model PV is observed PV minimum (lt
50).
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Observed and Model Simulated AAIW Salinity
minimum layer in Eastern Indian Ocean (100oE)
Salinity (psu). Observed depth and latitude
extent of AAIW salinity minimum is overlain on
model salintiy distribution.
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Summary of Climate Model Simulation of AAIW and
SAWM

• Climate models, except for HadCM, CSIRO and MRI,
  provide a reasonable simulation of SAMW and AAIW
  in the Southern Ocean. (Note Hadley Centre and
  CSIRO have further developed their climate
  models.)
• In the formation regions, models generally better
  simulate SAMW and AAIW in the eastern Pacific
  Ocean than in the eastern Indian Ocean.
• Models display limited equatorward extension of
  low potential vorticity layer and salinity
  minimum layer of SAMW and AAIW, respectively.
  Indicating models are too diffusive north of 40oS
• Large isopycnal depth errors north of SAF and
  subtropical oceans explained by incorrect density
  of model SAMW pycnostad and too warm upper ocean
  which both deepen model isopycnal layers.
• Error in simulation of SAMW/AAIW properties in
  formation regions may be due to biases in
  position and strength of wind stress, inadequate
  representation of sub-grid scale mixing and
  eddies. Errors in subtropical ocean possibly due
  to model representation of eastern and western
  boundary processes.

How can we improve model representation of SAMW
and AAIW?
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SAMW and AAIW Formation Processes
Eddies
Schematic diagram of processes involved in the
formation of SAMW and AAIW. Relative impact of
each process on water mass formation likely to be
different between formation site.
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Process Study SAMW and AAIW Formation in eastern
Pacific Ocean
Collaborative research with Lynne Talley and Teri
Chereskin, Rana Fine and Andrew Dickson
Austral Spring 2005
The aim of this project is to characterizing the
processes responsible for the formation of SAMW
and AAIW in the southeast Pacific Ocean.
Austral Summer 2006
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Process Study Formation and Subduction of SAMW
and AAIW in eastern Pacific Ocean
89oW
SAF
Austral Spring 2005
SAF
Austral Summer 2006
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Southeast Pacific Winter Mixed Layer and Near
Surface Salinity
Near Surface Salinity
MLD
2005
1980
MLD defined as Dr 0.03 kg m-3
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Diffusivity Estimates from Thorpe Scale
Thorpe Scale (LT) provide a length scale
associated with density overturns. It is defined
as the rms of vertical displacement of a
reordered density profile over a gravitationally
unstable patch.
A relationship exists between the Thorpe Scale
and Ozimdov Scale (Lo), where e Lo2N3 , such
that Lo aLT and k a2GNLT2 . e dissipation
rate to turbulent kinetic energy, N buoyancy
frequency, G mixing efficiency
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Mixing Analysis Thorpe Scale Lengths of
Turbulent Overturns and Winter Mixed Layer Depth
Contour plot potential density with 26.9, 27.02,
27.03 and 27.4 highlighted.
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Closing Comments

• The data provides the basis for understanding
  the processes leading to the formation of SAMW
  and AAIW.
• Explore the role of turbulent and convective
  mixing on formation of SAMW and AAIW in Southeast
  Pacific. What about advection on evolution of
  properties between winter and summer sections?
• Combined observational and modeling studies will
  help unravel the relative importance of processes
  involved in SAMW and AAIW formation in the
  Southeast Pacific and Indian Oceans.
• GFDL, CCSM, CNRM and MIROC_m(h) climate models
  best simulate current climate SAMW and AAIW.
  What about higher resolution numerical models?
• Quantifying current climate biases in the
  climate models provides useful knowledge for the
  assessment of possible future climates under IPCC
  forcing scenarios.

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Thank You