Ocean Mixed Layer Dynamics and its Impact on SST & Climate Variability

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Ocean Mixed Layer Dynamics and its Impact on SST
Climate Variability

• Michael Alexander
• Earth System Research Lab
• michael.alexander_at_noaa.gov
• http//www.cdc.noaa.gov/people/
• michael.alexander/presentations

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Ocean Mixed layer

• Turbulence creates a well mixed surface layer
  where temperature (T), salinity (S) and density
  (?) are nearly uniform with depth
• Primarily driven by vertical processes (assumed
  here) but can interact with 3-D circulation
• Density jump usually controlled by temperature
  but sometimes by salinity (especially in high
  latitudes)
• Often measured by the depth at which T is some
  value less than SST (e.g. ?T 0.5)
• Under goes large seasonal cycle
• This impacts the evolution of ocean temperature
  anomalies and has important biological
  consequences

surface
s
?
T
?TTb-Tm
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Vertical Flux Entrainment and MLD (h)

• Entrainment To pull or draw along after itself
• MLD Mixed Layer Depth or h
• When deepening
• dh/dt we
• we M B D / (Dr - S)
• Where
• M - Mechanical Turbulence (wind stirring)
• B - Buoyancy Forcing
• Net surface heating/cooling (Qnet)
• Precipitation Evaporation (P-E)
• D - Dissipation (eh)
• ?? - Density jump at base of the ML
• S - Shear across ML (not in all models)
• When Shoaling
• we 0 (no detrainment, h reforms closer to the
  surface)
• h M /(B D)

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Seasonal Cycle of Temp MLD Northeast Pacific
(50ºN, 145ºW)
MLD (h)
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Climatological Mixed Layer Depth (m)
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SST Tendency Equation

Integrated heat budget over the mixed layer
Variables v velocity (current in
ML) Tm mixed layer temp (SST) Tb temp
just beneath ML h mixed layer depth w
mean vertical velocity we entrainment
velocity Qnet net surface heat flux Qswh
penetrating shortwave radiation A
horizontal eddy viscosity coefficient ?
density of sea water C Specific heat of
sea water
Qnet
Tm SST
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Temperature change due to the surface heat flux

• Over March through August a location in the North
  Pacific typically receives 150 Wm-2 flux through
  the surface. Assuming a constant mixed layer
  depth of 50 m, and no other changes in the ocean
  how much will the SST change over that time?
• dTm/dt d(SST)/dt Qnet/?ch
• ?SST (Qnet/?ch) x ?t
• ? 1025 kg/m3 c 3850 Joules/(kg C)
• ?SST (150 Wm-2 / (1025 kg m-3 x 3850 Joules
  kg-1 C-1 x 50 m)) (184 days 86400 s day-1)
• SST 12.1C
• gt Check units (W J s-1)
• gt reasonable value for winter to summer change
  in SST

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Zonal Average Mean
Surface Heat Flux
Entrainment Flux
Standard Deviation
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Observed Standard Deviation of SST Anomalies (C)
March
August
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Processes for Generating SST Anomalies
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Simple model for generating SST
variabilitystochastic model
-?Tm
F
dTm/dt Qnet ?ch
dTm/dt F ?Tm ?ch
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Stochastic SST Anomaly model IIidealized forcing
and time series
Null Hypothesis for midlatitude SST variability
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Stochastic Model correspondence to the real
world?Observed and Theoretical Spectra for SST
anomalies (SSTA)at a location in the North
Pacific Ocean
Temperature Variance
Atm forcing
Atmospheric forcing and ocean feedback can be
estimated from data. Then can then develop
stochastic model and generate multiple time
series to look at spread
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Patterns of Surface Fluxes and SSTsexample
North Atlantic Oscillation
Contours are sea level pressure (SLP) vectors -
winds Shading left is SST anomalies, on right is
the Flux anomalies NAO north-south SLP anomaly
pattern over the Atlantic
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The Reemergence Mechanism

• Winter Surface flux anomalies
• Create SST anomalies which spread over ML
• ML reforms close to surface in spring
• Summer SST anomalies strongly damped by air-sea
  interaction
• Temperature anomalies persist in summer
  thermocline
• Re-entrained into the ML in the following fall
  and winter

Namias and Born 1970, 1974 Alexander and Deser
(1995, JPO) Alexander et al. 1999
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Reemergence in three North Pacific regions
Regression between SST anomalies in April-May
with monthly temperature anomalies as a function
of depth.
Regions
Alexander et al. (1999, J. Climate)
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Reemergence in the North Atlantic
Reg 2 - Northeast Atlantic (47)
Reg 1 - Subtropical Atlantic (48)
Timlin, Alexander, Deser, 2002, J.Climate
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Reemergence of SST Tripole
Watanabe and Kimoto (2000) Timlin et al. 2002,
Deser et al 2003, De Coetlogon and Frankignoul
2003 all in J. Climate
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Impact of reemergence on SST PersistenceAugmenti
ng the Stochastic SST model
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North Atlantic
Heff winter MLD for interannual variability in
a stochastic model
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Main Concepts

• Mixed Layers
• Processes that control its depth
• Wind stirring buoyancy forcing, density jump at
  base of ML
• Processes that control its temperature (SST)
• Surface heat flux
• Entrainment heat flux
• Mechanisms for the behavior of SST anomalies
• Stochastic model
• Reemergence
• Large scale patterns of atmospheric forcing
  organizes fluxes, shapes SST Anomaly and
  reemergence patterns
• Questions?

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2. Surface signature of reemergence in the
Labrador Sea
1. What is the oceanic reemergence?
Sea Surface Temperature
Level of significance
ERSSTv2 Datasets 1950-2003
Degrees Celcius
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Atmosphere forcing the ocean in winter NAO
the Atlantic SST tripole
March SST EOF1 (shade) Regressed JFM SLP (contour)
PC time series March SST (bars), JFM MSLP (line)
Correlation0.63
NCEP MSLP 1950-2003
e.g. Deser and Timlin (1997), J.Clim.
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Summary

• Forcing of SST (mixed layer temperature
• Net heat flux key term, Ekman transport
  entrainment also important
• SST anomalies larger in summer than winer due to
  shallow MLD
• Processes that impact extratropical SST
  variability
• Stochastic atmospheric forcing
• Reemergence
• Atmospheric Bridge
• Tropical Pacific gt Global SSTs
• Impacts in both winter and summer
• Influence of air-sea feedback on extratropical
  atmosphere complex
• Other Processes that influence SST variability
• Cloud - SST feedbacks
• Ocean currents Rossby waves in western N.
  Pacific
• Changes in the Thermohaline Circulation

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Additional Topics

• The flux components and their variability
• Schematic of the mixed layer model
• Pattern of atmospheric circulation (SLP) and the
  underlying fluxes)
• Basin-wide reemergence
• The Pacific Decadal Oscillation
• Wind generated Rossby waves and its relation to
  SSTs
• The Latif and Barnett mechanism for the PDO and
  problems with this mechanism

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Atmosphere-Ocean Ice Model

• Atmospheric GCM
• NCAR CAM2T42 resolution
• Ice
• Thermodynamic portion of NCAR CSIMv4
• Ocean
• Mixed layer Model (MLM)
• An individual column model with a uniform mixed
  layer
• Atop a layered model that represents conditions
  in the pycnocline
• Prognostic ML depth
• Same grids as the atmosphere (128 lon x 64 lat)
• 36 vertical levels (from 0m to 1500m depth)
• higher resolution close to surface and a
  realistic bathymetry
• Flux correction needed to get reasonable climate
• Cassou et al. 2007 J Clim Alexander et al. 2000
  JGR, Alexander et al 2002 J.Clim Gaspar 1988
  JPO

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Mixed Layer Ocean Model
Qcor
Qnet
h (MLD)
Qwe
Tm1
Tb1
Qswh
CA
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Mean ML Budget terms (Wm-2) in January From an
AGCM couple to a mixed layer ocean model
Surface Flux Qnet
Ekman Qek ?cvek(?Tm)
Entrainment Qwe ?cwe(Tb-Tm)
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Mean Mixed Layer Budget terms (Wm-2) in August
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Standard Deviation of the Mixed Layer Budget
Terms (Wm-2) in January
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Standard Deviation of Fluxes in AugustResults
from an AGCM- Ocean MLM
Qnet
W m-2
Qwe ?cwe(Tb-Tm) W m-2
Qwe / Qnet
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Fig11 JGR 2000
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Wind Generated Rossby Waves
Atmosphere
Ocean
ML
Thermocline
West
East

• After waves pass ocean currents adjust
• Waves change thermocline depth, if mixed layer
  reaches that depth, cold water can be mixed to
  the surface

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Observed Rossby Waves SST
Correlation Obs SST hindcast With thermocline
depth anomaly
KE Region 40N, 140-170E
March
Forecast equation for SST based on integrating
wind stress (curl) forcing and constant
propagation speed of the (1st Baroclinic) Rossby
wave
Schneider and Miller 2001 (J. Climate)
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Forecast Skill Correlation with Obs SST Wave
Model Reemergence
Wave Model
Reemergence
years
Schneider and Miller 2001 (J. Climate)
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Climatological heat fluxes August
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Zonal Average of the standard deviation of the
mixed layer budget terms
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Observed SST (?C) / SLP (mb) Warm-Cold (50-03)
DJF
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Evolution of the leading pattern of SST
variabilityas indicated by extended EOF analyses
No ENSO Reemergence
ENSO No Reemergence
Alexander et al. 2001, Prog. Ocean.
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Upper Ocean Temperature and mixed layer depth
El Niño La Niña model composite Central North
Pacific
Alexander et al. 2002, J. Climate
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ENSO SST MLD in Western N. Pacific Region
Niño Niña NCEP Ocean Temp White MLD
(1980-2001)
La Niña MLD
C
El Niño MLD