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Ocean Mixed Layer Dynamics and its Impact on SST & Climate Variability
Michael Alexander
Earth System Research [email protected]
http://www.cdc.noaa.gov/people/
michael.alexander/presentations
Ocean Mixed layer• Turbulence creates a well mixed
surface layer where temperature (T), salinity (S) and density (ρ) 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
T s ρ
∆T
∆T=Tb-Tm
Vertical Flux: Entrainment and MLD (h)Entrainment “To pull or draw along after itself”MLD – Mixed Layer Depth or h
When deepening: dh/t = we
we = M + B – D / (ρS)
WhereM - Mechanical Turbulence (wind stirring)B - Buoyancy Forcing
Net surface heating/cooling (Qnet)Precipitation – Evaporation (P-E)
D - Dissipation (h)ρ- Density jump at base of the MLS - Shear across ML (not in all models)
When Shoaling: we = 0 (no detrainment, h reforms closer to the surface)h = M /(B – D)
Seasonal Cycle of Temp & MLD Northeast Pacific (50ºN, 145ºW)
MLD (h)
Climatological Mixed Layer Depth (m)
SST Tendency Equation
Variablesv – velocity (current in ML)Tm – mixed layer temp (SST)Tb – temp just beneath MLh – mixed layer depthw – mean vertical velocitywe – entrainment velocity Qnet – net surface heat fluxQswh – penetrating shortwave radiationA – horizontal eddy viscosity coefficientρ – density of sea waterC – Specific heat of sea water
Integrated heat budget over the mixed layer:
Qnet
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/ρch
SST = (Qnet/ρch) x t
• ρ = 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 = 12.1°C
• => Check units (W = J s-1)
• => reasonable value for winter to summer change in SST
Zonal Average
Mean
StandardDeviation
Surface Heat Flux Entrainment Flux
Observed Standard Deviation of SST Anomalies (°C)
August
March
Processes for Generating SST Anomalies
Simple model for generating SST variability“stochastic model”
Heat fluxes associated with weather events,“random forcing”
Ocean response to flux back heatwhich slowly damps SST anomalies
SST anomalies formAir-sea interface
Fixed depth oceanNo currents
Bottom
dTm/dt = F – λTm
ρch
F λTm
dTm/dt = Qnet
ρch
Stochastic SST Anomaly model IIidealized forcing and time series
Null Hypothesis for midlatitude SST variability
Stochastic Model: correspondence to the real world?Observed and Theoretical Spectra for SST anomalies (SSTA)
at a location in the North Pacific Ocean
Te
mp
era
ture
Va
rian
ce
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
Period
1yr10 yr
No damping
SS
TA
Var
ianc
e
1 mo
Atm forcingTheoretical spectra (white line) of stochastic model
Observed
Spread (5%-95%)
Patterns of Surface Fluxes and SSTs:example North Atlantic Oscillation
Contours are sea level pressure (SLP); vectors - windsShading left is SST anomalies, on right is the Flux anomaliesNAO north-south SLP anomaly pattern over the Atlantic
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 +
Qnet’
MLD
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)
Reg 2 - Northeast Atlantic (47%)
Reemergence in the North Atlantic
Reg 1 - Subtropical Atlantic (48%)
Timlin, Alexander, Deser, 2002, J.Climate
Watanabe and Kimoto (2000); Timlin et al. 2002, Deser et al 2003, De Coetlogon and Frankignoul 2003 : all in J. Climate
Auto-correlation of EOF PC time series
Level ofsignificance
Degrees Celsius
Reemergence of theSST North Atlantic tripole
Leading EOF of March SST
ERSSTv2 Datasets [1950-2003]
Reemergence of SST Tripole
Impact of reemergence on SST Persistence:Augmenting the Stochastic SST model
Deser et al. 2003
Heat content (EM)
Obs (dashed)
EntrainingModel (EM)
SST (EM)SST (OBS)
North Atlantic
Heff = winter MLD for interannual variability in a stochastic model
Deser et al. 2003
Heat content (EM)
Obs (dashed)
EntrainingModel (EM)
SST (EM)
SST (OBS)
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?
1. What is the oceanic reemergence? 2. Surface signature of reemergence in the Labrador Sea
Sea SurfaceTemperature
e-folding = ~ 4 mths
Auto-correlation of the Labrador SST time series(all months considered), e.g. for lag=1, Jan50/Feb50/…/Dec00values are correlated with Feb50/Mar50/…/Jan01 values
e-folding = ~ 36 mths
e-folding = ~ 4 mths
Auto-correlation of the Labrador SST time series(Starting from March), e.g. for lag=1, March and Apriltime series are correlated, for lag =2 March and May etc.
Reemergence of the latewinter SST anomalies
a year after
Deser et al. 2003 (J.Clim)
ERSSTv2 Datasets [1950-2003]
Degrees Celcius
Level ofsignificance
March SST EOF1 (shade)Regressed JFM SLP (contour)
PC time series: March SST (bars), JFM MSLP (line)
NCEP MSLP [1950-2003]
Correlation=0.63
e.g. Deser and Timlin (1997), J.Clim.
Atmosphere forcing the ocean in winter: NAO & the Atlantic SST tripole
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 => 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
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
Atmosphere-Ocean Ice Model
Atmospheric GCM– NCAR CAM2–T42 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
Mixed Layer Ocean Model
h (MLD)
Tb1
Tm1
Qnet Qcor
Qswh
Qwe
CA
Mean ML Budget terms (Wm-2) in January From an AGCM couple to a mixed layer ocean model
EntrainmentQwe =
ρcwe(Tb-Tm)
EkmanQek =
ρcvek(Tm)
Surface FluxQnet
Mean Mixed Layer Budget terms (Wm-2)
in August
Standard Deviation of the Mixed
Layer Budget Terms (Wm-2)
in January
Standard Deviation of Fluxes in AugustResults from an AGCM- Ocean MLM
Qwe =
ρcwe(Tb-Tm)
W m-2
Qnet
W m-2
Qwe / Qnet
Wind Generated Rossby Waves
West East
Atmosphere
Ocean
Thermocline
ML
L
Rossby Waves
1) After waves pass ocean currents adjust2) Waves change thermocline depth, if mixed layer reaches that
depth, cold water can be mixed to the surface
Observed Rossby Waves & SST
Schneider and Miller 2001 (J. Climate)
March
KE Region: 40°N, 140°-170°E
SSTOBS
T400SSTfcst
Correlation Obs SST hindcast With thermocline depth anomaly
Forecast equation for SST based on integrating wind stress (curl) forcing and constant propagation speed of the (1st Baroclinic) Rossby wave
Forecast Skill: Correlation with Obs SST Wave Model & Reemergence
Wave Model Reemergence
years
Schneider and Miller 2001 (J. Climate)
Climatological heat fluxes August
Zonal Average of
the standard
deviation of the mixed
layer budget terms
DJF
JAS
Observed SST (C) / SLP (mb) Warm-Cold (50-03)
Evolution of the leading pattern of SST variabilityas indicated by extended EOF analyses
Alexander et al. 2001, Prog. Ocean.
No ENSO;Reemergence
ENSO;No Reemergence
Upper Ocean: Temperature and mixed layer depth
El Niño – La Niña model composite: Central North Pacific
Alexander et al. 2002, J. Climate
ENSO SST & MLD in Western N. Pacific Region
El Niño MLD
La Niña MLD
Niño – Niña: NCEP Ocean Temp & White MLD (1980-2001)
°C