Internal Variability in the Oceans and AirSea interactions

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Internal Variability in the Oceansand Air-Sea
interactions

• Raghu Murtugudde
• Earth System Science Interdisciplinary
  Center/METO
• University of Maryland, College Park, MD
• Markus Jochum
• MIT
• Cambridge, MA

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Internal Variability in the Oceans Western
Boundery Currents
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Kessler 2003
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Mesoscale Variability in the tropical Atlantic
Tropical Instability Waves
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Tropical Atlantic

• ITCZ tends to be sensitive to the weak meridional
  SST gradient during boreal spring months.

And the gradient can be reversed due to internal
variability in the ocean!
Jochum et al. 2004
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Meridional heat flux at 20oW.
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Correlation of SST anomalies at 20oW/0oN with
surrounding SSTAs and with anomalies at 0oN.
Its not advection..
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Mean SSTs and the Eddy Kinetic Energy for the
high resolution model
Jochum et al. 2004
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Enhanced resolution and resolving mesoscale
variability clearly impacts Model SST
simulations. But how is it accomplished?
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Heat Budgets? Better have an INTERACTIVE heat
flux formulation! Advective Atmospheric Mixed
Layer
.
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Cloud layer
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Turbulence Closure
wE dC0w0
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What do the heat budgets say?
Coarse resolution leads to excessive diffusion of
heat from the ITCZ to the Cold-tongue.
Jochum et al. 2004
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High resolution tells a different story!
Entrainment is enhanced slightly and yet
diffusion is reduced greatly.
Jochum et al. 2004
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Entrainment increases with a higher resolution
but there is a seasonality to it.
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Warmer SSTs even though entrainment
increases.because the latent and sensible heat
losses are reduced.
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With a non-local heat flux formulation, the heat
transports in the ocean and the atmosphere can
adjust realistically to SST variability.
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The TIWs are well-known in the equatorial
Pacific. Are they a source of stochasticity just
like MJOs? Do they have a strong-enough decadal
variability to impact mean conditions?
Jochum and Murtugudde 2004
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The energy, seasonality, and spectrum of TIW in
the model are in very good agreement with all
available observations.
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The variance of SST due to internal variability
is comparable to any other source of stochastic
forcing or changes due to STCs.
MORE IMPORTANTLY, the decorrelation length scales
are 1000Km!!!
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Both SST changes in the eastern equatorial
Pacific and the SSTgradient changes across the
equator are comparable to most interannual
signals.
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The zonal gradient also shows significant
internal variability. Potential for SST-wind
feedbacks?
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The story form the heat budget in the deep
tropical Pacific is the same as in the Atlantic!
TIWs are not mixing heat horizontally! Its not
the diffusion of heat.
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A NEW PARADIGM FOR THE ROLE OF TIWs on SSTs.
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Circulation schematic for Southwest Monsoon

• Great Whirl and Southern Gyre
• Upwelling concentrated in wedges and filaments
• cyclonic domes both sides of Sri Lanka
  upwelling?
• Indonesian Throughflow max.
• Ekman transport southward in both hemispheres,
  but northward winds on equ.
• Subduction in southern subtropics

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Circulation schematic for Northeast Monsoon

• Somali Current and East African Coast Current
  merge into SECC
• Doming NE of Madagascar open-ocean uwelling?
• Indonesian Throughflow min.
• some subduction in northern Arabian Sea
• Ekman transport northward in both hemispheres

In between monsoon seasons eastward wind stress
causesWyrtki Jets along equator, that send out
coastal Kelvin waves
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Mesoscale features in Indian Ocean SSTs are often
associated with high mean SSTs.
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The other tropical ocean. High mean SSTs and
high convective energy.
Jochum and Murtugudde 2004b
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Is it good news that the SST anomaly variance
associated with the internal variability is
restricted coastal regions and the southern
subtropics? What about Java? But the
decorrelation scales are only 400km.
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Variance of anomalies due to internal variability
can be as large as most Interannual anomalies. A
challenge for coupled climate models
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Heat flux convergence over 2oS-2oN,45oE-50oE
horizontal (black), vertical (red), net
armospheric (green) and total (blue) heat flux
convergences.
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Consequences for Interannual variability
Quantifying the impact of TIWs
Jochum and Murtugudde 2004c
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Coastal upwelling in the east is improved because
TIWs enhance zonal SST Gradient Zonal advection
into the cold-tongue clearly impacts the coastal
region.
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The warm pool gets warmer SSTs through zonal
advection from the cold-tongue.
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Biogeochemical Consequences of TIWs The 1998 El
Niño-La Niña Transition
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TIWs reduce the cold-bias in SST, restratify the
thermocline better in the east, And improve the
thermocline gradient over much of the equatorial
ocean.
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Is it all STCs or do we have a TIW driven
tropical mechanism for Decadal ENSO Variability??
Rothstein et al. 2000
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The thermocline differences are restricted to the
TIW scales.
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Even in the central-western Pacific, TIWs
determine the differences
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Wind-response to TIWs in a OGCM-Lindzen/Nigam
model Seasonality
Ballabrera et al. 2004
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When does the ocean mesoscale variability affect
coupled climate variability?