Ocean Processes and Pacific Decadal Climate Variability Michael Alexander Earth System Research Lab Physical Science Division NOAA

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Ocean Processes and Pacific Decadal Climate
Variability Michael AlexanderEarth System
Research LabPhysical Science DivisionNOAA
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Why should we look to the ocean for
low-frequency (gt 1 season) variability?

• Thermal Inertia
• 4 m of ocean holds as much heat as atmosphere
  above
• Water takes a long time to heat up and cool down
• Temperature anomalies once created persist
• Dynamical Processes
• Some very slow
• Currents slow (1 m/sec)
• Advection of temperature anomalies can take many
  years
• Adjustment of midlatitude currents (5 years -
  decades)
• Exchanges with the deep ocean can take decades to
  centuries!
• Important Implications
• Marine Ecosystems (fisheries)
• Atmospheric Circulation
• ( SSTs gt Atmosphere?)

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Midlatitude SST Variability

• There are many ways that SST anomalies form
• We will explore just a few mechanisms
• Ones that are part of larger Pacific climate
  signals
• Mechanisms for generating midlatitude SST
  anomalies
• Surface heat fluxes - Climate Noise
• Upper Ocean mixing processes
• Atmospheric Bridge Teleconnections with ENSO
• Changes in ocean currents
• Wind driven (through ocean Rossby waves)
• Thermal/salt driven Thermohaline (Atlantic) -
  next week

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Simple model for generating SST
variabilitystochastic model
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The Simple Oceans SST Anomaly Variability
Complex behavior with decadal anomalies!
SSTA
10 yrs
time
SSTn1 ? SSTn ? lconstant ? Random number
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Simple Ocean Model correspondence to the real
world?Observed and Theoretical Spectra for a
location in the North Atlantic Ocean
Observed OWS
Temperature Variance
Theoretical spectra of Simple ocean model
(Hz) is the frequency
1 year
1 month
period
Atmospheric forcing and ocean feedback estimated
from data
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Seasonal cycle of Temperature MLD in N.
PacificReemergence 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

MLD
Alexander and Deser (1995, JPO), Alexander et al.
(1999, J. Climate)
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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
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The Atmospheric Bridge
Meridional cross section through the central
Pacific
(Alexander 1992 Lau and Nath 1996 Alexander et
al. 2002 all J. Climate)
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Mechanism for Atmospheric Circulation Changes due
to El Nino/Southern Oscillation
Atmospheric wave forced by tropical
heating
Latent heat release in thunderstorms
Horel and Wallace, Mon. Wea Rev. 1981
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El Niño La Niña Composite
DJF SLP Contour (1 mb) FMA SST (shaded ºC)
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Ocean Surface Currents
Surface currents mainly driven by wind
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The Pacific Decadal Oscillation (PDO)
Leading Pattern (1st EOF) of North Pacific SST
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Pacific Decadal Atmospheric Variability
NP Index (Nov-Mar) 1900-2002
Trenberth and Hurrell (1994)
EOF1 SLP Pacific/Arctic PC Regressed on full
field Independent of the Atlantic

• Extratropical Signature
• Tropical Linkages

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Precipitation and Temperature Patterns Associated
with NP Index
Surface Air Temperature
Warm
Cold
180
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North Pacific Climate Indices (Winter)
1900
2000
25
47
77
SST PC 1
SLP
(- NP Index)
PRECIP
Alaska Japan
AIR T
Alaska/Canada
Deser et al. (J. Climate, 2004)
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What Causes the PDO? and Pacific Decadal
Variability in General?

• Random forcing by the Atmosphere
• Aleutian low gt underlying ocean
• Signal from the Tropics?
• Perhaps associated with decadal variability in
  the ENSO region
• Midlatitude Dynamics
• Shifts in the strength/position of the ocean
  gyres
• Could include feedbacks with the atmosphere

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Aleutian Low Impact on Fluxes SSTs in
(DJF)Leading Patterns of Variability AGCM-MLM
EOF 1 SLP (50)
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PDO or slab ocean forced by noise?
From David Pierce 2001, Progress in Oceanography
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Climate Indices
1900
2000
(Boreal Winter)
25
47
77
- NP Index
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Decadal variability in the North Pacific
tropical (ENSO) Connection?
Observed SST Nov-Mar (1977-88) (1970-76)
MLM SST Nov-Mar (1977-88) (1970-76)
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Wind Generated Rossby Waves
Atmosphere
Ocean
ML
Thermocline
West
East

1. After waves pass ocean currents adjust
2. 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
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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Ocean Response to Change in Wind Stress
SLP 1977-88 - 1968-76
Deser, Alexander Timlin 1999 J. Climate
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Response to Midlatitude SST Anomalies
CI 0.5C
2.5
SST Anomaly (C) specified as the Boundary
Condition in an AGCM
Peng et al. 1997 J Climate Peng and Whittaker
1999, J. Climate
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Response to Midlatitude SST anomalies
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PDO Multiple Causes?

• Newman, Compo, Alexander 2003, Schneider and
  Miller 2005, Newman 2006 (All in Journal of
  Climate)
• Interannual timescales
• Integration of noise (Fluctuations of the
  Aleutian Low)
• Response to ENSO (Atmospheric bridge)
• Decadal timescales ( of Variance)
• Integration of noise (1/3)
• Response to ENSO (1/3)
• Ocean dynamics (1/3)
• Predictable out to (but not beyond) 1-2 years
• We developed a statistical method gives skillful
  PDO prediction out 1 year
• Trend
• Most Prominent in Indian Ocean and far western
  Pacific
• Likely associated with Global warming

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Prediction of the PDO
1998 Transition?
Monthly values PDO Index
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Summary

• Climate noise
• Expect decadal variability when looking at SST
  time series
• Atmospheric Bridge
• Cause and effect well understood
• Tropical Pacific gt Global SSTs
• Influence of air-sea feedback on extratropical
  atmosphere complex
• PDO (1st EOF of North Pacific SST)
• Thermal response to random fluctuations in
  Aleutian Low
• A significant fraction of the signal comes from
  the tropics
• Extratropical ocean integrates (reddens) ENSO
  signal
• Decadal variability in tropics impact
  atmosphere ocean
• Ocean currents Rossby waves in western N.
  Pacific
• extratropical air-sea feedback modest amplitude
• Other Processes/modes of variability
• Other variability besdies PDO, focused on west
  Pacific
• Extratropical gt tropical interactions

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Extratropical gt Tropical Connections
Seasonal Footprinting Mechanism (SFM)
Subduction
Meridional cross section through the central
Pacific
(SFM Vimont et al. 2003 Subduction Schneider
et al. 1999 JPO)
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Seasonal Footprinting Mechanism
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Subduction and the Subtropical Cell
Ekman
Subtropical Cell
McPhaden and Zhang 2002 Nature
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Change in Subduction Rate
Transport at 9ºN 9ºS
Convergence SST
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Subduction
Central North Pacific
Colored contours -0.3C anomaly isotherms for 3
different pentads
Black lines mean isopycnal surfaces (lines of
constant density)
Averaged over 170ºW-145ºW
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Do subducting anomalies reach the equator and
influence ENSO?
a)
b)
c)
d)
Latitude
Year
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Additional Information

• Processes that influence SSTs
• PDO verses ENSO
• Reemergence as a function of time
• Ocean Dynamics
• Rossby waves,
• Ocean Rossby waves
• Latif Barnett Hypothesis for decadal
  variability
• Subduction

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SST Tendency Equatione.g. Frankignoul (1985,
Reviews of Geophysics)
Variables Tm mixed layer temp (SST) Tb
temp just beneath ML Qnet net surface
heat flux Qswh penetrating shortwave
radiation h mixed layer depth w
mean vertical velocity we entrainment
velocity v - velocity (current in ML)
vek Ekman vg - geostrophic A
horizontal eddy viscosity coefficient
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Process that Influence SST

Vek important on all time scales Vg associated
with eddies (50km) large-scale Rossby waves
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Model Experiments to Test Bridge Hypothesis
Specified SSTs
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Influence of Air-sea Feedback on the atmospheric
response to ENSO
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Atmospheric Response to ENSO over the North
Pacific
El Niño La Niña 30-day Running Mean Composite
500 mb height anomaly (176ºE-142ºW 32ºN-48ºN)
Aleutian Low
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Basin-wide Reemergence
Alexander et al. 2001, Progress in Oceanography
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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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Forecast Skill Correlation with Obs SST Wave
Model Reemergence
Wave Model
Reemergence
years
Schneider and Miller 2001 (J. Climate)
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PDO The Latif and Barnett Hypothesis

• Coupled atmosphere-ocean interaction in the
  extratropics causes variability with a period of
  20 years
• Key processes
• Atmosphere strongly responds to SST anomalies
  near Japan. Atmospheric circulation maintains SST
  through surface heat fluxes but drive changes in
  the ocean surface currents which reverse the SSTs
  5-10 years later. Time scale determined by
  oceanic Rossby waves.

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Mechanisms for North Pacific Decadal Variability

• Air-sea interaction within the North Pacific
  basin
• stochastic forcing (null hypothesis, simple slab)
• Ocean dynamics (Latif and Barnett 1994, 1996)
  Time scale set by changes in ocean currents
  (oceanic Rossby waves). Relies on strong
  atmospheric response to midlatitude SST
  anomalies.
• Tropical-extratropical interactions
• Subduction ocean transport from N. Pacific to
  tropics atmospheric teleconnections from
  Tropics to midlatitudes close the loop (Gu and
  Philander . Observations indicate this pathway is
  unlikely (Schneider et al., 1999).
• Air-sea interaction within the Tropical
  Indo-Pacific basin, with atmospheric
  teleconnections to the North Pacific as a
  by-product
• Tropical Ocean has ENSO Reemergence gt PDO
  (Null hypothesis II)
• Tropical ocean has a mechanism for decadal
  variability

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SLP SST Patterns of Pacific VariabilityWhat
process are involved?
ENSO
Regressions SLP Contour SST Shaded
Mantua et al. 1997, BAMS
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Schematic of the Latif and Barnett Hypothesis
Positive air - sea feedback
Warm SSTs
Rossby Waves
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Impact of Ocean Currents on the Atmosphere
Prescribed ocean heat flux convergence in a slab
ocean model coupled to a AGCM Mimics ocean heat
transport anomalies in Kuroshio region
60N
30N
15 Wm-2
eq
From Yulaeva et al., 2001, J Climate
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PDO Null hypothesis IIENSO Reemergence
Noise?
Model PDOn1 ?PDOn ?ENSOn1 ? ? and ?
are constants estimated from data, then ran model
1000 times
Newman et al, 2003, J. Climate
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Forecast of Annual Mean Anomalies PDO vs.
observed PDO
Correlation 0.74
a0.58 b0.58
Newman et al. 2003, J. Climate
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Skill of Seasonal Statistical PDO predictions by
verification season during 1971-2000
PDO (1st EOF of N. Pacific SST)
Nino 3.4
Correlation between prediction and observed time
series.
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PDO ENSO combined influence on SLP signal?
Gershunov and Barnett (1998, J. Climate)
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Testing the PDOs influence on the ENSO SLP signal
High-Low PDO, El Niño
High-Low PDO, La Niña
Obs
Pierce (2002, J. Climate)
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Winter Epoch Differences High Low SLP North
Pacific
Wet
Dry
minus
Land Precipitation
Deser et al. (In preparation)
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Winter Epoch Difference High-Low SLP North
Pacific
Land Precipitation
Marine Cloudiness
47-76 minus 77-97
180
180
Dry
Wet
Dry
Wet
Deser et al. (In preparation)