Mode water and eddies: Their role in North Pacific climate variability

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Mode water and eddies Their role in North
Pacific climate variability
CLIVAR Pacific Panel Meeting Honolulu, February
15-17, 2006
Toshio Suga Department of Geophysics, Graduate
School of Science, Tohoku University, Sendai,
Japan
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Outline

• Eddy induced transport of heat and thermocline
  water
• (Roemmich Gilson, 2001 Qiu Chen, 2005)
• Major contributor to time variability in STCC
  and KE regions
• Mode water and subtropical countercurrent
• (Aoki, Suga Hanawa, 2002)
• STCC caused by equatorward spreading of mode
  water
• Decadal variability in mode water formation
• (Qiu Chen, in press)
• Eddy variability controlling mode water
  formation
• WBC heat transport/content and ocean-atmosphere
  interaction
• (Kelly Dong, 2004)
• Heat content (mode water) anomalies causing
  air-sea flux variations

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Eddy transport of heat
Roemmich Gilson (2001)
High-resolution XBT (HRX) transect
rms variability of SSH baroclinic instability
in the STCC region (Qiu, 1999)
Total northward eddy transport 0.086 pW
(15 of total transport) (0.0054 PW x 14 eddies)
Heat transport per unit depth due to eddy field
Composite eddy
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Eddy transport of thermocline water
Roemmich Gilson (2001)
southward eddy transport of thermocline
water 3.9Sv
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Time variability of eddy transport
Roemmich Gilson (2001)
Eddy heat transport may be a major contributor
to time variability of northward heat transport.
Eddy transport of themocline (10º-22ºC) water
(solid). Mean transport of thermocline water
(dashed). No. of eddies from T/P data
(dotted). Eddy may cause interannual-to-decadal
fluctuation in thermocline water supply to
tropical ocean.
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Eddy transport of heat
Qiu Chen (2005)
Argo float data
ltTvgt
T
v
eddy heat flux 0.025 pW
(in the reference frame moving with the eddy)
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Qiu Chen (2005)

• Confinement of eddy heat transport to the
  seasonal thermocline suggests an effective depth
  He may be sought that relates the surface and
  depth-integrated eddy heat transport values

0
He ? vT(z)dz / vT(z0)
1500db
He 177m
(estimates based on individual Argo floats)
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Qiu Chen (2005)
Meridional eddy heat transport ?cpvTHe
Here, vT is calculated using concurrent
SSH/SST data of 1998-2003
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Meridional eddy heat transport by Stammer (1998)
vT -? dT/dy where T Levitus mean
temperature in the upper
1000m layer ? estimated
from T/P SSH data
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Qiu Chen (2005)
Meridional eddy heat transport ?cpvTHe
Stammer (1998)
Present study
zonal average flux (pw)
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Qiu Chen (2005)
Meridional eddy heat transport ?cpvTHe
Roemmich and Gilson (2001)
Present study
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Qiu Chen (2005)
Meridional eddy heat transport ?cpvTHe
Subtropical Front
KE Front
Temperature section along 144E
155ºE repeat section
144E section
Qiu (1999)
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Subtropical Mode Water and Subtropical
Cuntercurrent/Front
Aoki, Suga Hanawa (2002)
Composite of STCC
sq
U
PV
AOU
14
STMW and Subtropical Countercurrent/Front
Aoki, Suga Hanawa (2002)
STCC caused by southwestward spreading of STMW
consistent with theoretical
(Kubokawa, 1999) and OGCM (Kubokawa Inui, 1999)
argument
STMW ? STCC ? Eddy transport
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Decadal variability in mode water formation
Qiu Chen (in press)
Decadal signal in Subtropical Mode Water (STMW)
16º-18ºC layer
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Decadal variability in mode water formation
Qiu Chen (in press)
STMW thickness
Qnet anomaly not corresponding to STMW
variability
Eddy variability corresponding to STMW
variability large eddy variability ? thin STMW
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What do eddies do ?
Qiu Chen (in press)
KE
high eddy activity
low eddy activity
Fall stratification in the top 450m
High eddy variability causes stronger
stratification just before the cooling
season. preconditioning
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Decadal variability in mode water formation
Qiu Chen (in press)
Winter MLD is not sensitive to Qnet .
Eddy variability controlling STMW through changes
in preconditioned stratification (N ).
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WBC heat transport/storage and ocean-atmosphere
interaction
Kelly Dong (2004)
EOF of heat content (upper 400m)
weak AO (weak westerlies) ? negative HC anomalies
strong AO (strong westerlies) ? positive HC
anomalies
AO leading HC by 13 months
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WBC heat transport/storage and ocean-atmosphere
interaction
Kelly Dong (2004)

• Correlation 0.40 (Curl variations leading heat
  content by about one year)
• advection dominates heat content variations in
  the KE region
• Stronger westerlies spinning up the (barotropic)
  circulation
• Transporting more heat into western boundary
  current region
• .
• Anomalously warm water accumulated below ML
• (?increased stratification ? less STMW formation)
• Entrainment cooling of winter ML is decreased
• Anomalously high SST
• Anomalously large heat loss to the atmosphere
  H-Qnet

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WBC heat transport/storage and ocean-atmosphere
interaction
Kelly Dong (2004)
Surface heat flux anomaly for the year 2000 based
on a regression of NCEP/NCAR net heat flux onto
EOF1 of heat content anomalies
Heat flux anomalies associated with heat content
are of the order of 20 of the annual mean value.
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Summary Processes to be clarified through obs.
models
mentioned
not mentioned
atmosphere
northward heat transport
eddies
thermocline water supply to the tropical ocean
PV
baroclinic instability
STCC/subtropical front
Preconditioned stratification
heat
KE/recirculation
PV
Heat content
STMW
heat
heat
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Summary

• Eddy induced transport of heat and thermocline
  water
• (Roemmich Gilson, 2001 Qiu Chen, 2005)
• Major contributor to time variability in STCC
  and KE regions
• Mode water and subtropical countercurrent
• (Aoki, Suga Hanawa, 2002)
• STCC caused by equatorward spreading of mode
  water
• Decadal variability in mode water formation
• (Qiu Chen, in press)
• Eddy variability controlling mode water
  formation
• WBC heat transport/content and ocean-atmosphere
  interaction
• (Kelly Dong, 2004)
• Heat content (mode water) anomalies causing
  air-sea flux variations

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Stammer (1998)

• From mixing length hypothesis, eddy diffusivity
• ?2?KeTint
• where Ke eddy kinetic energy from
  satellite altimetry
• Tint integral timescale
  (estimated from auto-
• correlation of
  altimetric SSH data)
• ? scaling factor (0.005)
• Eddy heat transport
• vT -? dT/dy
• where T Levitus mean temperature in the
  upper
• 1000m layer

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dT/dy in 0-200m layer
dT/dy in 0-1000m layer
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Eddy and mode water formation
Qiu Chen (in press)
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WBC heat transport/storage and ocean-atmosphere
interaction
Kelly Dong (2004)
Advection is more important in determining heat
content.

• Positive heat content anomalies
• Small volume of STMW
• Large advection of heat increasing
  stratification (less STMW formation)

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Reemergence of winter SST anomalies
Hanawa Sugimoto (2004)
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Reemergence of winter SST anomalies
Hanawa Sugimoto (2004)