R/V Revelle over Kuroshio Extension, May

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Dynamic and Thermodynamic Variability of the
Kuroshio Extension System on Decadal Timescales
B. Qiu, S. Chen and P. Hacker Department of
Oceanography University of Hawaii at Manoa
R/V Revelle over Kuroshio Extension, May05
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Altimeter-derived rms sea surface height
variability (cm)
Schematic of NW Pacific Ocean Circulation
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Semi-monthly Kuroshio Extension paths (1.7m SSH
contours)
Stable yrs 1993-94, 2002-04
Unstable yrs 1996-2001, 2006-
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KE path length
Level of EKE
Stable yrs 1993-94, 2002-04
Unstable yrs 1996-2001, 2006-
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• Q1 What causes the transitions between the
  stable and unstable dynamic states of the KE
  system?

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PDO index
EKE level
Mesoscale EKE level in the KE region lags the PDO
index by 4 yrs
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Pacific Decadal Oscillations (Mantua et al. 1997)

• Center of action of wind forcing is in the
  eastern half of the N Pacific basin
• Positive (negative) phase of PDO generates ()
  local SSH through Ekman divergence (convergence)

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EKE level
SSHA along 34N
SSH field
PDO index
L
H
L
145E
165E
155E
135E
center of PDO forcing
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SSH variability vs wind stress forcing
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SSHA along 34N
PDO index
Wind-forced SSHA along 34N
L
H
L
center of PDO forcing
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• Q1 What causes the transitions between the
  stable and unstable dynamic states of the KE
  system?
• Q2 Does the nonlinear WBC dynamics play a role
  in the observed KE variability?

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Feedback of eddies to the modulating time-mean
flow

• Surface ocean vorticity equation

eddy-driven mean flow modulation

• Evaluate

mechanical feedback of eddies onto the
slowly-varying mean SSH field (e.g. Hoskins et
al. 1983, JAS)

• Regress S(x,y,T) field to the observed EKE time
  series

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Eddy-forced S(x,y,T) field regressed to the EKE
time series
contours mean SSH field

• anticyclonic forcing vs . cyclonic forcing
• In the upstream KE region, enhanced eddy
  variability works to increase the intensity of
  the southern RG.
• Enhanced eddy variability strengthens the two
  quasi-stationary meanders (cf. Rossby lee-wave
  dynamics)

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KE path length
Level of EKE
RG strength
Stable yrs 1993-94, 2002-04
Unstable yrs 1996-2001, 2006-
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Yearly-mean sea surface height field
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• Q1 What causes the transitions between the
  stable and unstable dynamic states of the KE
  system?
• Q2 Does the nonlinear WBC dynamics play a role
  in the observed KE variability?
• Q3 How does the decadal KE variability affect
  the regional upper ocean thermal structures?

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T, s? and PV sections across the KE jet along
146E
05/2004 KESS cruise
Subtropical Mode Water
potential temp 16-18C
potential density 25.2-25.5 s?
potential vorticity Qlt2x10-10
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T(z,t)
MLD
Q(z,t)
(based on available XBT, CTD, Argo, KESS data
inside RG)
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NCEP wintertime Qnet anomalies in the KE/RG region
less cooling
more cooling
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Late winter MLD vs. Qnet anomalies
less cooling
more cooling
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(No Transcript)
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Late winter MLD vs. Qnet anomalies
Late winter MLD vs. KE path length in the
previous year
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Pre-conditioning vs. convective depth

• Let the fall season upper ocean stratification
  be and let surface cooling be .

N
Q(t)lt0

• The convective depth in this case can be
  estimated from conservation of heat

D
z
Q(t)
D
T(z)
Convective depth (i.e., wintertime STMW
thickness) depends not only on the cumulative
heat loss, but also on the pre-conditioning
stratification.
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s? and PV sections across the KE jet along 146E
When KE is in the unstable state,
lateral/isopycnal mixing of high-PV KE water into
the RG can inhibit the STMW formation through
pre-conditioning.
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Quantifying the relative importance of
pre-conditioning vs air-sea flux forcing in
determining the late winter MLD
full forcing
oceanic forcing
atmos. forcing
Oceanic forcing 80 of the variance Atmospheric
forcing 13 of the variance

• Full forcing run Initialize 1-D PWP model with
  observed fall N(z) profile daily
  NCEP flux forcing
• Oceanic forcing run Surface flux forcing
  replaced by climatological fluxes
• Atmosphere forcing run Initial N(z) replaced by
  climatological fall profile

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Lateral Watermass Property Exchange Occurs
Efficiently
KESS period
KESS captured the transition from a stable to an
unstable state.
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PV values on 25.4 s? surface (250m, STMW core)
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T(z) and PV(z) evolution in the KE recirculation
gyre
mixed layer depth
subtropical mode water
KESS period
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Summary

• The observed Kuroshio Extension system oscillates
  between a stable and unstable dynamic state with
  a period of 10 years.
• Transitions between the two dynamic states are
  initiated by wind-induced SSH anomalies
  associated with the PDOs and originate in the
  central N. Pacific.
• When PDO forcing is , enhanced Ekman pumping
  generates SSHAs and causes a delayed weakening
  of the zonal KE jet and a southerly path shift.
• By interacting with the shallow Shatsky Rise, the
  southerly KE jet increases the eddy variability
  and this works to strengthen the RG and enhance
  the amplitude of the decadally-modulating KE
  system.
• Decadal KE variability effectively changes the
  regional upper ocean thermal structures through
  either direct property exchange or
  pre-conditioning.

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• Q1 What causes the transitions between the
  stable and unstable dynamic states of the KE
  system?
• Q2 Does the nonlinear WBC dynamics play a role
  in the observed KE variability?
• Q3 How does the decadal KE variability affect
  the regional upper ocean thermal structures?
• Q4 Is the decadal KE variability merely a
  forced response of the midlatitude basin-scale
  atmospheric forcing?

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Yearly SSH anomaly field in the North Pacific
Ocean


-
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Shatsky Rise
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EKE level
SSHA along 34N
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PDO-NPGO linear correlation -0.29
(monthly) -0.51 (interannual)
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PDO- vs NPGO-Forcing of SSHAs in a 1st-order AR
Model (1992-2008 AVISO period)
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2004
Kuroshio Extension System Study Observing
Array/Timeline
20 floats 28 floats
UH-KESS profiling float measurements
2005
1500db
5 days
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Distributions of semi-monthly profiling float
data
(5-day repeat all KESS float data are part of
the global Argo dataset)