Observational evidence for propagation of decadal spiciness anomalies in the North Pacific

1
Observational evidence for propagation of decadal
spiciness anomalies in the North Pacific

• Yoshi N. Sasaki, N. Schneider, N. Maximenko, and
  K. Lebedev
• International Pacific Research Center, University
  of Hawaii

2
Subduction of water mass
Qiu and Huang (1995)

• In the subtropical region, the mixed layer water
  in winter is subducted into the main thermocline
• Subduction plays an important role in the
  freshwater cycle

3
Decadal salinity variability
Lukas et al. 2008 from the Hawaii Ocean
Time-series (HOT)
PSS-78

• Spiciness (density compensated) anomaly
• Sa (x, y,?? ,t) S (x, y,?? ,t) Sclm (x, y,??)
• warm/salty or cool/fresh
• passive advection by current
• Subsurface spiciness signals show substantial
  decadal fluctuations

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Subduction of spiciness anomaly
?? 25.5 kg m-3
Yeager and Large 2004

• Several modeling studies showed propagation of
  spiciness signals from the subtropics to the
  equator
• It is difficult to identify basin-wide
  propagation of spiciness signals from
  observations due to the sparseness of salinity
  observations

5
Argo

• Argo observations provide a lot of T-S profiles
  with nearly global coverage from the early 2000s
• ? suitable for examining propagation of spiciness
  signals

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Purpose

• To show observational evidence for basin-wide
  propagation of spiciness anomalies in the North
  Pacific
• propagation speed?
• decay timescale?

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Data

• The Argo profile data are obtained from the US
  Argo Data Assembly Center
• The each Argo profile is linearly interpolated to
  isopycnal surfaces, and the variables are
  averaged into 3? ? 3? bins for each month
• We also employ a spatially interpolated dataset
  using the Variational Interpolation algorithm

Number of profiles on 2525.5??
8
Long-term mean on 2525.5??
Montgomery
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Salinity anomaly
2003
2004
2005
2006
?10 PSS-78
2007
2008
10
Salinity anomaly
2003
2004
2005
2006
?10 PSS-78
2007
2008
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Salinity anomaly
2003
2004
2005
2006
?10 PSS-78
2007
2008
12
Salinity anomaly
2003
2004
2005
2006
?10 PSS-78
2007
2008
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Hovmoller diagram
?103 km
?10 PSS-78
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Trajectory of particles

• Fifteen parcels are released around the
  cool/fresh spiciness anomaly positions in 2008,
  and their backward trajectories are calculated
  using the mean velocity field

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Trajectory of particles
16
Trajectory of particles
17
Trajectory of particles
18
Trajectory of particles
19
Trajectory of particles
20
Trajectory of particles
2004
2003
2005
2006
The propagation path and speed show good
agreement with advection by the mean geostrophic
current
2007
2008
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Hovmoller diagram
?103 km
?10 PSS-78
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Hovmoller diagram
?103 km
?10 PSS-78
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Spiciness on streamline coordinate

• The amplitude of the salinity anomaly of the
  spiciness signal decreases by about 80 from
  -0.15 in 2004 to -0.03 in 2008
• The anomalies are diffused in the course of
  propagation

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Summary

• Argo observations provide an evidence for
  propagation of the spiciness anomalies from the
  eastern subtropics to western tropics
• The propagation path and speed of the spiciness
  signals are good agreement with advection by the
  mean geostrophic current
• Sasaki, Y. N., N. Schneider, N. Maximenko and K.
  Lebedev, 2010 Observational evidence for
  propagation of decadal spiciness anomalies in the
  North Pacific. GRL, 37, L07708,
  doi10.1029/2010GL042716.