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

Observational evidence for Observational evidence for

propagation of decadal propagation of decadal

spiciness anomalies in the spiciness anomalies in the

North PacificNorth Pacific

Yoshi N. Sasaki, N. Schneider, N. Maximenko, and K. Lebedev

International Pacific Research Center, University of Hawaii

Subduction of water massSubduction of water mass

• 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

Qiu and Huang (1995)

Decadal salinity variabilityDecadal salinity variability

• 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

Lukas et al. [2008]

from the Hawaii Ocean Time-series (HOT)

[PSS-78]

Subduction of spiciness Subduction of spiciness anomalyanomaly

• 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

Yeager and Large [2004]

= 25.5 kg m-3

ArgoArgo

• Argo observations provide a lot of T-S profiles with nearly global coverage from the early 2000s

suitable for examining propagation of spiciness signals

PurposePurpose

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

– propagation speed?

– decay timescale?

DataData• 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 25–25.5

Montgomery

Long-term mean on 25–Long-term mean on 25–25.525.5

Depth

[m]

outcrop line

Salinity anomalySalinity anomaly

[10 PSS-78]

2003 2004

2005 2006

2007 2008

Salinity anomalySalinity anomaly

2003 2004

2005 2006

2007 2008

[10 PSS-78]

Hovmoller diagramHovmoller diagram

[10 PSS-78]

[103 km]

Trajectory of particles 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

Trajectory of particles Trajectory of particles

Trajectory of particlesTrajectory of particles

2003 2004

2005 2006

2007 2008

The propagation path and speed show good agreement with advection by the

mean geostrophic current

Hovmoller diagramHovmoller diagram

[10 PSS-78]

[103 km]

Spiciness on streamline Spiciness on streamline coordinatecoordinate

• 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

SummarySummary

• Argo observations provide an evidence for propagation of the spici

ness anomalies from the eastern subtropics to western tropics

• The propagation path and speed of the spiciness signals are good a

greement with advection by the mean geostrophic current

Sasaki, Y. N., N. Schneider, N. Maximenko and K. Lebedev, 2010: Observational e

vidence for propagation of decadal spiciness anomalies in the North Pacific. GR

L, 37, L07708, doi:10.1029/2010GL042716.

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

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