1. Decadal variability in the Indo-Pacific ocean inferred from satellite data and ECCO assimilation
Tong Lee
NASA Jet Propulsion Laboratory, California Institute of Technology
Michael McPhaden
NOAA Pacific Marine and Environmental Laboratory
collaborator
Reference for observational part: Lee and McPhaden (2008), GRL, Jan. 15 issue

2. Motivation
Linkages of DecVar of ocean circulation in the Indo-Pacific region not well understood, e.g., the meridional overturning circulations (MOCs) of the two oceans were studied separately (McPhaden & Zhang 2002/2004, Lee 2004).
Approach
Analyze satellite observations of sea surface height (SSH) and ocean surface wind in the past 15 years; infer ocean circulation changes.
SSH: TOPEX/Poseidon ( ) & JASON-1 (2001-present) altimeters;
Wind: ERS ( ) & QuikSCAT (1999-present) scatterometers.
Quantify ocean circulation changes using ECCO ( & ECCO-2 ( hi-res) ODA products.

3. SSH trend for 1993-2000 (T/P altimeter)
Decadal tendencies change sign around yr SSH in many regions forced by near-local wind (tilted arrows reflect ocean adjustment by Rossby waves)
cm/yr
SSH trend for (T/P altimeter)
dyn/cm2/yr
Wind stress curl trend for (ERS scat)
SSH trend for (T/P,Jason-1 altimeters)
Wind stress curl trend for (QuikSCAT)

4. Tropical Pacific leads other regions.
SSH time series (black curves) in key regions further illustrate the tendency change around 2000
Tropical Pacific leads other regions.
The role of ENSO residuals on decadal time scales?
Have implications to horizontal & meridional circulations in the Pac & Indian Oceans. MOC focus.

5. Schematics for zonally averaged view and horizontal view of time-mean Pacific & Indian-Ocean shallow MOCs 0N
Southeasterly trade wind
Westerly wind .
thermocline flow
Cross-equatorial cell
10S
20S
500 m
50 m
30S
Subtropical cell
Ekman flow
Indian Ocean
Trade wind
Ekman flow
Equatorward thermocline flow N
10N
Pacific Ocean
Divergence of Ekman flow
Convergence of w. boundary & interior pycnocline flow
Ekman outflow
Pycnocline inflow
Bird’s eye view
Wind data – upper branch. E-W SSH difference between coasts: thermocline flow

6. Implications of observed decadal wind & SSH to MOCs
Decadal change of zonal wind ~ meridional Ekman flow (upper branch of MOC): reflect atmospheric bridge (anomalous Walker Circulation).
Stronger tradewind
Anomalous Ekman
Weaker tradewind
Decadal change of E-W SSH difference ~ thermocline flow (lower branch of MOC): reflect oceanic linkage (wave-transmission from NW Pacific to SE Indian Oceans)
SE IO lags NW Pac 6-12 months
Anomalous pycnocline flow
Anomalous pycnocline flow

7. Volume (left) & temperature (right) fluxes of the Indonesian throughflow estimated by ECCO products
Acknowledge Michael Shodlok for computing fluxes from ECCO-2 product

8. Observed decadal change of Indian-Ocean trade wind & MOC strength estimated by ECCO assimilation
Estimated MOC

9. Comparison of SSH trend for 1993-2000 (indicative of DecVar): large-scale similarity
Altimeter data
ECCO-JPL estimate
Used to estimate gyre circulation changes
Although low-res model couldn’t resolve eddies, it represents large-scale trend relatively well.

10. Weakening Western Subarctic Gyre
Strengthening Kuroshio recirculation gyre
Strengthening North Central Pacific Gyre
Strengthening South Pacific Subtropical Gyre
Mean
Decadal change
Barotropic streamfunction: mean (left) & decadal change (lower)
NE corner consistent with Douglas et al using XBT, though referred to as interannual; S Pac consistent with Roemmich et al about
spinup

11. Summary
Near-coherent decadal tendency changes SSH & wind in the Indo-Pacific region at the end of the 20th century: broader than PDO.
Equatorial Pacific SSH, forced by local trade wind, leads the changes.
Anti-correlated changes of MOCs in the Pacific & Indian Oceans, thus opposite roles in regulating tropical heat content: atmospheric connection via Walker circulation & oceanic linkage by ITF.
Coherent variations of subtropical & subpolar gyres: atmospheric tele-connections.

12. Decadal signals from Reynolds SST
PDO index
Nino3.4 index

13. Tropical Pacific leads other regions.
SSH time series (black curves) in key regions further illustrate the tendency change around 2000
Tropical Pacific leads other regions.
The role of ENSO residuals on decadal time scales?
NINO3 3.4 index
PDO index
Have implications to horizontal & meridional circulations in the Pac & Indian Oceans. MOC focus.