Tong LeeNASA Jet Propulsion Laboratory, California Institute of Technology

Michael McPhadenNOAA Pacific Marine and Environmental Laboratory

Decadal variability in the Indo-Pacific ocean inferred from satellite data and ECCO assimilation

Reference for observational part: Lee and McPhaden (2008), GRL, Jan. 15 issue

MotivationLinkages 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 (1992-2005) & JASON-1 (2001-present) altimeters; Wind: ERS (1991-2001) & QuikSCAT (1999-present) scatterometers.

• Quantify ocean circulation changes using ECCO (http://www.ecco-group.org) & ECCO-2 (http://ecco2-org, hi-res) ODA products.

Decadal tendencies change sign around yr 2000. SSH in many regions forced by near-local wind (tilted arrows reflect ocean adjustment by Rossby waves)

SSH trend for 1993-2000 (T/P altimeter)

SSH trend for 2000-2006 (T/P,Jason-1 altimeters)

Wind stress curl trend for 1993-2000 (ERS scat)

Wind stress curl trend for 2000-2006 (QuikSCAT)

cm/yr

dyn/cm2/yr

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?

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

10S20S

500 m

50 m

30S

Subtropical cell

Ekman flow

Indian OceanTrade wind

Ekman flow Ekman flow

Equatorward thermocline flow

Equatorward thermocline flow

N10S 10N

Pacific Ocean

Divergence of Ekman flow

Convergence of w. boundary & interior pycnocline flow

Ekman outflow

Pycnocline inflow

Bird’s eye view

Implications of observed decadal wind & SSH to MOCsDecadal change of zonal wind ~ meridional Ekman flow (upper branch of MOC): reflect atmospheric bridge (anomalous Walker Circulation).

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)

Anomalous pycnocline flowAnomalous

pycnocline flow

Stronger tradewind

Anomalous Ekman

Anomalous Ekman

Weaker tradewind

Stronger tradewind

Weaker tradewind

Volume (left) & temperature (right) fluxes of the Indonesian throughflow estimated by ECCO products

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

Trade wind

Estimated MOC

Comparison of SSH trend for 1993-2000 (indicative of DecVar): large-scale similarity

Altimeter data

ECCO-JPL estimate

Used to estimate gyre circulation changes

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)

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.

PDO index

Nino3.4 index

Decadal signals from Reynolds SST

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