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Relation between PDO and large-scale ocean circulation variations

Relation between PDO and large-scale ocean circulation variations. ChuanLi Jiang Earth & Space Research, US Gary Lagerloef Earth & Space Research, US Scott Springer Earth & Space Researc h, US. Motivation.

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Relation between PDO and large-scale ocean circulation variations

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  1. Relation between PDO and large-scale ocean circulation variations ChuanLi JiangEarth & Space Research, US Gary LagerloefEarth & Space Research, US Scott Springer Earth & Space Research, US

  2. Motivation • Goal: do the oceanic dynamics play a role in the initiation and evolution of the Pacific Decadal Oscillation (PDO) (Clement et al., 2009, J.Climate)? • Hypothesis: with atmospheric stochastic forcing being strengthened and moved southward (40o to 35oN), Ekman transport, vertical mixing, surface heat fluxes intensified, gyre circulation moves southward; • Question 1: what physical processes contribute to the decadal new gyre (SSH field) re-establishment? • Question 2: what physical processes contribute to the decadal surface temperature (SST field) variations?

  3. PDO SSH index: upper ocean heat content variations (Cummins et al., 2005 GRL.) • PDO SSH altimetry index: robust indicator of PDO variations; consistent with JISAO SST index; • ECCO2index: agrees with altimetry index; • ESM2M index : erroneous regime shift 1997-1998;

  4. Data & Method • Time period: 1993-2012; • Lag regressions of the PDO-related variables against the PDO SSH index from 2 year before PDO (-2 year) to PDO peak (0 year); • Variables: altimetry SSH, Renolds SST (OI.v2), OSCAR satellite-derived ocean currents, OAFlux net heat fluxes, ECCO2 observational-model synthesisSSH (Stammer et al., 2002 EOS.), GFDL-ESM2M carbon/climate coupled model (similar climate fidelity as CM2.1) SSH (Dunne et al., 2012 J. Climate.); • GFDL-ESM2M historical (1993-2005), rcp85 (2006-2012);

  5. Lag regressions of altimetry SSH vs. ECCO2 Evolution (-2, -1, 0 year) of PDO in ECCO2 agrees with altimetry; differs near northeast coast;

  6. Lag regressions of altimetry SSH vs. ESM2M PDO in ESM2M seems to center near 50oN, far north than the altimetry PDO;

  7. Lag regressions of SSH, SST against PDO SSH index Evolution of SST similar to SSH, but differs near northeast coast; imply important role of subsurface processes;

  8. vs. Wind stress anomalies one to two years before seems to contribute to much of the PDO SSH tendency;

  9. vs. Net heat flux seems to explain much of PDO SST variations; not in the central Pacific;

  10. vs. Meridional advection of Ekman currents contribute to SST variations; explain some variations in the central Pacific;

  11. vs. Zonal advection of geostrophic currents one to two years prior seem to explain much of PDO SST variations in the central Pacific;

  12. Conclusions • 20yr altimetry PDO index: a robust PDO indicator; • GFDL-ESM2M regime shift near 1997-1998 issue; • Wind stress anomalies in the central north Pacific one to two years before the PDO mature state accounted for much of the SSH tendency: • Once SSH fields are re-established, surface net heat fluxes, zonal advection by geostrophic currents, meridionaladvection by Ekman currents appear to contribute to the PDO SST tendency:

  13. Future work • Role of the Rossby wave and the observed mixed layer depth in the PDO variations; • Role of the oceanic dynamics in ECCO2 in the initiation and evolution of the PDO? • Role of the oceanic physics in GFDL-ESM2M in the initiation and evolution of the PDO? What causes the erroneous regime shift near 1997-1998?

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