Xujing Jia Davis Graduate School of Oceanography, University of Rhode Island Lewis M. Rothstein

1. Numerical and Theoretical Investigations of North Pacific Subtropical Mode Water with Implications for Pacific Climate Variability Xujing Jia Davis Graduate School of Oceanography, University of Rhode Island Lewis M. Rothstein Graduate School of Oceanography, University of Rhode Island William K. Dewar Department of Oceanography, Florida State University, Dimitris Menemenlis Jet Propulsion Laboratory, California Institute of Technology

2. North Pacific Subtropical Mode Water (STMW) Schematic current patterns in western North Pacific Location: forms and resides south of Kuroshio Extension (KE) • Features: • - weakly stratified, low PV • - upper 500 m of the ocean • water column • inhabits thermostads • between 16 and 19C STMW formation region - salinity range of 34.65-34.8psu - potential density range of 24.8-25.7 kg/m^3 (Masuzawa, 1969; Suga et al., 1990; Eitarou et al., 2004)

3. Questions • STMW has known seasonal variability, but what is the variability of STMW on longer time scales (as revealed by models)? • Do models properly capture the seasonal variability of STMW volume? • What is the relationship (if any) between low frequency variability of STMW and known climate patterns in the Pacific? • Supporting dynamics?

4. “Top Down” Modeling • MITgcm: 3D, z-level, primitive equation OGCM (Marshall, 1997) • ECCO2: global-ocean and sea-ice simulation • North Pacific data from two ECCO2 MITgcm simulations are extracted for analysis: • Cube37 simulation: 28-year spin-up prior to its initial January 1992 conditions, 1992-2000 NCEP forcing converted to fluxes using model SST (Large et al, 1981,Menemenlis, 2005) • Cube76 simulation: driven by ERA40 atmospheric surface boundary conditions, followed by ECMWF analysis after August 2002 when the ERA40 reanalysis stops; weak relaxation to climatological seasonal cycle of sea surface salinity. • Common horizontal resolution 1/6o x 1/6o; 50 levels • Common temporal coverage: 1992, Jan – 2006 Mar (171 months) • Output has not been constrained by oceanic and sea-ice data

5. STMW Region & Definition - 130E~ 160W, 20N~ 40N and east of islands of Japan

6. Compare with Observations KESS, late May 2004 MITgcm (Cube37), May 2004 MITgcm (Cube76), May 2004 After Rainville,et al., 2007 Meridional cross section at 145 E of potential density, PV and zonal velocity in MITgcm experiments and observations. The stippled areas are the observed and modeled STMW.

7. Temporal Variability: STMW Volume Dominant signal is seasonal; Lower frequency variability apparent (Cube 37)

8. Seasonal Cycle

9. STMW Seasonal CycleThreeDistinct Periods STMW Volume Period I: STMW is exposed to atmos. forcing Period II: STMW is partially isolated from atmos. forcing Period III: STMW is completely isolated from atmos. forcing

10. Interannual Variability

11. STMW Interannual Variability (Cube 37)

12. Pacific Decadal Oscillation (PDO) SST Warm phase: Cooler SST in STMW region Cool Phase: Warmer SST in STMW region Warm Phase                                Cool Phase 4 year Cool Phase 1976/77 1998/99 cool phase warm phase www.jisao.washington.edu/pdo/

13. STMW Variability & PDO PDO index (top) and STMW volume in MITgcm simulations (bottom) STMW volume variability is correlated with the PDO index Co=0.69 (Cube37); Co=0.80 (Cube76)

14. Connection Between STMW & PDO:Large-scale Atmospheric Variability

15. Connection Between STMW & PDO:Large-scale Atmospheric Variations from NCEP 1st EOF (37.7%) 1st EOF (37.7%) More heat loss from ocean to atmos. Weaker Ekman pumping Year 1996, STMW maximum

16. Connection between STMW & PDO:Large-scale Atmospheric Variations from NCEP 1st EOF (37.7%) 1st EOF (37.7%) Less heat loss from ocean to atmos. Stronger Ekman pumping Year 1999, STMWminimum

17. Summary (MITgcm) • The dominant temporal pattern of STMW is seasonal; the annual cycle can be divided into formation, partial isolation and complete isolation periods that correspond to distinct stages of STMW evolution. Strong air-sea interaction is the main feature of the STMW formation period. • An interannual signal is clearly seen in STMW variability, and this lower frequency signal shows significant correlation with the PDO index • This likely results from variations in large scale atmospheric forcing: wind stress and/or surface heat flux

18. Planetary Geostrophic Theoretical FrameworkA Ventilated Thermocline Model of STMW(Dewar et al. 2005) • Modified LPS theory is used to describe STMW and its connections to large scale ocean/atmosphere circulation • An analytical model of STMW, based on ventilated thermocline theory. • Ventilated Pool Hypothesis: Assumes that all of the water in the pool (i.e. circulating) region is ventilated and, therefore, that all the Sverdrup transport is carried in the uppermost, outcropped layer. • A PGOM (Samelson & Vallis, 1997) numerically approximates the solutions to this theory and is used to describe/diagnose STMW characteristics/dynamics. • PGOM experiments were performed to help interpret the role of large-scale wind stress curl and the local heat flux in forcing STMW variability, as seen in the MITgcm simulations. • Reasonably simulates the analytical solutions of PG framework.

19. Planetary Geostrophic Theoretical FrameworkResults • The formation of a deep, vertically homogeneous, fluid layer in the northwest corner of the subtropical gyre that extends from the surface to the base of the ventilated thermocline. • This ventilated pool is the model analog of the observed STMW.

20. PGOM Model Description Designated “SV97”, adapted from Samelson and Vallis 1997, Dewar et al, 2005, based on PG Equations (non-dimensionalized): } Momentum Hydrostatic Continuity Thermal Salinity Eq of state

21. PGOM Model Domain • Domain - Non-dimensionalized box; the dimensional values are 5000 km horizontally, 5km vertically - Central latitude 35N, y=2500 km, zero Ekman pumping line at y~24N and y~46N • Resolution - 80 km horizontally - vertically stretched to give more resolution in the thermocline ………………… Zero Ekman Pumping 5000km 5km 5000km

22. Ekman Surface Layer • Upper B.C. on T and w: Simple ‘slab’ model of frictional surface B.L., fixed depth • The vertically integrated Ekman balance is assumed to hold: Ekman Surface Layer • TE is obtained from the vertically • integrated thermodynamic • equation Adapted from Welander, 1971

23. 2 Classes of PGOM Experiments • Steady forcing: To diagnose the characteristics of STMW circulation and upper ocean structure under constraints of the modified ventilated thermocline scheme. • Time-dependent forcing: To investigate dynamics associated with STMW variability for reasonably realistic - basin-scale wind forcing (in the form of Ekman pumping) - heat flux (isolated to the northwest corner of the STG)

24. PGOM Experiment 2 Design:Time Varying Forcing Heat flux varying experiment Wind stress curl varying experiment

25. Results: Time-dependent Ekman Pumping : Ekman pumping amplitude (x10-4cm/s) STMW volume (m3) x1015 O(1013 m3) O(1015 m3) . • Both seasonal and interannual variations are simulated • Stronger Ekman pumping, more mode water • Interannual change, with 1~2 year lag • The STMW volume is O(1015 m3) • The magnitude of the oscillation annually ~ O(1013 m3)

26. Results: Time-dependent Heat Flux : Air-sea heat flux coefficient STMW volume (m3) x1014 O(1014 m3) O(1015 m3) • Seasonal and Interannual variations simulated; • More heat loss, more STMW volume, with 1~2 year offset • The STMW volume is O(1015 m3) • The magnitude of the oscillation annually ~ O(1014 m3)

27. PGOM Results • PGOM experiments demonstrate that the interannual variability observed in the MITgcm simulation can be driven by variations in the large scale air-sea heat flux (zero lag) and wind stress patterns (2 year lag) seen in the NCEP reanalysis • The variations in air-sea heat flux play the more dominant role during the period in he late 1990s when STMW “discharges” its volume, with influence of one order of magnitude larger than the varying wind stress

28. Acknowledgements • Roger M. Samelson (OSU) • Geoffrey K. Vallis(GFDL) • Young-Oh Kwon (WHOI) • ECCO2: Estimating the Circulation and Climate of the Ocean, Phase II, which is sponsored by the NASA Modeling Analysis and Prediction (MAP) program

29. Numerical and Theoretical Investigations of North Pacific Subtropical Mode Water with Implications to Pacific Climate Variability Xujing Jia Davis Graduate School of Oceanography, University of Rhode Island Lewis M. Rothstein Graduate School of Oceanography, University of Rhode Island William K. Dewar Department of Oceanography, Florida State University, Dimitris Menemenlis Jet Propulsion Laboratory, California Institute of Technology

30. STMW Definition in PGOM In the region of and roughly in the subtropical gyre and east of the western boundary current.

31. Seasonal CycleMeridional cross sections

32. Seasonal CycleMeridional cross sections

33. Seasonal CycleMeridional cross sections

34. Seasonal CycleMeridional cross sections

35. Seasonal CycleMeridional cross sections

36. Seasonal CycleMeridional cross sections

37. Experiment 1: Constant Atmosphere Forcing Ekman pumping (WE) Initialization: from motionless, 18000 years integration diffusive time: T=H2/kv , H~3750 m, kv=2.5x10-5 m2/s

38. Experiment 1: Thermocline Structure at the center of the domain I. ventilated regime STMW W=0 II. diffusive regime T (C) Tz (C/m) Tzz (C/m2) W (m/s)

39. Experiment 1: Thermostad Zero Ekman Pumping line • A thermostad is found between the two Tz maximums, or between the two thermocline regimes • The isopycnal surfaces of the shallow themostad are in the ventilated thermocline • Around the bottom of the themostad, the isopycnal surfaces in the internal thermocline