1. Michael J. McPhaden, NOAA/PMEL Dongxiao Zhang, University of Washington and NOAA/PMEL Circulation Changes Linked to ENSO- like Pacific Decadal Variability Workshop on Low Frequency Modulation of ENSO 23 September 2003 Toulouse, France

2. Purpose 1)Review observations indicating a slowdown in the shallow meridional overturning circulation in the tropical Pacific from the 1970s to the 1990s coincident with the development of warm PDO conditions (McPhaden & Zhang, Nature, 2002). 2)Update these results for the late 1990s to the present. Circulation Changes Linked to ENSO- like Pacific Decadal Variability

3. PDO S. Hare, personal comm.

4. Relevance of PDO  Affects the climate of North America (Latif & Barnett, 1994; Cayan et al, 2001)  Affects marine ecosystems of the Pacific (Mantua et al, 1997; Hare and Mantua, 2000; Chavez et al, 2003)  May affect NAO and Atlantic deep meridional overturning circulation (Latif 2001; Hoerling et al, 2001)  Linked to decadal modulation of ENSO (Trenberth and Hurrell, 1997; Latif et al, 1997)

5. Relevance of PDO  Affects the climate of North America (Latif & Barnett, 1994; Cayan et al, 2001)  Affects marine ecosystems of the Pacific (Mantua et al, 1997; Hare and Mantua, 2000; Chavez et al, 2003)  May affect NAO and Atlantic deep meridional overturning circulation (Latif 2001; Hoerling et al, 2001)  Linked to decadal modulation of ENSO (Trenberth and Hurrell, 1997; Latif et al, 1997)

6. Theories and Hypotheses*  Mid-latitude ocean-atmosphere interactions (Latif & Barnett, 1994; Yuleava et al, 2001)  Tropical ocean-atmosphere interactions (Trenberth and Hurrell, 1994; Graham, 1994)  Tropical-extratropical ocean-atmosphere interactions (Gu and Philander, 1997; Kleeman et al, 1999)  Stochastically forced by atmospheric noise (Burgers, 1999; Barnett et al, 1999) * Reviewed by Miller and Schneider, 2000, Prog. Oceanogr.

7. Kleeman et al (1999) Hypothesis for Pacific Decadal Climate Variability: Decadal time scale tropical Pacific temperature anomalies are determined by the rate at which the subtropical cells transport thermocline water towards the equator (V’T) rather than by the transport of anomalously warm or cold thermocline water by the mean circulation (VT’).

8. Ocean Circulation Schematic

9. Mean Circulation in Pycnocline 9°N 9°S 14 Sv 7 Sv (Integrated over 22.5-26.5 kg m -3 )

10. Changes from 1970s to 1990s

11. Interior Ocean Pycnocline Transport Changes (22.5-26.5 kg m -3 ) McPhaden & Zhang, 2002

12. Pycnocline Convergence SST anomaly (9°N-9°S, 90°W-180°)

13. Surface Layer Divergence *Ekman transports computed from NCEP, ECMWF, COADS, FSU wind stresses; surface transport adjusted for geostrophic flow *

14. PDO positive phase Change in 1998? 0.84 -0.36

15. Perfect Ocean for Drought “The Perfect Ocean for Drought” (Hoerling & Kumar, Science, 2003) “…the modeling results offer compelling evidence that the widespread drought was strongly determined by the tropical oceans.”

16. Differences during last 10 yrs (SST, wind stress) Reynolds SST; ERS & Quikscat wind stress

17. Differences during last 10 yrs (SST, wind stress) Reynolds SST; ERS & Quikscat wind stress

18. Differences during last 10 yrs (SST, wind stress differences) Reynolds SST; ERS & Quikscat wind stress, TOPEX/Poseidon & Jason sea level

19. Differences during last 10 yrs (20C)

20. PV and Data Distribution Potential Vorticity (   = 25 kg m -3 ) CTD Casts to 900 m July 92- June 98 July 98- June 03 11,585 6,729

21. Interior Ocean Pycnocline Transport Changes

22. Pycnocline Convergence

23. Differences during last 10 yrs Surface layer divergence is Ekman divergence (based on ERS & Quikscat wind stress) reduced by geostrophic convergence in the surface layer.

24. Differences during last 10 yrs Western boundary current transport convergence computed assuming mass conservation (surface layer divergence minus pycnocline convergence) and Indonesian Throughflow decadal variations small.

25. Differences during last 10 yrs Equatorial upwelling computed assuming 20% of western boundary current transports in the surface layer, 80% in the pycnocline.

26. WBC compensation Western Boundary Current Compensation Heat and mass fluxes into and out of the interior tropical Pacific Ocean are partially compensated by flows in the western boundary currents on seasonal-to-interannual time scales (Cane and Sarachik, 1979; Springer et al, 1990).

27. Differences 1970s vs 2000s Differences between the 1970-1977 and 1998-2003 are smaller than differences between either of these periods and early-mid 1990s; differences between 1970-77 and 1998-2003 may also not be significant given computational uncertainties.

28. Differences during last 10 yrs (rainfall, winds) CMAP rainfall; ERS & Quikscat wind velocity

29. Differences during last 10 yrs (rainfall, winds) CMAP rainfall; ERS & Quikscat wind velocity

30. 1)The shallow meridional overturning circulation in the Pacific has accelerated since the late 1990s in concert with a cold phase shift in the PDO. Conclusions 2) Pycnocline, surface layer, and equatorial upwelling transports are presently comparable to those prior to the mid-1970s “regime shift.” 3) As on seasonal-to-interannual time scales, changes in interior ocean circulation on decadal time scales are partially compensated for by opposing changes in western boundary current transports (~1/3)

31. 4)Sudden reversal of tropical Pacific warming and associated circulation changes suggest that greenhouse gas forcing effects on the 1970s to 1990s warming trend were of secondary importance. Conclusions 5) Structural and dynamical similarities between ENSO and the tropical manifestations of the PDO make it very difficult to deconvolve cause and effect between the PDO and decadal modulation of ENSO.

32. El and La Interannual ENSO cycle and tropical Pacific manifestations of the PDO exhibit analogous patterns of variability and share similar dynamics, though with different time scales. PDO cold phase PDO warm phase

33. Gu and Philander

34. Deser et al

35. Ocean Circulation Schematic

36. Mean Circulation in Pycnocline

37. Data Sets Wind data sets:  COADS ship/buoy, 1945-93  NCEP reanalysis, 1958-99  ECMWF reanalysis and operational analyses, 1979-99  FSU ship/buoy, 1961-99

38. Meridional Structure

39. Mean Circulation in Pycnocline

40. 3-D Circulation

41. Ekman Transport 9°N 9°S

42. Differences 70s vs 90s Western boundary current transport convergence computed assuming mass conservation (surface layer divergence minus pycnocline convergence).

43. Differences 70s vs 90s Equatorial upwelling computed assuming 20% of western boundary current transports in the surface layer, 80% in the pycnocline.

44. Differences during last 10 yrs (Equatorial Section)