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

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

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’).

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

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

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

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

PDO positive phase Change in 1998?

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

Differences during last 10 yrs (20C)

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

Interior Ocean Pycnocline Transport Changes

Pycnocline Convergence

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.

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.

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

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).

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

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)

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.

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