michael j. mcphaden, noaa/pmel dongxiao zhang, university of washington and noaa/pmel circulation...
TRANSCRIPT
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
S. Hare, personal comm.
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)
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)
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.
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’).
Ocean Circulation Schematic
Mean Circulation in Pycnocline
9°N
9°S
14 Sv
7 Sv
(Integrated over 22.5-26.5 kg m-3)
Changes from 1970s to 1990s
Interior Ocean Pycnocline Transport Changes(22.5-26.5 kg m-3)
McPhaden & Zhang, 2002
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
*
positive phase
Change in 1998?
0.84
-0.36
“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.”
Reynolds SST; ERS & Quikscat wind stress
Reynolds SST; ERS & Quikscat wind stress
Reynolds SST; ERS & Quikscat wind stress, TOPEX/Poseidon & Jason sea level
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
Pacific Ocean Circulation Changes9°N to 9°S
13.0
49.7
36.7
42.4
28.1
57.9
29.8
51.9
0
10
20
30
40
50
60
70
PycnoclineConvergence
Surface LayerDivergence
WesternBoundaryCurrent
EquatorialUpwelling
Volume Transport (Sv)
1992-1998
1998-2003
Surface layer divergence is Ekman divergence (based on ERS & Quikscat wind stress) reduced by geostrophic convergence in the surface layer.
Pacific Ocean Circulation Changes9°N to 9°S
13.0
49.7
36.7
42.4
28.1
57.9
29.8
51.9
0
10
20
30
40
50
60
70
PycnoclineConvergence
Surface LayerDivergence
WesternBoundaryCurrent
EquatorialUpwelling
Volume Transport (Sv)
1992-1998
1998-2003
Western boundary current transport convergence computed assuming mass conservation (surface layer divergence minus pycnocline convergence) and Indonesian Throughflow decadal variations small.
Pacific Ocean Circulation Changes9°N to 9°S
13.0
49.7
36.7
42.4
28.1
57.9
29.8
51.9
0
10
20
30
40
50
60
70
PycnoclineConvergence
Surface LayerDivergence
WesternBoundaryCurrent
EquatorialUpwelling
Volume Transport (Sv)
1992-1998
1998-2003
Equatorial upwelling computed assuming 20% of western boundary current transports in the surface layer, 80% in the pycnocline.
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).
Pacific Ocean Circulation Changes9°N to 9°S
27.0
51.7
24.7
46.8
28.1
57.9
29.8
51.9
0
10
20
30
40
50
60
70
PycnoclineConvergence
Surface LayerDivergence
WesternBoundaryCurrent
EquatorialUpwelling
Volume Transport (Sv)
1970-1977
1998-2003
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.
CMAP rainfall; ERS & Quikscat wind velocity
CMAP rainfall; ERS & Quikscat wind velocity
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.
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
Ocean Circulation Schematic
Mean Circulation in Pycnocline
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
Meridional Structure
Mean Circulation in Pycnocline
Ekman Transport
9°N
9°S
Pacific Ocean Circulation Changes9°N to 9°S
27.0
51.7
24.7
46.8
14.0
40.7
26.7
35.4
0.0
10.0
20.0
30.0
40.0
50.0
60.0
PycnoclineConvergence
Surface LayerDivergence
WesternBoundaryCurrent
EquatorialUpwelling
Volume Transport (Sv)
1970-1977
1990-1999
Western boundary current transport convergence computed assuming mass conservation (surface layer divergence minus pycnocline convergence).
Pacific Ocean Circulation Changes9°N to 9°S
27.0
51.7
24.7
46.8
14.0
40.7
26.7
35.4
0.0
10.0
20.0
30.0
40.0
50.0
60.0
PycnoclineConvergence
Surface LayerDivergence
WesternBoundaryCurrent
EquatorialUpwelling
Volume Transport (Sv)
1970-1977
1990-1999
Equatorial upwelling computed assuming 20% of western boundary current transports in the surface layer, 80% in the pycnocline.
