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Michael J. McPhadenNOAA/PMEL
Seattle, Washington
Decadal Variability and Trends in Tropical Pacific SST and Their Relation to the Shallow
Meridional Overturning Circulation
CSIRO Marine and Atmospheric Research
29 August 2005
Aspendale, Australia
In collaboration with Dongxiao Zhang, U. of Washington
Coupled Model Evaluation Project (CMEP)
Purpose: to increase community-wide diagnostic research into the quality of climate model simulations
Efforts are intended to aid in understanding and assessing the uncertainty of the future climate change projections for the Intergovernmental Panel on Climate Change Assessment Report #4
Focus is on using existing observational datasets for evaluating 20th century simulations (20C3M)
Sponsored by U.S. CLIVAR Project Office
Relevance of Pacific Decadal Variability
Affects the climate of the Pacific basin (Latif & Barnett, 1994; Cayan et al, 2001)
Affects Pacific marine ecosystems and the global carbon cycle (Mantua et al, 1997; Hare & Mantua, 2000; Chavez et al, 2003; Peterson & Schwing, 2003) Linked to decadal modulation of ENSO (Trenberth & Hurrell, 1997; Latif et al, 1997; Power et al, 1999)
Tropical Pacific Ocean Circulation
Mean Circulation in Pycnocline
14 Sv (1 Sverdrup=106 m3 s-1)
7 Sv
(f∂/∂z)
(Integrated over 22.5-26.5 kg m-3)
9°N
9°S
XX
Pycnocline Volume Transport
McPhaden and Zhang, 2002, Nature
SST anomaly (9°N-9°S, 90°W-180°)
Changes from 1970s to 1990s
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 circulation cells transport thermocline
water towards the equator (V’T)
Greenhouse Gas-Induced Tropical Pacific Warming Trends?
Some computer models suggest anthropogenic forcing of sea surface temperature trends in the tropical Pacific (Meehl & Washington, 1996; Knutson and Manabe, 1998).
However, models are sensitive to specification of poorly understood physical processes, and not all models give the same results (Cane et al, 1997; Collins et al, 2005).
positive phase
Change in 1998-99
0.84
-0.36
Interior Ocean Pycnocline Transport Changes
McPhaden and Zhang, GRL, 2004
Mass Conservation
Upwelling=Net volume flux out in surface layer =Net volume influx in the pycnocline
20%
80%
1) Variations in meridional overturning circulation are directly linked to the decadal variability and trends in tropical SST
Conclusions (Observations)
4) Sudden reversal of tropical Pacific warming associated with circulation changes in late 1990s greatly reduced SST trend, calling into question the magnitude of the presumed anthropogenic influence on SST.
3) The shallow meridional overturning circulation in the Pacific accelerated in the late 1990s in concert with a cold phase shift in the PDO.
2) Changes in interior ocean circulation on decadal time scales are partially compensated for by opposing changes in western boundary current transports (~1/3)
Questions1) How well do global climate models simulate decadal
variability and trends like those observed in the tropical Pacific?
2) Can these models help us understand the relative importance of natural vs anthropogenic influences in the tropical Pacific over the past 50 years?
The Coupled Models
* MIROCH is eddy permitting* MRI is only model with flux correction.
Observations
Observations and Model 50 Year Mean Convergence: 9°N-9°S, 1950-99
Pycnocline transport=flow between base of mixed layer to 26.2 surface
Volume Transport Convergence Anomalies 9°S-9°N (6 yr lowpass)
Pycnocline Transport
Convergence Trend
Pycnocline Transport
Convergence Standard Deviation
Mean SST, 1950-99
Mean SST, 1950-99
SST EOF-1, 1950-99
SST EOF-1 and Index Time Series
SST and Transport Convergence Anomalies
SST standard deviation
(detrended)
Correlation between
detrended SST and
transport convergence
R=0.79 (95% @ 0.44)
SST Trends, 1950-99
SST Trends, 1950-99
Tropical Pacific SST
Trends
Pycnocline Transport
Trends
R=-0.46 (95% @ 0.44)
R=-0.24 (95% @ 0.46) without MIROCH
Global Air Temperature and Tropical SST
Global Air Temperature and Tropical SST
Tropical Pacific SST
Trends
Global Air Temperature
Trends
R=0.79 (95% @ 0.44)R=0.56 (95% @ 0.46)
Summary All the models produce a mean meridional pycnocline volume convergence
toward the equator from the subtropics, but the means are generally underestimated.
The models also exhibit decadal variations in pycnocline volume transport on decadal time scales over the last half century, but the magnitude of the variability is underestimated and western boundary current transports variations are not well represented in most of the models.
However, significant correlation exists between meridional transport convergence and tropical SST in the majority of the models, indicating an important role for ocean circulation in tropical Pacific SST variability on decadal time scales.
50 year long trends towards decreasing transport convergence as appear in the observations are generally poorly simulated. It is unclear why the models fail to simulate this trend.
Most models exhibit an SST trend over the latter half of the 20th century comparable to that observed. These trends are apparently not directly related to the meridional overturning circulation.
There is a suggestion that the simulated trends could be due to greenhouse gas forcing, using global air temperature trends as a proxy for that forcing. Examination of control runs is needed to verify this suggestion.
Summary All the models produce a mean meridional pycnocline volume convergence
toward the equator from the subtropics, but the means are generally underestimated.
The models also exhibit decadal variations in pycnocline volume transport on decadal time scales over the last half century, but the magnitude of the variability is underestimated and western boundary current transports variations are not well represented in most of the models.
However, significant correlation exists between meridional transport convergence and tropical SST in the majority of the models, indicating an important role for ocean circulation in tropical Pacific SST variability on decadal time scales.
50 year long trends towards decreasing transport convergence as appear in the observations are generally poorly simulated. It is unclear why the models fail to simulate this trend.
Most models exhibit an SST trend over the latter half of the 20th century comparable to that observed. These trends are apparently not directly related to the meridional overturning circulation.
There is a suggestion that the simulated trends could be due to greenhouse gas forcing, using global air temperature trends as a proxy for that forcing. Examination of control runs is needed to verify this suggestion.
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-decadal time scales (Cane & Sarachik, 1979; Springer et al, 1990; Lee & Fukumori, 2003; Cheng et al, 2005; Capotondi et al, 2005).
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
“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.”
June 1998-May 2002
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