Michael J. McPhaden NOAA/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 20 th 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=10 6 m 3 s -1 ) 7 Sv (  f∂  /∂z) (Integrated over kg m -3 ) 9°N 9°S X X

Background Pycnocline Volume Transport McPhaden and Zhang, 2002, Nature

Pycnocline Convergence 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).

PDO positive phase Change in

Interior Ocean Pycnocline Transport Changes McPhaden and Zhang, GRL, 2004

Pycnocline Convergence

Mass Conservation Upwelling=Net volume flux out in surface layer =Net volume influx in the pycnocline 20% 80%

Differences during last 10 yrs

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)

Questions 1)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, Pycnocline transport=flow between base of mixed layer to 26.2   surface

Volume Transport Convergence Anomalies 9°S-9°N (6 yr lowpass)

Volume Transport Time Series Pycnocline Transport Convergence Trend Pycnocline Transport Convergence Standard Deviation

Mean SST,

SST EOF-1,

SST EOF-1 and Index Time Series

SST and Transport Convergence Anomalies

SST Std Devs and Correlations with Transport SST standard deviation (detrended) Correlation between detrended SST and transport convergence

SST and Transport Std Dev Scatterplot R= )

SST Trends,

Tropical Pacific SST Trends Pycnocline Transport Trends

SST and Transport Trend Scatterplot R= )

SST and Transport Trend Scatterplot R= ) without MIROCH

Global Air Temperature and Tropical SST

Tropical Pacific SST Trends Global Air Temperature Trends

Pacific SST and Global AirT Scatterplot R= ) R= )

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.

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-decadal time scales (Cane & Sarachik, 1979; Springer et al, 1990; Lee & Fukumori, 2003; Cheng et al, 2005; Capotondi et al, 2005).

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

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

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.” June 1998-May 2002