1. Michael J. McPhaden & Dongxiao Zhang NOAA/PMEL Decadal Variability and Trends of the Pacific Shallow Meridional Overturning Circulation and Their Relation to Sea Surface Temperature  Coupled Model Evaluation Project (CMEP): 1) Diagnose the quality of climate model simulations; 2) Understand and assess the uncertainty of the future climate change projections.  Focus is on using existing observational datasets for evaluating 20th century simulations (20C3M). Second Session of the CLIVAR Global Observations and Synthesis Panel (GSOP-2) Scripps Institution of Oceanography 8-9 December 2006

2. 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) Kleeman et al (1999) Hypothesis for Pacific Decadal Climate Variability

3. Greenhouse Gas-Induced Tropical Pacific Warming Trends? Some previous generation computer models suggested 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).

4. Tropical SST and global warming Cloud-shielding thermostat over warm pool (Meehl and Washington 1986, 1996; Ramanathan and Collins 1991) Non-linear evaporative cooling (Knutson and Manabe 1995) El Nino-like trend: La Nina-like trend: Bjerknes feedback (Cane et al. 1997)

5. Meridional Structure Along 140°W 1950-1999 Temperature Salinity Density surface    kg m  Shallow Overturning in the Subtropical Cells

6. PDO Geostrophic streamlines and velocity (ref. 900m) Hydrographic Data: WOD2001, WOCE, TAO sections, ARGO profiles. Pycnocline transport = flow between base of mixed layer to 26.2   surface McPhaden and Zhang (2002,2004)

7. The Coupled Models (20 th century simulation forced by history of greenhouse gases, solar and volcanic activity) * MIROCH is eddy permitting * MRI is only model with flux correction.

8. 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 ± 10 Sv

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

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

11. Mean SST, 1950-99

13. SST and Transport Convergence Anomalies

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

15. Correlation maps Correlation maps (transport convergence vs. SST)

16. SST and Transport Std Dev Scatterplot R=0.79 (95% @ 0.44)

17. SST Trends, 1950-99 Tropical Pacific SST Trends Pycnocline Transport Trends

18. SST and Transport Trend Scatterplot R=-0.24 (95% @ 0.46) without MIROCH

19. Pacific SST and Global AirT Scatterplot R=0.79 (95% @ 0.44) R=0.58 (95% @ 0.46)

20. Pacific SST and Global AirT Scatterplot R=0.79 (95% @ 0.44) R=0.58 (95% @ 0.46)

21. Summary  There is a weakening trend of the shallow overturning (interior STC) of -11 Sv over the past 50 years, but the circulation fluctuates significantly on decadal time scales, with decade-to-decade variations of 8-12 Sv about the trend.  The models exhibit decadal variations in pycnocline volume transport, but the magnitude of the variability is underestimated. Models too sticky?  As in observations, 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.  Most models show no trend in transport convergence and underestimate the trend in the eastern tropical Pacific SST. None show significant cooling trend. These trends are apparently not directly related to the shallow overturning.  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.  Eddy permitting MIROCH is the only model that reasonably reproduces the observed trend in transport convergence and eastern tropical Pacific SST over the last half century. Is this because of its higher resolution?  If the observed trends and those simulated in the MIROCH model are ultimately related to greenhouse gas forcing, these results suggest that the changing shallow overturning may enhance warming that arises from anthropogenic forcing in the eastern tropical Pacific.

22. SST Trends, 1950-99