1. Variability of Pacific Pycnocline Overturning in a Coupled GCM Bill Merryfield and George Boer Gu, D. and S.G.H. Philander, 1997: Inter- decadal climate fluctuations that depend on exchanges between the tropics and extratropics. Science, 275, 805- 807. Kleeman, R., J.P. McCreary and B.A. Klinger, 1999: A mechanism for generating ENSO decadal variability. Geophys. Res. Lett., 26, 1743-1746. McPhaden, M.J. and D. Zhang, 2002: Slowdown of meridional overturning circulation in the upper Pacific Ocean. Nature, 415, 603-608. WCRP Informal Report No. 4, 2001: CLIVAR workshop on shallow tropical-subtropical overturning cells and their interaction with the atmosphere, Oct. 9-13, 2000, Venice. References 5. Conclusions The coupled model results suggest that  The STC slowdown seen by McPhaden & Zhang (2002) is caused by combined effects of natural decadal variability and global warming (mainly the former).  STC strength is tightly coupled to tropical easterly wind stress and El Niño-like var- iability  STC variations apparently driven by tropical decadal variability, rather than driving variability via ‘bridge’ to extratropics. The subtropical cells (STCs) of the North and South Pacific are shallow, wind-driven overturning circulations featuring poleward surface Ekman transports and equatorward flows within the pycnocline. Because STCs exchange heat and other water properties between tropics and subtropics, they have been proposed as a key element of decadal climate variability. Recently, McPhaden & Zhang (2002) reported that equatorward pycnocline transports in the Pacific declined substantially from the 1970s to the 1990s. Here we examine variability of Pacific pycnocline transport in the second-generation CCCma coupled global climate model (CGCM2), asking:  Are the observed changes a result of natural variability, global warming, or both?  Do the changes drive, or are they driven by decadal climate variability? 1. Motivation 2. STCs in the Coupled Model Model exhibits equatorward flow within sloping tropical pycnocline, much as observed. Fig. 1 Equatorward transports near 9 o N and 9 o S are computed as in McPhaden & Zhang (2002) Two models are considered: 1000-year control run, and 200-year warming run (1900-2100) forced by projected greenhouse gas and aerosol emissions. Control run transports vary decadally, though less strongly than observed. Fig. 2 Warming run transports decrease relative to control run by ~10% in the present epoch, ~40% by 2100. N PacificS Pacific Fig. 1 Meridional velocity at  9 o latitude equatorward poleward v v 4. vT / or Tv / ? Fig. 5 Equatorward pycnocline transport and SST index NINO3 (note inverted scale) correlation = -0.88 At least two hypotheses have been advanced for how STCs might induce decadal climate variability through meridional heat flux changes (vT) / = v / T + vT / + v / T / Gu & Philander (1997) proposed that temp- erature changes of water advected from subtropics modulate equatorial SST via vT /. Kleeman et al. proposed that variations in STC strength instead modulate equatorial SST via v / T. Fig. 6 shows the relative contributions of these terms to changes in meridional heat transport that occur in the coupled model under global warming: the v / T term clearly dominates. (To cause a 10% change in vT via vT / would require T / ~ 30 o C.) Fig. 2 Decadal pycnocline transports Control run Warming run McPhaden & Zhang 9 o N 9 o S Fig. 4 Meridional velocity at  9 o regressed against decrease in pycnocline transport N Pacific S Pacific v equatorward poleward Bill Merryfield and George Boer, Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, P.O. Box 1700, University of Victoria, Victoria, B.C. V8W 2Y2, CANADA ; Bill.Merryfield@ec.gc.ca, George.Boer@ec.gc.ca 3. Analysis of Model Results Fig 3 ( ) shows SST, wind stress , and wind stress curl  regressed against decrease in equatorward pycnocline transport: (a) Control run: decreased pycnocline transport is associated with El Niño-like pattern of higher equatorial SST and reduced tropical easterly  (b) Warming run also shows El Niño-like pattern, superimposed on global warming trend (note translation in color scale.) (c) McPhaden & Zhang SST changes are broadly consistent with control run. (d) Control run  changes at  9 o are concentrated at longitudes where meridional velocity changes are largest (arrows; Fig. 4. Fig. 5 confirms apparent link between pycnocline transport and El Niño-like variability: time series of transport and the NINO3 are highly anti- correlated (r= -0.88). Environment Canada Environnement Canada Canadian Centre for Climate Modelling and Analysis Fig. 3 Surface fields regressed against decrease in pycnocline transport (a) Control run: SST ( o C/Sv),  (N m -2 /Sv) (b) Warming run: SST ( o C/Sv),  (N m -2 /Sv) (c) McPhaden & Zhang: SST ( o C/Sv) (d) Control run:  (N m -3 /Sv) Meteorological Service of Canada Service météorologique du Canada poleward v v * Note expanded scale for vT / vT / (vT) / vT v / T * Fig. 6 Meridional heat transport changes at 9 o N under global warming in coupled model equatorward poleward Differences (Warming) – (Control) years 2041-2050