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Variability of Pacific Pycnocline Overturning in a Coupled GCMBill 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 9o N and 9o 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 Pacific S Pacific
Fig. 1 Meridional velocity at 9o latitude
equatorwardequatorward poleward polewardv v
4. vT/ or Tv/ ?
Fig. 5 Equatorward pycnocline transportand 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/ ~ 30oC.)
Fig. 2 Decadal pycnocline transportsControl run Warming run McPhaden & Zhang
9o N
9o S
Fig. 4 Meridional velocity at 9o regressed against decrease in pycnocline transport
N Pacific S Pacific
vequatorwardequatorward 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 ; [email protected], [email protected]
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 (notetranslation in color scale.)
(c) McPhaden & Zhang SST changes are broadly consistent with control run.
(d) Control run changes at 9o 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 (oC/Sv), (N m-2/Sv) (b) Warming run: SST (oC/Sv), (N m-2/Sv)
(c) McPhaden & Zhang: SST (oC/Sv) (d) Control run: (N m-3/Sv)
Meteorological Service of CanadaService météorologique du Canada
poleward vv
* Note expanded scale for vT/
vT/ (vT)/
vT v/ T
*
Fig. 6 Meridional heat transport changes at 9oN under global warming in coupled model
equatorwardequatorward poleward poleward
Differences (Warming) – (Control) years 2041-2050