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 ; 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 (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