The effect of doubled CO2 and model basic state biases on the monsoon-ENSO system: the TBO and changing ENSO regimes

1
The effect of doubled CO2 and model basic state
biases on the monsoon-ENSO system the TBO and
changing ENSO regimes

• A. G. Turner,
• P. M. Inness J. M. Slingo

1 Background Integrations of the Met Office
model HadCM3 at 1xCO2 show systematic model
biases to have a detrimental impact on the
monsoon-ENSO system and its predictability
(Turner et al. 2005). Such systematic biases
could cause uncertainties in future climate
simulations (Federov Philander 2000). An
annual cycle of heat flux adjustments (FA) is
applied to the equatorial Indo-Pacific in a 2xCO2
run of HadCM3 to reduce the biases. This reveals
a greater climate change response than the
uncorrected model (Turner et al. 2006).
Additionally, HadCM3FA at 2xCO2 features distinct
ENSO regimes (Fig. 1), which feature several
similarities with theoretical studies (Jin 1997
Wang et al. 1999). The irregular regime features
strong stochastic forcing whilst the biennial
events are part of a self-sustained limit cycle
in a more strongly coupled ocean-atmosphere
system.
Fig. 1 Niño-3 SSTA in HadCM3FA 2xCO2. Irregular
and biennial ENSO regimes are selected for
comparison.
2 Regime characteristics Similarities can also
be drawn between the regimes and observed ENSO
modes. The irregular regime features mild El
Niño events located centrally and lasting several
years (Fig. 2), akin to local-modes in
observations (e.g. 2002). In the biennial
regime, ENSO dominates over the annual cycle of
cold upwelling in the east (Figs. 2,3), and more
evidence of Kelvin wave propagation is revealed
(Fig. 2), similar to observed basinwide
thermocline modes (e.g. 1997).
Central Pacific El Niño
Fig. 4 Annual mean biennial minus irregular SST
composite. Speckling indicates 95 significance.
Annual cycle and ENSO compete to dominate East
Pacific.
Fig. 3 Normalized power spectra of Niño-3 SST in
HadCM3FA 2xCO2.
Fig. 5 Lag correlation of Niño-3 SST with
Trans-Niño Index in HadCM3(FA) integrations at
1x, 2xCO2.
Basinwide eastward propagation
The tendency toward eastward propagating
basinwide modes can be related to slackening of
the zonal SST gradient (Fig. 4), favouring
vertical motion of the thermocline over zonal
advection of SST anomalies (Federov Philander
2001). This tendency is increased both by 2xCO2
forcing and flux adjustment (Fig. 5, shown using
the Trans-Niño Index of Trenberth Stepaniak
2001).
Fig. 2 Equatorial Pacific 20ºC isotherm depth
anomalies (dam), as a proxy for heat content.
3 Reasons for the biennial tendency The biennial
tendency in HadCM3 2xCO2 is in contrast with
observed basinwide El Niño events which are often
of 4-5 year period (Guilyardi 2006). Its cause
can be understood by considering interannual
variability in the Asian summer monsoon, to which
ENSO is tied. The dynamical monsoon index
(Webster Yang 1992) allows selection of strong
(weak) years of the tropospheric biennial
oscillation (TBO), i.e., years which are stronger
(weaker) than preceding and following years (Fig.
6.). A composite evolution of strong minus weak
seasons shows strong coupling between the Asian
monsoon and the Indian Ocean dipole and ENSO
(Fig. 7). The decay of the Indian Ocean dipole
to a basinwide anomaly of the same sign as ENSO
is essential in reversing ENSO phase in the
Pacific (Kug and Kang 2006). Strong monsoon
forcing is increased by both 2xCO2 and flux
adjustment, acting to increase coupling between
the Indian and Pacific Oceans.
Fig. 7 Strong minus weak composite evolution of
precip (contours), SST (shading) and 850hPa winds.
Fig. 6 Dynamical monsoon index of HadCM3FA 2xCO2.
4 Implications Whilst the presence of biennial
oscillations of the monsoon-ENSO system and
strong coupling of the Indo-Pacific may suggest
increased predictability, such extreme seasons
are detrimental to the populations of South Asia.
The existence of regime changes on interdecadal
timescales suggests there may be greater
uncertainty in projections of future climate.
5 References Federov Philander (2000) Science
288 Federov Philander (2001) J. Clim. 14 Jin
(1997) J. Atmos. Sci. 54 Guilyardi (2006) Clim.
Dyn. 26 Kug Kang (2006) Geophys. Res. Lett.
33 Trenberth Stepaniak (2001) J. Clim. 14
Turner, Inness Slingo (2005) Q. J. R. Meteorol.
Soc. 131 Turner, Inness Slingo (2006)
Submitted, Q. J. R. Meteorol. Soc. Wang,
Barcilon Fang (1999) J. Atmos. Sci. 56 Webster
Yang (1992) Q. J. R. Meteorol. Soc. 118.
A.G. Turner is supported via an EU-ENSEMBLES
grant at NCAS-Climate, University of Reading, UK
a.g.turner_at_rdg.ac.uk