Transcript ENSO-Monsoon relationships in current and future climates

The University of Reading
Department of Meteorology
ENSO-Monsoon relationships in current and future climates
Earley Gate, Reading, RG6 6BB, UK
Andrew Turner, Pete Inness and Julia Slingo
[email protected]
past
Previous work (Turner et al., 2004) has shown that warming
(improving) the basic state in the tropical Pacific of a coupled GCM
produces a better monsoon climate and a stronger, better timed
monsoon-ENSO teleconnection (Fig. 1).
Fig.1 Lag correlations between Nino-3 region SSTs and summer
(a) Dynamical Monsoon Index (b) Indian Rainfall.
present
The SPEEDY model (Molteni, 2003) provides a quick way of
running simple experiments with reasonable skill.
• The Simplified Parametrizations, primitivE-Equation DYnamics
model is designed to run on only a few vertical levels, T30 L8
resolution in this configuration.
45mins/year runtime on a
workstation as an AGCM, forced by the HadISST dataset.
• Main errors with the model include low speed jets (Fig. 3) and
upper tropospheric warm bias (Fig. 4), likely related to poor longwave scheme.
Fig. 3 JJA 925hPa u-wind error Fig. 4 JJA zonal temp. error
• The tropical Pacific is too cool in HadCM3 and is warmed
using a system of limited-area flux adjustments, devised by
Inness et al. (2003).
• The revised model, HadCM3FA, has a better climate in the
tropical Pacific Ocean (less confined warm pool, reduced zonal
temperature gradient, weaker trade winds).
• Indian monsoon climate is improved, having less precipitation
and a weaker flow.
Fig. 2 Power spectra of Nino• ENSO amplitude increases
3 region SSTs.
(Fig. 2); the combination of
this enlarged amplitude and
stronger teleconnection (Fig.
1) increases the monsoon
variability.
• Enhanced biennial power in
HadCM3FA, likely due to
stronger coupling.
Is the strengthened teleconnection due to a more realistic basic
state and better ENSO evolution, or simply to the much larger
ENSO amplitude?
• SPEEDY monsoon simulation gets the gross features but fails in
the detail. Monsoon winds are too weak, lacking stationary wave
patterns, and precipitation is wrongly distributed.
• East African highlands are important in focussing and
maintaining the Somali Jet (Rodwell and Hoskins, 1995; Slingo et
al., 2004). Their poor representation at T30 resolution impacts on
the monsoon flow.
• Envelope orography (Wallace et al., 1983) better emphasises the
East African Highlands and makes some improvement to the
Somali Jet.
The monsoon-ENSO teleconnection is also
strengthened (Fig. 5), but still mis-timed.
Fig. 5 Lag correlations
between Nino-3 region
SSTs and summer DMI
future
Once the SPEEDY model is tuned correctly it will be used to
evaluate the effect of ENSO amplitude on the monsoon-ENSO
teleconnection. Other considerations include coupling with an
Indian Ocean mixed-layer model, and use of a ‘bucket’ soilmoisture scheme to improve monsoon precipitation distribution.
• Run SPEEDY with idealised ENSO forcing (based on Spencer et
al., 2004).
Fig. 6 Idealised ENSO cycle of
SSTs, to be applied to SPEEDY
• SST anomalies of varying
amplitude (Fig. 6) will be
applied to a background
climatology of current and
also future climates, further
testing the role of the basic
state.
The changing nature of the monsoon-ENSO teleconnection in
recent years (Fig. 7) together with a likely warmer basic state after
climate change provide motivation for further work with the
Unified Model.
Fig. 7 21-year moving
correlation
between
summer
Nino-3
temperatures and AllIndia Rainfall
• Integrations will be made using HadCM3 at 2x and 4x CO2, to
look at the changing nature of the monsoon-ENSO teleconnection
with climate change, as well as changes to the monsoon variability
and climate.
References: Inness, P. M., Slingo, J. M., Guilyardi, E. and Cole, J. (2003), “Simulation of the Madden-Julian Oscillation in a coupled general circulation model. Part II: The role of the basic state” J. Clim. 16: 365-382; Molteni (2003), “Atmospheric simulations using a GCM with simplified physical
parametrizations. I: model climatology and variability in multi-decadal experiments” Clim. Dyn. 20: 175-191; Rodwell, M. J. and Hoskins, B. J. (1995), “A Model of the Asian Summer Monsoon. Part II: Cross-Equatorial Flow and PV Behavior” J. Atmos. Sci. 52: 1341-1356; Slingo, J. M., Spencer, H.,
Hoskins, B., Berrisford, P. and Black, E. (2004), “The Meteorology of the Western Indian Ocean, and the influence of the East African Highlands” submitted, Proc. Roy. Soc; Spencer, H., Slingo, J. M. and Davey, M. K. (2004), “Seasonal predictability of ENSO teleconnections: the role of the remote
ocean response” Clim. Dyn. 22: 511-526; Turner, A. G., Inness, P. M. and Slingo, J. M. (2004), “The Role of the Basic State in Monsoon Prediction” submitted, Q. J. R. Meteorol. Soc.; Wallace, J. M., Tibaldi, S. and Simmons, A. J. (1983), “Reduction of systematic forecast errors in the ECMWF model
through the introduction of an envelope orography” Q. J. R. Meteorol. Soc. 109: 683-717.