Biennial and interdecadal variations in the monsoon-ENSO system of a coupled GCM under

doubled CO2 conditions

Andrew Turner, P.M. Inness & J.M. Slingo

IUGG meeting: JPS001 Interannual and interdecadal climate variability

Introduction

Notable tendency for biennial oscillation of the monsoon-ENSO system in this coupled GCM.Dynamical monsoon index*

Niño-3 SSTA

* P.J. Webster & S. Yang (1992). QJRMS 118: 877—926.

Outline

Introduction

Model framework

Scientific questions

Characteristics of each regime

Reasons for the overall biennial tendency

The regimes as part of the TBO

Future work

Model set-up

Hadley Centre coupled model HadCM3 run at high vertical resolution (L30) which better represents intraseasonal tropical convection1 and has an improved atmospheric response to El Niño2.

1P.M. Inness, J.M. Slingo, S. Woolnough, R. Neale, V. Pope (2001). Clim. Dyn. 17: 777--793.

2H. Spencer, J.M. Slingo (2003). J. Climate 16: 1757--1774.

3A.G. Turner, P.M. Inness, J.M. Slingo (2005). QJRMS 131: 781-804.

4A.G. Turner, P.M. Inness, J.M. Slingo (2007a). QJRMS, accepted

Integration shown is 95-year run using equatorial Indo-Pacific flux adjustments (HadCM3FA3,4) under 2xCO2.

HadCM3FA 2xCO2 ENSO

ENSO at 2xCO2 in HadCM3FA

Why the overall biennial tendency?

Why are there distinct regime shifts?

ENSO characteristics

Biennial regime features large amplitude events strongly phase locked to the seasonal cycle.

Biennial power exceeds annual cycle .

Niño-3 power spectra (normalized to annual cycle)

Phase-lockingNiño-3 anomaly index

ENSO propagation

Irregular regime shows signature of longer duration El Niño events based in the central Pacific.

Biennial regime shows more evidence of basinwide, eastward propagation at depth, consistent with thermocline mode events.

irregular biennial

Anomalous depth of equatorial 20°C isotherm

ENSO propagation #2

Lag correlations of the Trans-Niño Index1 with Niño-3 show strong eastward propagation of SST anomalies during biennial regime, consistent with thermocline mode events.

Tendency towards eastward propagation occurs both with 2xCO2

2 and with flux adjustments.

1K.E. Trenberth, D.P. Stepaniak (2001). J. Climate 14: 1697-1701.

2E. Guilyardi (2006). Clim. Dyn. 26: 329-348.

HadCM3 1xCO2 HadCM3FA 1xCO2

HadCM3 2xCO2 HadCM3FA 2xCO2

Summary of regime characteristics

Low amplitude, irregular ENSO, annual cycle dominates.

ENSO more central, consistent with S-mode.

Large amplitude, periodic, strong phase-locking, ENSO dominant mode.

ENSO peaks in east, with eastward propagation, consistent with T-mode.

Consistent with irregular and self-excited modes in Jin’s recharge oscillator* as coupling strength is increased.

Short biennial period in contrast to observed T-mode ENSO (4-5 years) and at odds with longer period as air-sea coupling is increased in Zebiak-Cane models.

Irregular regime Biennial regime

*F-F. Jin (1997). J. Atmos. Sci. 54: 811-829.

Explanation for the overall biennial tendency of HadCM3FA

The tendency cannot simply be related to differences in the structure of ENSO in the Pacific.Capotondi et al. (2006) relate ENSO period in coupled GCMs to two measurements:

1. the meridional extent of the zonal windstress response to ENSO SST variations

2. The longitudinal position of the centre of action of ENSO

( ) ( )p y yT T a L L b C C

Meridional width of zonal average taux regressed onto Niño-3

– little change in HadCM3FA.

EOF1 of SSTA at 2xCO2

– FA moves this further east. HadCM3

HadCM3FAdifference

Explanation for the overall biennial tendency of HadCM3FA #2

A key mechanism for biennial ENSO is monsoon wind forcing in West Pacific1, eg, strong monsoon forcing adjusting the WPA2.

Inclusion of ASM heating anomalies in the Zebiak-Cane model leads to increased feedbacks between the Indo-Pacific3.

Extension of Jin’s recharge oscillator4 to the Indian Ocean shows that increased coupling between the two basins significantly shortens the period of oscillation.

Strongly coupled El Niño events terminate more rapidly than uncoupled events5 (SINTEX CGCM).

1K-M. Kim, K-M. Lau (2001). GRL 28: 315-318.2K-M. Lau, H.T. Wu (2001). J. Climate 14: 2880-2895.3C. Chung, S. Nigam (1999). J. Climate 12: 2787-2807.4J-S. Kug, I-S. Kang (2006). J. Climate 19: 1784-1801.5J-S. Kug, T. Li, S-I. An, I-S. Kang, J-J. Luo, S. Masson, T. Yamagata (2006). GRL 33.

Strong Indo-Pacific coupling is implicated in the biennial tendency.

Dynamical monsoon index used to generate composite evolution of strong minus weak events.

Explanation for the overall biennial tendency of HadCM3FA #3

Biennial minus irregular SST during ENSO onset years (SON)

The TBO

The TBO and biennial ENSO

The TBO and irregular ENSO

Explanation for the overall biennial tendency in HadCM3FA

Strong Indo-Pacific coupling is implicated, relating to increased variability of the Asian-Australian monsoon on interannual timescales.

Indian Ocean dipole central to the mechanism, its decay to a basinwide SST anomaly instrumental in causing ENSO phase change.

Coupling between monsoon, IOD and ENSO is strengthened by both 2xCO2 and flux adjustments.

Summary

ENSO behaviour in HadCM3FA 2xCO2 features distinct irregular and biennial regimes, with notable biennial tendency.

Some consistency with ENSO modes based on air-sea interaction and those dependent on basinwide ocean wave coupling.

Increased Indo-Pacific coupling and monsoon-IOD-ENSO interactions implicated in biennial tendency.

The monsoon-ENSO teleconnection

rainfallDMI

ENSO regimes have dramatic impact on teleconnection.

Much greater monsoon predictability during the biennial regime.

Further questions

• Realism of regime changes uncertain, but they have potential to have dramatic impacts on remote teleconnections.

• Reasons for changes between regimes not yet elucidated, possibly:– Interactions with the annual cycle in east

Pacific.– Changes to meridional circulations in the

subtropical Pacific.

Thank you!