The effect of doubled CO 2 and model basic state biases on the monsoon-ENSO system

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

Part I: mean and interannual variability

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

LASG seminar 17 October 2008

Introduction

Asian summer monsoon is vital to the lives of more than 2 billion people across south and east Asia through agricultural and, increasingly, industrial users of water.

How characteristics of the mean monsoon and its variation on different timescales may change in the future is a key goal of climate research.

Changing predictability of the monsoon through its teleconnection to ENSO must also be addressed.

Outline (part I)

Introduction

Model framework

Climate change and the mean monsoon

Interannual variability

How do systematic model biases affect the result?

The monsoon-ENSO teleconnection

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.

Control (1xCO2) and future climate (2xCO2) integrations used to test the impact of increased GHG forcing. Further integration of each climate scenario to test the role of systematic model biases.

2xCO2 response of HadCM3

Summer climate of HadCM3 2xCO2 Response to 2xCO2

The monsoon in IPCC AR4 models

Annamalai et al. (2007):Of the six AR4 models which reasonably simulate

the monsoon precipitation climatology of the 20th century, all show general increases in seasonal rainfall over India in the 1pctto2x runs (including HadCM3 L19).

H. Annamalai, K. Hamilton, K. R. Sperber (2007). J. Climate 20: 1071--1092

Interannual variability

Interannual variability is projected to increase at 2xCO2 (+24% using Webster-Yang dynamical index*).

seasonal rainfall PDF

Increased likelihood of very wet seasons.

DMI (JJAS)

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

1xCO2

2xCO2

Interannual variability

Greater difference between extreme monsoon seasons at 2xCO2.

1xCO2

2xCO2

strong-weak monsoon precip and 850hPa wind

Model set-up

Hadley Centre coupled model HadCM3 run at higher 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.

Control (1xCO2) and future climate (2xCO2) integrations used to test the impact of increased GHG forcing. Further integration of each climate scenario to test the role of systematic model biases.

Systematic biases in HadCM3

Summer climate of HadCM3 1xCO2 HadCM3 minus observations

Monsoon-ENSO teleconnection: lag-correlations

The monsoon-ENSO teleconnection is weak and mis-timed in HadCM3.

DMI (JJAS) vs. Niño-3

Flux adjustments at 1xCO2

Flux adjustments are calculated by relaxing Indo-Pacific SSTs back toward climatology in a control integration.

The heat fluxes required for the relaxation are saved and meaned to form an annual cycle.

Annual cycle applied to the equatorial band of a new integration*.

Annual Mean

Amplitude of annual cycle

* After: P.M. Inness, J.M. Slingo, E. Guilyardi, J. Cole (2003). J. Climate 16: 365-382.

Systematic biases in HadCM3& their reduction in HadCM3FA

Maritime Continent cooled; cold tongue warmed

Coupled response: reduced trade wind errors and monsoon jet

Reduced convection over Maritime Continent & other precip errors opposed

HadCM3 minus observations HadCM3FA minus HadCM3

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

Flux adjustments at 2xCO2

Assume systematic biases will still be present in the future climate.

Assume that the adjustments necessary to correct these biases will be the same.

Same annual cycle of flux adjustments used at 2xCO2 (in common with previous studies where adjustments were necessary to combat drift, eg in HadCM2*).

* M. Collins (2000). J. Climate 13: 1299-1312.

2xCO2 response of HadCM3

Summer climate of HadCM3 2xCO2 Response of HadCM3 2xCO2

2xCO2 response of HadCM3FA

Summer climate of HadCM3FA 2xCO2 Response of HadCM3FA to 2xCO2

Monsoon precipitation response

Systematic bias seems to mask full impact of changing climate

Taken from A.G. Turner, P.M. Inness, J.M. Slingo (2007a). QJRMS 133.

Monsoon-ENSO teleconnection: lag-correlations

Flux adjustments have dramatic impact on the teleconnection, particularly when measured by Indian rainfall.

The impact of increased GHG forcing is less clear but the teleconnection is generally robust.

DMI Indian rainfall

Monsoon-ENSO teleconnection: moving correlations

Variations of correlation strength in models are of similar amplitude to those seen in observations despite fixed CO2 forcing.

See also AR4 models in Annamalai et al. (2007).

HadISST vs. All-India gauge data

rainfallDMI

Summary: part I

Projections of the future climate show enhanced mean monsoon consistent with other modelling studies.

Interannual modes of variation are more intense at 2xCO2, potentially leading to greater impacts of the monsoon on society.Systematic model biases may be masking the true impact of increased GHG forcing.The monsoon-ENSO teleconnection, useful for seasonal prediction, remains robust. Indeed model error has more impact than climate change.Large amplitude variations occur in the modelled monsoon-ENSO teleconnection despite fixed CO2 forcing.

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

Part II: the TBO and changing ENSO regimes

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

LASG/IAP seminar 17 September 2008

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 (part II)

Introduction

Scientific questions

Characteristics of each regime

Reasons for the overall biennial tendency

The regimes as part of the TBO

Future work

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!

Email: [email protected]

Web: www.met.rdg.ac.uk/~sws05agt

Part I: Q.J.R.Meteorol.Soc. (2007) 133: 1143—1157

Part II: Q.J.R.Meteorol.Soc. (2007) 133: 1159—1173