The effect of doubled CO2 on intraseasonal behaviour of the Asian Summer Monsoon

Andrew Turner & Julia Slingo

RMetS Conference 2007: 3-6 September

Introduction

Active and break events on intraseasonal timescales represent the largest variations of the Asian summer monsoon. Many studies have made projections of monsoon rainfall in the future climate and its interannual variations, however little is known about the possible impact of climate change on intraseasonal phenomena such as active-break cycles and extreme precipitation events.This study examines active-break cycles of the monsoon and associated heavy rainfall events in a coupled GCM.

Outline

Introduction

Model set-up & observed datasets

Climate change and the mean monsoon

Extremes of precipitation and their impact

Active-break cycles and climate change

Summary & future work

Model set-up & observed datasets

Hadley Centre coupled model HadCM3 run at high vertical resolution (L30), better representing intraseasonal tropical convection1 and having 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.

3M. Rajeevan, J. Bhate, J.D. Kale, B. Lal (2006). Curr. Sci. 91: 296—306.

Control (1xCO2) and future climate (2xCO2) integrations used to test the impact of increased GHG forcing.

Mean climate comparisons with CMAP data (1979-2004) and IMD 1° gridded dataset (1953-2004) based on 1803 stations3.

Mean precipitation climate & change

CMAP IMD

HadCM3 1xCO2 2xCO2 minus 1xCO2

Mean precipitation & climate change

Mean monsoon remains robust at 2xCO2 (in the 6 of 18 AR4 runs that reasonably simulate the seasonal cycle1).

HadCM3 shows increases to the mean monsoon of around 5% at 2xCO2

2.

1H. Annamalai, K. Hamilton, K.R. Sperber (2007). J. Clim. 20: 1071—1092.

2A.G. Turner, P.M. Inness, J.M. Slingo (2007a). QJRMS. 133: 1143—1157.

Increases are mainly occurring during mid-to-late season.

Rainfall distribution

Rainfall averaged over Indian land surface; bin size 0.5mm.Model has tendency to drizzle (common in many convection schemes) and thus under-represents moderate rainfall (grid sizing plays a minor role too).Model doesn’t represent extreme tail (not shown) again due to grid.

At 2xCO2 there is a clear tendency for reduced frequency of moderate events and increased heavy events.

Heavy precipitation extremes

Heavy (95th) and very heavy (99th) percentiles of precipitation are calculated for 1xCO2 and 2xCO2 climates.

Extreme precipitation increases are larger than those in the seasonal mean.

2xCO2 minus 1xCO22xCO21xCO2

95th

99th

Heavy precipitation extremes #2

Alternatively, the probability of reaching 1xCO2 defined rainfall percentiles at 2xCO2 can be assessed.

By definition, 95th (99th) percentile occurs on 5% (1%) of days at 1xCO2, thus at 2xCO2 the chance of reaching these precipitation extremes almost doubles (especially north India).

95th 99th

Heavy precipitation extremes #3

Upper extremes of precipitation are also assessed for each grid point in the tropics and Indian land surface, following Allen & Ingram1.

1M.R. Allen, W.J. Ingram (2002). Nature 419: 224—232.

Maximum precipitation intensity in the tropics increases broadly in line with Clausius-Clapeyron and measured (tropical) climate sensitivity. 14% 2.27K 6.5% / K

tropics India

Construction of active-break composites

Active-break events defined when area-averaged all-India rainfall falls outside ±σ from the mean seasonal cycle, for a period of at least five consecutive days1.Composites generated of anomalies to the seasonal cycle in both integrations.Various other indices also tested with similar results (core monsoon region only, OLR dipole index2).

1After V. Krishanmurthy, J. Shukla (2007) J. Clim. 20: 3—20, amongst others.

2G.A. Vecchi, D.E. Harrison (2002). J. Clim. 15: 1485—1493.

Composite evolution of active-break cycles in precipitation at 1xCO2

Evolution of active phase shown here only

Change to evolution of break anomalies relative to the annual cycle

Clear significant intensification of break events, relative to the seasonal cycle at 2xCO2.

This is index invariant and also applies to active events.

Absolute precipitation at lag-zero in active-break events.

Clear intensification of absolute precip in active events at 2xCO2.

Whilst break anomalies to the seasonal cycle intensify, when the increased seasonal cycle is taken into account, absolute levels of precipitation during break events stay broadly similar.

1xCO

22xC

O2

difference

active break

Summary

Projections of the future climate show robust-to-enhanced mean monsoon consistent with other modelling studies.Extremes of subseasonal precipitation increase beyond changes to the mean, and contribute a greater proportion of the seasonal total.Maximum precipitation intensity over India and tropics-wide increases broadly inline with climate sensitivity and the Clausius-Clapeyron relation.Active-break events are intensified relative to the seasonal cycle but no changes to the number of break days occurs.

Future work

Analyse maximum length of consecutive dry days and break events.

More in-depth study of the evolution of active-break cycles using local mode analysis or CCA.

Thank you!