The effect of doubled CO 2 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 convection 1 and having an improved atmospheric response to El Niño 2. 1 P.M. Inness, J.M. Slingo, S. Woolnough, R. Neale, V. Pope (2001). Clim. Dyn. 17: H. Spencer, J.M. Slingo (2003). J. Climate 16: M. Rajeevan, J. Bhate, J.D. Kale, B. Lal (2006). Curr. Sci. 91: Control (1xCO 2 ) and future climate (2xCO 2 ) integrations used to test the impact of increased GHG forcing. Mean climate comparisons with CMAP data ( ) and IMD 1° gridded dataset ( ) based on 1803 stations 3.

Mean precipitation climate & change CMAPIMD HadCM3 1xCO 2 2xCO 2 minus 1xCO 2

Mean precipitation & climate change Mean monsoon remains robust at 2xCO 2 (in the 6 of 18 AR4 runs that reasonably simulate the seasonal cycle 1 ). HadCM3 shows increases to the mean monsoon of around 5% at 2xCO H. Annamalai, K. Hamilton, K.R. Sperber (2007). J. Clim. 20: A.G. Turner, P.M. Inness, J.M. Slingo (2007a). QJRMS. 133: 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 doesnt represent extreme tail (not shown) again due to grid. At 2xCO 2 there is a clear tendency for reduced frequency of moderate events and increased heavy events.

Percentage of wet days in season Wet days defined above 0.1mm/day threshold. Model simulates similar number of wet days per season as observations (not shown), illustrating discrete nature of monsoon rain. Doubled CO 2 climate suggests slightly fewer wet days, especially over southeast states. HadCM3 1xCO 2 2xCO 2 minus 1xCO 2

Precipitation intensity Defined as total seasonal precip over number of wet days. Pattern of precipitation intensity change as CO 2 doubles is qualitatively similar to the mean change but larger in amplitude, consistent with decreased number of wet days. 2xCO 2 minus 1xCO 2 HadCM3 1xCO 2

Heavy precipitation extremes Heavy (95th) and very heavy (99th) percentiles of precipitation are calculated for 1xCO 2 and 2xCO 2 climates. Extreme precipitation increases are larger than those in the seasonal mean. 2xCO 2 minus 1xCO 2 2xCO 2 1xCO 2 95th 99th

Heavy precipitation extremes #2 Alternatively, the probability of reaching 1xCO 2 defined rainfall percentiles at 2xCO 2 can be assessed. By definition, 95 th (99 th ) percentile occurs on 5% (1%) of days at 1xCO 2, thus at 2xCO 2 the chance of reaching these precipitation extremes almost doubles (especially north India). 95th99th

Heavy precipitation extremes #3 Upper extremes of precipitation are also assessed for each grid point in the tropics and Indian land surface, following Allen & Ingram 1. 1 M.R. Allen, W.J. Ingram (2002). Nature 419: Maximum precipitation intensity in the tropics increases broadly in line with Clausius-Clapeyron and measured (tropical) climate sensitivity. tropicsIndia

Importance of extreme precipitation Rain on only 1-in-10 (20) days contributes typically 40% (25%) of total seasonal rainfall. At 2xCO 2, these percentiles increasingly contribute, indicating greater reliance on heavy precipitation. The contribution of those days featuring more than the 90 th or 95 th precipitation percentile to the total seasonal rainfall is calculated. 1xCO 2 2xCO 2 -1xCO 2 95th 99th

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 days 1. 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 index 2 ). 1 After V. Krishanmurthy, J. Shukla (2007) J. Clim. 20: 320, amongst others. 2 G.A. Vecchi, D.E. Harrison (2002). J. Clim. 15:

Composite evolution of active-break cycles in precipitation at 1xCO 2 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 2xCO 2. 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 2xCO 2. 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 2 2xCO 2 difference activebreak

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!