Multidecadal simulations of the Indian monsoon in SPEEDY- AGCM and in a coupled model Annalisa Bracco, Fred Kucharski and Franco Molteni The Abdus Salam International Center for Theoretical Physics Physics of Weather and Climate Section

Focus on: the role of the SST anomalies on Indian Ocean dynamics the role of the SST anomalies on Indian Ocean dynamics the role of CO 2 forcing the role of CO 2 forcing the role of interactive SSTs (mixed layer ocean) the role of interactive SSTs (mixed layer ocean) the role of the ocean the role of the ocean

SPEEDY an AGCM of intermediate complexity T30, 8 σ layers model T30, 8 σ layers model hydrostatic spectral dynamical core hydrostatic spectral dynamical core convection: max-flux scheme activated by conditional instability convection: max-flux scheme activated by conditional instability computationally very fast computationally very fast

series of 50-ys ensemble with SPEEDY forced with HadISST SST (35 members) SPEEDY forced with HadISST SST (35 members) SPEEDY + observed SST + CO 2 increase (35 members) SPEEDY + observed SST + CO 2 increase (35 members) SPEEDY + climatological SST in the Indian Ocean + observed SST elsewhere (+ CO 2 ) SPEEDY + climatological SST in the Indian Ocean + observed SST elsewhere (+ CO 2 ) (8 members each) (8 members each) SPEEDY + mixed layer ocean (+ CO 2 ) SPEEDY + mixed layer ocean (+ CO 2 ) (8 members each) (8 members each) SPEEDY + OGCM in the tropical Pacific and Indian ocean (+ CO 2 ) … 8 members SPEEDY + OGCM in the tropical Pacific and Indian ocean (+ CO 2 ) … 8 members

SST trend

Precip and wind climatology

Precip trends

EOFs precip 24.6% 19.3% 10.4% 14.1% 10.7% 21.3%

Coupled model set-up Atmosphere model: SPEEDY 8  layers in the vertical, spectral truncation at wavenumber 30 Atmosphere model: SPEEDY 8  layers in the vertical, spectral truncation at wavenumber 30 Ocean models: Ocean models: 1. Slab ocean anomaly model (passive mixed layer with observed mean temperatures). 2. Slab ocean in “q-flux” configuration + anomalous Ekman transports, anomalous wind-driven turbulent mixing and anomalous barotropic transport 3. MICOM (Miami Isopycnic Coordinate Ocean Model)

Preliminary results Speedy (8 layers) + MICOM in the tropical Pacific and Indian Ocean (30N-30S), 20 layers, 1 o resolution in the horizontal, + climatological SST elsewhere, + Land model Speedy (8 layers) + MICOM in the tropical Pacific and Indian Ocean (30N-30S), 20 layers, 1 o resolution in the horizontal, + climatological SST elsewhere, + Land model

VARIABILITY

CO2 –noCO2 run DJF JJA

EOFs precip coupled model 21.3% 13.9% 12.7% 10.7% 7.5% 7.1%

12.9% 7.2% 13.1% 7.2%

coupled model atmos cm forced

Conclusions in SPEEDY warming induces a weakening of the monsoon circulation in SPEEDY warming induces a weakening of the monsoon circulation the weakening trend is not observed with the mixed layer and in the coupled runs the weakening trend is not observed with the mixed layer and in the coupled runs with prescribed SSTs precipitations are localized over the ocean (too south) with prescribed SSTs precipitations are localized over the ocean (too south) better agreement with obs in the western IO than in the eastern side better agreement with obs in the western IO than in the eastern side low level westerlies too weak over the Arabian Sea. Not true for the coupled runs low level westerlies too weak over the Arabian Sea. Not true for the coupled runs Larger variability in the uncoupled runs forced by the coupled SST Larger variability in the uncoupled runs forced by the coupled SST