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Multidecadal Simulations of the Indian Monsoon in SPEEDY-AGCM and a Coupled Model

This study investigates the role of SST anomalies, CO2 forcing, and interactive SSTs on the dynamics of the Indian Ocean and the Indian monsoon. The results show the impact of different model configurations on precipitation trends and variability.

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Multidecadal Simulations of the Indian Monsoon in SPEEDY-AGCM and a Coupled Model

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  1. 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

  2. Focus on: • the role of the SST anomalies on Indian Ocean dynamics • the role of CO2 forcing • the role of interactive SSTs (mixed layer ocean) • the role of the ocean

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

  4. series of 50-ys ensemble with • SPEEDY forced with HadISST SST (35 members) • SPEEDY + observed SST + CO2 increase (35 members) • SPEEDY + climatological SST in the Indian Ocean + observed SST elsewhere (+ CO2) (8 members each) • SPEEDY + mixed layer ocean (+ CO2) (8 members each) • SPEEDY + OGCM in the tropical Pacific and Indian ocean (+ CO2) … 8 members

  5. SST trend

  6. Precip and wind climatology

  7. Precip trends

  8. 10.4% 24.6% EOFs precip 14.1% 19.3% 21.3% 10.7%

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

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

  11. VARIABILITY

  12. CO2 –noCO2 run 1950-2000 DJF JJA

  13. 10.7% 21.3% EOFs precip coupled model 7.5% 13.9% 12.7% 7.1%

  14. 7.2% 12.9% 7.2% 13.1%

  15. coupled model atmos cm forced

  16. Conclusions • 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 • with prescribed SSTs precipitations are localized over the ocean (too south) • 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 • Larger variability in the uncoupled runs forced by the coupled SST

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