Abstract
The leading modes of daily variability of the Indian summer monsoon in the climate forecast system (CFS), a coupled general circulation model, of the National Centers for Environmental Predictions (NCEP) are examined. The space–time structures of the daily modes are obtained by applying multi-channel singular spectrum analysis (MSSA) on the daily anomalies of rainfall. Relations of the daily modes to intraseasonal and interannual variability of the monsoon are investigated. The CFS has three intraseasonal oscillations with periods around 106, 57 and 30 days with a combined variance of 7%. The 106-day mode has spatial structure and propagation features similar to the northeastward propagating 45-day mode in the observations except for its longer period. The 57-day mode, despite being in the same time scale as of the observations has poor eastward propagation. The 30-day mode is northwestward propagating and is similar to its observational counterpart. The 106-day mode is specific to the model and should not be mistaken for a new scale of variability in observations. The dominant interannual signal is related to El Niño-Southern Oscillation (ENSO), and, unlike in the observations, has maximum variance in the eastern equatorial Indian Ocean. Although the Indian Ocean Dipole (IOD) mode was not obtained as a separate mode in the rainfall, the ENSO signal has good correlations with the dipole variability, which, therefore, indicates the dominance of ENSO in the model. The interannual variability is largely determined by the ENSO signal over the regions where it has maximum variance. The interannual variability of the intraseasonal oscillations is smaller in comparison.
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Acknowledgments
This research was supported by grants from the National Science Foundation (ATM-0332910, ATM-0830062, ATM-0830068), the National Oceanic and Atmospheric Administration (NA04OAR4310034, NA09OAR4310058), and the National Aeronautics and Space Administration (NNG04GG46G, NNX09AN50G). The authors thank Kathy Pegion for sharing the model data and Ben Kirtman for helpful discussions. This work formed a part of the Ph.D. thesis of Deepthi Achuthavarier submitted to George Mason University.
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Achuthavarier, D., Krishnamurthy, V. Daily modes of South Asian summer monsoon variability in the NCEP climate forecast system. Clim Dyn 36, 1941–1958 (2011). https://doi.org/10.1007/s00382-010-0844-9
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DOI: https://doi.org/10.1007/s00382-010-0844-9