Dynamics of the Indian monsoon and ENSO relationships in the SINTEX global coupled model

International audienceThis paper uses recent gridded climatological data and a coupled General CirculationModel (GCM) simulation in order to assess the relationships between the interannualvariability of the Indian Summer Monsoon (ISM) and the El Niño-Southern Oscillation(ENSO). The focus is on the dynamics of the ISM-ENSO relationships and the abilityof a state-of-the-art coupled GCM to reproduce the complex lead-lag relationshipsbetween the ISM and ENSO.The coupled GCM is successful in reproducing the ISM circulation and rainfallclimatology in the Indian areas even though the entire ISM circulation is weakerrelative to that observed. In both observations and in the simulation, ISM rainfallanomalies are significantly associated with fluctuations of the Hadley circulation and200 hPa zonal wind anomalies over the Indian Ocean. A quasi-biennial time-scale isfound to structure ISM dynamical and rainfall indices in both cases. Moreover, ISMindices have a similar interannual variability in the simulation and observations.The coupled model is less successful in simulating the annual cycle in the tropicalPacific. A major model bias is the eastward displacement of the western North PacificInter Tropical Convergence Zone (ITCZ), near the dateline, during northern summer.This introduces a strong semi-annual component in Pacific Walker circulation indicesand central equatorial Pacific sea surface temperatures. Another weakness of thecoupled model is a less-than-adequate simulation of the Southern Oscillation due to anerroneous eastward extension of the Southern Pacific Convergence Zone (SPCZ) yearround.Despite these problems, the coupled model captures some aspects of theinterannual variability in the tropical Pacific. ENSO events are phase-locked with theannual cycle as observed, but are of reduced amplitude relative to the observations.Wavelet analysis of the model Niño34 time series shows enhanced power in the 2-4year band, as compared to the 2-8 year range for observations during the 1950-2000period.The ISM circulation is weakened during ENSO years in both the simulation and theobservations. However, the model fails to reproduce the lead-lag relationship betweenthe ISM and Niño34 SSTs. Furthermore, lag correlations show that the delayedresponse of the wind stress over the central Pacific to ISM variability is insignificant inthe simulation. These features are mainly due to the unrealistic interannual variabilitysimulated by the model in the western North Pacific. The amplitude and even the signof the simulated surface and upper-level wind anomalies in these areas are notconsistent with observed patterns during weak/strong ISM years. The ISM and westernnorth Pacific ITCZ fluctuate independently in the observations, while they arenegatively and significantly correlated in the simulation. This isolates the PacificWalker circulation from the ISM forcing. These systematic errors may also contributeto the reduced amplitude of ENSO variability in the coupled simulation. Most of theunrealistic features in simulating the Indo-Pacific interannual variability may be tracedback to systematic errors in the base state of the coupled model