A simple approach to quantifying the noise–ENSO interaction. Part II: the role of coupling between the warm pool and equatorial zonal wind anomalies
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Stochastic forcing has been used conceptually to explain ENSO irregularity. More recently, the concept of state-dependent (multiplicative) stochastic forcing has been explored as an explanation of a number of ENSO properties. By calculating the state-dependence factor of ENSO zonal wind stress noise forcing on SST, we are able to separate the additive and multiplicative components of the wind stress noise forcing of ENSO. Spatially, the months with large additive or multiplicative components all resemble previous studies on westerly wind bursts. They differ from each other in that the wind stresses are significantly stronger during months with a large multiplicative noise component. It is further shown that when the multiplicative noise component is large, there have been large values of the wind stress noise in the preceding months. This is not true of the months when the additive component is large. The multi-month growth of the wind stress from the multiplicative noise process is shown to be related to an eastward migration of the western Pacific Warm Pool, which is coupled to the wind stress through convection. This process is shown to be significantly weakened in a climate model when the ocean and atmosphere are uncoupled.
KeywordsENSO Westerly wind bursts State-dependent stochastic forcing
This work was supported by NSF Grant AGS-1034798, Department of Energy Grant DE-SC0005110 and NOAA Grant NA10OAR4310200. The authors would like to express thanks to C. Karamperidou and A. Timmermann for their helpful discussions and three anonymous reviewers for their comments.
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