Zonal Momentum Budget of the Madden–Julian Oscillation: The Source and Strength of Equivalent Linear Damping

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Abstract
Linear, dissipative models with resting base states are sometimes used in theoretical studies of the Madden–Julian oscillation (MJO). Linear mechanical damping in such models ranges from nonexistent to strong, since an observational basis for its source and strength has been lacking. This study examines the zonal momentum budget of a composite MJO over the equatorial western Pacific region, constructed using filtering and regression techniques from 15 yr (1979–93) of daily global reanalysis data. Two different reanalyses (NCEP–NCAR and ERA-15) give qualitatively similar results for all major terms, including the budget residual, whose structure is consistent with its interpretation as eddy momentum flux convergence (EMFC) in convection. The results show that the MJO is a highly viscous oscillation, with a 3–5-day equivalent linear damping time scale, in the upper as well as lower troposphere. Upper-level damping is mainly in the form of large-scale advection terms, which are linear in MJO amplitu... Abstract Linear, dissipative models with resting base states are sometimes used in theoretical studies of the Madden–Julian oscillation (MJO). Linear mechanical damping in such models ranges from nonexistent to strong, since an observational basis for its source and strength has been lacking. This study examines the zonal momentum budget of a composite MJO over the equatorial western Pacific region, constructed using filtering and regression techniques from 15 yr (1979–93) of daily global reanalysis data. Two different reanalyses (NCEP–NCAR and ERA-15) give qualitatively similar results for all major terms, including the budget residual, whose structure is consistent with its interpretation as eddy momentum flux convergence (EMFC) in convection. The results show that the MJO is a highly viscous oscillation, with a 3–5-day equivalent linear damping time scale, in the upper as well as lower troposphere. Upper-level damping is mainly in the form of large-scale advection terms, which are linear in MJO amplitu...