What does the mean mean? Step-like moisture dependence, MJO dynamics, and the relevance of composites

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Monday, 5 January 2015
Brian Mapes, Univ. of Miami/RSMAS, Miami, FL; and W. Hannah

The MJO is commonly thought to involve storage and discharge of moisture or, almost equivalently, column-integrated moist enthalpy. However, the dependence of convection of column moisture is observed to be very nonlinear: step-like, with the 50mm precipitable water (PW) contour roughly bounding the region where deep precipitating convection can occur over the Indo-Pacific warm pool from day to day. There is a spatial plateau (a mode in the PDF) with convection-permitting values >50 mm. MJO suppressed and active periods differ largely by the horizontal area of this plateau. These nonlinear dynamics make simple linear averaging of moisture fields ill-advised, yet much of MJO research involves such averaging, in composites and/or in latitudinal belt averages. Several artificial consequences will be illustrated, by working from full-resolution animations of events toward the blurry averaged views common in much of the literature on MJO research.