Convective momentum transport associated with the Madden-Julian oscillation based on a reanalysis dataset

Abstract

A better understanding of multiscale interactions within the Madden-Julian oscillation (MJO), including momentum exchanges, is critical for improved MJO prediction skill. In this study, convective momentum transport (CMT) associatedwith theMJO is analyzed based on theNOAAClimate Forecast SystemReanalysis (CFSR). A three-layer vertical structure associated with the MJO, as previously suggested in the mesoscale momentum tendency profile based on global cloud-resolving model simulations, is evident in the subgrid-scale momentum tendency from the CFSR. Positive (negative) subgrid-scale momentum tendency anomalies are found near the surface, negative (positive) anomalies are found in the low to midtroposphere, and positive (negative) anomalies in the upper troposphere are found within and to the west (east) of the MJO convection. This tends to damp theMJOcirculation in the free atmosphere,while enhancing MJOwinds near the surface. In addition, it could also reduce theMJO eastward propagation speed and lead to the backward tilt with height in the observed MJO structure through a secondary circulation near the MJO center. Further analyses illustrate that this three-layer vertical structure in subgrid-scale momentum tendency largely balances the grid-scale momentum transport of the zonal wind component u, mainly through the transport of seasonal mean u by the MJO-scale vertical motion. Synoptic-scale systems, which were previously proposed to be essential for the u-momentumtransport of theMJO, however, are found to play aminor role for the total grid-scalemomentum tendency. The abovemomentumtendency structure is also confirmedwith theECMWFanalysis for the Year of Tropical Convection (YOTC) that lends confidence to these above results based on the CFSR.