Planetary Scale Kelvin Wave Propagation from the Stratosphere as a Potential Explanation of the Association between the MJO and the QBO

Previous works have shown that the northern winter Madden Julian oscillation (MJO) tends to be stronger during easterly phases of the quasi-biennial oscillation (QBO) in the lower stratosphere. Simple model analyses have suggested that downward propagating Kelvin waves associated with the MJO are more coherent across the tropopause during easterly QBO. Reanalysis and observational data have shown such waves near the Maritime Continent during easterly QBO in December. Other authors have shown that downward propagating temperature signals that might be consistent with Kelvin waves increase tropopause level instability during easterly QBO, which would strengthen MJO convection. This work applied linear regression against an MJO index during easterly and westerly phases of the QBO to diagnose the vertical structure of the MJO-associated geopotential height and wind over Africa east to the Central Pacific basin to assess the geographical distribution of this vertically propagating signal. Results show a more coherent downward propagating Kelvin wave signal from the lower stratosphere during the easterly phase of the QBO over Africa east to the Maritime Continent. A wavenumber frequency spectrum analysis of lower tropospheric zonal wind shows fast planetary scale Kelvin waves during westerly QBO, which shift toward lower frequencies during easterly QBO, suggesting that advection by the background wind renders planetary scale Kelvin waves more resonant with the MJO during easterly QBO.