The Tropical–Extratropical Interaction between High-Frequency Transients and the Madden–Julian Oscillation

The interaction between high-frequency transient disturbances and convection, and the Madden–Julian Oscillation (MJO), is investigated using NCEP–NCAR reanalysis and satellite outgoing longwave radiation data for 15 northern winters. During the phase of the MJO with enhanced convection over the East Indian Ocean and Indonesia, and suppressed convection over the South Pacific convergence zone, both the Asian–Pacific jet and the region of upper-tropospheric tropical easterlies over the warm pool are displaced westward. These changes in the basic state lead to a weaker or ‘‘leakier’’ waveguide in the Asian–Pacific jet, with a westwarddisplaced ‘‘forbidden’’ region of tropical easterlies, such that high-frequency transient waves propagate equatorward into the deep Tropics over the central Pacific near the date line. As these waves induce convection in the region of ascent and reduced static stability ahead of the upper-level cyclonic disturbances, there is an enhancement of high-frequency convective variability over the central Pacific intertropical convergence zone during this phase of the MJO. This enhanced high-frequency convective variability appears to project back onto intraseasonal timescales and forms an integral part of the slowly varying diabatic heating field of the MJO. In the opposite phase of the MJO, the Asian–Pacific jet is extended eastward and there is an almost continuous waveguide across the Pacific. Together with the expanded forbidden region of tropical easterlies over the warm pool, this leads to a more zonal propagation of high-frequency transients along the waveguide with less equatorward propagation, and hence reduced high-frequency convective variability over the tropical central Pacific. There is also evidence of high-frequency waves propagating into the Indian Ocean region at the beginning of the MJO cycle, which may be important in the initiation of intraseasonal convective anomalies there.