Tropical and mid-latitude 45-day perturbations over the Western Pacific during the northern summer

The tropical Madden-Julian oscillation (MJO), which is of convective origin, systematically propagates northward in the Western North Pacific summer monsoon region (5°-20°N, 110°-160°E). The northward propagating MJO, which is statistically significant, displays strong horizontally and vertically asymmetric circulation. The low-level cyclonic response is pronounced due to the presence of convergent, cyclonically sheared mean monsoon flow, while the upper-level anticyclonic counterpart is weak due to the influence of the divergent, anticyclonically sheared upper-level mean monsoon flow. The upper tropospheric MJO response is largely associated with divergent winds, as manifested by strong equatorward outflows that export anticyclonic vorticity into the upper troposphere of the Southern Hemisphere. The summer mean monsoon flow is responsible for reinforcing the vertical asymmetry of the MJO circulation induced by convective heating, and for generating a barotropic component in the baroclinic MJO through the mean monsoon flow-MJO interaction. The mid-latitude intraseasonal oscillation (ISO) displays a barotropic wave train structure along a great circle traversing the North Pacific, and exhibits the largest variability in the westerly jet exit region of the North Pacific (40°-50°N, 180°-150°W). This phenomenon is attributed to the combination of two different dynamic processes. While the MJO is propagating northward, the barotropic MJO component generated by the mean monsoon flow-MJO interaction produces a significant tropical-extratropical interaction on the intraseasonal time scale. Namely, when the 45-day convective activity becomes strongest near the Philippines, the barotropic MJO component acts as an origin of the barotropic Rossby wave dispersion emanating out of the convective forcing, eventually contributing to the development of an extratropical ISO. The enstrophy budget analysis shows, on the other hand, that the largest contribution to ISO is the in situ barotropic mean flow-ISO interaction in and around the jet exit.

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