Role of Subtropical Rossby Waves in Amplifying the Kelvin-Mode Circulation Component of the Madden-Julian Oscillation

The composite structure of the Madden-Julian Oscillation (MJO) has long been known to feature pronounced Rossby gyres in the subtropical upper troposphere, whose existence can be interpreted as the linear response to convective heating anomalies in the presence of a subtropical westerly jet. Less is known, however, about whether these forced gyre circulations have any subsequent effects on the tropics. In this work, we perform idealized modelling experiments to explore the role of the subtropics in maintaining the MJO's Kelvin-mode circulation. We use a non-linear spherical shallow water model with different imposed background jet profiles and an assumed MJO-like thermal heat source. Results show that a stronger jet leads to a stronger Kelvin-mode response in the tropics, together with stronger divergence anomalies in the vicinity of the forcing. To understand this behavior, we perform additional experiments in which a localized vorticity forcing is imposed in the extratropics, without any thermal forcing in the tropics. We find that the extratropical forcing is able to trigger equatorial Kelvin waves, provided the jet is of sufficient amplitude. A closer analysis of the vorticity and energy budget reveals that the zonal-mean zonal wind shear plays a key role in amplifying the Kelvin-mode divergent winds near the equator. These results help to explain observations showing that the MJO during boreal winter tends to be more active during periods when the Asian jet is stronger than normal.