Development of convection along the SPCZ within a Madden-Julian oscillation

A subtropical Rossby wave propagation mechanism is proposed to account for the poleward and eastward progression of intraseasonal convective anomalies along the South Pacific Convergence Zone (SPCZ) that is observed in a significant proportion of Madden-Julian Oscillations (MJOs). Large scale convection, associated with an MJO, is assumed to be already established over the Indonesian region. The latent heating associated with this convection forces an equatorial Rossby wave response with an upper tropospheric anticyclone, centred over or slightly to the west of the convection. Large potential vorticity (PV) gradients, associated with the subtropical jet and the tropopause, lie just poleward of the anticyclone and large magnitude PV air is advected equatorwards on the eastern side of the anticyclone. This ``high'' PV air, or upper tropospheric trough, is far enough off the equator that it has associated strong horizontal temperature gradients, and it induces deep ascent on its eastern side, at a latitude of about 15-30\degr. If this deep ascent is over a region susceptible to deep convection, such as the SPCZ region, then convection may be forced or triggered. Hence convection develops along the SPCZ as a forced response to convection over Indonesia. The response mechanism is essentially one of subtropical Rossby wave propagation. This hypothesis is based on a case study of a particularly strong MJO in early 1988, and is tested by idealised modelling studies. The mechanism may also be relevant to the existence of the mean SPCZ, as a forced response to mean Indonesian convection.