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 obs erved in a significant proportion of Madden-Julian oscillations (MJOs) . Large-scale convection, associated with an MJO, is assumed to be alr eady established over the Indonesian region. The latent heating associ ated with this convection forces an equatorial Rossby-wave response wi th an upper-tropospheric anticyclone centred over, or slightly to the west of, the convection. Large potential-vorticity (PV) gradients, ass ociated with the subtropical jet and the tropopause, lie just poleward of the anticyclone, and large magnitude PV air is advected equatorwar ds on the eastern side of the anticyclone. This 'high' PV air, or uppe r-tropospheric trough, is far enough off the equator that it has assoc iated strong horizontal temperature gradients, and it induces deep asc ent on its eastern side, at a latitude of about 15-30 degrees. If this deep ascent is over a region susceptible to deep convection, such as the SPCZ, then convection may be forced or triggered. Hence convection develops along the SPCZ as a forced response to convection over Indon esia. The response mechanism is essentially one of subtropical Rossby- wave propagation. This hypothesis is based on a case study of a partic ularly strong MJO in early 1988, and is tested by idealized modelling studies. The mechanism may also be relevant to the existence of the me an SPCZ, as a forced response to mean Indonesian convection.