The extratropical response to a developing MJO: forecast and climate simulations with the DREAM model
Résumé
The Madden Julian oscillation (MJO) has a systematic influence on the state of the extratropical atmosphere. The physical mechanism involves teleconnections from tropical deep convective heat sources through wave propagation. MJO heating resembles a propagating dipole that cycles through phases on a timescale similar to the response timescale. This complicates the attribution of the response to a well identified phase in the tropical heat source. In this paper the Dynamical Research Empirical Atmospheric Model (DREAM) is used to examine the extratropical response to a moving cyclic MJO-like tropical heat source based on outgoing long-wave radiation data. A set of ensemble forecast experiments and consistent stationary wave solutions are analysed, together with longer equilibrium integrations, to investigate how the extratropical response relates to the phase of the MJO heating. We find that the response is modified by the propagation of the source in a way that depends on the initial phase. It also displays both stationary and transient nonlinearity which can account for an asymmetric response to opposite-phase dipole heat sources. The solution for Indian Ocean heating/West Pacific cooling is more zonally oriented and weaker than the solution for the opposite phase. If the MJO heat source is specified as a continuous cycle, it modifies the time-mean extratropical circulation and generates remote teleconnections. The timing and starting phase of MJO events can greatly modify the relationship between MJO phase and extratropical response, especially when an unforced "resting" period is inserted between simulated MJO cycles.