Equatorial intraseasonal variability: is there a causal connection from the extratropics?

Propagating intraseasonal modes that resemble the Madden Julian oscillation can be found in numerical model simulations with varying degrees of success, and a number of studies have attributed their generation and maintenance to a range of physical mechanisms. One of the recurring questions is whether variability in the extratropics has an influence, and whether it is necessary or sufficient to induce tropical variability on the same timescales.

Tropical modelling experiments designed to asses the influence of boundary conditions can encounter problems of interpretation. Does the variability imposed at the boundary actually originate from the extratropics ? How far from the equator should the boundaries be placed ? In this study we first use a dry global model with the extratropics nudged at various timescales and latitude ranges. An assessment is made of where the extratropics should start in order to avoid contamination in our measures of tropical variability.

We then use the WRF model in channel mode with a selection of boundary conditions, filtered to isolate the intraseasonal signal. We find that our configuration of WRF is capable of producing a propagating Indo-Pacific signal. Differences between twin experiments are then examined to reveal the part of the model variability that is formally independent of the boundary conditions, and yet still consistent a realistic prescription of extratropical variability. Propagating tropical signals persist in the difference fields to varying degrees, depending on the timescales present in the boundary conditions.