Extratropical Influence on Tropical Intraseasonal Variability

Abstract:

Intraseasonal variability in the tropics is characterized by propagating signals that interact with convection. Their signature in space-time spectra can be associated with Rossby and Kelvin waves and the MJO. The fidelity of current model simulations of these modes depends on multiple factors, including resolution, convection, simulation of mean winds, SST distributions and atmosphere-ocean coupling.

In this presentation I will concentrate on the potential role of influence from the extratropics. Previous work shows evidence of coherence between tropical and midlatitude propagating signals, indicating either global modes or tropical-extratropical coupling. Some evidence has also been found for explicit extratropical influence. Regional modelling experiments will be presented that are designed to asses the importance of incoming midlatitude signals. However, our experiments 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 ? How do we design an experiment that isolates the internal tropical variability from the boundary influence ?

Results will be shown from the WRF model in tropical channel mode with a selection of boundary conditions. We find that our configuration of WRF produces a clearly propagating dynamical signal in the Indo-Pacific region with relatively poor coupling to convection. Differences between twin experiments are examined to reveal the part of the model variability that is formally independent of the boundary conditions. Experiments are also shown with filtered boundary conditions to asses the importance of intraseasonal timescales. The runs presented are long enough (20-years) to provide a generic view of the variability. Our results suggest that most of the local propagation characteristics are inherent to the tropical system, but that the initiation of active phases of the MJO is subject to extratropical influence.