The Role of MJO Propagation, Lifetime, and Intensity on Modulating the Temporal Evolution of the MJO Extratropical Response

The Madden-Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability. Many studies have found that the MJO has significant impacts on the weather in the extratropics, affecting temperature, precipitation, extratropical cyclones and extreme events. Given that the MJO can be predicted out to several weeks, these impacts have implications on subseasonal to seasonal predictability in the mid-latitudes. Hence it is important to better understand the link between the MJO and its extratropical impacts.

Since the MJO can act as a tropical heat source that excites Rossby Waves, midlatitude weather is modulated by the MJO due to the Rossby Waves that propagate into the midlatitude and modulate the midlatitude circulation. The heat sources of individual MJO events are different since each event has different eastward propagation speed, lifetime, intensity, and structure. The background flow is also different for each event. These can result in different evolution of the Rossby Waves excited by the heat sources, and different extratropical response for each MJO event.

In this study, the role of MJO propagation speed, lifetime, and intensity on modulating the structure and temporal evolution of the MJO extratropical response has been systematically explored by using an idealized nonlinear GCM. By adding the MJO associated heating into the GCM as forcing, the extratropical response that has been found in the Reanalysis is well captured by the model, especially over the Pacific and North America. Sensitivity experiments have been performed with MJO events that have different idealized propagation speeds, lifetimes, and intensities. The results show that in order to excite a strong extratropical response, the MJO has to propagate through specific MJO phases. The intensity, timing and duration of the extratropical response, especially over North America, also strongly depend on when the MJO is initiated and when the MJO decays. The extratropical impacts of slow propagating MJO and fast propagating MJO also have significant differences, especially on intensity and duration. Not surprisingly, stronger MJO events induce stronger extratropical responses compared to weaker MJO events.