The Madden-Julian Oscillation (MJO) and the Weather of Washington State
The weather of the tropics on time scales of weeks to a couple of months is related to a phenomenon known as the Madden-Julian Oscillation (MJO). The MJO modulates the distribution of deep cumulus convection, and the attendant atmospheric circulation on scales of thousands of kilometers in a band along the equator. The perturbations in the convection and winds in this band travel from west to east, and typically take 40-70 days to circle the globe. They tend to be manifested most prominently from the Indian Ocean into the central Pacific Ocean. The periodicity and strength of the MJO varies; it is considered active roughly one-half the time. The MJO is of particular interest to the weather and climate community because of its remote impacts on higher latitudes.
The MJO impacts the interactions between the tropical and mid-latitude portions of the atmospheric flow aloft. The deep convection with the MJO in the tropics can both trigger disturbances with a sort of a ripple effect extending into higher latitudes, and can influence the propagation and reflection of disturbances originating at higher latitudes. The mechanisms at work here are very complicated and not fully understood, but in a broad sense, appear to be similar to those that operate in association with El Nino/Southern Oscillation (ENSO) in terms of the latter’s impacts on the weather on seasonal time scales. It bears noting that just like ENSO, the MJO more predisposes, rather than dictates, weather patterns at higher latitudes. That being said, statistically robust relationships have been documented between the MJO and the atmospheric circulation over the North Pacific Ocean, and with the precipitation over the western U.S. during wintertime, among other aspects of the tangible weather. For example, relatively heavy rainfall and floods are favored in Washington state during early winter (Oct-Dec) when the MJO is in the part of its cycle when deep convection is enhanced near the dateline; it tends to be drier than normal during the opposite phase of the MJO when convection is strong in the Indian ocean.
The dynamics of the MJO, and its influences on the higher-latitude weather are subjects of active research. From a Pacific Northwest perspective, it would be interesting and valuable to determine the degree to which the MJO could be used to help anticipate not just flooding rains but other kinds of episodic events such as arctic-air outbreaks and summertime heat waves. It may also play a role in seasonal transitions, with a notable example here being the timing of the typically rather sudden switch in coastal winds from southerlies to upwelling-favorable northerlies. In parallel, there are efforts underway to improve operational predictions of the MJO itself. It appears that these predictions may soon be feasible with time horizons of a few weeks. Reliable forecasts of the MJO, coupled with statistical relationships based on past occurrences of the MJO, would provide the potential for outlooks beyond the 1-2 week range for which numerical weather prediction (NWP) models have useful skill.