Per the Climatic Data Center, the MJO (Madden-Julian Oscillation) is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall then generally becomes nondescript as it moves over the cooler ocean waters of the eastern Pacific (except over the region of warmer water off the west coast of Central America) but occasionally reappears at low amplitude over the tropical Atlantic and higher amplitude over the Indian Ocean. The wet phase of enhanced convection and precipitation is followed by a dry phase where thunderstorm activity is suppressed. Each cycle lasts approximately 30–60 days. Because of this pattern, the MJO is also known as the 30–60 day oscillation, 30–60 day wave, or intraseasonal oscillation.
We lean on the current and forecast phases of the MJO frequently in building our medium and long range forecasts. At times the MJO can take more control of the pattern than others. A great example of that can be seen playing out in front of us over the next couple weeks.
Note the current GEFS forecast plot. Though the MJO is expected to move into Phases 4 and 5, the amplitude isn’t nearly as great as when it was moving through Phases 8, 1, and 2 over the past few weeks. This means we have to also start paying attention to other global teleconnections to gain more insight as to what will take place with the weather pattern in the short to medium term.
At other times, especially this time of year and into the winter, the MJO can be so amplified, it’ll take over as the primary driver of the pattern.
Note the way the European monthly re-amplifies things as we head into the 2nd half of September. This is what’s tipping us off to the warmth that likely looms as we traverse the last couple weeks of the month.
