Tropical rainfall exhibits strong variability on sub-seasonal time scales. These fluctuations in tropical rainfall often go through an entire cycle in 30-60 days, and are referred to as "intraseasonal oscillations" or the "Madden-Julian Oscillation" or "MJO".
The MJO is a naturally occurring component of our coupled ocean-atmosphere system. It significantly affects the atmospheric circulation throughout the global Tropics and subtropics, and also strongly affects the wintertime jet stream and atmospheric circulation features over the North Pacific and western North America (through its effect on the state of the Pacific North American (PNA) oscillation. As a result, it has an important impact on storminess and temperatures over the U.S. It can impact heavy precipitation events on the west coast with a stream of storms that move from near Hawaii to the west coast called the Pineapple Express. Some of these storms historically have been ferocious with heavy rains and strong winds. Not all MJOs are strong enough to produce these events.
THE MJO AND THE TROPICS
During the summer the MJO has a modulating effect on hurricane activity in the Indian Ocean, the western and eastern Pacific and Atlantic basin.
The MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Oceans. 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 less organized as it moves over the cooler ocean waters of the eastern Pacific although the waters north of the equator are frequently warm enough to support storm development especially in El Nino years. The MJO then reappears over the tropical Atlantic and Indian Ocean. Each cycle lasts approximately 30-60 days
There is strong year-to-year variability in MJO activity, with long periods of strong activity followed by periods in which the oscillation is weak or absent. This interannual variability of the MJO is partly linked to the ENSO cycle. Strong MJO activity is often observed during weak La Niña years or during ENSO-neutral years, while weak or absent MJO activity is typically associated with strong El Niño episodes.
TRACKING THE MJO
One source for tracking the actual MJO is the Climate Prediction Centers daily updates.
This version has the 200mb velocity potential anomalies. Green areas depict areas where rising motion is favored, brown areas where descending air is favored aloft.
Obviously during the active hurricane season, when an area favoring rising air moves over sufficiently warm tropical waters with low level convection, activity is more likely to increase and in the absence of shear, has a better chance to develop.
CPC provides an example of the movement of a series of MJOs and the point of origin of tropical systems by phase. Note that most tropical systems form in the favored region and the activity shifts east with time. It should be noted the MJO is not an on-off switch but a tool that suggests enhanced or diminished probabilities and can be used to identify upcoming active periods.
The MJO sometimes stalls or weakens or suddenly regenerates making forecasting challenge. There are some MJO forecast models that project future states of the MJO. But they too have their troubles at times with the phenomena.
CPC has put together a useful page that shows some MJO products and the effects at various stages as well as links to forecasts.
The MJO continues in a favorable mode for activity the next week or so as it was when Bertha was born and further development is possible. That would put the likely next favorable period in late August to mid-September, the peak season for the Atlantic.
Below is the NCEP ensemble forecast for the velocity potential for the week ending July 26th. The blue areas here are the favored areas for convection, the red areas with subsidence aloft suppressing activity. Note the lingering blue in the far eastern Atlantic and the emergence of a new MJO in the Indian Ocean that by late August should be firing up the Atlantic. That is if this MJO develops and behaves as it usually does.
This and other model forecasts can be found at the CDC site here, another excellent source for MJO status and forecasts.