What Is The Madden Julian Oscillation (MJO) & How Does It Affect Tropical Cyclone Development
I have been asked to explain what the Madden Julian Oscillation is; especially since I refer to it so often during the Hurricane season. So, here is an explanation of the MJO and how it impacts the hurricane season:
The Madden Julian Oscillation is the biggest contributor to the variability we see in the tropical atmosphere (between sunshine and calm weather and frequent showers and thunderstorms with gusty winds) throughout the year. It is basically a link between the atmospheric circulation and tropical thunderstorm activity and travels from west to east year-round at a forward speed of between 9 and 18 mph. The overall circulation pattern that is the Madden Julian Oscillation shows itself in the form of higher than average rainfall and thunderstorm activity.
The MJO cycle between suppressed rainfall and higher than average rainfall usually lasts between 30 and 60 days and travels from west to east across the entire tropical region of the Globe. Think of it as an area of concentrated energy and upward motion that is surrounded by sinking air. So, when the upward motion pulse of the MJO reaches your area, it will be accompanied by frequent thunderstorms and above average rainfall. Once the upward motion pulse passes you, it will be followed by downward sinking air which will lead to dry, sunny weather and suppressed thunderstorm activity. Again, this cycle runs every 30 to 60 days with 45 days being the mid-point. This means that roughly every 45 days, an upward motion pulse of the MJO should reach a specific point in the tropics across the Globe. So, say for instance one cycle of the upward motion of the MJO impacts the western Caribbean around the late part of May and early June. You should expect the next cycle of the upward motion pulse of the MJO in the western Caribbean about 45 days later or around the middle of July.
How Does The Madden Julian Oscillation Affect Tropical Cyclone Development? It is well understood that there are periods of enhanced/suppressed tropical cyclone activity within any one hurricane season. There is quite a bit of evidence that the Madden Julian Oscillation controls whether we are in a period of enhanced activity or suppressed activity. The reason why is that MJO provides the large scale environmental conditions necessary for tropical cyclone development.
So, when we are in a upward motion phase of the MJO, it leads to widespread upward moving air, enhanced thunderstorm activity and a higher chance for tropical cyclone development. On the other end of the scale, when there is a downward motion phase of the MJO, it leads to widespread sinking air and thus an environment that is not favorable for tropical cyclone development.
During any hurricane season, the MJO upward pulse is constantly pushing eastward across the tropics. As this MJO upward motion pulse moves eastward, the favored area for tropical cyclone activity will shift eastward from the western Pacific to the eastern Pacific and finally to the Atlantic Basin.
There is an inverse relationship between tropical cyclone activity in the Pacific Ocean and the Atlantic Ocean. When the Pacific is active with tropical cyclone activity, the Atlantic is usually quiet and vice versa. The main reason for this appears to be the phase of the Madden Julian Oscillation, which is usually in opposite modes between the two basins at any given time (upward motion in the Pacific, downward motion in the Atlantic).
It should be emphasized that the Madden Julian Oscillation is only one of many factors that contribute to the development of tropical cyclones. Sufficiently warm ocean waters, low wind shear and plenty of mid-level moisture is also needed for tropical cyclones to form and persist (explained thoroughly HERE). With that said, the Madden Julian Oscillation also influences these conditions that are favorable or not favorable for tropical cyclone formation. We are constantly monitoring the MJO throughout the hurricane season to try and predict periods of enhanced tropical cyclone activity or suppressed tropical cyclone activity.
Rob Lightbown of Crown Weather