The US National Weather Service forecast ten to 17 named storms with wind speeds of over 74mph this hurricane season (June to November), compared to 15 last year. There have only been four so far but they come in clusters; in 2008 four hurricanes formed in the week of 25 August and none in the three following weeks.
One reason for clustering is the Madden-Julian Oscillation or MJO, a pattern of rising and sinking air which circles the globe eastward in equatorial regions at about 11mph. This produces weather in cycles of 45 to 60 days. The MJO’s two-week enhanced phase increases rainfall and decreases vertical wind shear, the difference in wind speed with altitude. Both these factors help in hurricane formation, and the combination means that major hurricanes are five times as likely to form during the enhanced phase of the MJO.
In the suppressed phase of the MJO, sinking air reduces the likelihood of thunderstorms forming or growing, as these feed on moisture rising upwards from the sea.
The MJO is only one factor in hurricane formation. The sea temperature, moisture levels and existing winds also play a part. Hurricanes are inherently unpredictable, but understanding effects like the MJO may help us anticipate their formation and respond in good time.
