Modulation of eastern Pacific and Gulf of Mexico hurricanes by the Madden-Julian Oscillation

Eastern Pacific tropical cyclones are strongly modulated by the Madden-Julian Oscillation (MJO). Over twice as many named tropical systems (hurricanes and tropical storms) accompany eastern Pacific MJO 850 mb westerly anomalies than easterly anomalies, and the systems that do exist are stronger. Hurricanes are over four times more numerous during westerly phases of the MJO than easterly phases. Equatorial MJO Kelvin waves propagating eastward from convective regions of the western Pacific Ocean alter dynamical conditions over the eastern Pacific Ocean. Convection locally amplifies the wind anomalies over the eastern Pacific. Periods of equatorial 850 mb westerly wind anomalies associated with the MJO are accompanied by cyclonic low-level relative vorticity anomalies, surface convergence, barotropic eddy kinetic energy conversion, and near-zero vertical wind shear over the eastern Pacific hurricane region, conditions that are favor able for tropical cyclone formation.

The MJO also modulates tropical storm and hurricane activity over the Gulf of Mexico and western Caribbean Sea. When MJO wind anomalies in the lower troposphere of the eastern Pacific are westerly, Gulf of Mexico and western Caribbean hurricane genesis is four times more likely than when the MJO winds are easterly.