El Niño‐Southern Oscillation, the Madden‐Julian Oscillation and Atlantic basin tropical cyclone rapid intensification

Abstract

Both El Niño‐Southern Oscillation (ENSO) and the Madden‐Julian Oscillation (MJO) have previously been documented to impact Atlantic basin tropical cyclone (TC) activity through alterations in large‐scale fields such as vertical wind shear, mid‐level moisture, sea level pressure and sea surface temperature. Atlantic TC activity has been shown to be enhanced when La Niña conditions are present in the tropical Pacific, while activity is reduced when El Niño conditions occur. Atlantic TC activity is enhanced when the convectively active phase of the MJO is over Africa and the western Indian Ocean (Phases 1–2), while TC activity is suppressed when the convectively active phase of the MJO is over the tropical Pacific (Phases 6–7). These relationships are shown to extend to Atlantic basin rapid intensification (RI) events (typically defined as intensification of 30 knots or greater in 24 h), with nearly three times as many RI events in La Niña years when compared with El Niño years. In addition, approximately four times more RI episodes occur when the MJO exceeds one standard deviation in Phases 1–2 than when the MJO exceeds one standard deviation in Phases 6–7. Storms forming in Phases 1–2 are twice as likely to undergo at least one RI episode during their lifetime as storms forming in Phases 6–7. Even stronger relationships are seen when the MJO and ENSO are considered in combination.