Regional‐scale ocean wave variability associated with El Niño–Southern Oscillation‐Madden‐Julian Oscillation combined activity

The significant wave height ( Hs) variability caused by wind anomalies associated with the co‐occurrence of the Madden‐Julian Oscillation (MJO) and El Niño–Southern Oscillation (ENSO) was investigated in the New Zealand region. For this purpose, Hs and wind anomalies composites were created using 23 years (1979–2002) of modelled data during November–March periods, when simultaneous ENSO‐MJO phase pairs are potentially most active. The results show striking features: El Niño‐related wave conditions (which consist of increased Hs along the west and south coasts of New Zealand) are reinforced during MJO phase 8, whereas the wave conditions associated with La Niña (which consist of larger Hs along the north coast) are enhanced during MJO phase 6; Similar wave anomalies are generated during opposing ENSO phases (La Niña and El Niño) when these are combined with MJO phases 3 and 5, respectively; The majority of statistically significant Hs anomalies disappear from the study area during El Niño‐MJO phase 6, El Niño‐MJO phase 2, and La Niña‐MJO phase 4, showing the neutralizing nature of some phase combinations; Finally, negative Hs anomalies are experienced off the country's west coast during El Niño‐MJO phase 4, in contrast to the positive anomalies expected during El Niño events. These results clearly show the importance of remote forcing to wave anomalies in the New Zealand region, and highlight the need to assess atmospheric and oceanic conditions considering multiple climate oscillations. Significant wave height ( Hs) anomalies are substantially modified during simultaneous ENSO‐MJO phase pairs (as shown by the statistically significant Hs daily anomaly composites in the image) relative to those anomalies expected during periods in which only the ENSO is active (e.g., increased Hs occurs along the west and south coasts of New Zealand during El Niño). Changes in Hs anomalies comprise reinforcement, nullification, and even sign reversal. Some phase combinations are associated with hazardous conditions along the New Zealand coastline.