The Role of Equatorially Forced Waves in Triggering Benguela Niño/Niña as Investigated by an Energy Flux Diagnosis

The Benguela Niño/Niña events manifest the anomalous signals of sea surface temperature (SST) in the upwelling region off the west coast of Africa. These events are triggered by the interannual modulation of either equatorial waves or local atmospheric forcing. In the present study, the mechanism that equatorial waves induce the coastal SST anomaly is investigated in terms of the transfer episodes of wave energy by both numerical experiments and reanalysis data. The result of numerical experiments suggests that most of the coastal events can be reproduced by subseasonal wind forcing with an interannual modulation that excites oceanic waves of the first three baroclinic modes. The transfer routes of wave energy illustrate the role of wave dynamics that explains how the interannual variability of SST in the equatorial Atlantic is connected with that in coastal regions. The linearly superposed sign-indefinite potential energy flux owing to waves manifests its capability of sufficiently displacing the thermocline so as to trigger the coastal events. The diagnosis of wave energy for reanalysis data further confirms that there are clear wave energy routes from the equatorial Atlantic to the coastal region, along which different source regions for waves in the first-four modes are found, jointly contributing to the 2019 Niño event.