[AOS22-05] Influences of the Interdecadal Pacific Oscillation on the locally amplified Ningaloo Niño
Keywords:coastal niño, interdecadal variation, Indian Ocean
Ningaloo Niño is associated with warm sea surface temperature (SST) anomalies off the west coast of Australia. Although recent increase in the occurrence of Ningaloo Niño is attributed to the swing to the negative phase of the Interdecadal Pacific Oscillation (IPO), its mechanism is not completely understood. In particular, the role of local air-sea interaction that has been shown to play an important role in the development of the Ningaloo Niño/Niña was not examined. Using observation/reanalysis data and outputs from a coupled general circulation model (CGCM), influences of the IPO on the frequency of the Ningaloo Niño/Niña and their mechanisms are investigated in this study. Furthermore, outputs from a regional ocean model and an atmospheric general circulation model (AGCM) are analyzed to examine their detailed mechanisms.
It is found that the Ningaloo Niño occurs more frequently and the ratio of the locally amplified events increases during the negative IPO in both observation and the coupled model. The increased occurrence of the locally amplified Ningaloo Niño during the negative IPO is due to positive interdecadal SST anomalies to the northwest of Australia that promote deep atmospheric convection and anomalous cyclone off the west coast of Australia through a Matsuno-Gill response. Based on a mixed layer heat budget analysis using outputs from a regional ocean model, it is shown that an enhanced warming of anomalously thin mixed layer by the shortwave radiation contributes dominantly to the interdecadal SST anomalies. The negative mixed layer depth anomalies are suggested to be generated by anomalously weak winds associated with the negative IPO. In addition, the importance of SST anomalies over the southern Indian Ocean is confirmed by a sensitivity experiment of an AGCM.
It is found that the Ningaloo Niño occurs more frequently and the ratio of the locally amplified events increases during the negative IPO in both observation and the coupled model. The increased occurrence of the locally amplified Ningaloo Niño during the negative IPO is due to positive interdecadal SST anomalies to the northwest of Australia that promote deep atmospheric convection and anomalous cyclone off the west coast of Australia through a Matsuno-Gill response. Based on a mixed layer heat budget analysis using outputs from a regional ocean model, it is shown that an enhanced warming of anomalously thin mixed layer by the shortwave radiation contributes dominantly to the interdecadal SST anomalies. The negative mixed layer depth anomalies are suggested to be generated by anomalously weak winds associated with the negative IPO. In addition, the importance of SST anomalies over the southern Indian Ocean is confirmed by a sensitivity experiment of an AGCM.