Impact of ENSO on the Entrance of the Indonesian Throughflow: The Oceanic Wave Propagation

A regional ocean model simulation for the period from 2000 through 2016 is used to study oceanic wave propagation in the Indonesian seas. The model simulation compares favorably with the interannual variability of the satellite derived sea level anomaly and the observed currents in the entrance passages of ITF. Lag correlation analysis suggests that the interannual sea level anomaly and opposite responses of the velocity above and below the thermocline in the western (the Sulawesi Sea and the Makassar Strait) and eastern (the Halmahera Sea) pathways of ITF are associated with the ENSO induced Rossby waves from the equatorial Pacific rather than the local wind forcing. In particular, during La Nina years ENSO induced anomalies traveling through the western pathway are comparable to those through the traditional eastern waveguide, suggesting the importance of both pathways in the propagation of ENSO signals in the Indonesian seas. Different roles of the western and eastern pathways in transmitting the ENSO signals into the Indonesian seas are associated with the nonlinear processes in the Sulawesi Sea, which appears to be linked with different states of the western boundary currents during the warm and cold ENSO phases. Plain Language Summary Impacts of ENSO on the Indonesian seas that connect the Pacific and Indian Oceans at low latitudes, play an important role in interactions between the two oceans. Traditional understanding of the propagation of ENSO signals into the Indonesian seas is through the Halmahera Sea (i.e., the eastern pathway). In this work, we investigated the interannual sea level and current variations in the Indonesian seas using observations and a regional ocean model. We found both the eastern and western (via the Sulawesi Sea then the Makassar Strait) pathways have significant responses to ENSO signals via the propagating Rossby and Kelvin waves. More ENSO induced oceanic waves propagate through the western pathway during the La Nina years than the El Nino years concurrent with the penetrating state of the western boundary currents and weakened nonlinear processes in the Sulawesi Sea.

Automated summary

This study examines the oceanic wave propagation in the Indonesian seas from 2000 to 2016 using a regional ocean model simulation. The model simulation agrees well with the interannual variability of sea level anomaly and observed currents in the entrance passages of the Indonesian Throughflow (ITF). Lag correlation analysis suggests that the interannual sea level anomaly and opposing velocity responses above and below the thermocline in the western and eastern pathways of the ITF are associated with ENSO-induced Rossby waves from the equatorial Pacific. The study also highlights the importance of both the eastern and western pathways in the propagation of ENSO signals in the Indonesian seas, with different roles attributed to the nonlinear processes in the Sulawesi Sea.