Chapter 8 Variability of the Indonesian Throughflow: A Review and Model-to-Data Comparison

Abstract

Existing observations of ocean temperature, sea level and transport within the Indonesian seas and south-east Indian Ocean are reviewed and compared with a simulation. The dynamical mechanisms by which the Indonesian Throughflow (ITF) interacts with the surrounding oceans in the model and real oceans are discussed. The focus is on the variability on timescales ranging from the intraseasonal (<3 months) to the interannual (a few years). A lagged multiple regression technique is applied to anomalies of sea level from a mean seasonal cycle for observed and modeled fields. The results reveal that the region comprises the intersection of two ocean wave guides where sea level and upper ocean temperature anomalies can be largely understood in terms of Kelvin and Rossby waves generated by remote zonal winds along the Indian and Pacific equators, with local wind forcing appearing to play a minor role in generating interannual variability. Variations in zonal Pacific equatorial winds force a response along the Arafura/Australian shelf break through Pacific equatorial Rossby waves exciting coastally trapped waves off the western tip of Irian Jaya, which propagate westward into the Banda Sea and poleward along the Australian west coast. Equatorial Kelvin waves excited by Indian Ocean winds propagate eastward and scatter into coastal waves along the Sumatra–Java–Lesser Sunda coasts to penetrate the western internal seas of Indonesia. While the model captures the variability generated by the equatorial Indian Ocean winds, its pathway eastward into the Indonesian region is prevented by a dynamically “wide” Lombok Strait, which carries too much of the main throughflow and allows too little wave energy to pass directly into the Banda Sea. The model response to El Nino-southern oscillation (ENSO) wind changes is too weak as expressed by its sea-level response in the western Pacific. The subsequent transmission of this ENSO signal into the ITF region and along the west coast of Australia is also much weaker in the model than observed. Generally, the model/data comparisons suggest that too little energy gets scattered into the model coastal wave guide. An assessment of whether this difference is a model resolution or model dissipation issue (and they are related) needs further work.