Indonesian Throughflow Variability and Linkage to ENSO and IOD in an Ensemble of CMIP5 Models

Abstract

Abstract Understanding variability of the Indonesian Throughflow (ITF) and its links to El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD), and how they are represented across climate models constitutes an important step toward improved future climate projections. These issues are examined using 20 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and the SODA-2.2.4 ocean reanalysis. It is found that the CMIP5 models overall simulate aspects of ITF variability, such as spectral and vertical structure, that are consistent with the reanalysis, although intermodel differences are substantial. The ITF variability is shown to exhibit two dominant principal vertical structures: a surface-intensified transport anomaly (ITFM1) and an anomalous transport characterized by opposing flows in the surface and subsurface (ITFM2). In the CMIP5 models and reanalysis, ITFM2 is linked to both ENSO and the IOD via anomalous Indo-Pacific Walker circulation. The driver of ITFM1 however differs between the reanalysis and the CMIP5 models. In the reanalysis ITFM1 is a delayed response to ENSO, whereas in the CMIP5 models it is linked to the IOD associated with the overly strong IOD amplitude bias. Further, the CMIP5 ITF variability tends to be weaker than in the reanalysis, due to a tendency for the CMIP5 models to simulate a delayed IOD in response to ENSO. The importance in considering the vertical structure of ITF variability in understanding ENSO and IOD impact is further underscored by the close link between greenhouse-forced changes in ENSO variability and projected changes in subsurface ITF variability.