ABSTRACT

The role of the Drake Passage (DP) and Indonesian Throughflow (ITF) in controlling regional mean climate and rainfall is examined using a set of coupled ocean-atmosphere general circulation models. This first involves exploring a series of climate model simulations employing different geometries in the Drake Passage gap. A profound global climate response is obtained, although the depth of the DP sill turns out to be a critical parameter in controlling the scale of the response. Other parameters that affect the climate sensitivity to Southern Ocean gateway changes include the rate of poleward atmospheric moisture transport in the Southern Hemisphere, and the prescribed atmospheric greenhouse gas concentrations. In the second part of the talk, experiments employing both a closed and open ITF are equilibrated to steady state and then 200 years of natural climatic variability is assessed within each model run, with a particular focus on the Indian Ocean region. Opening of the ITF results in a mean Pacific-to-Indian throughflow of 21 Sv, which advects warm West Pacific waters into the East Indian Ocean. This warm signature is propagated westward by the mean ocean flow, however it never reaches the west Indian Ocean, as an ocean-atmosphere feedback in the tropics generates a weakened trade wind field that is reminiscent of the negative phase of the Indian Ocean Dipole (IOD). This is in marked contrast to past ocean-only studies. The coupled feedback leads to cooler conditions over the west Indian Ocean, and an anomalous zonal atmospheric pressure gradient that enhances the advection of warm moist air toward south Asia and Australia. This leaves the African continent significantly drier, and much of Australia and southern Asia significantly wetter, in response to the opening of the ITF. Given the substantial variability in the ITF, the likelihood that it will change in the future, and the restriction of the throughflow in past climate eras, this could have important implications for past, present and future regional rainfall patterns for Indian Ocean rim nations.