title: Nonlinear processes reinforce extreme Indian Ocean Dipole events.
abstract: Under global warming, climate models show an almost three-fold increase in extreme positive Indian Ocean Dipole (pIOD) events by 2100. These extreme pIODs are characterised by a westward extension of cold sea surface temperature anomalies (SSTAs) which push the downstream atmospheric convergence further west. This induces severe drought and flooding in the surrounding countries, but the processes involved in this projected increase have not been fully examined. Here we conduct a detailed heat budget analysis of 19 models from phase 5 of the Coupled Model Intercomparison Project and show that nonlinear zonal and vertical heat advection are important for reinforcing extreme pIODs. Under greenhouse warming, these nonlinear processes do not change significantly in amplitude, but the frequency of occurrences surpassing a threshold increases. This is due to the projected weakening of the Walker circulation, which leads to the western tropical Indian Ocean warming faster than the east. As such, the magnitude of SSTAs required to shift convection westward is relatively smaller, allowing these convection shifts to occur more frequently in the future. The associated changes in wind and ocean current anomalies support the zonal and vertical advection terms in a positive feedback process and consequently, moderate pIODs become more extreme-like.
authors.affiliation: 1] CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia [2] School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Ng
authors.affiliation: CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia.
authors.forename: Wenju
authors.initials: W
authors.lastname: Cai
authors.affiliation: School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia.
authors.forename: Kevin
authors.initials: K
authors.lastname: Walsh
authors.affiliation: Australian Research Council Centre of Excellence for Climate System Science, University of New South Wales, Sydney, New South Wales, Australia.
authors.forename: Agus
authors.initials: A
authors.lastname: Santoso