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.
Highlights
Examination of the two nonlinear terms and their individual components reveals that the increased occurrence of extreme events is largely due to changes in the mean state
During extreme pIOD events, nonlinear zonal and vertical advections stimulate a positive feedback process along the equator that reinforces the convection shift. This leads to stronger anomalous easterly winds which in turn drive upwelling equatorial Kelvin waves that increase upwelling off the Sumatra-Java coast
An extreme pIOD is defined when the EOF1 time series is greater than one standard deviation and the EOF2 time series (PC2) is greater than 0.5 standard deviations[8]. This nonlinear relationship between EOF1 and EOF2 means that strong nIOD events cannot be defined using this approach
Summary
An ensemble of 19 CMIP5 models shows that nonlinear zonal and vertical advection are important processes during extreme pIODs, yet under increasing greenhouse gases, the amplitude of these two processes does not change considerably. Examination of the two nonlinear terms and their individual components reveals that the increased occurrence of extreme events is largely due to changes in the mean state. During extreme pIOD events, nonlinear zonal and vertical advections stimulate a positive feedback process along the equator that reinforces the convection shift.
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