Editing IPCC:AR6/WGI/Chapter-4 (section)

==== 4.5.3.3 Indian Ocean Basin and Dipole Modes ====

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In the mid- to long-term, projected climate mean state changes in the tropical Indian Ocean are expected to resemble a positive IOD state, with faster warming in the west compared to the east ( [[#Cai--2013|Cai et al., 2013]] ; X.-T. [[#Zheng--2013|]] [[#Zheng--2013|Zheng et al., 2013]] ). However, it was argued that this projected mean state change could be due to the large mean state biases in the simulated current climate and potentially not a realistic outcome (G. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ). Mean state biases also lead to lack of consensus on projected equatorial Indian Ocean SST variability and equatorial modes of climate variability independent of the IOD ( [[#DiNezio--2020|DiNezio et al., 2020]] ). If mean state change will indeed resemble a positive IOD state, however, this would lead to a reduction in the amplitude difference between positive and negative IOD events, but with no robust change in IOD frequency ( [[#Cai--2013|Cai et al., 2013]] ). For a small subset of CMIP5 models that simulate IOD events best, a slight increase in IOD frequency was found under the CMIP5 RCP4.5 scenario ( [[#Chu--2014|Chu et al., 2014]] ).

However, it was also found that the frequency of extreme positive IOD events, which exhibit the largest climate impacts, might increase by a factor of about three under the CMIP5 RCP8.5 scenario ( [[#Cai--2014b|Cai et al., 2014b]] ). Partially consistent with the above result, a more recent study by [[#Cai--2021|Cai et al. (2021)]] , based on CMIP5 RCP8.5 and CMIP6 SSP5-8.5 simulations, shows a robust increased SST variability of large positive IOD events, but a decreased variability of moderate IOD events. An approximate doubling of these extreme positive IOD events was still found for global warming of 1.5°C warming above pre-industrial levels, without a projected decline thereafter ( [[#Cai--2018b|Cai et al., 2018b]] ). These results depend, however, on the realism of the projected mean state change in the Indian Ocean (G. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ).

To conclude, the forced change in IOD in mid- and long-term future remains uncertain due to limited lines of evidence and its dependence on model mean biases. However, there is ''low confidence'' that the frequency of extreme positive IOD events will increase under the high-emissions scenario of SSP5-8.5.

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