Editing IPCC:AR6/WGI/Chapter-2 (section)

=== 2.4.3 Indian Ocean Basin and Dipole Modes ===

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The AR5 did not provide an assessment of the Indian Ocean Dipole (IOD) records based on paleo reconstructions. For the instrumental era, AR5 reported that there were no trends in the IOD behaviour. However, the strength of the Indian Ocean Basin-wide (IOB) mode, quantified by the basin-scale averaged SST index, increased in all assessed periods except 1979–2012, but the AR5 neither quantified trends nor provided a confidence statement.

For the LGM, enhanced equatorial Indian Ocean productivity in marine sediment records was associated with strengthened westerly jets, in line with a shallower central-western Indian Ocean thermocline and stronger negative IOD events ( [[#Punyu--2014|Punyu et al., 2014]] ). The LGM Indian Ocean basin was substantially modified by the exposure of the tropical shelves ( [[#DiNezio--2018|DiNezio et al., 2018]] ) and this has been associated with an Indian Ocean ‘El Niño‘ ( [[#Thirumalai--2019|Thirumalai et al., 2019]] ). [[#Wurtzel--2018|Wurtzel et al. (2018)]] contend that during the LDT, including the Younger Dryas event, Indian Ocean precipitation did not mirror the zonal asymmetry observed in Indian Ocean SSTs in the Holocene based upon a speleothem record from Sumatra, but instead reflected shifts in moisture transport pathways and sources. Using Seychelles corals, representing the western pole of the IOD, spanning from the MH to present, [[#Zinke--2014|Zinke et al. (2014)]] identified changes in seasonality, with the lowest seasonal SST range in the MH and then again around 2 ka, while the largest seasonal range occurred around 4.6 ka and then again during the near-present (1990–2003). Reconstructions from fossil corals for the eastern Indian Ocean point to stronger negative IOD SST anomalies due to the enhanced upwelling and cooling driven by a stronger monsoon with enhanced anomalous easterly winds in the eastern Indian Ocean during the MH ( [[#Abram--2020b|Abram et al., 2020b]] ). [[#Niedermeyer--2014|Niedermeyer et al. (2014)]] from the analysis of stable isotopes in terrestrial plant waxes, suggest that the period 6.5 ka to 4.5 ka was characterized by an anomalously positive IOD mean state. During various parts of the Holocene, periods of a mean positive IOD-like state were associated with increased IOD variability, including events that exceed the magnitude of the strongest events during the instrumental period ( [[#Abram--2020a|Abram et al., 2020a]] ).

From the coral δ <sup>18</sup> O record from the Seychelles over 1854–1994, [[#Du--2014|Du et al. (2014)]] showed a 3–7 year dominant period associated with the IOB in response to ENSO forcing. They identified multi-decadal variability in the IOB, with more active IOB phases during 1870–1890, 1930–1955, and 1975–1992, while decadal variability in the IOB dominated during 1940–1975 ( [[#Du--2014|Du et al., 2014]] ). ''Evidence'' for changes in IOB characteristics during earlier periods (e.g., MH, LGM) is ''limited'' .

The role of decadal to multi-decadal variability has recently emerged as an important aspect of the IOD with many indications of the effects of Pacific Ocean processes on IOD variability through atmospheric and oceanic mechanisms ( [[#Dong--2016|Dong et al., 2016]] ; [[#Krishnamurthy--2016|Krishnamurthy and Krishnamurthy, 2016]] ; [[#Zhou--2017|Zhou et al., 2017]] ; [[#Jin--2018|Jin et al., 2018]] ). Positive events in the 1960s and 1990s were linked to a relatively shallow eastern Indian Ocean thermocline, and a primarily negative IOD state in the 1970s and 1980s was related to a deeper thermocline ( [[#Ummenhofer--2016|Ummenhofer et al., 2016]] ). Positive IOD events may have increased in frequency during the second half of the 20th century ( [[#Abram--2020a|Abram et al., 2020a]] , b). Earlier observations of apparent changes in the frequency and/or magnitude of the IOD events are considered unreliable, particularly prior to the 1960s ( [[#Hernández--2020|Hernández et al., 2020]] ). Although the seasonal evolution and the type of ENSO ( [[#2.4.2|Section 2.4.2]] ) may influence the character of the IOD ( [[#Guo--2015|Guo et al., 2015]] ; [[#Zhang--2015|Zhang et al., 2015]] ; [[#Fan--2016|Fan et al., 2016]] ), the occurrence of some IOD events may be independent of ENSO ( [[#Sun--2015|Sun et al., 2015]] ).

To summarize, there is ''low confidence'' in any multi-decadal IOD variability trend in the instrumental period due to data uncertainties especially before the 1960s. In addition to data uncertainties, understanding of the IOB variability during the instrumental period is also limited by large-scale warming of the Indian Ocean. Neither the IOD nor the IOB have exhibited behaviour outside the range implied by proxy records ( ''low confidence'' ).

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