Editing IPCC:AR6/WGI/Chapter-1 (section)

==== 1.5.2.3 Ocean Reanalyses ====

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Since AR5, ocean reanalyses have improved due to: increased model resolution ( [[#Zuo--2017|Zuo et al., 2017]] ; [[#Lellouche--2018|Lellouche et al., 2018]] ; [[#Heimbach--2019|Heimbach et al., 2019]] ); improved physics ( [[#Storto--2019|Storto et al., 2019]] ); improvements in the atmospheric forcing from atmospheric reanalyses (see [[#1.5.2.1.3|Section 1.5.2.1.3]] ); and improvements in the data quantity and quality available for assimilation (e.g., [[#Lellouche--2018|Lellouche et al., 2018]] ; [[#Heimbach--2019|Heimbach et al., 2019]] ), particularly due to Argo observations (Annex I; [[#Zuo--2019|Zuo et al., 2019]] ).

The first Ocean Reanalyses Intercomparison project (ORA-IP; [[#Balmaseda--2015|Balmaseda et al., 2015]] ) focussed on the uncertainty in key climate indicators, such as ocean heat content ( [[#Palmer--2017|Palmer et al., 2017]] ), thermosteric sea level ( [[#Storto--2017|Storto et al., 2017]] , 2019), salinity ( [[#Shi--2017|Shi et al., 2017]] ), sea ice extent ( [[#Chevallier--2017|Chevallier et al., 2017]] ), and the AMOC ( [[#Karspeck--2017|Karspeck et al., 2017]] ). Reanalysis uncertainties occur in areas of inhomogeneous or sparse observational data sampling, such as for the deep ocean, the Southern Ocean, and western boundary currents ( [[#Lellouche--2018|Lellouche et al., 2018]] ; [[#Storto--2019|Storto et al., 2019]] ). Intercomparisons have also been dedicated to specific variables such as mixed-layer depths ( [[#Toyoda--2017|Toyoda et al., 2017]] ), eddy kinetic energy, globally ( [[#Masina--2017|Masina et al., 2017]] ) and in the polar regions ( [[#Uotila--2019|Uotila et al., 2019]] ). [[#Karspeck--2017|Karspeck et al. (2017)]] found disagreement in the AMOC variability and strength in reanalyses over observation-sparse periods, whereas [[#Jackson--2019|Jackson et al. (2019)]] reported a lower spread in AMOC strength across an ensemble of ocean reanalyses of the recent period (1993–2010), linked to improved observation availability for assimilation. Reanalyses also have a larger spread of ocean heat uptake than data-only products and can produce spurious overestimates of heat uptake ( [[#Palmer--2017|Palmer et al., 2017]] ), which is important in the context of estimating climate sensitivity ( [[#Storto--2019|Storto et al., 2019]] ). The ensemble approach for ocean reanalyses provides another avenue for estimating uncertainties across ocean reanalyses ( [[#Storto--2019|Storto et al., 2019]] ).

While there are still limitations in their representation of oceanic features, ocean reanalyses add value to products based only on observation, and are used to inform assessments in AR6 (Chapters 2, 3, 7 and 9). Reanalyses of the atmosphere or ocean alone may not account for important atmosphere–ocean coupling, motivating the development of coupled reanalyses ( [[#Laloyaux--2018|Laloyaux et al., 2018]] ; [[#Schepers--2018|Schepers et al., 2018]] ; [[#Penny--2019|Penny et al., 2019]] ), but these are not assessed in AR6.

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