Editing IPCC:AR6/WGI/Chapter-3 (section)

==== 3.5.1.1 Sea Surface and Zonal Mean Ocean Temperature Evaluation ====

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In CMIP3 and CMIP5 models, large SST biases were found in the mid- and high latitudes ( [[#Flato--2013|Flato et al., 2013]] ). In CMIP6, the Northern Hemisphere mid-latitude surface temperature biases appear to be marginally improved in the multi-model mean when contrasted to CMIP5 despite large biases remaining in a few models (Figures 3.23a and 3.24). There is a decreased spread of the zonal mean SST error between 50°N and 30°S, relative to CMIP5 (Figure 3.24a). On the other hand, the Southern Ocean’s warm surface temperature bias remains (Figure 3.23a; [[#Beadling--2020|Beadling et al., 2020]] ), and is on average larger in CMIP6 than in CMIP5 models (Figures 3.23a and 3.24). This warm bias is often associated with persistent overlying atmospheric cloud biases ( [[#Hyder--2018|Hyder et al., 2018]] ). Several other large biases also appear to remain largely unresolved in CMIP6, particularly warm biases in excess of 1°C along the equatorial eastern continental boundaries of the tropical Atlantic and Pacific Oceans (Figure 3.23a).

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[[File:75e0df52ad6c606b3aafe157c00f8761 IPCC_AR6_WGI_Figure_3_23.png]]

Figure 3.23 | '''Multi-model mean bias of (a) sea surface temperature and (b) near-surface salinity, defined as the difference between the CMIP6 m''' '''ulti-mo''' '''del mean and the climatology from the World Ocean Atlas 2018.''' The CMIP6 multi-model mean is constructed with one realization of 46 CMIP6 historical experiments for the period 1995–2014 and the climatology from the World Ocean Atlas 2018 is an average over all available years (1955–2017). Uncertainty is represented using the advanced approach: No overlay indicates regions with robust signal, where ≥66% of models show change greater than the variability threshold and ≥80% of all models agree on sign of change; diagonal lines indicate regions with no change or no robust signal, where &lt;66% of models show a change greater than the variability threshold; crossed lines indicate regions with conflicting signal, where ≥66% of models show change greater than the variability threshold and &lt;80% of all models agree on sign of change. For more information on the advanced approach, please refer to Cross-Chapter Box Atlas.1. Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

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'''Figure 3.24 |''' '''Biases in zonal mean and equatorial sea surface temperature (SST) in CMIP5 and CMIP6 models.''' CMIP6 (red), CMIP5 (blue) and HighResMIP (green) multi-model mean '''(a)''' zonally averaged SST bias; '''(b)''' equatorial SST bias; and '''(c)''' equatorial SST compared to observed mean SST (black line) for 1979–1999. The inter-model 5th and 95th percentiles are depicted by the respective shaded range. Model climatologies are derived from the 1979–1999 mean of the historical simulations, using one simulation per model. The Hadley Centre Sea Ice and Sea Surface Temperature version 1 (HadISST) ( [[#Rayner--2003|Rayner et al., 2003]] ) observational climatology for 1979–1999 is used as the reference for the error calculation in (a) and (b); and for observations in (c). Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

Overall, the simulated and observed trends in SST patterns are generally consistent for the historical period ( [[#Olonscheck--2020|Olonscheck et al., 2020]] ). The CMIP6 models generally represent the observed pattern of trends better than the CMIP5 models, and observed trends fall within the range of simulated trends over a larger area for CMIP6 models than for CMIP5 models ( [[#Olonscheck--2020|Olonscheck et al., 2020]] ).

The CMIP5 multi-model mean zonally averaged subsurface ocean temperature showed warm biases between 200 m and 2000 m (mid-depth) over most latitudes, with exceptions in the Southern Ocean (&gt;60°S, 100–2000 m) and upper (0–400 m) Arctic Ocean. Cold biases were simulated near the surface (0–200 m) at most latitudes ( [[#Flato--2013|Flato et al., 2013]] ). CMIP6 biases are broadly consistent with those reported in CMIP5 for the near-surface (&lt;200 m) and mid-depth (200–2000 m) ocean ( [[#Voldoire--2019b|Voldoire et al., 2019b]] ; [[#Beadling--2020|Beadling et al., 2020]] ; [[#Zhu--2020|]] [[#Zhu--2020|Y. Zhu et al., 2020]] ). The warm bias begins between 100 and 400 m depth in all three basins, however, it is most prominent in the Atlantic Ocean, with a maximum magnitude in the equatorial latitudes, as in CMIP5 (Figure 3.25). In the Pacific, the large warm biases are mostly seen in the subtropical regions (30°N–60°N and 30°S–60°S). The cool near surface tropical bias is most prominent in the Pacific Ocean and also present in the Atlantic, with a smaller magnitude (Figure 3.25). Relative to CMIP5, the most prominent difference is an increase to the mid-depth (300–2000 m) warm bias in CMIP6 and a change in sign of the bias from cold to warm for the Southern Ocean mid-depth (&gt;60°S) from CMIP5 to CMIP6 (Figure 3.25). Compared to CMIP3 and CMIP5, there is improved agreement between most CMIP6 models and observations in their representation of the zonal mean temperature of the upper 100 m of the Southern Ocean ( [[#Beadling--2020|Beadling et al., 2020]] ).

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'''Figure 3.25 |''' '''CMIP6 potential temperature and salinity biases for the global ocean, Atlantic Ocean, Pacific Ocean and Indian Ocean.''' Shown in colour are the time-mean differences between the CMIP6 historical multi-model climatological mean and observations, zonally averaged for each basin (excluding marginal and regional seas). The observed climatological values are obtained from the World Ocean ( [[IPCC:Wg1:Chapter:Atlas|Atlas]] 2018 (WOA18, 1981–2010; Prepared by the Ocean Climate Laboratory, National Oceanographic Data Center, Silver Spring, MD, USA), and are shown as labelled black contours for each of the basins. The simulated annual mean climatologies for 1981 to 2010 are calculated from available CMIP6 historical simulations, and the WOA18 climatology utilized synthesized observed data from 1981 to 2010. Output from a total of 30 available CMIP6 models is used for the temperature panels (left column) and 28 models for the salinity panels (right column). Potential temperature units are °C and salinity units are the Practical Salinity Scale 1978 [PSS-78]. Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

Focusing on the deep ocean (&gt;2000 m), the CMIP6 ensemble mean shows a prominent and consistent warm bias (Figure 3.25), in all basins except the equatorial and northern Pacific, which contrasts to a cold bias seen in CMIP5 ( [[#Flato--2013|Flato et al., 2013]] ). We note that while an updated observational temperature dataset is used in this assessment (WOA09 was used in AR5, while WOA18, 1981–2010 is used in AR6), the deep-ocean warm bias remains and is approaching double the magnitude (about 0.5°C) of the equivalent CMIP5 multi-model mean bias, a feature which is particularly prominent in the Atlantic and southern Indian Oceans. Increased horizontal resolution as well as the choice of the vertical coordinate are reported to partly improve these biases in some models ( [[#Adcroft--2019|Adcroft et al., 2019]] ; [[#Rackow--2019|Rackow et al., 2019]] ; [[#Hewitt--2020|Hewitt et al., 2020]] ).

Since AR5, there has been growing evidence that the representation of mean surface and deeper ocean temperatures in coupled climate models can be improved by increasing the horizontal resolution both in the ocean and the atmosphere (e.g., [[#Small--2014|Small et al., 2014]] ; [[#Hewitt--2016|Hewitt et al., 2016]] ; [[#Iovino--2016|Iovino et al., 2016]] ; [[#Roberts--2019|Roberts et al., 2019]] ). At an ocean resolution of around 1°, which is typical of CMIP6 models, some processes are parameterized rather than explicitly resolved, leading to a compromise in their dynamical representation. An increase in the model resolution allows for processes to be explicitly resolved, and can for example, enhance the simulation of eddies, thus improving simulated vertical eddy transport, and reducing temperature drifts in the deeper ocean ( [[#Griffies--2015|Griffies et al., 2015]] ; [[#von%20Storch--2016|von Storch et al., 2016]] ). For some models, the mean absolute error in ocean temperature below 500 m is smaller in the high resolution version compared to the low resolution version, particularly in eddy-active regions such as the North Atlantic ( [[#Rackow--2019|Rackow et al., 2019]] ). Increasing the horizontal resolution of individual climate models often leads to an overall decrease in the surface temperature biases over regions where they persisted through earlier CMIP generations, such as the central and western equatorial Pacific, as well as the North and tropical Atlantic (Figure 3.3e; [[#Roberts--2019|Roberts et al., 2019]] ; [[#Hewitt--2020|Hewitt et al., 2020]] ). Despite this, as a group the four HighResMIP models included in Figures 3.3e and 3.24 do not on average show smaller SST biases than the CMIP6 multi-model mean, demonstrating the importance of factors other than resolution in contributing to SST biases.

In summary, there is little improvement in the multi-model mean sea surface and zonal mean ocean temperatures from CMIP5 to CMIP6 ( ''medium confidence'' ). Nevertheless, the CMIP6 models show a somewhat more realistic pattern of SST trends ( ''low confidence'' ).

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