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

==== 4.5.3.5 Pacific Decadal Variability ====

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The AR5 assessed that there is ''low'' ''confidence'' in projections of future changes in Pacific decadal variability (PDV) due to the inability of CMIP5 models to represent the connection between PDV and Indo-Pacific SST variations. Because the PDV appears to encompass the combined effects of different dynamical processes operating at different time scales, representation of PDV in climate models remains a challenge ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.6|Section 3.7.6]] ) and its long-term evolution under climate change uncertain.

In addition to uncertainty from the future evolution of the mechanisms that determined the PDV, it is also unclear how the background state in the Pacific Ocean will change due to time-varying radiative forcing, and how this change will interact with variability at interannual and low-frequency time scales ( [[#Fedorov--2020|Fedorov et al., 2020]] ). Recent research suggests that the PDV will have a weaker amplitude and higher frequency with global warming ( [[#Zhang--2016|Zhang and Delworth, 2016]] ; [[#Xu--2017|Xu and Hu, 2017]] ; [[#Geng--2019|Geng et al., 2019]] ). The former appears to be associated with a decrease in SST variability and the meridional gradient over the Kuroshio-Oyashio region, with a reduction in North Pacific wind stress and meandering of the subpolar/subtropical gyre interplay ( [[#Zhang--2016|Zhang and Delworth, 2016]] ). The latter is hypothesized to rely on the enhanced ocean stratification and shallower mixed layers of a warmer climate, which would increase the phase speed of the westward-propagating oceanic waves, hence shortening the decadal to inter-decadal component ( [[#Goodman--1999|Goodman and Marshall, 1999]] ; [[#Zhang--2016|Zhang and Delworth, 2016]] ; [[#Xu--2017|Xu and Hu, 2017]] ). The weakening of the PDV in a warmer climate may reduce the internal variability of global mean surface temperature, to which PDV seems associated ( [[#Zhang--1997|Zhang et al., 1997]] ; [[#Kosaka--2016|Kosaka and Xie, 2016]] ; [[#Geng--2019|Geng et al., 2019]] ). Thus, a weaker and higher frequency PDV could reduce the contribution of internal variability to the GSAT trend and eventually lead to a reduced probability of surface-warming hiatus events.

In summary, based on CMIP5, there is ''medium confidence'' that a weaker and higher frequency PDV is expected under global warming.

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