Editing IPCC:AR6/WGI/Chapter-7 (section)

===== 7.4.4.2.2 Tropical Pacific temperature gradients in past high-CO 2 climates =====

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The AR5 stated that paleoclimate proxies indicate a reduction in the longitudinal SST gradient across the equatorial Pacific during the Mid-Pliocene Warm Period (MPWP; [[#Masson-Delmotte--2013|Masson-Delmotte et al., 2013]] ; see Cross-Chapter Box 2.1 and Cross-Chapter Box 2.4 in this Report). This assessment was based on SST reconstructions between two sites situated very close to the equator in the heart of the western Pacific warm pool and eastern Pacific cold tongue, respectively. Multiple SST reconstructions based on independent paleoclimate proxies generally agreed that during the Pliocene the SST gradient between these two sites was reduced compared with the modern long-term mean ( [[#Wara--2005|Wara et al., 2005]] ; [[#Dekens--2008|Dekens et al., 2008]] ; [[#Fedorov--2013|Fedorov et al., 2013]] ).

Since AR5, the generation of new SST records has led to a variety of revised gradient estimates, specifically the generation of a new record for the warm pool ( [[#Zhang--2014|Zhang et al., 2014]] ), the inclusion of SST reconstructions from sites in the South China Sea as warm pool estimates ( [[#O’Brien--2014|O’Brien et al., 2014]] ; [[#Zhang--2014|Zhang et al., 2014]] ), and the inclusion of several new sites from the eastern Pacific as cold tongue estimates ( [[#Zhang--2014|Zhang et al., 2014]] ; [[#Fedorov--2015|Fedorov et al., 2015]] ). Published estimates of the reduction in the longitudinal SST difference for the Late Pliocene, relative to either Late Quaternary (0–0.5 million years ago) or pre-industrial values, include 1°C to 1.5°C ( [[#Zhang--2014|Zhang et al., 2014]] ), 0.1°C to 1.9°C ( [[#Tierney--2019|Tierney et al., 2019]] ), and about 3°C ( [[#Ravelo--2014|Ravelo et al., 2014]] ; [[#Fedorov--2015|Fedorov et al., 2015]] ; [[#Wycech--2020|Wycech et al., 2020]] ). All of these studies report a further weakening of the longitudinal gradient based on records extending into the Early Pliocene. While these revised estimates differ in magnitude due to differences in the sites and SST proxies used, they all agree that the longitudinal gradient was weaker, and this is supported by the probabilistic approach of [[#Tierney--2019|Tierney et al. (2019)]] . However, given that there are currently relatively few western equatorial Pacific records from independent site locations, and due to uncertainties associated with the proxy calibrations ( [[#Haywood--2016a|Haywood et al., 2016a]] ), there is only ''medium confidence'' that the average longitudinal gradient in the tropical Pacific was weaker during the Pliocene than during the Late Quaternary.

To avoid the influence of local biases, changes in the longitudinal temperature difference within Pliocene model simulations are typically evaluated using domain-averaged SSTs within chosen east and west Pacific regions and as such there is sensitivity to methodology. Unlike the reconstructed estimates, longitudinal gradient changes simulated by the Pliocene Model Intercomparison Project Phase 1 (PlioMIP1) models do not agree on the change in sign and are reported as spanning approximately –0.5°C to +0.5°C by [[#Brierley--2015|Brierley et al. (2015)]] and approximately –1°C to +1°C by [[#Tierney--2019|Tierney et al. (2019)]] . Initial PlioMIP Phase 2 (PlioMIP2) analysis suggests responses similar to PlioMIP1 ( [[#Feng--2019|Feng et al., 2019]] ; [[#Haywood--2020|Haywood et al., 2020]] ). Models that include hypothetical modifications to cloud albedo or ocean mixing are required to simulate the substantially weaker longitudinal differences seen in reconstructions of the Early Pliocene ( [[#Fedorov--2013|Fedorov et al., 2013]] ; [[#Burls--2014a|Burls and Fedorov, 2014a]] ).

While more western Pacific warm pool temperature reconstructions are needed to refine estimates of the longitudinal gradient, several Pliocene SST reconstructions from the east Pacific indicate enhanced warming in the centre of the eastern equatorial cold tongue upwelling region ( [[#Liu--2019|Liu et al., 2019]] ). This enhanced warming in the east Pacific cold tongue appears to be dynamically consistent with reconstruction of enhanced subsurface warming ( [[#Ford--2015|Ford et al., 2015]] ) and enhanced warming in coastal upwelling regions, suggesting that the tropical thermocline was deeper and/or less stratified during the Pliocene. The Pliocene data therefore suggest that the observed cooling trend over the last 60 years in parts of the eastern equatorial Pacific ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.1.1|Section 9.2.1.1]] and Figure 9.3; [[#Seager--2019|Seager et al., 2019]] ), whether forced or due to internal variability, involves transient processes that are probably distinct from the longer-time scale process ( [[#Burls--2014a|Burls and Fedorov, 2014a]] , b; [[#Luo--2015|Luo et al., 2015]] ; [[#Heede--2020|Heede et al., 2020]] ) that maintained warmer eastern Pacific SST during the Pliocene.

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