Patterns and Mechanisms of Northeast Pacific Temperature Response to Pliocene Boundary Conditions

Abstract

Ocean‐atmosphere dynamics in the north Pacific play an important role in the global climate system and influence hydroclimate in western North America. However, changes to this region's mean climate under increased atmospheric greenhouse gas concentrations are not well understood. Here we present new alkenone‐based records of sea surface temperature (SST) from the northeast Pacific from the mid‐Piacenzian warm period (approximately 3.3–3.0 Ma), an interval considered to be an analog for near‐future climate under middle‐of‐the‐road anthropogenic emissions. We compare these and other alkenone‐based SST records from the north Pacific to fully‐coupled climate model simulations to examine the impact of mid‐Pliocene CO2 and other boundary conditions on regional climate dynamics and to explore factors governing model disagreement about regional temperature patterns. Model performance varies regionally, with Community Earth System Model 1.2 (CESM 1.2) and CESM2 performing best in regions with greater warming like the California Margin, though these models underestimate the warming evidenced in our new proxy record and others from the region. Single forcing simulations reveal a strong influence for prescribed land surface changes and higher CO2 levels on coastal warming patterns along the California Margin in CESM2. Furthermore, differences in shortwave and longwave radiation and circulation between the models, likely related to changes in the atmospheric component of the model, may play a key role in the ability of models to capture regionally‐varying patterns of Pliocene warmth. Regional patterns of temperature change inferred from geochemical records could therefore help to understand the impacts of different model parameterization schemes on regional climate patterns.

Expansion and Intensification of the North American Monsoon During the Pliocene

“…Disentangling causality in a short instrumental record is challenging, and the cause of MHW‐related SST anomalies, which result from changes in surface radiation, may differ from the cause of the California margin warming in the Pliocene, which could also involve ocean dynamical adjustments (Brennan et al., 2022; Fedorov et al., 2013; Ford et al., 2015; Myers et al., 2018). Despite this, there are clear parallels between our conceptual model of Pliocene NAM changes and MHW‐related NAM changes.…”

“…In contrast, our results suggest that the Pliocene featured enhanced California margin warming compared to the tropical eastern Pacific, amplifying monsoon strength. This enhanced warming in Pliocene simulations may reflect the long‐term influence of Earth system processes related to the cryosphere and vegetation, or long‐term adjustments of deep ocean dynamics that alter SST patterns (Brennan et al., 2022; Fedorov et al., 2013; Feng et al., 2020; Ford et al., 2015; Tierney et al., 2019). In fact, a growing body of literature suggests that the equilibrium response of SST and hydroclimate to greenhouse boundary conditions, especially those that involve long‐term Earth system feedbacks as likely occurred during the Pliocene, may differ fundamentally from responses to transient warming (Burls & Fedorov, 2017; Feng et al., 2022; Sniderman et al., 2019; Zappa et al., 2020).…”

“…The Plio‐Pleistocene portion of the core from DSDP 475 is predominantly composed of hemipelagic muds, transitioning to diatomaceous muds in the mid‐ to early‐Pliocene section, showing evidence of a consistent marine setting for this site from the early Pliocene through the Pleistocene (Curray et al., 1982). Age control primarily comes from biostratigraphic tiepoints (Brennan et al., 2022).…”

Pleistocene shifts in Great Basin hydroclimate seasonality govern the formation of lithium-rich paleolake deposits

“…However, this high latitude warming does not likely reflect a canonical warm PDO-like pattern, which would involve warm SST anomalies extending down the west coast of North America. Farther south, sites on the northern California Margin do not show an excursion to warmer temperatures in this time period (LaRiviere et al, 2012;Brennan et al, 2022). Instead, sites like 1012, 1014, and 1010 show intensifying orbital-scale variability without any evidence of a mean shift to warmer values between 2.0 and 2.6 Ma (Dekens et al, 2007;Brierley et al, 2009).…”

“…Proxy evidence from the Pliocene suggests that the NAM was stronger between 3.5 and roughly 2.0 Ma, and could have contributed to increased lake levels and more mesic vegetation in the southwest (Bhattacharya et al, 2022). Modeling experiments with Pliocene boundary conditions have also helped clarify how other phenomena like atmospheric rivers respond to changes in topography and geography, as well as altered SST patterns (Menemenlis et al, 2021;Brennan et al, 2022).…”