A Pliocene Precipitation Isotope Proxy‐Model Comparison Assessing the Hydrological Fingerprints of Sea Surface Temperature Gradients

Abstract

Global warming is predicted to exaggerate the modern patterns of precipitation minus evaporation, such that the tropics get wetter and the subtropics get drier under what is referred to as the "thermodynamic effect" (Held & Soden, 2006;Seager et al., 2010). Assuming relative humidity remains roughly the same, an assumption that does not hold well over land (Byrne & O'Gorman, 2015), the Clausius-Clapeyron relation predicts 7% more water vapor per degree Celsius of warming. If atmospheric circulation remains unchanged, this thermodynamic effect results in the more efficient transport of moisture from the subtropics into the tropics. However, a slight reduction of large-scale circulation strength, the "dynamic effect," is predicted to partially counteract this thermodynamic effect (Held & Soden, 2006;Seager et al., 2010). Changes to the hydrological cycle predicted by climate models appear dominated by the thermodynamic effect in response to near future warming (Seager et al., 2010) and an abrupt quadrupling of pre-industrial CO 2 levels (Burls & Fedorov, 2017). Results from Phase 6 of the Coupled Model Intercomparison Project (CMIP6) predict global monsoon precipitation will increase by the end of the twenty-first century (Wang et al., 2021). CMIP6 results also predict that regional changes in precipitation over the oceans will be affected by the uneven heating of the ocean surfaces (Xie, 2020).A complementary perspective on how the hydrological cycle might change under global warming can be gained by examining past warm climates, such as the Pliocene. The Pliocene (5.3-2.6 million years ago; Mya) had a similar continental configuration and included times when atmospheric pCO 2 approached modern values (∼400 ppm) (Martínez-Botí et al., 2015). Global mean surface temperature (GMST) estimates from reconstructions of deep ocean temperature indicate an early (∼4-5 Mya) Pliocene GMST about 3°C warmer than pre-industrial, cooling by about 1-2°C in the late (∼3 Mya) Pliocene (Hansen et al., 2013). These GMST estimates make both

Plio‐Pleistocene Southwest African Hydroclimate Modulated by Benguela and Indian Ocean Temperatures

“…The preindustrial control simulation features fixed SSTs and sea ice derived from the 1970-1980 climatological average (Hurrell et al, 2008). Simulated preindustrial δD p patterns agree with the modern SWA isoscape (Kaseke et al, 2016;Knapp et al, 2022b). The second is an early (5-4 Ma) "Pliocene-like" simulation, with SSTs and sea ice fields taken from an idealized experiment with perturbed cloud albedo (Burls & Fedorov, 2014).…”

“…The second is an early (5–4 Ma) “Pliocene‐like” simulation, with SSTs and sea ice fields taken from an idealized experiment with perturbed cloud albedo (Burls & Fedorov, 2014). All other boundary conditions, including paleogeography, are set to modern values, allowing us to isolate the influence of SST patterns on regional hydroclimate (Knapp et al., 2022b).…”

“…Model simulations used in this paper are previously published in Knapp et al. (2022b) and available at https://doi.org/10.5281/zenodo.6953979 (Knapp et al., 2022a). The PlioMIP2 simulations analyzed in this study that are part of the Climate Model Intercomparison Project 6 have been deposited to the Earth System Grid Federation.…”

“…Newly generated data sets from ODP 1081 are available on the NOAA/NCEI Paleoclimatology Database (https:// www.ncei.noaa.gov/access/paleo-search/study/37258) (Rubbelke et al, 2023). Model simulations used in this paper are previously published in Knapp et al (2022b) and available at https://doi.org/10.5281/zenodo.6953979 (Knapp et al, 2022a). The PlioMIP2 simulations analyzed in this study that are part of the Climate Model Intercomparison Project 6 have been deposited to the Earth System Grid Federation.…”

Plio-Pleistocene evolution of westerly moisture transport into the northern tropical Andes

“…We investigated the influence of equatorial Pacific sea surface temperatures on water isotopes in precipitation in the northern tropical Andes with the isotope-enabled Community Atmospheric Model 5 (iCAM5 Brady et al, 2019;Knapp et al, 2022b). The simulation was initialized using sea surface temperature fields from a Community Earth System Model version 1 simulation that best-captured proxy sea surface temperature reconstructions in the equatorial Pacific (Burls and Fedorov, 2014), particularly the reduced zonal temperature gradient of the early Pliocene (Knapp et al, 2022b). The applied sea surface temperature field coheres with Mg/Ca sea surface temperature reconstructions that suggest a weakened zonal gradient in sea surface temperatures in the tropical Pacific before 3 Ma (i.e.…”