Multiple lines of evidence suggest that southwestern North America (SWNA), like many subtropical continents, was much wetter during the Pliocene epoch, a climate interval featuring reduced ice volume and CO 2 concentrations above preindustrial levels (Figure 1). Sedimentological data document widespread perennial and ephemeral lakes in southern California and Arizona (Pound et al., 2014;Ibarra et al., 2018) (Figure 1), and palynological and macrobotanical evidence from southern California suggests expanded tree cover and the presence of species that today only grow in regions with mesic conditions and summer rainfall (Ballog & Malloy, 1981;Remeika et al., 1988). Faunal remains from Baja California contain Crocodylus spp. fossils, which require freshwater habitats, further suggesting increased water resources in regions that are arid at present (Miller, 1980;Salzmann et al., 2009). At face value, this evidence for a wet Pliocene is at odds with the theoretical and model-derived prediction that regions like SWNA, and subtropical continents more broadly, will continue to dry in coming centuries as a result of elevated greenhouse gases (Byrne & O' Gorman, 2015;Seager et al., 2010).
Earth's hydrological cycle is expected to intensify in response to global warming, with a 'wet-gets-wetter, dry-gets-drier' response anticipated. The subtropics (˜15-30°N/S) are predicted to become drier, yet proxy evidence from past warm climates suggests these regions may be characterised by wetter conditions. Here we use an integrated data-modelling approach to reconstruct global-and regional-scale rainfall patterns during the early Eocene (˜48-56 million years ago), with an emphasis on the subtropics. Model-derived precipitation-evaporation (P -E ) estimates in the tropics (0-15°N/S) and high latitudes (>60°N/S) are positive and increase in response to higher temperatures, whereas model-derived P -E estimates in the subtropics (15-30°N /S) are negative and decrease in response to higher temperatures. This is consistent with a 'wet-gets-wetter, dry-gets-drier' response. However, some DeepMIP model simulations predict increasing -rather than decreasing -subtropical precipitation at higher temperatures (e.g., CESM, GFDL). Using moisture budget diagnostics we find that the models with higher subtropical precipitation are characterised by a reduction in the strength of subtropical moisture circulation due to weaker meridional temperature gradients. These model simulations (e.g., CESM, GFDL) agree more closely with various proxy-derived climate metrics and imply a reduction in the strength of subtropical moisture circulation during the early Eocene. Although this was Posted on 23 Nov 2022 -CC-BY 4.0 -https://doi.org/10.1002/essoar.10512308.1 -This a preprint and has not been peer reviewed. Data may be preliminary.insufficient to induce subtropical wetting, if the meridional temperature was weaker than suggested by the DeepMIP models, this may have led to wetter subtropics. This highlights the important role of the meridional temperature gradient when predicting past (and future) rainfall patterns.
Earth's hydrological cycle is expected to intensify in response to global warming, with a "wet-gets-wetter, dry-gets-drier" response anticipated over the ocean. Subtropical regions (∼15°-30°N/S) are predicted to become drier, yet proxy evidence from past warm climates suggests these regions may be characterized by wetter conditions. Here we use an integrated data-modeling approach to reconstruct global and zonal-mean rainfall patterns during the early Eocene (∼56-48 million years ago). The Deep-Time Model Intercomparison Project (DeepMIP) model ensemble indicates that the mid-(30°-60°N/S) and high-latitudes (>60°N/S) are characterized by a thermodynamically dominated hydrological response to warming and overall wetter conditions. The tropical band (0°-15°N/S) is also characterized by wetter conditions, with several DeepMIP models simulating narrowing of the Inter-Tropical Convergence Zone. However, the latter is not evident from the proxy data. The subtropics are characterized by negative precipitation-evaporation anomalies (i.e., drier conditions) in the DeepMIP models, but there is surprisingly large inter-model variability in mean annual precipitation (MAP). Intriguingly, we find that models with weaker meridional temperature gradients (e.g., CESM, GFDL) are characterized by a reduction in subtropical moisture divergence, leading to an increase in MAP. These model simulations agree more closely with our new proxy-derived precipitation reconstructions and other key climate metrics and imply that the early Eocene was characterized by reduced subtropical moisture divergence. If the meridional temperature gradient was even weaker than suggested by those DeepMIP models, circulation-induced changes may have outcompeted thermodynamic changes, leading to wetter subtropics. This highlights the importance of accurately reconstructing zonal temperature gradients when reconstructing past rainfall patterns. Plain Language SummaryAs the world warms, the atmosphere is able to hold more moisturehowever, this moisture will not fall evenly across the globe. Some regions are expected to become wetter, whereas other regions will become drier. This is the basis of the familiar paradigm "wet-gets-wetter, dry-gets-drier" and is largely supported by future model projections. However, evidence from the geological record contradicts this hypothesis and suggests that a warmer world could be characterized by wetter (rather than drier) subtropics. Here, we use an integrated data-modeling approach to investigate the hydrological response to warming during an ancient warm interval (the early Eocene, 56-48 million years ago). We show that models with weaker latitudinal temperature gradients are characterized by a reduction in subtropical CRAMWINCKEL ET AL.
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
<p>The Pliocene epoch offers insights into future climate change, with near-modern atmospheric pCO<sub>2</sub> and global mean surface temperature estimated to be 3-4&#176;C above pre-industrial. The discrepancy in the hydrological response seen between simulations of future global warming and early Pliocene simulations is hypothesized to result from reduced SST gradients in the early Pliocene. However, the interpretation of Pliocene SST proxies is still debated, generating uncertainty about the reduced gradient scenario. One avenue toward reducing uncertainty in Pliocene warming patterns is to establish the degree of dynamical consistency between Pliocene SST reconstructions and hydrological cycle reconstructions. To this end, hydrological cycle reconstructions are needed in regions where water isotopic signals are predicted to be uniquely sensitive to Pliocene SST gradient changes. Here, we seek to identify these regions using an isotope-enabled GCM, iCAM5, to model the distribution of water isotopes in precipitation in response to four climatological SST and sea-ice fields representing modern, abrupt 4xCO&#173;&#173;<sub>2</sub>, late Pliocene and early Pliocene climates. We identify two regions with distinct precipitation isotope fingerprints resulting from early Pliocene SST gradients. The first region, the Indo-Pacific warm pool, is characterized by isotopic enrichment due to weakened convection and a reduced amount effect. The second region, the Sahel, is characterized by isotopic depletion due to more intense and widespread precipitation. A model-proxy comparison with available precipitation proxies in Africa provides promising initial results. However, additional proxy reconstructions are needed in both target regions to provide robust tests of dynamical consistency with current early Pliocene SST reconstructions.</p>
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