A model–model and data–model comparison for the early Eocene hydrological cycle

Abstract

Abstract. A range of proxy observations have recently provided constraints on how Earth's hydrological cycle responded to early Eocene climatic changes. However, comparisons of proxy data to general circulation model (GCM) simulated hydrology are limited and inter-model variability remains poorly characterised. In this work, we undertake an intercomparison of GCM-derived precipitation and P − E distributions within the extended EoMIP ensemble (Eocene Modelling Intercomparison Project; Lunt et al., 2012), which includes previously published early Eocene simulations performed using five GCMs differing in boundary conditions, model structure, and precipitation-relevant parameterisation schemes. We show that an intensified hydrological cycle, manifested in enhanced global precipitation and evaporation rates, is simulated for all Eocene simulations relative to the preindustrial conditions. This is primarily due to elevated atmospheric paleo-CO2, resulting in elevated temperatures, although the effects of differences in paleogeography and ice sheets are also important in some models. For a given CO2 level, globally averaged precipitation rates vary widely between models, largely arising from different simulated surface air temperatures. Models with a similar global sensitivity of precipitation rate to temperature (dP∕dT) display different regional precipitation responses for a given temperature change. Regions that are particularly sensitive to model choice include the South Pacific, tropical Africa, and the Peri-Tethys, which may represent targets for future proxy acquisition. A comparison of early and middle Eocene leaf-fossil-derived precipitation estimates with the GCM output illustrates that GCMs generally underestimate precipitation rates at high latitudes, although a possible seasonal bias of the proxies cannot be excluded. Models which warm these regions, either via elevated CO2 or by varying poorly constrained model parameter values, are most successful in simulating a match with geologic data. Further data from low-latitude regions and better constraints on early Eocene CO2 are now required to discriminate between these model simulations given the large error bars on paleoprecipitation estimates. Given the clear differences between simulated precipitation distributions within the ensemble, our results suggest that paleohydrological data offer an independent means by which to evaluate model skill for warm climates.

Atmospheric circulation and hydroclimate impacts of alternative warming scenarios for the Eocene

“…As discussed in Huber and Caballero (2011), the coupled model reproduces the available proxy temperature reconstructions reasonably well, with no appreciable bias in either the global-mean temperature or the mean equator-pole temperature difference. As shown in Carmichael et al (2016) (where this simulation is referred to as CCSM_H-16x), the precipitation field is also in broad agreement with a global compilation of precipitation proxies, albeit with some mismatches particularly in the Southern Hemisphere high latitudes. This provides confidence that the GHG simulation is a reasonable approximation to the Eocene climate at least in the annual mean.…”

“…This must be considered a substantial increase: Carmichael et al (2016, their Figure 2c) show a robust linear scaling of precipitation with temperature of around 0.06 mm day −1 K −1 , implying that a precipitation increase of this magnitude would require a CO 2 -driven warming in excess of 7 K (comparable to that across the Paleocene-Eocene Thermal Maximum (PETM) hyperthermal event; see Pagani et al, 2014). For comparison, a present-day simulation using CAM3 (Collins et al, 2006a) has a global-mean temperature around 15 K cooler and precipitation around 0.9 mm day −1 lower than the GHG simulation, in agreement with the scaling above.…”

Depositional Evolution of the Western Amundsen Basin, Arctic Ocean: Paleoceanographic and Tectonic Implications

“…The high sedimentation rates may also be linked to increased pelagic deposition associated with high biological productivity (Stein et al, ) that characterizes the early–mid‐Eocene greenhouse climate conditions (Zachos et al, ). Moreover, an intensified hydrological cycle (Carmichael et al, ; Pagani et al, ) resulting in episodic fresh water accumulation (Brinkhuis et al, ) apparently enabled high biological productivity as evidenced by the large quantities of the freshwater fern Azolla in the central Arctic (Brinkhuis et al, ; Speelman et al, ; van der Burgh et al, ) and in adjacent regions (e.g., Collinson et al, ).…”

Orbital forcing of terrestrial hydrology, weathering and carbon sequestration during the Palaeocene-Eocene Thermal Maximum

“…• Dramatic increases in sedimentation rates during the main phase of the PETM are interpreted to represent significant changes in regional hydrology, most likely an increase in the magnitude and frequency of extreme rainfall and runoff events, potentially within an overall lower mean annual precipitation regime (Carmichael et al, 2017;Carmichael et al, 2016). These changes in sedimentation accumulation rates -and inferred perturbations to hydrology -persist throughout the PETM interval, 20 only returning towards pre-event levels during the rapid phase of CIE recovery.…”