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A Model-Model and Data-Model Comparison for the Early Eocene Hydrological CycleA 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.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Carmichael, Matthew J.
(Bristol Univ. United Kingdom)
Lunt, Daniel J.
(Bristol Univ. United Kingdom)
Huber, Matthew
(New Hampshire Univ. Durham, NH, United States)
Heinemann, Malte
(Kiel Univ. Germany)
Kiehl, Jeffrey
(University Corp. for Atmospheric Research Boulder, CO, United States)
LeGrande, Allegra
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Loptson, Claire A.
(Bristol Univ. United Kingdom)
Roberts, Chris D.
(MET Office (Meteorological Office) Exeter, United Kingdom)
Sagoo, Navjit
(Bristol Univ. United Kingdom)
Shields, Christine
(University Corp. for Atmospheric Research Boulder, CO, United States)
Date Acquired
February 28, 2017
Publication Date
February 25, 2016
Publication Information
Publication: Climate of the Past
Volume: 12
Issue: 2
Subject Category
Geosciences (General)
Meteorology And Climatology
Report/Patent Number
Funding Number(s)
Distribution Limits
Boundary conditions
Atmospheric general circulation models
Hydrological cycle
Climate change

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