Abstract
The wetness of high-latitude land surfaces is strongly dependent on the difference between precipitation (P) and evapotranspiration (ET). If climate models are to capture the trajectory of surface wetness in high latitudes, they must be able to simulate the seasonality and variations of the surface moisture fluxes, as well as the sensitivities to the variations to the drivers. In this study, a combination of regional climate model output and eddy covariance measurements from flux tower locations in Alaska is used to evaluate model simulations of the surface moisture fluxes and their variations. In particular, we use the model output and the field measurements to test the hypothesis that temperature (T) is the key driver of variations of ET in tundra regions underlain by permafrost, while precipitation plays a greater role in boreal forest areas. Although the model’s hydrologic cycle is stronger (larger P, larger ET) relative to the in situ measurements at all the sites, the prominent seasonal cycles of P, T, and ET are captured by the model. The tower measurements from all sites show a short period (one or two months) of negative P-ET during summer, indicative of surface drying, although the model does not show this period of drying at the inland tundra site. At all the tundra sites, both the flux tower data and the model simulations show that daily and warm-season totals of ET are largely temperature-driven. Daily ET shows a weak negative correlation with precipitation in the measurements and in the model results for all the sites. Precipitation is the main driver of year-to-year variations of the seasonally integrated net moisture flux at all the sites, implying that precipitation will be at least as important as temperature in the future trajectory of surface wetness.
Highlights
Some of the largest and most consequential uncertainties in the trajectory of the Arctic climate system are associated with the hydrology of the Arctic terrestrial surface
We use the model output and the field measurements to test the hypothesis that temperature (T) is the key driver of variations of ET in tundra regions underlain by permafrost, while precipitation plays a greater role in boreal forest areas
The results in the preceding sections lead to the following conclusions:. Both P and ET are considerably larger in the model for all sites, indicating that the model’s hydrological cycle is stronger than the observed. This over-simulation of the high-latitude hydrologic cycle is consistent with the known tendency of global models to simulate more precipitation than is observed in northern high latitudes [28] [29], subject to uncertainties in the observational data
Summary
Some of the largest and most consequential uncertainties in the trajectory of the Arctic climate system are associated with the hydrology of the Arctic terrestrial surface. The results of these studies highlight the uncertainty in the trajectory of surface wetness in Arctic land areas in the present generation of global climate models. Because direct measurements of these quantities are greatly limited compared to T, P, and ET, the diagnostic evaluation in the present study focuses on P and T as key drivers Against this backdrop of prior studies, hypotheses and data constraints, we address two key questions: 1) How well are the surface moisture fluxes over timescales of days to seasons captured by a state-of-the-art regional climate model?.
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