Humidity Trends Imply Increased Sensitivity to Clouds in a Warming Arctic
Data from three Arctic measurement sites show how clouds, temperature, and water vapor impact the Arctic surface energy budget.
Amplified warming of the Arctic and coinciding decreases in sea ice are driven in part by perturbations to the surface energy budget. Using data from the Department of Energy’s Atmospheric Radiation Measurement (ARM) Climate Research Facility site at Barrow, Alaska, and two other Arctic measurement sites, scientists studied the relationships among temperature, water vapor, and surface infrared cloud radiative effect (CRE; a measure of cloud insulating properties) in the Arctic. They found that the unique temperature and humidity ranges observed in the Arctic lead to compensating variations in the infrared fluxes at mid- and far-infrared regions of the spectrum. This observation is in contrast to lower latitudes, where CRE is expected to decrease with increasing temperature at constant relative humidity, illustrating a unique sensitivity of the polar regions to climate change.
This work presents the novel finding that at constant relative humidity, CRE does not decrease with increasing temperature or water vapor amount as expected, but is approximately constant for temperatures and water vapor amounts characteristic of the Arctic. This stability is disrupted if relative humidity varies. These findings explain why observed seasonal and regional variability in Arctic CRE do not display the same relationships as seen in the midlatitudes and tropics and have important implications for future changes in cloud feedbacks. Model simulations of future climate indicate that as sea ice decreases resulting in increasing areas of open water in the Arctic, autumn temperatures are expected to increase more than the water vapor amount increases, leading to reductions in relative humidity and increased CRE in this season. These findings illustrate the importance of being able to predict future co-variability of temperature and humidity, as well as changes in cloud cover, to understand how cloud impacts may vary with climate change.
Using observational data, scientists derived three-hour averages of CRE at stations representative of different Arctic regions—Barrow, Alaska; Eureka, Canada; and Summit, Greenland. The observed values of precipitable water vapor (PWV) at these locations span a large range from less than 0.1 cm in winter at Summit to ˜2 cm in summer at Barrow. Over the range of Arctic conditions, CRE in the atmospheric window region (mid-infrared) increases with temperature and PWV while CRE in the far-infrared decreases. When summed, the compensation of the two spectral regions obscures the dependence on temperature and humidity between ˜230 and 280 K, and, thus, explains the lack of correlation in CRE shown in the observations. These compensating flux variations are unique to the temperature and humidity ranges observed in the Arctic. Conversely, CRE increases with temperature below ˜230 K and decreases above ˜280 K. To investigate the consequences of this compensation using an idealized framework, scientists performed radiative transfer calculations with radiosoundings acquired at Barrow and Summit. Owing to the compensation described previously, when summed, CRE has values of constant flux that closely follow the Clausius-Clapeyron relationship. Thus, temporal or spatial variations in temperature and PWV within the Arctic temperature range do not change CRE as long as the variability is consistent with the Clausius-Clapeyron relationship. However, deviations from this relationship will either increase or decrease CRE over a range of ˜40 W m-2 at a given temperature. These results explain some of the observed variability in Arctic CRE observed in other studies.
Output from a reanalysis product (ERA-Interim) and a climate model (Community Earth System Model-Large Ensemble or CESM-LE) is used to provide a conceptual understanding of how future changes in the Arctic system might have an impact on its sensitivity to CRE. Positive anomalies in CRE in both ERA-Interim and CESM-LE emerge in autumn and early winter in the early 2000s. The largest anomalies are projected by CESM-LE to appear after 2040 in autumn. This result is associated with temperature increases in autumn outpacing the expected water vapor increases via the Clausius-Clapeyron relationship. A similar but smaller signal is observed in spring, in part because of less cloud cover and generally thinner clouds during that season.
University of Colorado
C.J.C. acknowledges funding from the Arctic Research Program of the National Oceanic and Atmospheric Administration’s Climate Program Office and Cooperative Institute for Research in Environmental Sciences Visiting Fellowship Program. V.P.W. acknowledges funding from National Science Foundation (NSF) grants ARC-0856773, PLR-1414314, and PLR-1420932. P.M.R. acknowledges funding from NSF award ARC-1108451, USACH-DICYT 041331CC_DAS, and FONDECYT 1151034. M.D.S. acknowledges funding from NSF PLR-1314156 and U.S. Department of Energy DE-SC0011918. Barrow observations are from the U.S. Department of Energy Atmospheric Radiation Measurement Program obtained at the North Slope of Alaska site.
C. J. Cox, V. P. Walden, P. M. Rowe, and M. D. Shupe, “Humidity trends imply increased sensitivity to clouds in a warming arctic.” Nature Communications 6(10117), (2015). [DOI: 10.1038/ncomms10117]