The Spectral Dimension of Arctic Outgoing Longwave Radiation and Greenhouse Efficiency Trends from 2003-2016

Fourteen years of spectral fluxes derived from collocated AIRS and CERES observations are used in conjunction with AIRS retrievals to examine the trends of zonal-mean spectral outgoing longwave radiation (OLR) and greenhouse efficiency (GHE) in the Arctic. AIRS retrieved profiles are fed into a radiative transfer model to generate synthetic clear-sky spectral OLR. Trends are derived from the simulated clear-sky spectral OLR and GHE and then compared with their counterparts derived from collocated observations. Spectral trends in different seasons are distinctively different. March and September exhibit positive trends in spectral OLR over the far-IR dirty window and mid-IR window region for most of the Arctic. In contrast, spectral OLR trends in July are negative over the far-IR dirty window and can be positive or negative in the mid-IR window depending on the latitude. Sensitivity studies reveal that surface temperature contributes much more than atmospheric temperature and humidity to the spectral OLR and GHE trends, while the contributions from the latter two are also discernible over many spectral regions (e.g., trends in the far-IR dirty window in March). The largest increase of spectral GHE is seen north of 80◦N in March across the water vapor v2 band and far-IR. When the secular fractional change of spectral OLR is less than that of surface spectral emission, an increase of spectral GHE can be expected. Spectral trend analyses reveal more information than broadband trend analyses alone.