Arctic warming in ERA-Interim and other analyses

Near-surface and lower-tropospheric warming of the Arctic over the past 35 years is examined for several datasets. The new estimate for the near surface reported by Cowtan and Way in 2014 agrees reasonably well with the ERA-Interim reanalysis for this region. Both provide global averages with a little more warming over recent years than indicated by the widely used HadCRUT4 dataset, which has sparse coverage of the high Arctic. ERA-Interim is more sensitive than the Cowtan and Way estimate to the state of the underlying Arctic Ocean. Observational coverage of the Arctic varies considerably over the period. Surface air-temperature data of identified types are generally fitted well by ERA-Interim, especially data from ice stations, which appear of excellent quality. ERA-Interim nevertheless has a warm wintertime bias over sea-ice. Mean fits vary in magnitude as coverage varies, but their overall changes are much smaller than analysed temperature changes. This is also largely the case for fits to data for the free troposphere. Much of the information on trends and low-frequency variability provided by ERA-Interim comes from its background forecast, which carries forward information assimilated from a rich variety of earlier observations, rather than from its analysis of surface air-temperature observations. ERA-Interim agrees quite well with the new JRA-55 reanalysis, and with the MERRA reanalysis until recent years when MERRA exhibits weaker surface warming. Temperatures vary coherently between the surface and middle troposphere, with largest amplitude at the surface except in summer, when air temperatures are constrained by sea-ice and open-sea temperatures that differ little from 0 degrees C. Much of the recent near-surface warming of the Arctic is associated with reduced cold-season sea-ice cover, with low temperatures over ice replaced by much higher ones over open sea. This occurs primarily in a relatively well-observed region around the northernmost islands of Europe and western Asia.