We map the timing of the seasonal melt-freeze transitions across the pan-Arctic region land and sea ice north of 60°N from microwave instrument QuikSCAT (1999-2009), eleven CMIP5 GCMs (1850-2100), and two reanalyses (1979-2011, 1871-2010). We evaluate these results against surface observations (1979-2009). In order to retrieve transitions from microwave measurements, we exploit the abrupt changes in the time series associated with major melt-freeze events using a sophisticated algorithm. A running median and a threshold estimates the major melt-freeze events in data from GCMs, reanalyses, and observations.
From the evaluation we conclude that the edge-detector algorithm applied to QuikSCAT measurements successfully captures the seasonal melt-freeze transitions; the bias from observations is typically a few days. ERA-Interim is the best performing dataset over the full region, closely followed by several GCMs. In general, the seasonal transitions are better represented over the landmass and old ice, than in the marginal seas where seasonal ice is abundant. According to the RCP8.5 projection during the 21st century, melt and freeze onset will be displaced by ~10 days over land, while over sea ice, the freezeup will be delayed by ~90 days as old ice retreats. This is a consequence of freezeup occurring 1-2 months later over seasonal ice than old ice, indicated by results from all datasets.
Seasonal melt-freeze transitions have been studied in the past, but the studies typically focus on one transition, domain, or data source. The variety of datasets we use provides spatiotemporal evolution serving as a foundation for hydrological applications: e.g., for improving the monitoring of melt-water runoff and river discharge in spring; to assess or complement operational snow and sea-ice cover products; and to study mechanisms that onset major melt and freeze events. Moreover, the results can be used for model development in this sensitive and important region. We currently work on prolonging our transition map from microwave measurements, employing the ASCAT instrument presently in orbit. Preliminary results are ready and look promising; we expect to have more developed results shortly. Additionally, we aim to further study the mechanisms in play, on a large scale.