 in the 1980s. The thinner, younger ice is more vulnerable to completely melting in the summer, resulting in lower minimum ice extents.
Sea Ice Thickness
Satellite remote sensing and regular airborne survey programs continued to record changes in Arctic sea-ice thickness and volume. In 2017, the ESA CryoSat-2 radar altimeter mission completed its 7th year of operation since its launch in 2010, providing sea-ice thickness between October and April (Laxon et al., 2013) from freeboard observations. The CryoSat-2 freeboard measurements expand the data record of satellite and submarine based observations that document the decline in sea-ice thickness in the last decades (Kwok and Rothrock, 2009; Lindsay and Schweiger, 2015).
In spring 2017, CryoSat-2 products from the Alfred Wegener Institute (AWI) indicated a pattern of ice thickness change that was more variable (Fig. 4a) compared to previous years (2011-2016) in the observational period (Fig. 4b). The April 2017 thickness anomaly (Fig. 4c) shows below average thicknesses in the multiyear ice region north of the Canadian Archipelago, the Chukchi Sea and the shelf regions of the East Siberian Sea. Above-average thicknesses were observed in the Beaufort Sea and the eastern part of the central Arctic Ocean. The magnitude of these anomalies was generally below 1 meter.
(a) Sea-ice thickness from CryoSat-2 radar altimeter data for April 2017, (b) mean April data record, for the period 2011-2016, and (c) the April 2017 sea-ice thickness anomaly. (Data source: AWI CryoSat sea ice product, www.seaiceportal.de
The CryoSat-2 sea-ice volume estimates (Fig. 5) for the central Arctic Ocean showed a continued decline from 2014 through 2017. The April 2017 sea-ice volume of 13.19 ± 1.15 thousand cubic kilometers ranks as the third lowest spring volume after April 2012 (13.14 ±1.27) and 2013 (12.56 ± 1.21) in the AWI CryoSat-2 data record. The difference between the three lowest volume estimates lies within the observational uncertainties.
Fig. 5. (a) Monthly time series of sea ice volume from CryoSat-2 measurements for the months November (minimum volume) to April (maximum volume). Number in the markers indicate the ranking of each month for the seven years of the data record, with 1 representing the smallest volume and 7 the largest. (b) The shaded area denotes the region where ice volume was determined.
Snow on Sea Ice
Snow plays several critical roles in the mass balance of Arctic sea ice. During the winter growth season, snow insulates the ocean from the atmosphere, dampening heat fluxes and reducing sea ice growth. In spring, snow reflects more than 80% of the incoming solar radiation, delaying ice melt. Snow can also contribute to ice growth after flooding and refreezing, leading to formation of snow ice (Granskog et al., 2017). As the melt season progresses, freshwater from snowmelt enables the formation of ponds on the sea ice.
Prior to the 1990s, observations of snow on Arctic sea ice were limited to in situ measurements. Warren et al. (1999) compiled many of these observations into a climatology. Since the 1990s, new approaches to measure snow accumulation have rapidly emerged, including improved instruments for in situ and autonomous observations and remote sensing techniques for retrieving snow depth. Field observations from recent years continue to underscore significant regional and inter-annual variability in Arctic snow on sea ice conditions. Figure 6 shows the historical snow depth climatology (background) plus a compilation of recent airborne snow depth measurements collected between March and May in 2009-2012 and 2014-2015 (lines), and in situ measurements made in 2015 and 2017 (individual points). Mean snow depths range from ~0.05 m to 0.55 m as shown. Compared to the climatology, these data indicate that there has been an overall decrease in snow depths in the western Arctic at the end of winter, potentially associated with later sea ice formation in the autumn and the shift from MY to FY ice. (Webster et al., 2014). Airborne observations confirm that snow on first ye