Arctic sea surface height variability and change from satellite radar altimetry and GRACE, 2003–2014

Abstract

Arctic sea surface height (SSH) is poorly observed by radar altimeters due to the poor coverage of the polar oceans provided by conventional altimeter missions and because large areas are perpetually covered by sea ice, requiring specialized data processing. We utilize SSH estimates from both the icecovered and ice-free ocean to present monthly estimates of Arctic Dynamic Ocean Topography (DOT) from radar altimetry south of 81.58N and combine this with GRACE ocean mass to estimate steric height. Our SSH and steric height estimates show good agreement with tide gauge records and geopotential height derived from Ice-Tethered Profilers. The large seasonal cycle of Arctic SSH (amplitude 5 cm) is dominated by seasonal steric height variation associated with seasonal freshwater fluxes, and peaks in October-November. Overall, the annual mean steric height increased by 2.2 6 1.4 cm between 2003 and 2012 before falling to circa 2003 levels between 2012 and 2014 due to large reductions on the Siberian shelf seas. The total secular change in SSH between 2003 and 2014 is then dominated by a 2.1 6 0.7 cm increase in ocean mass. We estimate that by 2010, the Beaufort Gyre had accumulated 4600 km 3 of freshwater relative to the 2003-2006 mean. Doming of Arctic DOT in the Beaufort Sea is revealed by Empirical Orthogonal Function analysis to be concurrent with regional reductions in the Siberian Arctic. We estimate that the Siberian shelf seas lost 180 km 3 of freshwater between 2003 and 2014, associated with an increase in annual mean salinity of 0.15 psu yr 21 . Finally, ocean storage flux estimates from altimetry agree well with high-resolution model results, demonstrating the potential for altimetry to elucidate the Arctic hydrological cycle.

Arctic Ocean surface geostrophic circulation 2003–2014

“…We use the monthly Arctic DOT estimates from Envisat (2003Envisat ( -2011 and CryoSat-2 (2012CryoSat-2 ( -2014 produced by Armitage et al (2016). These estimates of DOT combine sea surface height (SSH) from the open ocean and ice-covered ocean (via leads) to estimate basin-wide DOT up to 81.5 • N (see Armitage et al, 2016 for full details of the data processing).…”

“…These estimates of DOT combine sea surface height (SSH) from the open ocean and ice-covered ocean (via leads) to estimate basin-wide DOT up to 81.5 • N (see Armitage et al, 2016 for full details of the data processing). To estimate monthly geostrophic currents the following steps are taken.…”

“…The West Spitsbergen Current is not fully resolved in Fig. 3a and b as it is only ∼ 100 km wide and the Gaussian smoothing function applied to the DOT has a standard deviation of 100 km (Armitage et al, 2016).…”

“…3c-h). We chose these time periods because the DOT in the Beaufort Sea showed a marked increase between 2006 and 2008 before decreasing slightly after 2011 (Armitage et al, 2016), and we wish to investigate the impact of these changes on geostrophic currents. To quantify these changes we examine time series of the monthly mean geostrophic current speed normal to three gates in the Beaufort Sea, chosen to characterise important aspects of the BG circulation: (1) (Fig.…”

“…Hence, previous efforts have been limited to long-term means (Farrell et al, 2012), intermittent seasonal means (Kwok and Morison, 2011) and, more recently, to the ice-covered portion of the Arctic only since the launch of CryoSat-2 in 2011 (Kwok and Morison, 2015;Mizobata et al, 2016). Time-variable satellite gravimetry has been used to study circulation variability (Volkov and Landerer, 2013;Peralta-Ferriz et al, 2014); however, this only captures variations in DOT due to ocean mass variability, missing the majority of DOT variability in the Arctic Ocean (Armitage et al, 2016). In this context, we calculate monthly geostrophic currents using monthly, satellite-derived estimates of DOT from the ice-covered and ice-free portions of the Arctic Ocean between 2003 and 2014, to create the longest record of extensive Arctic surface circulation to present.…”