Andy Ridout

Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum

September 2007 marked a record minimum in sea ice extent. While there have been many studies published recently describing the minimum and its causes, little is known about how the ice thickness has changed in the run up to, and following, the summer of 2007. Using satellite radar altimetry data, covering the Arctic Ocean up to 81.5 degrees North, we show that the average winter sea ice thickness anomaly, after the melt season of 2007, was 0.26 m below the 2002/2003 to 2007/2008 average. More strikingly, the Western Arctic anomaly was 0.49 m below the six-year mean in the winter of 2007/2008. These results show no evidence of short-term preconditioning through ice thinning between 2002 and 2007 but show that, after the record minimum ice extent in 2007, the average ice thickness was reduced, particularly in the Western Arctic. Citation: Giles, K. A., S. W. Laxon, and A. L. Ridout (2008), Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum, Geophys. Res. Lett., 35, L22502, doi: 10.1029/2008GL035710.

Controls on ERS altimeter measurements over ice sheets: Footprint‐scale topography, backscatter fluctuations, and the dependence of microwave penetration depth on satellite orientation

We consider the reliability of radar altimeter measurements of ice sheet elevation and snowpack properties in the presence of surface undulations. We demonstrate that over ice sheets the common practice of averaging echoes by aligning the first return from the surface at the origin can result in a redistribution of power to later times in the average echo, mimicking the effects of microwave penetration into the snowpack. Algorithms that assume the topography affects the radar echo shape in the same way that waves affect altimeter echoes over the ocean will therefore lead to biased estimates of elevation. This assumption will also cause errors in the retrieval of echo-shape parameters intended to quantify the penetration of the microwave pulse into the snowpack. Using numerical simulations, we estimate the errors in retrievals of extinction coefficient, surface backscatter, and volume backscatter for various undulating topographies. In the flatter portions of the Antarctic plateau, useful estimates of these parameters may be recovered by averaging altimeter echoes recorded by the European Remote Sensing satellite (ERS-1). By numerical deconvolution of the average echoes we resolve the depths in the snowpack at which temporal changes and satellite travel-direction effects occur, both of which have the potential to corrupt measurements of ice sheet elevation change. The temporal changes are isolated in the surface-backscatter cross section, while directional effects are confined to the extinction coefficient and are stable from year to year. This allows the removal of the directional effect from measurement of ice-sheet elevation change.

Arctic Ocean gravity field derived from ICESat and ERS-2 altimetry : Tectonic implications

A new, detailed marine gravity field for the persistently ice-covered Arctic Ocean, derived entirely from satellite data, reveals important new tectonic features in both the Amerasian and Eurasian basins. Reprocessed Geoscience Laser Altimeter System (GLAS) data collected by NASAs Ice Cloud and land Elevation Satellite (ICESat) between 2003 and 2005 have been combined with 8 years worth of retracked radar altimeter data from ESAs ERS-2 satellite to produce the highest available resolution gravity mapping of the entire Arctic Ocean complete to 86 degrees N. This ARCtic Satellite-only (ARCS) marine gravity field uniformly and confidently resolves marine gravity to wavelengths as short as 35 km. ARCS relies on a Gravity Recovery and Climate Experiment (GRACE)-only satellite gravity model at long (> 580 km) wavelengths and plainly shows tectonic fabric and numerous details imprinted in the Arctic seafloor, in particular, in the enigmatic Amerasian Basin (AB). For example, in the Makarov Basin portion of the AB, two north-south trending lineations are likely clues to the highly uncertain seafloor spreading history which formed the AB.

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

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 ice-covered and ice-free ocean to present monthly estimates of Arctic Dynamic Ocean Topography (DOT) from radar altimetry south of 81.5°N 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 ± 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 ± 0.7 cm increase in ocean mass. We estimate that by 2010, the Beaufort Gyre had accumulated 4600 km3 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 km3 of freshwater between 2003 and 2014, associated with an increase in annual mean salinity of 0.15 psu yr−1. 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 sea ice freeboard from AltiKa and comparison with CryoSat‐2 and Operation IceBridge

Satellite radar altimeters have improved our knowledge of Arctic sea ice thickness over the past decade. The main sources of uncertainty in sea ice thickness retrievals are associated with inadequate knowledge of the snow layer depth and the radar interaction with the snow pack. Here we adapt a method of deriving sea ice freeboard from CryoSat-2 to data from the AltiKa Ka band radar altimeter over the 2013–14 Arctic sea ice growth season. AltiKa measures basin-averaged freeboards between 4.4 cm and 6.9 cm larger than CryoSat-2 in October and March, respectively. Using airborne laser and radar measurements from spring 2013 and 2014, we estimate the effective scattering horizon for each sensor. While CryoSat-2 echoes penetrate to the ice surface over first-year ice and penetrate the majority (82 ± 3%) of the snow layer over multiyear ice, AltiKa echoes are scattered from roughly the midpoint (46 ± 5%) of the snow layer over both ice types.

Meteorological Origin of the Static Crossover Pattern Present in Low-Resolution-Mode CryoSat-2 Data Over Central Antarctica

The most effective way of determining the rate of elevation change of the Earths large ice sheets using radar altimeters is to examine the difference in the elevation measured on ascending and descending orbits. This crossover difference has a static and time-varying component, and by isolating the time-varying part, one can construct a time series of the ice sheet elevation change. The static component of the crossover difference arises as a result of an anisotropic dependence of the extinction coefficient on the angle between the radar polarization and wind-induced features of the firn. Here, the static crossover difference observed by CryoSat-2 over the Antarctic ice sheet is examined, and a simple model is developed to explain the observed pattern. There is an excellent agreement between the modeled results and the observations, calling into question the results of previous studies of the same phenomenon with different radar altimeters.

Variability of the Ross Gyre, Southern Ocean: drivers and responses revealed by satellite altimetry

Year‐round variability in the Ross Gyre (RG), Antarctica, during 2011–2015, is derived using radar altimetry. The RG is characterized by a bounded recirculating component and a westward throughflow to the south. Two modes of variability of the sea surface height and ocean surface stress curl are revealed. The first represents a large‐scale sea surface height change forced by the Antarctic Oscillation. The second represents semiannual variability in gyre area and strength, driven by fluctuations in sea level pressure associated with the Amundsen Sea Low. Variability in the throughflow is also linked to the Amundsen Sea Low. An adequate description of the oceanic circulation is achieved only when sea ice drag is accounted for in the ocean surface stress. The drivers of RG variability elucidated here have significant implications for our understanding of the oceanic forcing of Antarctic Ice Sheet melting and for the downstream propagation of its ocean freshening footprint.

An Assessment of State‐of‐the‐Art Mean Sea Surface and Geoid Models of the Arctic Ocean: Implications for Sea Ice Freeboard Retrieval

State-of-the-art Arctic Ocean mean sea surface (MSS) models and global geoid models (GGMs) are used to support sea ice freeboard estimation from satellite altimeters, as well as in oceanographic studies such as mapping sea level anomalies and mean dynamic ocean topography. However, errors in a given model in the high frequency domain, primarily due to unresolved gravity features, can result in errors in the estimated along-track freeboard. These errors are exacerbated in areas with a sparse lead distribution in consolidated ice pack conditions. Additionally model errors can impact ocean geostrophic currents, derived from satellite altimeter data, while remaining biases in these models may impact longer-term, multi-sensor oceanographic time-series of sea level change in the Arctic. This study focuses on an assessment of five state-of-the-art Arctic MSS models (UCL13/04, DTU15/13/10) and a commonly used GGM (EGM2008). We describe errors due to unresolved gravity features, inter-satellite biases, and remaining satellite orbit errors, and their impact on the derivation of sea ice freeboard. The latest MSS models, incorporating CryoSat-2 sea surface height measurements, show improved definition of gravity features, such as the Gakkel Ridge. The standard deviation between models ranges 0.03-0.25 m. The impact of remaining MSS/GGM errors on freeboard retrieval can reach several decimeters in parts of the Arctic. While the maximum observed freeboard difference found in the central Arctic was 0.59 m (UCL13 MSS minus EGM2008 GGM), the standard deviation in freeboard differences is 0.03-0.06 m.

Estimating snow depth over Arctic sea ice from calibrated dual-frequency radar freeboards

Snow depth on sea ice remains one of the largest uncertainties in sea ice thickness retrievals from satellite altimetry. Here we outline an approach for deriving snow depth that can be applied to any coincident freeboard measurements after calibration with independent observations of snow and ice freeboard. Freeboard estimates from CryoSat-2 (Ku-band) and AltiKa (Ka-band) are calibrated against data from NASA’s Operation IceBridge (OIB) to align AltiKa to the snow surface and CryoSat-2 to the 5 ice/snow interface. Snow depth is found as the difference between the two calibrated freeboards, with a correction added for the slower speed of light propagation through snow. We perform an initial evaluation of our derived snow depth product against OIB snow depth data by excluding successive years of OIB data from the analysis. We find a root-mean-square deviation of 7.7, 5.3, 5.9 and 6.7 cm between our snow thickness product and OIB data from the springs of 2013, 2014, 2015 and 2016 respectively. We further demonstrate the applicability of the method to ICESat and Envisat, offering promising potential for 10 the application to CryoSat-2 and ICESat-2, when ICESat-2 is launched in 2018.

Elevation changes in Antarctica 1992–1996: Implications for Ice Sheet Mass Balances

The uncertainties in the mass balance of the ice sheet of Antarctica of 5×1014 kg yr-1 are the largest uncertainties in the causes of the observed rise in sea level. The uncertainty in the mass balance of Antarctica has increased in time as the estimated role of ice shelf bottom melting has grown. The past few years have seen dramatic improvements in the force modelling of altimeter satellites and improved correction of the surface and volume scattering contributions of ice sheet altimeter echoes. In consequence, we have been able to constrain the elevation change 1992–1996 of 60% of the Antarctic ice sheet to 0,5+0,7 cm yr-1. The possible sources of error from the satellite orbit, the time-variant scattering at the ice sheet surface, and the echo travel-time corrections are all examined, and it is concluded these are too small to be significant to the five-year elevation trend. The relationship between the fluctuations of elevation and mass is examined through numerical modelling of the densification of firn at the surface of the ice sheet and it is concluded that, at a single location, time-variant densification, due to fluctuations in surface mass balance and surface density, may cause the elevation time-series to misrepresent the mass change by as much as +2 cm yr-1. However, evidence from deep Antarctic cores, and from atmospheric models, point to a spatial correlation length of these surface fluctuations of less than 1.000 km. On this basis, we estimate the elevation time series to track the mass fluctuations to within +0,6 cm yr-1. We thus conclude that 60% of the sheet has been in balance to within 7% in the period 1992–1996. Extended over Antarctica this is a mass balance uncertainty of 140 Gt yr-1, a substantial reduction on the previous estimate.