Armitage, TWK; (2016) Studies of the Arctic Ocean from satellite radar altimetry. Doctoral thesis , UCL (University College London).

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Abstract

Satellite radar altimetry is a powerful tool for studying the sea ice and physical oceanography of the Arctic, a remote but important component of the global climate system. However, challenges associated with the interpretation of the satellite retrieval and observational gaps remain. This thesis consists of a series of studies, focussed on the Arctic Ocean, that develop and utilise specialised radar altimetry data processing over sea ice. The interferometric capability of CryoSat- 2 (CS2) is exploited to examine the issue of off nadir ranging to leads. Leads can dominate waveforms even if they are more than 1500m away from nadir, biasing sea surface height (SSH) estimates low. Tuning data editing criteria can reduce the number of leads originating from high off nadir angles, however it is not pos- sible to completely remove the range error without the use of CS2 interferometric mode. Sea ice freeboard is derived from Ka-band radar altimeter data for the first time and is compared against CS2 and airborne data to investigate radar pene- tration into the snow pack. The AltiKa dominant scattering horizon lies roughly halfway between the air-snow and snow-ice interfaces. Over first year ice, CS2 freeboard is consistent with ranging to the snow-ice interface, but over multi-year ice the dominant scattering horizon lies, on average, 18% of the snow layer depth above the snow-ice interface, leading to an overestimate of multi-year ice thickness that could explain some of the difference between ice volume estimates from CS2 and models. A waveform model fit is applied to CS2 data to estimate significant wave height in the ice-free Arctic Ocean. Data from summer 2012, the summer of record open water area in the central Arctic, show that fully developed wind seas, with wave heights up to 4–5m, developed in the western Arctic Ocean. A time series of monthly Arctic SSH is constructed between 2003–14 and combined with GRACE ocean mass data to estimate steric height. A large seasonal cycle of steric height dominates Arctic SSH variability, primarily caused by seasonal freshwater fluxes. The steric height seasonal cycle on the Siberian shelves, which annually receive ∼2,000km3 of river runoff, reflects a ±2psu seasonal cycle of salinity. Principle component analysis reveals that nonseasonal SSH variability is dominated by doming of SSH in the Beaufort Gyre, which was concurrent with SSH reductions on the Siberian shelves. An increase in ocean mass dominated the total change in SSH between 2003–14, whilst steric height increased between 2003-11 but fell to ca. 2003 levels by 2014 due to large reductions in the East Siberian and Laptev Seas. The Beaufort Gyre accumulated 4,700km3 of freshwa- ter by 2010 relative to the 2003–06 mean, but between 2012–14 Beaufort Gyre freshwater storage dropped by ∼2,000km3.

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