Gravity and Topography of the Polar Oceans from Satellite Altimetry: Geoscience results from ERS-1 to CryoSat
McAdoo, David1; Farrell, Sinead1; Ridout, Andy2; Giles, Katharine2; Zwally, H. Jay3
1University of Maryland, ESSIC, UNITED STATES; 2University College London, CPOM, UNITED KINGDOM; 3NASA, Goddard, UNITED STATES
This paper describes some of the geodynamic and oceanographic research on the polar oceans begun over 20 years ago by Seymour Laxon, and continued under his vision and leadership until the present. Spaceborne altimeters began profiling the ocean surface in the 1970’s. They yielded data which was used in the 1980s and early 1990s to compute precise marine gravity fields over the non-polar oceans - gravity fields which produced a strong confirmation of plate tectonics. However, because altimetric height trackers on board the SEASAT, Geosat and ERS-1/-2 satellites were optimized for open, ice-free ocean, the computation of accurate altimetric marine gravity over the ice-covered polar seas proved impossible prior to 1993. But starting in 1993, with Laxon’s pioneering, innovative work reprocessing microwave waveforms from ERS-1 and doing concomitant gravity computations, this polar gap in accurate marine gravity over icy seas was mostly filled. The resulting ERS-1 gravity fields revealed previously uncharted tectonic fabric in the Arctic as well as Antarctic seafloor. These tectonic revelations were especially important for the Amerasian portion of the Arctic whose geologic history were and are the most poorly understood of the Earth’s ocean basins. In the Arctic basin, tectonic features newly delineated by this ERS gravity include continental-basin margins as well as mid-ocean ridges. The most important of these features is the north-south trending linear negative anomaly in the middle of the Canada Basin now understood to be the site of a buried, extinct mid-ocean ridge or spreading center along which substantial rotational opening of the (Arctic) Amerasia Basin occurred via seafloor spreading between 132 Ma and 125 Ma. This polar marine gravity work has continued now for nearly 20 years with emphasis remaining on the Arctic Ocean basin. Improvements include the incorporation of more recent altimeter data to produce updated Arctic marine gravity fields. These more recent data include ERS-2, Envisat, and ICESat altimetry. This led to the recently published (2013) ARCtic Satellite-only (ARCS-2) marine gravity field computed using reprocessed ICESat sea surface elevations and retracked Envisat sea surface heights. ARCS-2 is derived exclusively from satellite data, is independent of surface gravity observations, and extends north to 86N - farther north than previous altimetric marine gravity fields. Also ARCS-2 improves, by a factor of two, the spatial resolution over previous marine gravity fields in many areas of the high Arctic. The ARCS-2 field has better spatial resolution than previous fields which aids in tracing tectonic fabric. ARCS-2 also has increased precision. However, precise airborne gravity collected during NASA’s 2010 and 2011 IceBridge campaigns were used to validate models such as ARCS-2 as well as EGM2008 and to demonstrate that errors over the Arctic in current geoid/gravity models (e.g., EGM 2008) are still - notwithstanding great strides made in our knowledge of Arctic gravity - large enough limit our ability to map the fine-scale details of mean dynamic topography of the Arctic Ocean. With Seymour’s encouragement and leadership we recently began developing an ARCtic Satellite-only (ARCS-3) altimetric marine gravity field which incorporates SIRAL altimeter data from CryoSat-2 into the solution. We will present the ARCS-3 field and demonstrate the improvements in spatial resolution and precision as well as the geoscience impacts that result from employing CryoSat data. We note that Seymour’s research provided major impetus for the CryoSat mission.