Altimetric marine gravity fields in polar regions: History, status and future prospects
David McAdoo(1) and Seymour Laxon(2)
1335 East-West Highway,
Silver Spring 20910 ,
(2) University College London, Gower Street, London WC1E 6BT, United Kingdom
Altimetric contributions to mapping the global marine gravity field have, over the past 27 years, been substantial and of great importance to geophysicists. Beginning with SEASAT (1978), spaceborne radar altimeters including Geosat (1985-1989) , ERS-1 (1991-2000) , ERS-2 (1995-present) and Envisat(2002-present) have profiled sea surface topography with sufficient precision to permit computation of accurate marine gravity (to several mGals) therefrom over nearly all the world's oceans (e.g., Sandwell and Smith, 1997). Computation of marine gravity from altimetric sea surface profiles is possible because ocean topography closely conforms, in the mean, to a level surface, or geoid. However, because height trackers on board these satellite altimeters are optimized for open ocean - as opposed to ice-covered ocean - operation, the computation of polar marine gravity proved very difficult. These on-board trackers produce unacceptable height errors of order 10 m. In 1993, work (Laxon and McAdoo, 1994; McAdoo and Laxon, 1997) began on the reprocessing of ERS-1 return echoes and subsequent computation of detailed marine gravity in ice-covered polar seas. The resulting gravity fields revealed previously uncharted tectonic fabric, e.g., (1) a lineated low down the middle of the Arctic Ocean's Canada Basin locating the extinct spreading ridge which gave rise to the Basin's seafloor; (2) gravitational expression of fracture zones tracing early seafloor spreading associated with the separation of the New Zealand continent from West Antarctica.
Substantial improvements in altimetric polar gravity fields and associated maps of mean sea surface topography are now underway. Incorporation of acquired and future retracked Envisat RA-2 data will substantially improve polar gravity fields - particularly in Antarctic seas. CryoSat's SAR/Interferometric Radar Altimeter (SIRAL) data will enable a more precise separation of the sea ice "noise" from the gravitational "signal" possible heretofore from conventional, pulse-limited altimetry. Also, CryoSat data will nearly fill (to 88N) the polar "hole" (latitudes > 82N) or coverage gap in existing Arctic marine altimetric gravity fields. Also future joint estimation of altimetric sea surface topography and polar geoids - which incorporate GRACE and GOCE gravity - should prove very valuable for the polar oceanographer.