High-latitude Oceanography from Space Using ICESat, CryoSat-2, and GRACE with Implications for Future Use of ICESat-2 and GRACE-FO

ICESat and CryoSat2 dynamic ocean topography (DOT) combined with GRACE ocean bottom pressure (OBP) have uniquely allowed us to measure interannual variations in circulation and freshwater content over the whole Arctic Ocean and Sub-Arctic Seas. Gradients in DOT constitute the pressure gradients that drive surface geostrophic current. Measurements ofhydrography-derived dynamic heights (DH) relative to a 500-m level of no motion have been used successfully to validate the spatial variation of DOT from ICESat and CryoSat-2. Further, trends in GRACE OBP plus DH at the locations of repeat hydrographic stations in the Arctic Ocean and Sub-Arctic Seas agree with trends in ICESat and CryoSat-2 DOT, leading to the use of trends in satellite-derived DOT-OBP as a proxy for steric height and total freshwater content. Trends in circulation and freshwater content derived in this way from ICESat DOT and GRACE OBP from 2004-2009 have revealed a cyclonic shift in circulation and diversion of Eurasian runoff to the Canada Basin associated with a 2007 increase in the winter Arctic Oscillation (AO) index. Changes in DOT and OBP over the Arctic Ocean and sub-Arctic seas between the ICESat (2004-2009) and CryoSat-2 (2011-2015) periods show a marked increase in Arctic Ocean DOT that is supported by the in situ hydrography plus OBP in the Beaufort Sea and Transpolar Drift. However, the satellite-derived DOT indicates the DOT increase is greatest along the Eurasian side of the basin consistent with a shift to a more anticyclonic regime as a lagged response to the record low winter 2009-2010 AO.

Comparison of DOT from ICESat and CryoSat-2 require intermediate comparisons with in situ DH plus GRACE OBP. These comparisons show a common DOT v. DH+OBP relationship in the Arctic Ocean and for CryoSat-2 DOT in the open ocean Sub-Arctic Seas, but ICESat DOT in the open ocean falls up to 20-cm below the common relationship. This bias is strongly correlated with significant wave height (SWH) in the Sub-Arctic seas from the ECMWF reanalysis, leading to the first consistent finding of the SSB correction for a laser altimeter. We will explore how this finding has highlighted the importance of and strong potential for estimating SSB in ICESat-2 DOT so that the full potential of this new high-resolution photon counting lidar will be realized over the open ocean.