Altimetry of the Arctic Ocean and Subpolar Seas: Investigating Changes in Circulation and Dynamic Topography

Author:

Sinead Farrell - (University of Maryland, College Park)

Abstract:

Spaceborne altimeters have been profiling the Arctic Ocean and subpolar seas since 1991, initially via the radar altimeter on-board the European Space Agency’s (ESA) ERS-1 satellite. Between 1991 and 2012, radar altimeters onboard ERS-1, -2 and Envisat measured polar ocean topography to a latitudinal limit of 81.5 N. Current and future altimetry missions afford us the chance to extend this coverage poleward; for example NASA’s ICESat provided seasonal coverage of the Arctic to 86 N between 2003 and 2009, while ESA’s CryoSat-2, launched in 2010, provides almost complete coverage to 88 N. Near complete coverage (to 88 N) will be maintained by NASA’s upcoming ICESat-2 mission, carrying the Advanced Topographic Laser Altimeter System (ATLAS), which is due for launch in late 2017. Dedicated polar missions, such as CryoSat-2 and ICESat-2, together with data from the European Sentinel-3 and CNES/ISRO SARAL/AltiKa satellites, provide us a key opportunity to investigate and track both the seasonal and inter-annual variability of important parameters of the polar climate system, including sea surface topography, dynamic ocean topography, and geostrophic circulation.

Although sea surface topography is an important parameter in the climate system, and is accurately measured in the global oceans, our knowledge is limited in the Arctic Ocean and subpolar seas due to the constant presence of sea ice. Additional waveform processing is required to separate measurements of sea-ice elevation from sea surface height (SSH), typically acquired in leads/cracks in the ice cover. While knowledge of SSH has obvious applications in cryospheric studies (i.e. its utility for deriving sea ice freeboard, and hence ice thickness), it also has geodetic and oceanographic applications, such as in the analysis of dynamic ocean topography and geostrophic circulation, and as a means of deriving the marine gravity field. For example, the time invariant component of Arctic SSH, the mean sea surface, was first measured using ERS-2 data, while the mean dynamic ocean topography has been extracted from measurements made by Envisat and ICESat. More recent analyses of polar altimetry data has revealed an accumulation of freshwater in the Beaufort Gyre during the 2000s and a subsequent intensification of geostrophic currents on the periphery of the Gyre, relative to the climatological average.

Understanding the impact of these recent changes on the regional sea level budget, the North Atlantic Meridional Overturning Circulation, and sea ice dynamics (drift and deformation of the Arctic sea ice cover) is critical. We therefore propose a suite of analyses, utilizing CryoSat-2 and Sentinel-3 SSH datasets, augmenting these with historical data from ICEsat and Envisat, and in the future with measurements collected by ICESat-2’s laser altimeter. Our research seeks to exploit these polar ocean altimeter data to better understand variability in Arctic Ocean circulation, at monthly, seasonal, and yearly time-scales.

The proposed research is responsive to the A.11 solicitation, directly addressing three of the eight research topics for 2017-2020:

• Studies of high-resolution merged altimetric data sets to examine the role of Arctic circulation
• Investigations using gravity mission data together with altimeter data for improving the understanding of the mean ocean circulation
• Complementary studies on sea ice monitoring and the marine geoid

The investigators have the relevant capabilities to successfully conduct these studies having recently published on topics including the Arctic mean dynamic topography and the marine gravity field. The PI is a science investigator on the CryoSat-2 mission, and is a member of both the NASA ICESat-2 and Operation IceBridge science teams. The team has extensive experience in the use of ICESat, Envisat, and CryoSat-2 data for accurate measurement of the sea surface topography of the polar oceans and subpolar seas.