Sea Ice Thickness and Dynamic Topography of the Arctic Ocean from Satellite Altimetry
Farrell, Sinead L.1; McAdoo, David C.2; Ridout, Andrew L.3; Thomas, Sam4; Wingham, Duncan J.1; Jay Zwally, H.5
1University of Maryland, UNITED STATES; 2University of Maryland, ESSIC, UNITED STATES; 3CPOM, UCL, UNITED KINGDOM; 4Centre for Polar Observation & Modelling, UNITED KINGDOM; 5NASA Goddard Space Flight Center, UNITED STATES
The sea ice cover of the Arctic Ocean plays an important role in our climate system. Sea ice, covered by fresh snow, reflects more solar radiation back to space than the open ocean. As the sea ice cover declines more solar radiation can be absorbed by the ocean, increasing temperatures further, and melting additional sea ice. This positive feedback mechanism is thought to be largely responsible for the enhanced global warming that climate models predict will occur in the polar regions, in particular in the Arctic. Sea ice also acts as a barrier between the atmosphere and the ocean, modulating the transfer of heat and energy between the two. Changes to the sea ice cover may therefore affect oceanic circulation and hence the storage and distribution of fresh water in the Arctic Ocean. When sea ice forms it releases salt into the water column, which makes the water dense and causes it to sink. Conversely when sea ice melts it releases fresh water into the ocean. This dense water formation helps to drive the global ocean conveyor belt, which transports heat from lower latitudes towards the north. This paper summarises our understanding of the evolution of Arctic sea ice thickness and dynamic topography over the past 20 years. Advances in this field are thanks to the pioneering work of Professor Seymour Laxon who solved the problem of distinguishing ice floes from the ocean, in satellite radar altimetry signals. We review the progress in mapping Arctic sea ice thickness from the early 1990s through to the present day using satellite altimeters. We describe the first basin-scale winter sea ice thickness estimates derived from the European Space Agency (ESA) satellites ERS-1 and ERS-2, and discuss how the interannual variability in thickness was found to be highly correlated with the length of the summer melt season. Following the former-record sea ice extent minimum in 2007, we show the circumpolar-wide thinning of the Arctic ice pack, based on data from ESA’s Envisat and NASA’s ICESat satellites. We will discuss the first measurements of Arctic sea ice thickness derived from CryoSat-2, validated using independent airborne and in situ data. Combining the CryoSat-2 data with earlier measurements from ICESat, we show the sea ice thickness time-series spanning the last decade that reveals a decline in sea ice volume of 36% in autumn, and 9% in winter, between 2003 and 2012. We next turn to a discussion of the dynamic topography of the Arctic Ocean. Altimeter observations of leads between sea ice floes are more than just a stepping-stone to measuring sea ice thickness; they provide details of the local sea surface height. These measurements are key to deriving both the dynamic ocean topography and marine gravity field of the Arctic. Dr. Katharine Giles was the first to demonstrate that these measurements may also be used to investigate how winds affect the newly-exposed Arctic Ocean. Using the time-variant dynamic topography derived from ERS-2 and Envisat data Katharine demonstrated that the accumulation of fresh water in the western Arctic between 1995 and 2010 was due to increased anti-cyclonicity in the wind field. We also describe the mean dynamic topography of the Arctic Ocean derived from a combination of the Envisat and ICESat data, that reveals the major gyres and currents of the Arctic basin. Finally we update this work with recent results demonstrating the latest efforts to map the mean dynamic topography and geostrophic currents of the Arctic Ocean using CryoSat-2.