J Alec Casey

Comparing Springtime Ice-Algal Chlorophyll a and Physical Properties of Multi-Year and First-Year Sea Ice from the Lincoln Sea

With near-complete replacement of Arctic multi-year ice (MYI) by first-year ice (FYI) predicted to occur within this century, it remains uncertain how the loss of MYI will impact the abundance and distribution of sea ice associated algae. In this study we compare the chlorophyll a (chl a) concentrations and physical properties of MYI and FYI from the Lincoln Sea during 3 spring seasons (2010-2012). Cores were analysed for texture, salinity, and chl a. We identified annual growth layers for 7 of 11 MYI cores and found no significant differences in chl a concentration between the bottom first-year-ice portions of MYI, upper old-ice portions of MYI, and FYI cores. Overall, the maximum chl a concentrations were observed at the bottom of young FYI. However, there were no significant differences in chl a concentrations between MYI and FYI. This suggests little or no change in algal biomass with a shift from MYI to FYI and that the spatial extent and regional variability of refrozen leads and younger FYI will likely be key factors governing future changes in Arctic sea ice algal biomass. Bottom-integrated chl a concentrations showed negative logistic relationships with snow depth and bulk (snow plus ice) integrated extinction coefficients; indicating a strong influence of snow cover in controlling bottom ice algal biomass. The maximum bottom MYI chl a concentration was observed in a hummock, representing the thickest ice with lowest snow depth of this study. Hence, in this and other studies MYI chl a biomass may be under-estimated due to an under-representation of thick MYI (e.g., hummocks), which typically have a relatively thin snowpack allowing for increased light transmission. Therefore, we suggest the on-going loss of MYI in the Arctic Ocean may have a larger impact on ice–associated production than generally assumed.

Towards the retrieval of multi-year sea ice thickness and deformation state from polarimetric C- and X-band SAR observations

In situ and airborne observations of sea ice properties are compared to polarimetric C- and X-band synthetic aperture radar images acquired in the Lincoln Sea in 2012 and 2013. A decision-tree classification algorithm is developed to separate level and deformed ice, as well as first- and multi-year ice (MYI), using parameters of the Freeman-Durden Decomposition. Preliminary qualitative and quantitative evaluations of the algorithm indicate it has considerable promise for the separation of these ice types. For the MYI class, correlations between backscatter and ice thickness were moderate to strong for the 2012 field observations but were weak for the 2013 field observations. Further research is required to determine whether or not MYI thickness can be inverted from polarimetric C- and X-band SAR data.

Comparison of in situ and airborne measurements of multiyear sea ice thickness with dual-frequency, polarimetric SAR observations

First results are presented from a comparison of in situ and airborne measurements of sea ice thickness to C- and X-band polarimetric synthetic aperture radar images acquired over the Lincoln Sea in March-May 2012. Operation IceBridge data indicate a moderate correlation between ice surface roughness and thickness in this multi-year ice regime, motivating efforts to derive ice thickness from SAR data. C-band backscatter shows moderate correlations to in situ and airborne ice thicknesses (r ≈ 0.6). At X-band correlations with airborne ice thickness data are much lower. For all polarizations the relationship with ice thickness levels off at thicknesses greater than 5 m, where ice is heavily deformed. Current research efforts are focused on exploiting polarimetric parameters to discriminate thick deformed ice to support future efforts to develop a thickness retrieval algorithm for undeformed ice up to 5 m thick.

Characteristics of CryoSat-2 signals over multi-year and seasonal sea ice

CryoSat-2 Level 1B and Level 2 data are compared with coincident airborne laser scanner and airborne electromagnetic induction surveys conducted in the Baltic Sea in March 2011 and in the Lincoln Sea in March and April 2012. Across-track snagging caused range retrieval errors in all examined tracks. The L2 surface height profiles are very noisy due to poor retracker performance. Cryosat-2 L2 data are produced by tracking the strongest return in the Level 1B waveforms, not the first return. When a strong reflector is present off-nadir, the retracker incorrectly selects this surface as the surface location of the nadir point. The surface height profile is smoothed by tracking the first return point instead of the peak power return. Ice thickness data produced by retracking the CryoSat-2 L1B data showed good correlation with airborne electromagnetic induction measurements. CryoSat-2 and NASA Operation Ice Bridge data exhibited similar latitudinal freeboard gradients. Accurate measurements of sea ice freeboard or thickness changes can be achieved using appropriate spatial and temporal averaging.