The polar oceans are experiencing some of the most rapid environmental changes on Earth, including changes in ocean circulation and freshwater distribution. These changes can manifest as changes in sea surface height (SSH) and dynamic ocean topography (DOT). Measuring and monitoring basin-scale variability in the SSH of the ice-covered oceans has proven challenging because the surface of these oceans is only exposed within narrow openings in the sea ice (e.g., leads and polynyas), requiring high spatial resolution and bespoke measurement techniques.
Here, we use high-resolution laser altimetry measurements of SSH over the Arctic and the Southern Oceans, routinely collected by NASA’s Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) since its launch in September 2018. These SSH estimates are then used to study DOT of both oceans, which represents the deviation of the SSH from the height of the local geoid (e.g., EGM2008). Spatial gradients in DOT can be used to estimate surface ocean geostrophic circulation.
The ICESat-2 mission is now approaching three years of operation, and offers the opportunity to study SSH/DOT over three full seasonal cycles (2018-2021) across both polar oceans at unprecedented spatial resolution. ICESat-2 has proven to be very successful in measuring and tracking monthly and seasonal SSH variations in the Arctic Ocean (Bagnardi et al., 2021).
With the ultimate goal of extending time-varying DOT estimates to the last decade, we make an attempt at reconciling the ICESat-2 SSH and DOT estimates with those obtained using independent data from ESA’s CryoSat-2 satellite radar altimeter. CryoSat-2 has been acquiring unfocussed synthetic aperture radar (SAR) altimetry data over the polar regions since 2010.
In particular, we focus on semisynchronous along-track measurements from the CRYO2ICE orbit alignment campaign over the Arctic Ocean. Especially during the summer months of 2020 and 2021, the CRYO2ICE data provide overlapping measurements of SSH over hundreds of kilometers and offer the unique opportunity to directly compare high-resolution, along-track SSH profiling from laser and radar altimeters.
For ICESat-2, we use SSH estimates from specular leads recorded in the Level 3A sea ice data products (ATL07 and ATL10). These data provide along-track measurements for six individual ground tracks and up to 16 satellite passes per day over both polar oceans. For CryoSat-2, we use SSH retrievals from both Level 2 data and Level 1b waveform data (both archived at ESA’s CryoSat-2 Science Server) to which we apply a recently updated waveform inversion method to determine the retracking corrections.
In our attempt to best reconcile DOT estimates from the two altimetry missions, we reference heights to the same surface (e.g., mean-tide geoid) and perform an assessment of all time-dependent geophysical corrections (e.g., tides) by estimating differences at orbit cross-overs.
We make use of the most recent ICESat-2 data (release 005), which adopted improved range bias corrections with respect to previous data releases. Using these new data, we re-assess the range biases between the six beams (centimeter-level in previous releases) with the aim of taking full advantage of ICESat-2’s six-beam configuration when estimating SSH and DOT.
Overall, we find strong agreement between the two sensors (centimeter-level differences) in both semisynchronous along-track estimates from the CRYO2ICE overlaps in the Arctic Ocean and basin-scale gridded DOT estimates across both polar oceans.
Bagnardi, M., Kurtz, N. T., Petty, A. A., & Kwok, R. (2021). Sea surface height anomalies of the Arctic Ocean from ICESat-2: A first examination and comparisons with CryoSat-2. Geophysical Research Letters, 48, e2021GL093155. https://doi.org/10.1029/2021GL093155