Sea ice characteristics and volume flux in the western Ross Sea through high-resolution satellite imagery and altimetry

• Main Author: Farooq, Usama
• Format: Article in Journal/Newspaper
• Language: English
• Published: University of Canterbury 2021
• Subjects: Antarctic; Southern Ocean; The Antarctic; Ross Sea; McMurdo Sound; Terra Nova Bay; Ross Ice Shelf; Asar; Antarc*; Antarctica; Ice Shelf; Sea ice
• Online Access: https://dx.doi.org/10.26021/11598; https://ir.canterbury.ac.nz/handle/10092/102487

Sea ice and the related processes in formation, transformation, and melt, play a significant role in the global climate, considerably influencing Earth’s energy budget and global ocean circulation. Sea ice motion in combination with thickness determines the transportation of fresh ice to the areas of sea ice decay. The sea ice area has been regularly monitored using satellite remote sensing but the volume is still uncertain. This is especially true for the Southern Ocean. The reason behind this is that the thickness of sea ice is not extensively explored. The observation of Antarctic sea ice mass balance therefore needs further investigation. To understand changes in sea ice mass balance it is essential to observe sea ice thickness and dynamics. Due to the inaccessibility and extreme weather of Antarctica, the field measurements of sea ice kinematics and thickness are spatially and temporally sparse. This study aims to improve the knowledge of sea ice mass balance by advancing the understanding of sea ice dynamics and thickness by using satellite remote sensing. In the Antarctic, Ross Sea ice extent has been increasing over the past 40 years of satellite records but the trend substantially declined from 2014 and again started to increase in 2017. The western Ross Sea region, including three main polynya areas in McMurdo Sound, Terra Nova Bay, and in front of the Ross Ice Shelf, has experienced a significant increase in sea ice extent, and is the area under investigation. The sea ice morphology in this region ranges from simple land-fast sea ice to complex pack ice. To pursue this, sea ice drift is assessed in the first part of this study using sequential high- resolution (150 m) Advanced Synthetic Aperture Radar (ASAR) images from the Envisat satellite from 2002 to 2012. The output motion vectors were validated with manually drawn vectors. The low correlations and high directional differences are found between high- resolution velocity vectors and NSIDC low-resolution sea ice motion product. The high- resolution product is able to identify short-term spatial variability, and the low-resolution data underestimates (~47%) the actual sea ice velocities in this near-coastal region. In the second part of this study, sea ice drift data at high spatial resolution and Antarctic Mesoscale Prediction System (AMPS) surface wind model outputs are used to explore atmosphere-sea ice interactions. This data is used to quantify the relationship between the winds and sea ice drift and observes the average and annual anomalies across the region. Four drift parameters were selected (mean drift, the correlation between drift and wind, drift to wind scaling factor, and the directional drift constancy) to perform an unsupervised k-means classification to automatically distinguish six zones of distinctive sea ice characteristics solely based on ice drift and wind information. Results indicated that sea ice moves at a rate ranging from 0.41% to 2.24% of the wind speed in different zones. We also found that the directional constancy of sea ice drift is closely related to the wind fields. The classification illustrated the significance of localized wind-driven sea ice drift in this coastal area resulting in zones of convergence, shear, and free drift. It was identified that large- scale sea ice motion is predominantly wind-driven over much of the study area while ocean currents play only a minor role. In the last part of this study, satellite-derived radar and laser altimeter data were utilized to estimate the sea ice thickness of pack ice. Ice, Cloud, and land Elevation Satellite‐1 (ICESat-1) laser, CryoSat-2 radar, and ICESat-2 laser altimeter data were analyzed to develop a long time series and understand the annual variations in sea ice thickness. A significant contrast in sea ice thickness distribution from east to west was observed in each year reflecting thicker ice in the western part of the study area. The sea ice thickness is correlated with the sea ice drift to obtain a relationship between sea ice drift and thickness for each classified zones solely derived from the sea ice drift to wind relationship. The result depicted that the thick sea ice contributing to the low sea ice motion and thin sea ice resulted in high sea ice motion. Using a combination of sea ice thickness data and motion data, the sea ice volume export from the western Ross Sea is estimated to be 93.8 ± 37.9 km3/month for the spring months (September, October, and November). The large variation is likely a consequence of both the natural variability and the applied method based on limitations in data availability. Improved continuous satellite image coverage is still desired to capture drift variations shorter than 24 hours. This high-resolution satellite study contributes to the current understanding of the sea ice mass balance in the western Ross Sea and reveals pathways for further urgently needed improvements.