Seal data show strong, bursty heat transport under East Antarctic summer sea ice

Why it matters

The results suggest that how much heat reaches Antarctic sea ice depends on small-scale ocean processes, not only on how much warm deep water is present. Warm deep water may thus help sea ice loss in the melt season too, not just by delaying winter ice growth. The findings are from one animal track and have not been peer reviewed.

What they did

An instrumented elephant seal recorded temperature and salinity profiles over more than 10 weeks, from mid-summer to early autumn 2024, in the East Antarctic marginal ice zone. The profiles were spaced a median of 669 m apart. The authors computed sharpness of density fronts, then estimated vertical velocities with a quasi-geostrophic omega equation, using strain from satellite altimetry. They multiplied these by temperature anomalies to get vertical heat transport. They compared a late-summer thick ice-front period with an autumn thin-ice period, and ran a Monte Carlo test of the uncertainty.

Key findings

• In the thick ice-front period, vertical heat transport was intermittent and went both up and down, with local events exceeding 1,000 W m-2.
• Vertical velocities in that period often exceeded ±20 m per day and locally approached ±50 m per day.
• Strong upward events often matched upward shifts of warm modified Circumpolar Deep Water toward the mixed layer.
• The thin-ice autumn period had weaker fronts, weaker vertical velocities (at most ±15 m per day) and mean heat transport generally within ±10 W m-2.
• Mean heat transport in the thick-ice period peaked at about 50 W m-2 near 200 m depth. The downward mean at about 300 m is less certain.

Limitations

• The data come from a single seal track in East Antarctica over late summer and early autumn. The vertical velocities and heat transports are inferred, not measured directly.
• The authors did not account for horizontal ice or ocean heat advection, so they cannot close the heat budget or show how much basal melt resulted.
• The sign of the mean heat transport at depth is poorly constrained, and there were few deep profiles. The feedback with melt is a possibility, not shown directly.

Glossary

• modified Circumpolar Deep Water (mCDW): A warm, salty subsurface water mass that acts as a heat reservoir beneath Antarctic sea ice.
• Submesoscale front: A sharp, kilometer-scale boundary between water masses of different density that drives strong vertical motion.
• Quasi-geostrophic omega equation: An equation used to estimate vertical velocity from the density structure and the large-scale strain in the flow.
• Marginal ice zone: The transition region between open ocean and compact sea ice.