The original motive for the sensors was to provide information about animal behaviour but the physical data they have collected has already been valuable in studies of Antarctic bottom water formation, the global heat budget, Southern Ocean frontal structure and sea ice formation.
Argo float and seals provide different styles of profiling. Argo floats (in standard configuration) transmit a 2000m deep profile, with high vertical resolution, every 10 days whereas the SealCTDs transmit a profile every 6 hours. Each CTD sensor records data from every dive but selects the ascent profile from the deepest dive over the last 6 hours. In order to prolong battery life and to ensure the whole profile is transmitted before the seal dives again, the profile is also compressed (by calculating break-points) before transmission. The SealCTD (or tag) is glued onto the animal's head and drops off during their moult.
The temperature and salinity profiles (top right) during a female Elephant Seal’s journey from Kerguelen Island demonstrate the high temporal and spatial resolution possible of the upper 500 metres of the ocean. During her two-month journey this seal travelled through at least four distinct regions before losing her tag in the melting ice. The time series are also plotted in 10-day sections with the seal’s location indicated.
Some SealCTDs have been deployed in the Great Australian Bight (GAB) and southern New South Wales. The Sea Lions in the GAB appear to have a different profiling strategy compared to their cousins in Antarctica. One GAB Sea Lion spent 5 months of the 2015/2016 summer repeating a transect across the shelf from the head of the Bight. His travels document the degree of uplifted water coming onto the shelf along the bottom and also the development of a deep water salinity maximum (bottom right).
This article, written by Madeleine Cahill, originally appeared as a news item on IMOS OceanCurrent.