Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctic Bottom Water - Freshening and Warming 1980s-2000s

In this study freshening and warming of Antarctic Bottom Water (AABW) that occurred between the 1980s and the 2000s was assessed for the relative contribution of changes of water mass and isotherm heave. The analysis involved conductivity-temperature-depth measurements, that were highly accurate, full depth, ship-based, that were taken along repeated oceanic sections around the Southern Ocean. Within the South Pacific Ocean and the South Indian Ocean fresher varieties of AABW were present in the 2000s compared to the 1990s, the strongest freshening being found in the newest waters adjacent to the Antarctic continental slope and rise which indicates a recent shift in the salinity of AABW that was produced in this region. In the Weddell Sea bottom waters exhibit water-mass freshening that is significantly less than in the other 2 southern basins, though a volume decrease of the oldest, deepest waters is observed throughout the entire Southern Ocean. A salinification and warming on isobaths, from the bottom up to the shallow potential temperature maximum, is caused by this isotherm heave. The water-mass freshening of AABW, that occurred in the Indian and Pacific sectors, is equivalent to a freshwater flux of 73 ± 26 Gt/yr, which is roughly half of the estimated recent mass loss of the West Antarctic Ice Sheet. Below 2,000 m and south of 30^o S integrated isotherm heave equates to a net uptake of heat of 34 ± 3 TW of excess energy that is entering the deep ocean from deep ocean loss of volume of the AABW and 0.37 ± 15 mm/yr of sea level rise that results from thermal expansion.

Rise from Below²  

Purkey & Johnson estimated the thermal expansion and changes in bottom water distribution resulted in a local mean sea-level rise of about 0.5 mm/yr, a rise that is not insignificant when compared to the global mean of 3 mm/yr. These deep water property changes and geometry should also affect sea level in the Southern Ocean south of 30^o S, but most efforts to measure the rise of sea level focus on changes that are occurring in the heat content of the upper ocean and meltwater from terrestrial glaciers and ice sheets.

Sources & Further reading

1. Purkey, Sarah G., and Gregory C. Johnson. "Antarctic Bottom Water Warming and Freshening: Contributions to Sea Level Rise, Ocean Freshwater Budgets, and Global Heat Gain*." Journal of Climate (2013).
2. Newton, Alicia, 2013, News & Views, Oceanography: Rise from Below, Nature Geoscience, Vol. 6, April 2013.