The Abyssal Ocean's Contributions to the Global Energy and Sea Level Budgets Between the 1990s and 2000s
Purkey, Sarah Michelle Goldstein
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Over the past three decades, Antarctic Bottom Water (AABW), a cold, dense water-mass produced around Antarctica that feeds the bottom limb of the Meridional Overturning Circulation (MOC), has warmed, freshened, and declined in volume. We quantify water-property changes in the deep Southern Ocean and the abyssal global ocean and evaluate the relative contribution of these abyssal changes to the global energy and sea level rise budgets. We find a strong warming trend throughout the deep (greater than 1000 m) Southern Ocean and abyssal (greater than 4000 m) global oceans, equivalent to a heat flux of 0.095 (± 0.062) W m <super>-2</super> applied over the entire surface area of the Earth, a statistically significant fraction of the present global energy budget. The observed warming pattern is also consistent with a global-scale contraction of AABW, suggesting a slowdown of the bottom limb of the MOC. In addition, freshening of 0.02 PSS-78 per decade is observed in the AABW directly downstream from formation sites along the Antarctic continent, with freshening rates roughly a tenth of this in the deep interior of basins adjacent to Antarctica in the Indian and Pacific sectors of the Southern Ocean. The fresh water flux required to account for the observed freshening of AABW in these two basins is 73 (± 26) GT yr <super>-1</super>. Finally, the full-depth steric contributions to SLR calculated along the repeated hydrographic sections are used to assess regional and global rates of SLR owing to mass addition through a full depth SLR budget and compared to mass addition estimated directly using data from the Gravity Recovery And Climate Experiment (GRACE) from 2003-2013. These two independent methods both find a global mean rate of mass addition of 1.5 (± 0.4) mm yr <super>-1</super> over their respective periods with large regional variability with higher rates of mass addition in the North Pacific, South Atlantic, and Indo-Atlantic sector of the Southern Ocean, possibly associated with recent changes in the gravity field from ice loss in these two regions.
- Oceanography