Trends in the deep Southern Ocean (1958–2010): Implications for Antarctic Bottom Water properties and volume export

Abstract

[1] Regional formation of deep and bottom water masses around the Antarctic continental shelf is one of the most important processes contributing to variability of the global meridional overturning circulation deep cell. Southern Ocean hydrographic data collected during the years 1958-2010 indicate that dense shelf waters cooled and freshened during that period. In the surrounding open ocean, Antarctic Bottom Water (AABW) warmed, with no evidence of salinity change. As a result of source-water property changes, AABW exported from the Southern Ocean to the deep world ocean became lighter over the period analyzed. The average rate of density change within the areas that experienced statistically significant change was À0.0019 kg m À3 yr À1 . For the last 20 years of the analysis, a negative AABW volume anomaly (relative to the half-century average, 1958-2010) was indicated, possibly due to production of a lighter AABW variety or to changes in formation rates. Over the entire five decades, the upper isopycnal of the AABW layer deepened at a rate of À8.1 m yr À1 . Changes in fundamental hydrographic properties such as these can have important implications for long-term global ocean circulation and climate.

Assessment of the structure and variability of Weddell Sea water masses in distinct ocean reanalysis products

“…In concordance with these findings, a decrease in the WSBW's contribution (∼ 20 %) to the total water mass mixture in the Weddell Basin occurred in the 1980s and 1990s near the Greenwich Meridian and at the tip of the Antarctic Peninsula (Kerr et al, 2009a). More recently, Azaneu et al (2013) fully investigated the most complete Southern Ocean data set available and found a reduction in the volume of AABW in addition to warming and decreasing density from 1958 to 2011 in the deep and bottom layers south of 60 • S. Despite the reported freshening of the AABW layer at the Drake Passage during the period 1993-2010 (Jullion et al, 2013), no sign of this freshening trend was found by Azaneu et al (2013) in WSDW/WSBW layers in the last 50 years in the inner Weddell Sea. In a global context, the AABW's layer in the global basins underwent a contraction from the 1980s to the 2000s (Purkey and Johnson, 2012).…”

“…All reanalyses examined showed a γ n decreasing trend for WOCE SR4 (except for the SODA product; Table 4). AABW observations showed a warming trend for the global AABW exported from the Southern Ocean and a reduction in its volume (e.g., Johnson, 2010, 2012;Azaneu et al, 2013). The latter may be a consequence of the downward trend in AABW density, although no sign of freshening has been found in the inner Weddell Sea (Azaneu et al, 2013).…”

“…AABW observations showed a warming trend for the global AABW exported from the Southern Ocean and a reduction in its volume (e.g., Johnson, 2010, 2012;Azaneu et al, 2013). The latter may be a consequence of the downward trend in AABW density, although no sign of freshening has been found in the inner Weddell Sea (Azaneu et al, 2013). Recently, Jullion et al (2013) reported a significant freshening of AABW of −0.004 decade −1 in the Drake Passage, with no significant decrease in its thickness.…”

“…Over the past few decades, changes in the thermohaline properties of AABW source waters have been reported, such as freshening of the dense waters in the shelf regions (e.g., Hellmer et al, 2011;Azaneu et al, 2013) and long-term warming of WDW within the Weddell Sea (e.g., Robertson et al, 2002;Smedsrud, 2005). In turn, WSBW in the inner Weddell Sea also experienced warming during the second half of the 1990s (Fahrbach et al, 2004.…”

On deep convection events and Antarctic Bottom Water formation in ocean reanalysis products

“…However, since no real open ocean polynya has been reported for this period, a critical analysis of the model mechanisms of heat exchange between the surface waters and sea ice is required in the future to efficiently understand the role of WDW in open ocean polynya establishment. In addition, since bottom layer warming and intermediate layer cooling are the possible mechanisms that diminished stratification in ECCO2, further evaluation of the causes of those trends (Azaneu et al, 2013) is needed to understand the primary factors leading to the weak ocean surface stratification. Finally, the appearance of a real open ocean polynya in October 2016 and September 2017 in the Weddell Sea will also furnish a study scenario to understand heat exchange between sea ice and warmer waters, and to compare with ocean reanalysis products.…”