The lower limb of the Global Meridional Overturning Circulation (GMOC) is supplied by the sinking of the heavy water mass that forms around the Antarctica, known as the Antarctic Bottom Water (AABW). Repeat hydrographic data along the Atlantic, Pacific and Indian Ocean sections observed during the mid 1990s, 2000s and 2010s indicate that the volume of AABW decreased drastically (i.e., AABW became warmer and fresher) during the last three decades. This suggests a weakening GMOC in the lower limb, and also a shift in the global ocean and atmosphere energy balance. A recent study published in the Nature Climate Change analysed observations of AABW in the Scotia Sea, the pathway from the Weddell Sea to the Atlantic Ocean, to show that the the volume of AABW has partially recovered during 2014-2018. However, this study did not offer a clear mechanism responsible for the recently increased AABW volume. More specifically, the study found no clear signal in wind stress or its curl over the Weddell sea where a significant portion of the AABW is formed.
Figure 2 from Abrahamsen et al. (2019): Area of Lower Weddell Sea Deep Water (LWSDW) from hydrographic sections. a,b, Graphs showing LWSDW (γn= 28.31 kg m−3) areas from a section in the eastern Scotia Sea (a) and from the western Scotia Sea section (b). The blue bars are calculated from the measured properties; the red bars are compensated for the temperature anomalies on the γn= 28.31 kg m−3 surface; the green bars are compensated for salinity anomalies; and the black bars are compensated for both temperature and salinity anomalies.
Abrahamsen, E. P., Meijers, A. J., Polzin, K. L., Garabato, A. C. N., King, B. A., Firing, Y. L., … & Meredith, M. P. (2019). Stabilization of dense Antarctic water supply to the Atlantic Ocean overturning circulation. Nature Climate Change, 1-5, https://doi.org/10.1038/s41558-019-0561-2