Decadal changes in stratification of the abyssal oceans and implications for hydraulic control of abyssal channel flow
Image credit: S.TanAbstract
The observed bottom-intensified warming in the Antarctic Bottom Water (AABW) over the past few decades suggests a reduction in density stratification. By examining the hydrographic data from the WOCE/CLIVAR/GO-SHIP repeat sections over the past three decades, we found an overall decreasing trend in stratification in all the southern hemisphere’s abyssal ocean basins, particularly in the near-bottom layer that is denser than 46.0 sigma4. The deep-reaching reduction in stratification is most evident in abyssal ocean basins south of 20 °S, suggesting that it is an effect of the warming AABW. The AABW is carried northward by the lowest limb of the meridional overturning circulation that flows through abyssal passages as it travels from one basin to the next. In these passages, the flow may be hydraulically controlled, with the deepest part of the AABW being blocked by sills. The largest warming rate of the AABW is found not at the seabed, but hundreds of to a thousand meters above the seabed, upstream of three abyssal passages: the Samoan Passage in the Southern Pacific Ocean, the Vema Channel connecting the Argentine and the Brazil Basins, and a deep fracture zone canyon in the eastern Brazil Basin. As a consequence, there is an increase/decrease in stratification below/above the level of the warming rate maxima, which is found near (usually below) the strongly stratified interface above the abyssal layer. The peak of N2 has decreased by up to 20% upstream of these three passages over the past three decades, which changes the upstream condition of the hydraulic control in the channel. To explore how the change in upstream stratification affects hydraulic control of abyssal channel flow, we propose an idealized hydraulic theory that allows for nonuniform N2 to investigate the critical flow transport as a function of the shape and magnitude of N2 profiles.
