GRACE Satellite Observations of Antarctic Bottom Water Transport Variability

Antarctic Bottom Water (AABW) formation and transport constitute a key component of the global ocean circulation. Direct observations suggest that AABW volumes and transport rates may be decreasing, but these observations are too temporally or spatially sparse to determine the cause. To address this problem, we develop a new method to reconstruct AABW transport variability using data from the GRACE (Gravity Recovery and Climate Experiment) satellite mission. We use an ocean general circulation model to investigate the relationship between ocean bottom pressure and AABW: we calculate both of these quantities in the model, and link them using a regularized linear regression. Our reconstruction from modeled ocean bottom pressure can capture 65%–90% of modeled AABW transport variability, depending on the ocean basin. When realistic observational uncertainty values are added to the modeled ocean bottom pressure, the reconstruction can still capture 30%–80% of AABW transport variability. Using the same regression values, the reconstruction skill is within the same range in a second, independent, general circulation model. We conclude that our reconstruction method is not unique to the model in which it was developed and can be applied to GRACE satellite observations of ocean bottom pressure. These advances allow us to create the first global reconstruction of AABW transport variability over the satellite era. Our reconstruction provides information on the interannual variability of AABW transport, but more accurate observations are needed to discern AABW transport trends. Plain Language Summary Ocean circulation moves heat and carbon around the globe. Changes in the way this circulation moves heat and carbon influence future climate. One part of this ocean circulation is Antarctic Bottom Water, which forms around Antarctica and flows north along the ocean floor into the Pacific, Atlantic and Indian Oceans. Observations of Antarctic Bottom Water are sparse. Those which exist suggest that the volume of Antarctic Bottom Water is declining, but are insufficient to explain why this is happening. We design a new method to try and measure Antarctic Bottom Water transport. The physical equations describing fluid flows suggest gravity signals measured by satellites might be useful. To establish how useful this data is, we simulate the observations of these satellites in an ocean model. We also calculate the transport of Antarctic Bottom Water in the model. This means we