Showing papers by "Eberhard Fahrbach published in 2023"

The Weddell Gyre: what drives a long-term increase in surface nutrients?

Abstract: The Weddell Gyre plays a role in connecting the deep ocean to the surface through upwelling, and also in feeding heat towards the Antarctic ice shelves, regulating the density of water masses that feed the deepest limb of the global overturning circulation. Using Argo floats freely drifting throughout the Weddell Gyre, we describe its horizontal circulation as an elongated double-gyre system, with stronger transports in the east than in west, impacting water property distribution. The eastern sub-gyre region is also associated with stronger upwelling rates than in the west, as shown by radionuclide concentrations. To gain insight to long-term changes in the Weddell Gyre, nutrient concentrations can also be investigated as oceanic tracers. We determine long-term trends in surface silicates, a necessary nutrient for silicifying phytoplankton, from ship-based measurements since 1996, and find that the strongest increase is found in the central western sub-gyre region. In association with the eastern sub-gyre, long-term trends along the Prime Meridian are strongest (albeit weaker than in the central western sub-gyre) in the westward flowing southern limb of the gyre, downstream of Maud Rise. We hypothesize that there are different dynamical drivers, such as wind-driven upwelling (west) and turbulent mixing (east), which cause the positive silicate trends in the east versus the west, which are investigated accordingly.

The Southern Ocean supergyre: a unifyingdynamical framework identified by machinelearning

Abstract: The Southern Ocean closes the global overturning circulation and is key to the regulation of carbon and heat, biological production, and sea level. However, the dynamics of the general circulation and upwelling pathways remain poorly understood. Here, a unifying framework is proposed invoking a semi-circumpolar `supergyre' south of the Antarctic circumpolar current: a massive series of  ‘leaking’ sub-gyres spanning the Weddell and Ross seas that are connected and maintained via rough topography that acts as scaffolding. The supergyre framework challenges the conventional view of having separate circulation structures in the Weddell and Ross seas and suggests a limited utility for climate applications of idealized models and conventional zonal averaged frameworks. Machine learning was used to reveal areas of coherent driving forces within a vorticity-based analysis. Predictions from the supergyre framework are supported by available observations and could aid observational and modelling efforts of the climatically key region undergoing rapid change.