Characteristics and Variability of Antarctic Intermediate Water in the UKESM1-0-LL CMIP6 model

<p>Antarctic Intermediate Water (AAIW) is the dominant intermediate water mass in the Southern Hemisphere. AAIW plays a key role in the hydrological cycle and also contributes to the replenishment of nutrients at low latitudes. It is characterised by a mid-depth salinity minimum. Although its salinity minimum signature can be clearly identified, the formation mechanisms and how its properties evolve with climate change are unclear.</p><p>The aim of this study is to assess the ability of the UKESM1-0-LL CMIP6 model to represent the key characteristics and variability of AAIW and to evaluate its evolution under radiative forcing (with the SSP5-8.5 and SSP2-4.5 scenarios).</p><p>A diagnostic is developed to identify the core of AAIW in the different basins and scenarios. AAIW can be identified in the UKESM1-0-LL model but it is lighter than in observations. The Pacific, Atlantic and Indian type of AAIW have core density values of 26.5 kg/m<sup>3</sup>, 26.6 kg/m<sup>3</sup> and 26.9 kg/m<sup>3</sup> respectively. AAIW presents different properties across each basin with different depth, temperature and salinity properties. The Pacific type of AAIW is lighter and fresher than the Atlantic and Indian types of AAIW. Under radiative forcing, it is found that AAIW shoals and becomes warmer. The largest changes in temperature, salinity and density are found in the Pacific. The outcrop location of the salinity minimum remains constant in the different scenarios in spite of the surface conditions changing with climate change.</p><p>A change in depth could have major implications on the overturning circulation. Ongoing and future work focuses on identifying which mechanisms need to be improved in CMIP6 models to reduce the bias observed in AAIW.</p>

Water Mixing and Circulation within the South Atlantic Basin Constrained by Seismic Reflection Images

<p>South Atlantic water masses and circulation significantly influence the dynamics and water mass structure of the Atlantic Meridional Overturning Circulation (AMOC). Previous research in the South Atlantic has mostly focused on energetic regions such as the Brazil/Malvinas Confluence Zone along the western boundary and the Agulhas retroflection to the east. However, it is also important to understand water circulation and diapycnal mixing within the South Atlantic Basin (SAB). Previous studies have observed low salinity patches of the Antarctic Intermediate Water within the western side of the SAB at 30<sup>o</sup> S, but the temporal variability of the scales, locations and structures of these low salinity patches are still uncertain. Former studies also show an increased level of mixing within the SAB above the Mid-Atlantic Ridge, but did not evaluate mixing on smaller scales such as mesoscale and sub-mesoscale.</p><p>Here we present a water mass structure analysis at 30<sup>o</sup> S from Rio Grande Rise to the Mid-Atlantic Ridge by using Seismic Oceanography (SO). SO is being applied around the world to image mesoscale water mass structures using the seismic reflection method. Reflections in the seismic images are essentially temperature gradients that are proxies for isopycnal surfaces. We paid particular attention in seismic processing to imaging of structures that characterize the boundary between water masses. We imaged the upper South Atlantic Central Water, and identified discontinuous water boundaries (about 150 km long) between the Antarctic Intermediate Water and the North Atlantic Deep Water that could correspond to the intermittent appearance of low salinity patches. We combine seismic images with previous hydrographic measurements to investigate the temporal change of these low salinity patches. We use a horizontal slope spectra to quantify mixing rate from tracked seismic horizons to evaluate mesoscale and sub-mesoscale mixing events such as internal waves and eddies. Through SO, we hope to better constrain South Atlantic circulation and contribute to the understanding of AMOC as a whole.</p>

Opening the window to the Southern Ocean: The role of jet dynamics

The surface waters of the Southern Ocean act as a control valve through which climatically important tracers such as heat, freshwater, and CO2 are transferred between the atmosphere and the ocean. The process that transports these tracers through the surface mixed layer into the ocean interior is known as ocean ventilation. Changes in ocean ventilation are thought to be important for both rapid transitions of the ocean’s global overturning circulation during the last deglaciation and the uptake and storage of excess heat and CO2 as a consequence of anthropogenic climate change. I show how the interaction between Southern Ocean jets, topographic features, and ocean stratification can lead to rapid changes in Southern Ocean ventilation as a function of wind stress. For increasing winds, this interaction leads from a state in which tracers are confined to the surface mixed layer to a state in which tracers fill the ocean interior. For sufficiently high winds, the jet dynamics abruptly change, allowing the tracer to ventilate a water mass known as Antarctic Intermediate Water in the mid-depth Southern Ocean. Abrupt changes in Antarctic Intermediate Water ventilation have played a major role in rapid climate transitions in Earth’s past, and combined with the results presented here, this would suggest that jet dynamics could play a prominent role in contributing to, or even triggering, rapid transitions of the global climate system.