Neil James Fraser

The Atlantic meridional overturning circulation (AMOC) is sustained by changes in seawater density, but the link between those local density changes and the global-scale overturning circulation has not been fully resolved, in part due to ambiguity in the barotropic component of the flow. We define a new depth coordinate that compensates for the barotropic vortex stretching that balances planetary vorticity changes associated with meridional motion. In this framework, the divergence of barotropic flow is identically zero, and geostrophic divergence is determined solely by zonal density gradients. As a result, the geostrophic component of the Eulerian overturning circulation can be diagnosed directly from the density field without the need for a barotropic reference velocity or assumed level of no motion. We apply this framework to diagnose AMOC structure in a high-resolution numerical model. This application provides a direct link between density structure and overturning, clarifying how diapycnal processes at high latitudes influence Eulerian overturning at lower latitudes.

Small, frequent calving events dominate the behavior of most Arctic marine‐terminating glaciers, yet their oceanographic impacts remain largely unquantified. We present the first direct observations of internal waves generated by modest ice‐fall calving at Kronebreen, Svalbard. High‐resolution current meter and microstructure measurements show that each event excites weakly nonlinear multi‐modal internal wave packets with ∼4 m vertical displacements and mode 1 propagation speeds of 0.35–0.42 ms ⁻¹ . These waves enhance shear and turbulence in the upper 15 m and modify local stratification. Idealized numerical simulations reproduce the observed wave characteristics and confirm their sensitivity to fjord stratification. Given the high frequency of small calving events across Arctic glaciers, calving‐generated internal waves may represent an under‐recognized mechanism contributing to submarine melt and fjord‐scale mixing.

The Atlantic meridional overturning circulation (AMOC) is expected to decline dramatically over the 21st century, with severe impacts for Northern Hemisphere climate. After 20 years of sustained monitoring in the subtropics, a detectable AMOC weakening trend is now beginning to emerge. However, continuous observations at subpolar latitudes are currently too short-lived to determine any weakening signal above the large-amplitude interannual variability. Here, we introduce a new subpolar observing configuration, SCOTIA (Scotland–Canada overturning array), combining parts of the existing OSNAP mooring array with scattered CTD and Argo data, to extend the record of subpolar AMOC backward in time to cover the subtropical monitoring period, 2004–2024. SCOTIA facilitates a rigorous comparison of the decadal-scale variability in transports and overturning at subpolar and subtropical latitudes. Our results show subpolar AMOC varies on pentadal to decadal timescales with an amplitude comparable to that observed in the subtropics. Anomalously high overturning during 2016–2020 was driven by increased southward transports in the density classes associated with Labrador Sea Water. We find no statistically significant trend in subpolar AMOC during the period 2004–2024.

In the flat-bottomed open ocean, the seawater density distribution yields alone the existence of geostrophic baroclinic currents and steric sea-level spatial changes. However, at basin margins, the bathymetry exerts a considerable control on both these quantities. Indeed, the steric sea level vanishes at the coast where depth is zero. Also, continental slopes are vorticity barriers hindering convergence (divergence) of baroclinic transport toward the coast and accumulation (removal) of water there. In the limit of no temporal development, how the coastal sea level is impacted by open-ocean density changes is hence nontrivial and must involve ageostrophic mechanisms. Here, we focus on bottom friction as one such process, provide derivations extending the arrested topographic wave theories to a fully baroclinic ocean, and discuss an application for an eastern boundary margin (representative of the Rockall Slope Current region, for example). We demonstrate that open-ocean density changes yield important joint effect of baroclinicity and relief (JEBAR) along-slope currents, which generate cross-slope Ekman currents due to friction with the seabed. The latter are associated with divergence and convergence leading to sea-level changes above the slope, mediated to the coast via coastally trapped waves. Through this process, not only the coastal sea level is modified but also the along-slope currents are slowed down to well-known asymptotical solutions. Hence, our results link modern theoretical developments in sea-level research and past analytical studies of slope currents. Our effort describes the fundamental notions, and we anticipate it paves the way for more sophisticated works.

Sea level is rising globally, threatening the world coastlines. In this context, it is of paramount importance to understand the physical mechanisms driving spatiotemporal coastal sea-level changes. The adaptation of the coastal sea level to seawater density changes in the open ocean remains, for example, rather poorly understood. The present talk is a contribution towards this understanding. In the flat-bottomed open ocean, density horizontal gradients yield alone the presence of geostrophic baroclinic circulation and spatial variations in steric sea level. At the margins of oceanic basins, the situation is very different. The presence of continental slopes is a vorticity barrier hindering baroclinic geostrophic transport towards the coast and accumulation or removal of water there. In addition, the steric sea level vanishes at the coast where the seafloor depth is zero. In the low-frequency limit, how coastal sea level can be impacted by open ocean density spatiotemporal changes is hence non-trivial and must involve ageostrophic mechanisms. Here, we show that seawater density gradients generate large along-slope currents because of the Joint Effect of Baroclinicity and Relief (JEBAR). The latter currents are slowed down by bottom friction, which in the process transmits the sea level – originally of open ocean and steric origin – to the coast as manometric changes. The framework used is the Arrested Topographic Wave theory extended to a baroclinic ocean.

The Atlantic meridional overturning circulation (AMOC) is expected to decline dramatically over the 21st century, with severe impacts for northern hemisphere climate. After 20 years of sustained monitoring in the subtropics, a detectable AMOC weakening trend is now beginning to emerge. However, continuous observations at subpolar latitudes are currently too short-lived to determine any weakening signal above the large-amplitude the interannual variability. Here, we introduce a new subpolar observing array, SCOTIA (Scotland-Canada overturning array), combining parts of the existing OSNAP mooring array with scattered CTD and Argo data, to extend the record of subpolar AMOC backward in time to cover the subtropical monitoring period, 2004 to 2024. SCOTIA facilitates a rigorous comparison of the decadal-scale variability in transports and overturning at subpolar and subtropic latitudes. Our results show subpolar AMOC varies on pentadal to decadal timescales with an amplitude comparable to that observed in the subtropics. Anomalously high overturning during 2016–2020 was driven by increased southward transports in the density classes associated with Labrador Sea Water. We find no statistically significant trend in subpolar AMOC during the period 2004 to 2024.

The Atlantic meridional overturning circulation (AMOC) is expected to decline dramatically over the 21st century, with severe impacts for northern hemisphere climate. After 20 years of sustained monitoring in the subtropics, a detectable AMOC weakening trend is now beginning to emerge. However, continuous observations at subpolar latitudes are currently too short-lived to determine any weakening signal above the large-amplitude the interannual variability. Here, we introduce a new subpolar observing array, SCOTIA (Scotland-Canada overturning array), combining parts of the existing OSNAP mooring array with scattered CTD and Argo data, to extend the record of subpolar AMOC backward in time to cover the subtropical monitoring period, 2004 to 2024. SCOTIA facilitates a rigorous comparison of the decadal-scale variability in transports and overturning at subpolar and subtropic latitudes. Our results show subpolar AMOC varies on pentadal to decadal timescales with an amplitude comparable to that observed in the subtropics. Anomalously high overturning during 2016–2020 was driven by increased southward transports in the density classes associated with Labrador Sea Water. We find no statistically significant trend in subpolar AMOC during the period 2004 to 2024.

The Atlantic meridional overturning circulation (AMOC) is expected to decline dramatically over the 21st century, with severe impacts for northern hemisphere climate. After 20 years of sustained monitoring in the subtropics, a detectable AMOC weakening trend is now beginning to emerge. However, continuous observations at subpolar latitudes are currently too short-lived to determine any weakening signal above the large-amplitude the interannual variability. Here, we introduce a new subpolar observing array, SCOTIA (Scotland-Canada overturning array), combining parts of the existing OSNAP mooring array with scattered CTD and Argo data, to extend the record of subpolar AMOC backward in time to cover the subtropical monitoring period, 2004 to 2024. SCOTIA facilitates a rigorous comparison of the decadal-scale variability in transports and overturning at subpolar and subtropic latitudes. Our results show subpolar AMOC varies on pentadal to decadal timescales with an amplitude comparable to that observed in the subtropics. Anomalously high overturning during 2016–2020 was driven by increased southward transports in the density classes associated with Labrador Sea Water. We find no statistically significant trend in subpolar AMOC during the period 2004 to 2024.

The Atlantic meridional overturning circulation (AMOC) is expected to decline dramatically over the 21st century, with severe impacts for northern hemisphere climate. After 20 years of sustained monitoring in the subtropics, a detectable AMOC weakening trend is now beginning to emerge. However, continuous observations at subpolar latitudes are currently too short-lived to determine any weakening signal above the large-amplitude the interannual variability. Here, we introduce a new subpolar observing array, SCOTIA (Scotland-Canada overturning array), combining parts of the existing OSNAP mooring array with scattered CTD and Argo data, to extend the record of subpolar AMOC backward in time to cover the subtropical monitoring period, 2004 to 2024. SCOTIA facilitates a rigorous comparison of the decadal-scale variability in transports and overturning at subpolar and subtropic latitudes. Our results show subpolar AMOC varies on pentadal to decadal timescales with an amplitude comparable to that observed in the subtropics. Anomalously high overturning during 2016–2020 was driven by increased southward transports in the density classes associated with Labrador Sea Water. We find no statistically significant trend in subpolar AMOC during the period 2004 to 2024.

The Atlantic meridional overturning circulation (AMOC) is expected to decline dramatically over the 21st century, with severe impacts for northern hemisphere climate. After 20 years of sustained monitoring in the subtropics, a detectable AMOC weakening trend is now beginning to emerge. However, continuous observations at subpolar latitudes are currently too short-lived to determine any weakening signal above the large-amplitude the interannual variability. Here, we introduce a new subpolar observing array, SCOTIA (Scotland-Canada overturning array), combining parts of the existing OSNAP mooring array with scattered CTD and Argo data, to extend the record of subpolar AMOC backward in time to cover the subtropical monitoring period, 2004 to 2024. SCOTIA facilitates a rigorous comparison of the decadal-scale variability in transports and overturning at subpolar and subtropic latitudes. Our results show subpolar AMOC varies on pentadal to decadal timescales with an amplitude comparable to that observed in the subtropics. Anomalously high overturning during 2016–2020 was driven by increased southward transports in the density classes associated with Labrador Sea Water. We find no statistically significant trend in subpolar AMOC during the period 2004 to 2024.