Abstract's details
Vertical velocities and deep subduction events from coincident SWOT and glider observations in the Southern Ocean
Event: 2025 SWOT Science Team Meeting
Session: Oceanography: Velocities
Presentation type: Oral
The FOCUS (Fine-scale Observations of the Antarctic Circumpolar Current Under SWOT) project is a regional validation study conducted in support of the SWOT mission. FOCUS targeted an energetic region of the Antarctic Circumpolar Current thought to contribute disproportionately to meridional heat transport and ocean ventilation. A five-week cruise on the R/V Investigator collected data with a towed profiler and deployed multiple ocean gliders, floats, and drifters.
Through a combination of high-resolution in situ observations from the FOCUS campaign and observations collected from SWOT, we describe an approach to estimate vertical velocities, and potentially vertical tracer transport, using the quasi-geostrophic (QG) omega equation. First, we show that there is a tight relationship between local sea surface height (SSH) and vertical density profiles as collected from a pair of ocean gliders. This relationship holds at O(10-km) scales, and allows for the inference of a three-dimensional interior density field (to a depth of 1000 m) from two-dimensional SSH fields from SWOT. The reconstructed density field is combined with surface velocity estimates and the thermal wind relationship to produce fields of buoyancy and velocity gradients, which then enable an estimate of ocean vertical velocities using the QG omega equation.
A strong cyclonic eddy, sampled by the FOCUS gliders, has horizontal upper ocean velocities that exceed 1 m/s and a Rossby number > 1. The QG omega equation predicts vertical velocities associated with this cyclone that exceed many hundreds of meters per day and extend to depths of 800 m. These elevated velocities are confined to sharp density gradients around the periphery of the eddy. The magnitude of the vertical velocity is larger and the spatial distribution is more compact as compared to vertical velocities inferred from altimetry using surface QG (SQG) approaches in previous studies. Enhanced vertical velocities coincide with glider-observed deep tracer anomalies, such as optical backscatter, consistent with rapid subduction events. The QG omega approach is also tested using numerical model output, sampled similarly to the glider and SWOT observations, which confirms that these estimates capture a large fraction of the total vertical velocity. Finally, we explore the ability to extend the QG omega approach to other regions of the Southern Ocean using Argo float and other hydrographic data.
Back to the list of abstractThrough a combination of high-resolution in situ observations from the FOCUS campaign and observations collected from SWOT, we describe an approach to estimate vertical velocities, and potentially vertical tracer transport, using the quasi-geostrophic (QG) omega equation. First, we show that there is a tight relationship between local sea surface height (SSH) and vertical density profiles as collected from a pair of ocean gliders. This relationship holds at O(10-km) scales, and allows for the inference of a three-dimensional interior density field (to a depth of 1000 m) from two-dimensional SSH fields from SWOT. The reconstructed density field is combined with surface velocity estimates and the thermal wind relationship to produce fields of buoyancy and velocity gradients, which then enable an estimate of ocean vertical velocities using the QG omega equation.
A strong cyclonic eddy, sampled by the FOCUS gliders, has horizontal upper ocean velocities that exceed 1 m/s and a Rossby number > 1. The QG omega equation predicts vertical velocities associated with this cyclone that exceed many hundreds of meters per day and extend to depths of 800 m. These elevated velocities are confined to sharp density gradients around the periphery of the eddy. The magnitude of the vertical velocity is larger and the spatial distribution is more compact as compared to vertical velocities inferred from altimetry using surface QG (SQG) approaches in previous studies. Enhanced vertical velocities coincide with glider-observed deep tracer anomalies, such as optical backscatter, consistent with rapid subduction events. The QG omega approach is also tested using numerical model output, sampled similarly to the glider and SWOT observations, which confirms that these estimates capture a large fraction of the total vertical velocity. Finally, we explore the ability to extend the QG omega approach to other regions of the Southern Ocean using Argo float and other hydrographic data.