Projects
Balanced and unbalanced signals in SWOT data
Principal Investigator: Jörn Callies (California Institute of Technology)
We plan to investigate the origin of SWOT’s ocean signals to understand what they can tell us about the dynamics of meso- and submesoscale turbulence and the tracer exchange this turbulence effects between the surface and interior ocean. We pursue two specific goals. First, we aim to understand the balanced dynamics underlying the signal that dominates the SWOT data from the extratropics at relatively large scales (greater than about 40 km wavelength). Second, we aim to determine the origin of the clearly distinct signal that begins to appear at smaller scales and dominates down to the resolution scale (4 km wavelength).
The majority of the work will be directed at the signal dominating at large scales, which conforms to expectations for balanced turbulence derived from theory, in situ observations, and numerical simulations. In high-energy regions, this balanced signal can be isolated using a simple statistical approach, and we aim to further develop this approach and apply it across the world ocean. Once the balanced signal has been extracted, this will enable the first global study of submesoscale turbulence from observations.
Our preliminary analysis reveals intense cyclones and fronts that challenge our current interpretation of altimetry data based on geostrophic balance and quasi-geostrophic dynamics. We therefore plan to develop an interpretation of the balanced signal in SWOT data that is instead based on semi-geostrophic theory. This theory captures ageostrophic effects while continuing to allow for a straightforward interpretation of altimetry data and retaining much of the conceptual simplicity of quasi-geostrophic dynamics. We aim to leverage this interpretive framework to understand cyclone–anticyclone asymmetries, quantify the stretching and vertical transport implied by the evolution observed during SWOT’s rapid-repeat phase, and to work towards a diagnosis of the scale transfers of kinetic energy, including the downscale transfers produced by ageostrophic advection during frontogenesis.
To enable full use of SWOT’s data, we additionally aim to investigate the origin of the small-scale signal that is clearly not balanced. Our preliminary analysis indicates that this signal is much more stable geographically than the balanced one, so the internal-wave continuum might be a major contender for explaining it. We show that the spectral slope, geographic variations, and seasonal trends of the small-scale signal, however, are inconsistent with expectations for the surface signature of the internal-wave continuum. The geographic variations and seasonal trends instead match those of the surface gravity wave field, suggesting that some process related to surface waves might dominate SWOT’s signal at small scales. We plan to further explore the phenomenology of this signal to identify the process underlying it, with a goal to understand the governing physics and potentially to uncover currently buried signals from internal waves and balanced motion.
