High-wavenumber Variability of Sea Surface Height: Evaluating Sub-100-km Scales with Altimetry, ADCP, and Model Output

Sarah Gille (Scripps Institution of Oceanography, UC San Diego, United States)

CoAuthors

Saulo Soares (Scripps Institution of Oceanography, UC San Diego, United States); Teresa Chereskin (Scripps Institution of Oceanography, UC San Diego, United States); Eric Firing (University of Hawaii, USA); Julia Hummon (University of Hawaii, United States); Yao Yu (Scripps Institution of Oceanography, UC San Diego, United States); David T. Sandwell (Scripps Institute of Oceanography, University of California San Diego, USA)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Salient results from the 2017-2020 OSTST PIs

Presentation type: Type Forum

New satellite altimeters resolve variability in sea surface height on spatial scales less than ~100 km. One challenge in examining new sea surface height products is understanding what governs ocean variability on these newly accessible small scales. Our work has drawn on in situ observations from shipboard Acoustic Doppler Current Profilers (ADCPs) along with high-resolution model output from a 1/48 degree MITgcm simulation (llc4320) to probe the critical mechanisms responsible for ocean variability on scales less than 100 km.

In published work focused on the California Current region (Chereskin et al, 2019), we use a Helmholtz decomposition to examine the transition from balanced (geostrophic) flow to unbalanced (ageostrophic) flow. In ADCP data this transition occurs at length scales of 70 km, while in model output the transition is around 125 km. ADCP and model spectra are consistent with 3 different altimeter data sets in the balanced regime but diverge at smaller scales. These results imply that length scales less than 70-100 km resolved by new altimeters become increasingly influenced by unbalanced motions associated with submesoscale turbulence and waves and are dominated by unbalanced motions at the smallest scales.

We have extended this work to the tropical Pacific to examine a different regime of submesoscale and mesoscale ocean variability. The tropical eastern Pacific analysis has required recovering and providing final data quality control for previously unprocessed ADCP data. Results from shipboard data have been compared with analogous analyses from model output. Both model and data suggest that in the tropics the transition length scales can be longer than in the California Current, and this is particularly true in the deep tropics. The observations also imply less seasonality than the model suggests, with different underlying mechanisms. (See presentation by Saulo Soares for details.)

At the small-scale end of the wavenumber spectrum, we have drawn on ICESat-2 laser altimetry data, which has a footprint (~15 m) small enough to resolve individual surface waves. ICESat-2 shows that long-period swell is highly energetic and dominates the sea surface height spectrum at scales between 300 and 1000 m. When wave crests align with the satellite ground track, the ground speed of ICESat-2 results in swell waves aliasing to length scales of 50-100 km. ICESat-2 thus provides insights into small-scale sea surface height variability but also presents challenges due to its small footprint. (See presentation by Yao Yu for details.)
 
Sarah Gille
Scripps Institution of Oceanography, UC San Diego
United States
sgille@ucsd.edu