Coastal to open ocean exchange in the California Current System

Author:

Sarah Gille - (University Of California, San Diego)

Co-Investigator(s):

Julia Hummon (University of Hawaii, Honolulu)
Saulo Soares (University Of California, San Diego)
Steven Howell (University of Hawaii, Honolulu)
Teresa Chereskin (University Of California, San Diego)

Abstract:

Sentinel 6/Jason-CS will usher in a new era of satellite altimetry, taking advantage of synthetic aperture radar (SAR) techniques to obtain high-resolution sea surface height (SSH) measurements along the well-established TOPEX/Poseidon/Jason ground tracks. Together with other recent altimeters, including Jason-3, CryoSat-2, AltiKa, ICESat-2, and Sentinel-3, these measurements offer new opportunities to evaluate small-scale mesoscale processes typical of coastal zones. The proposed work uses the southern California Current System (CCS) as a test case to investigate the observability of small-scale eddies from satellite altimetry and to assess the role of these eddies in mediating cross-shelf exchanges between the coast and the open ocean. Our objectives include (1) taking advantage of high-resolution in situ observations from Acoustic Doppler Current Profilers (ADCPs), High-Frequency coastal radars (HFRs), and moored sensors to evaluate the fidelity of new SAR-mode altimeter observations in representing eddy-scale processes; (2) evaluating the resolution obtained by retracked coastal altimeter products that extend over the 25+ year duration of the radar altimeter record; and (3) assessing the impact that mesoscale eddies have on the exchanges between the deep ocean and coastal waters, as well as their impact on coastal circulation variability.

Understanding exchanges between the coastal ocean and the deep ocean is key to quantifying the transport of heat, nutrients, and planktonic life forms, as well as the export of pollutants such as plastic. Mesoscale eddies are an important mechanism mediating these exchanges. However, quantification of these exchanges lacks widespread observational verification, largely because of a lack of sustained observations that resolve coastal-scale eddies. Altimetric measurements of SSH provide information about geostrophic circulation. Our recent work has underscored the magnitude of submesoscale SSH variability, associated with ageostrophic motions such as internal waves, fronts, and filaments, which may play a key role in mediating exchanges between the coastal and deep ocean.

We focus on the CCS because it is regularly sampled by the quarterly surveys of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) and the California Current Ecosystem (CCE) LTER, as well as by gliders (California Underwater Glider Network), moorings, and a large network of coastal HFRs. SWOT Calibration and Validation will also take place in the region, offering enhanced sampling during our study period. Our objectives are to:

1. Evaluate high-resolution altimeter products as measures of SSH and upper ocean currents taking advantage of in situ observations from the CCS region.
2. Estimate the lateral fluxes between the coastal and deep ocean using altimetry, in situ observations and numerical model output.
3. Use altimetry, including high-resolution coastal products, to extend flux estimates in time (over the record) and in space (closer to the coast and along coast).
4. Determine the fraction of transport that is geostrophically balanced, using the Helmholtz decomposition applied to the above data sets. As part of this work, evaluate the use of the Helmholtz decomposition in coastal settings, where assumptions may be violated.

Results gleaned from the California Current assessment can be extended to other coastal regimes where fewer in situ observations are available, and a final component of our proposed work will assess the viability of assessing coastal-to-open-ocean exchanges in other regions. Our research group brings to this work experience with high-latitude exchanges around Greenland and Antarctica, where cross-shelf exchanges can govern ocean interactions with ice sheets and have potential repercussions for sea level rise, and we will evaluate whether methods tested in the California Current can be applied in these high-latitude regimes.

Supported by NASA