West Coast
Fig. 4. This longwave infrared image of sea surface temperature near Point Sal on 11 September 2017 at 10:41 UTC shows a curving wake (indicated by the white dashed line) caused by flow separation in the lee of Point Sal. Credit: Mike Kovatch, Ken Melville, and Luc Lenain

Untangling a Web of Interactions Where Surf Meets Coastal Ocean

In 2017, an ocean research team launched an unprecedented effort to understand what drives ocean currents in the overlap regions between surf zones and continental shelves.

By James Lerczak, John A. Barth, Sean Celona, Chris Chickadel, John Colosi, Falk Feddersen, Merrick Haller, Sean Haney, Luc Lenain, Jennifer MacKinnon, James MacMahan, Ken Melville, Annika O’Dea, Pieter Smit, and Amy Waterhouse  4 hours ago

Winds and waves drive the coastal ocean’s waters to flow and mix. So do differences in temperature, salinity, the topography of the seafloor, and a host of other factors. All these factors overlap and interact in complex patterns that influence where ocean creatures make their homes and where waterborne materials, both natural and human made, are dispersed along our coasts. The coastal physical oceanography community has made great strides in understanding the dynamics that drive water motions and density distributions in the coastal ocean. They have also worked to demonstrate the importance of these dynamics to coastal communities and ecosystems.

In the inner shelf region, where the continental shelf and shore regions overlap and their processes interact, challenges to our understanding persist.Over the past several decades, oceanographers have undertaken large field experiments to quantify coastal dynamics and their impacts. Often, these studies have been partitioned into specific regions of the coastal ocean and focused on specific processes: wind effects over the continental shelf or wave effects close to shore, for example. However, in the inner shelf region, where the continental shelf and shore regions overlap and their processes interact, challenges to our understanding persist [Lentz and Fewings, 2012].

In the summer and fall of 2017, a group of researchers sponsored by the U.S. Office of Naval Research (ONR) undertook an unprecedented seagoing and numerical ocean modeling experiment. The Inner Shelf Dynamics Experiment investigated the nonlinear, interacting processes that drive currents and transport in this important coastal region.

Studying Sea and Shore and Where They Overlap

The Coastal Ocean Dynamics Experiment (CODE) of the early 1980s was a major collaborative effort to explore wind-driven circulation on the continental shelf in northern California [Beardsley and Lentz, 1987]. These experiments produced a data set unprecedented for its time, and they inspired and motivated many field and numerical experiments on stratified wind-driven flows over midcontinental shelves (water depths around 50–100 meters).

Closer to shore, wave dynamics and wave-driven transport have been studied in great detail by the nearshore science community. In particular, a suite of experiments, including Duck94 and SandyDuck, at the U.S. Army Corps of Engineers Field Research Facility in Duck, N.C., was seminal in expanding knowledge of surf zone dynamics [e.g., Long and Sallenger, 1995].

The less explored inner shelf, with typical water depths ranging from 5 to 50 meters, is the region where the surf zone meets and interacts with the coastal ocean. Within the surf zone, breaking waves dominate the dynamics and can drive large wave-averaged flows, such as rip currents. On the density-stratified continental shelf, several mechanisms compete to drive currents, including wind forcing, bathymetric influences, tides, submesoscale eddies, and shoaling and breaking nonlinear internal bores and waves.

At the inner shelf, the dynamics that typify both the nearshore and continental shelf are in play. These overlapping dynamics lead to highly nonlinear, interacting processes that regulate the alongshore and across-shore transport of water, water properties (e.g., temperature), and waterborne materials (e.g., sediment, dissolved gases, plankton, and contaminants). These inner shelf processes vary over a wide range of spatial and temporal scales, and their interactions are poorly understood. In addition, interactions between currents and variable coastal bathymetric features (e.g., headlands) enhance the complexity of transport.

The Inner Shelf Dynamics Experiment aims to understand the interacting nonlinear dynamics of the inner shelf and identify and quantify the processes that drive the exchange of water properties and waterborne materials across this region over a range of temporal and spatial scales.

This ONR Departmental Research Initiative is centered around an extensive, multi-institutional field experiment coordinated with numerical modeling efforts to study a 50-kilometer stretch of the central California coast that straddles Point Sal and includes the region offshore of Vandenberg Air Force Base (Figure 1).

Map of the Inner Shelf Dynamics Experiment study site and X band radar measurements
Fig. 1. (a) Map of the Inner Shelf Dynamics Experiment study site, showing locations of moorings and bottom landers and measurement footprints of coastal X band and coherent radar systems. Contour lines represent water depth in meters. (b) Composite image of X band radar ocean surface measurements (time averaged to remove surface gravity waves) showing surface signatures of inner shelf processes, including coherent internal bore fronts and high-frequency internal waves. Credit: (a) Jim Lerczak; (b) Sean Celona

Our overarching goals are the following:

  • improving our understanding of inner shelf hydrodynamics
  • developing and improving the predictive capability of a range of numerical models to simulate the three-dimensional circulation, density, and surface wave field across the inner shelf
  • coupling a suite of remote sensing platforms with in situ measurement arrays to produce a synoptic description of inner shelf processes across the study region

Sensors at Sea and in the Sky

During the field component of the experiment, which took place from late August to November 2017, we obtained a diverse and unprecedented suite of in situ and remote sensing measurements.

Moorings and Landers. We installed a broad array of 176 mooring and bottom lander platforms to make in situ time series measurements that spanned the continental shelf to the nearshore in water depths ranging from 150 to 6 meters (Figures 1a and 2). These measurements included temperature, salinity, velocity, surface wave, turbulence, and meteorological measurements. The array focused on three regions with different bathymetric features: a region with a fairly straight, planar beach (Oceano); a coastal headland (Point Sal); and a region between two coastal capes (Vandenberg).

Time series of density anomaly and cross-shore current as a function of depth at the Oceano array.
Fig. 2. Twelve-hour time series of density anomaly (observed density minus 1,000 kilograms per cubic meter; contoured) and cross-shore current (color shaded) as a function of depth from the mooring-lander pair at a water depth of 50 meters at the Oceano array. A sharp internal bore front arrives at this location at 14:30 Coordinated Universal Time (UTC). A packet of high-frequency internal waves arrives at 22:00 UTC. Credit: Jim Lerczak

Shipboard Surveys. We conducted coordinated shipboard surveys during three intensive operation periods. We used three ships: R/Vs Sally Ride, Oceanus, and R. G. Sproul (the Sproul was funded with University of California Ship Funds). We also used four boats: R/Vs Kalipi, Sally Ann, Sounder, and Sand Crab.

Read full article . . .