• Speaker: Sung Yong Kim, Ph.D., Prof. (Korea Advanced Institute of Science and Technology -KAIST-)

 Image: An example of the hourly cascade surface current maps observed by an array of high-frequency radar network on (a) the U.S. West Coast, (b) Southern California, and (c) southern San Diego. The grid resolutions of surface current maps are 6 km for Figures 1a and 1b and 1 km for Figure 1c. The HFR sites are indicated with balloons, and their latency in the near-real-time status is shown as colors (e.g., green, yellow, red, and gray). About 100 km diameter mesoscale counterclockwise eddy appears off Point Arena along with the equatorward wind event off Oregon coast in (a), and about 15 km counterclockwise diameter eddy was observed in the Santa Barbara Channel in (b). The white circles with red dots indicate the NDBC wind buoys. As a reference, major coastal regions are denoted by abbreviated two-letter names from south to north: San Diego (SD), Long Beach (LB), Santa Monica (SM), San Buenaventura (VT), Santa Barbara (SB), Point Conception
(PC), Port San Luis (SL), Ragged Point (RP), Monterey Bay (MB), Gulf of the Farallones (GF), San Francisco (SF), Point Reyes (PR), Point Arena (PA), Shelter Cove (SC), Trinidad (TN), Crescent City (CC), Cape Blanco (CB), Winchester Bay (WB), Newport (NP), and Loomis Lake (LL).

Esporles, January 23, 2020.

This talk presents various aspects of coastal surface circulation based on multiyear observations of high-frequency radar-derived surface currents off the U.S. West coast.

The dynamics of the surface currents are governed by tides, winds, Coriolis force, low-frequency pressure gradients (less than 0.4 cycles per day (cpd)), and nonlinear interactions of those forces.

Alongshore surface currents show poleward propagating signals with phase speeds of O(10) and O(100 to 300) km day−1 and time scales of 2 to 3 weeks.

The signals with slow phase speed are only observed in southern California. It is hypothesized that they are scattered and reflected by shoreline curvature and bathymetry change and do not penetrate north of Point Conception.

The seasonal transition of alongshore surface circulation forced by upwelling-favorable winds and their relaxation is captured in fine detail.

Submesoscale eddies, identified using flow geometry, have Rossby numbers of 0.1 to 3, diameters in the range of 10 to 60 km, and persistence for 2 to 12 days.

The HFR surface currents resolve coastal surface ocean variability continuously across scales from submesoscale to mesoscale (O(1) km to O(1000) km). Their spectra decay with k−2 at high wave number (less than 100 km) in agreement with theoretical submesoscale spectra below the observational limits of present-day satellite altimeters.

In addition, the diagnostic characterizations of recent submesoscale observations of surface currents and chlorophyll concentrations are discussed.

Date and Time: Thursday, January 23, 10:00-11:00h

Place: IMEDEA Seminar Room

Source: IMEDEA (CSIC-UIB) and SOCIB