OCS Study

BOEMRE 2010-031

Inner-Shelf Surface Currents and

Characteristic Flow Patterns in the

Santa Barbara Channel

Final Technical Summary
OCS Study

BOEMRE 2010-031

Inner-Shelf Surface Currents and

Characteristic Flow Patterns in the

Santa Barbara Channel

Final Technical Summary

Author

J. Carter Ohlmann

Principal Investigator

Prepared under BOEMRE Cooperative Agreement

M05AC12301

by

J. Carter Ohlmann 

ERI/UCSB

University of California

Santa Barbara, CA 93106
Disclaimer

This report has been reviewed by the Pacific Outer Continental Shelf Region, Bureau of 

Ocean Energy Management, Regulation and Enforcement, U.S. Department of the 

Interior and approved for publication. The opinions, findings, conclusions, and 

recommendations in this report are those of the authors, and do not necessarily reflect the 

views and policies of the Bureau of Ocean Energy Management, Regulation and 

Enforcement. Mention of trade names or commercial products does not constitute an 

endorsement or recommendation for use. This report has not been edited for conformity 

with Bureau of Ocean Energy Management, Regulation and Enforcement editorial 

standards.

Availability of Report

Extra copies of the report may be obtained from:

U.S. Dept. of the Interior

Bureau of Ocean Energy Management, Regulation and Enforcement

Pacific OCS Region

770 Paseo Camarillo

Camarillo, CA 93010

Phone: 805-389-7621

Suggested Citation

The suggested citation for this report is:

Ohlmann, J.C., 2010. Inner-shelf surface currents and characteristic flow patterns in the 

Santa Barbara Channel. BOEMRE OCS Study 2010-031. Earth Research Institute, 

University of California, Santa Barbara, California. BOEMRE Cooperative Agreement

M05AC12301, 45 pages.
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Technical Summary

Study Title: Relationships between Inner-Shelf Surface Currents and Large-Scale 

Characteristic Flow Patterns in the Santa Barbara Channel

Report Title: Inner-Shelf Surface Currents and Characteristic Flow Patterns in the Santa 

Barbara Channel

Contract Number: M05AC12301

Sponsoring OCS Region: Pacific

Applicable Planning Area: Central and Southern California

Fiscal Years of Project Funding: FY2005-FY2010

Completion Date of the Report: October 2010

Costs: $259,685

Cumulative Project cost: $259,685

Principal Investigator: Carter Ohlmann

Key Words: ocean circulation, surface currents, inner-shelf, model validation, wavedriven currents

Background and Objectives:

During the 1990’s and early 2000’s, the BOEMRE sponsored the Santa Barbara Channel 

– Santa Maria Basin Coastal Circulation Study (SBC-SMB CCS), a large observational 

program aimed at improving the understanding and predictability of large-scale

circulation patterns within the study region. A key result of the SBC-SMB CCS has been 

identification of the roles of local winds and a generally poleward pressure gradient in 

forcing surface currents (Auad and Hendershott 1997), and the definition of characteristic 

circulation patterns for the Channel. Patterns were initially defined as “Upwelling”, 

“Cyclonic”, “Relaxation”, “Propagating Cyclones”, “Flood East” and Flood West” 

(Harms and Winant 1998), and later reduced to “Upwelling”, “Surface Convergence” and 

“Relaxation” (Winant et al. 2003). The characteristic patterns are extremely important 

because they provide a simple model for the large-scale circulation in the SBC-SMB. 

Data used in the SBC-SMB CCS do not resolve currents inshore of the 100 m isobath. 

Thus, results of that study hold for basin scale currents, and are not necessarily indicative 

of flows over the inner-shelf.

This project seeks to expand results of the SBC-SMB CCS by focusing on ocean 

circulation over the inner shelf. Particular questions to be addressed are:

• What are the characteristic patterns of circulation over the inner-shelf? 

• How do the inner-shelf patterns relate to the basin-scale patterns reported by Winant et 

al. (2003)? 

• What are the dominant forcing mechanisms?

• How well to modeled trajectories agree with observations?
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Subsequent to the proposed work being funded, significant coastal ocean observing 

system infrastructure was deployed in the study region. The observational array attracted 

modeling efforts. The proposed work plan evolved to take advantage of the added 

measurements, and contribute to the modeling studies. Results of the funded work thus 

extend beyond what was proposed. In addition to the proposed statistical analysis of inner 

shelf flows, the work revisits the historical SBC-SMB CCS drifter data over the inner 

shelf in conjunction with model results, and investigates the role of wave driven flows at 

the surf zone interface. Manuscripts on each of these three topics (tasks) are in various 

stages of the publication process and comprise this final report.

Description:

The inner-shelf circulation was studied through three distinct tasks that naturally follow 

the successful SBC-SMB CCS. 

Task 1: Lagrangian Assessment of Simulated Surface Current Dispersion in the Coastal 

Ocean

Model surface current trajectories computed from ROMS, a state-of-the-art ocean 

circulation model, were validated with historical drifter trajectories. The analysis required 

defining and implementing a quantitative comparison metric specific to Lagrangian data. 

Results show that model trajectories launched over the continental shelf throughout the 

SBC generally agree with observations.

Task 2: Wave-driven inner-shelf motions in Southern California

Drifting buoys that moved onshore generally appeared to decelerate as they reached the 

surf zone. This task tests the hypothesis that observed decelerations are a result of wave 

forced undertow. An undertow model gives decelerations that agree with observations. 

Results indicate that Eulerian observations may not necessarily capture all components of 

Lagrangian flows that move passive tracers.

Task 3: Circulation patterns over the inner-shelf of the Santa Barbara 

Channel

Relationships among moored velocity observations located on the inner-shelf were 

investigated. Results indicate that circulation patterns over the inner-shelf cannot be 

mostly explained by a few coherent statistical modes or patterns, as is the case for 

currents further offshore.

Significant Results: 

Task 1: Lagrangian Assessment of Simulated Surface Current Dispersion in the Coastal 

Ocean

A metric for quantitative assessment of modeled trajectories based on the two 

dimensional Kolmogorov-Smirnov statistical test is presented. The metric is used to show 

that ROMS model trajectories launched over the continental shelf throughout the SBC 

generally agree with observations with a few exceptions in specific regions.

Task 2: Wave-driven inner-shelf motions in Southern California

Drifting buoys that move onshore generally decelerate as they reached the surf zone. It is 

shown with model results that changes in the profiles of Stokes drift and undertow flow 

with water depth can give rise to enhanced offshore directed undertow flow, relative to 
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shoreward directed Stokes drift, of 2 cm/s over the depth of the drifter drogue (0.5 to 1.5 

meters beneath the surface) that may explain the observed decelerations. Thus, during 

conditions of weak winds and strong wave forcing, the ability of a water parcel to cross 

the interface of the outer edge of the surf zone may have significant depth dependence.

Task 3: Circulation patterns over the inner-shelf of the Santa Barbara 

Channel

Circulation patterns over the inner-shelf of the Santa Barbara Channel cannot be mostly 

explained by a few coherent statistical modes or patterns, as is the case for currents 

further offshore. Thus, the circulation modes presented in Winant et al. (2003) are not 

necessarily applicable to the inner-shelf.