Project Summary

This project examined how changing winds along the California coast generate

ocean currents that flow along the coastline. Typically winds blow strongly

southward along California’s coast to produce currents that also flow southward.

These currents form the California Current System and are part of a large, clockwise

circulation pattern in the North Pacific Ocean. Similar currents flowing toward the

equator along the eastern sides of ocean basins occur in the South Pacific, and

Atlantic Oceans. Together they are called eastern boundary current systems.

Most studies of ocean circulation along the California coast have focused on times

when southward winds prevail because these cause deep waters to rise to the

surface near shore in a process called upwelling. Upwelling is very important

because it supplies nutrients to surface waters which in turn support highly

productive ecosystems. These ecosystems benefit society by supporting productive

fisheries. Similar productive fisheries are found in the other eastern boundary

current systems.

 

But southward winds and upwelling are not the entire story of ecosystems along the

California coast. In this project we focused on times when winds weaken and cause

currents near shore to reverse and flow northward, or poleward. In contrast to

upwelling conditions that bring cooler waters, these poleward-flowing currents

bring warmer waters to nearshore areas. The warm waters accumulate behind

coastal headlands such as the one formed by Point Conception and Point Arguello in

southern California. After winds relax these waters flow around the headland and

move poleward. The Pt. Conception-Pt. Arguello headland is particularly large so the

poleward flows are very strong here. Similar poleward flows occur in the North

Atlantic along the coast of Portugal where the warm waters originate in the Gulf of

Cadiz.

 

These reversed currents are important because they transport many small marine

animals and their larvae northward along the coast. As they travel, many of these

animals find suitable habitats on shallow reefs or closer to shore in tide pools and

estuaries. They remain behind after southward winds and currents return. In this

way reversed coastal currents bring southern species northward along the

California coast to support the diversity of coastal marine ecosystems found here.

A goal of our project was to measure the size of these flows, to learn how they

propagate, and to determine the flow patterns they produce. An interesting feature

of these flows occurs near shore. On the inner part of the poleward currents, there is

often well-defined front end or “nose” that moves along the coast at about a walking

pace, typically 10 to 30 kilometers (6-19 miles) per day. This is unlike most ocean

currents that are continuous with no clearly defined front end. On a human scale the

poleward currents are large. They extend offshore up to 30 kilometers offshore (18

miles) and can be hundreds of kilometers long as they move northward along the

California coast.

 

In the nose regions of these currents, seawater flows toward coastline in wide

curving arcs. The seawater in the currents brought to the coastline is noticeably

warmer than the waters they displace, up to 4 °C (7 °F). If you were swimming in the

ocean, say off a beach on the central California coast, during the arrival of one of

these flows would notice such a large temperature increase. You might also notice

yourself being gently pushed toward the shore.

 

The large headland formed by Points Conception and Arguello that was the center of

our study area is shown in Figure 1. The headland shifts the orientation of the

coastline from roughly north-south to east-west. When upwelling winds prevail,

warm water is found to the east and south of Pt. Conception in an area called the

Southern California Bight. Figure 2 shows the pattern of surface currents during 5-9

June 2012 (black arrows) when strong, upwelling winds were blowing toward the

southeast along the coast (red arrows). This is a typical summertime current

pattern in the region. Water temperatures were cold along the coast north of Pt.

Arguello, about 10 °C (50 °F). Figure 3 shows currents a few days later when winds

relaxed during 10-11 June. Current patterns shifted dramatically. Southward

currents near Pt. Conception and Pt. Arguello now turned westward and northward

as they flowed up the central California coast. The surface current patterns of

Figures 2 and 3 were observed using a network of oceanographic radars (small

yellow triangles). We observed many such changes in current patterns during our

study.

 

We also studied the pattern of changing atmospheric conditions that give rise to

wind relaxations. To do this we used winds measured from NOAA meteorological

buoys and from satellites. The wind relaxations observed in our study area are part

of a very large atmospheric pattern that evolves over about a 10-day period along

the west coast of the U.S.

Figure captions:

Figure 1. Study area near Point Conception, California

The study area was centered along the south-central California coast on the

headland of Pt. Conception and Pt. Arguello. The area encompassed the Santa

Barbara Channel and the Northern Channel Islands. The Pacific Ocean lies to the

west and the greater Southern California Bight lies to the east and south.

Figure 2. Surface currents during strong winds

Surface currents in the study area during 5-9 June 2012 when strong winds

prevailed. Currents north off Pt. Arguello and Pt. Conception flowed southward.

Some of the flow near recirculated back into the Santa Barbara Channel along the

coasts of the islands. Currents were measured from oceanographic radars (small

yellow triangles along the coast).

Figure 3. Surface currents during a wind relaxation

Surface currents in the study area during 10-11 June 2012 during a wind relaxation.

In contrast to Figure 1, currents north off Pt. Arguello and Pt. Conception flowed

westward and northward. A counter-clockwise flowing eddy was also present in the

channel.

 

Conclusions & future directions

1. Strong poleward pressure gradients force poleward jet-like flows upwind along the California coast north of Pt. Conception.

2. The poleward jets extend to the seafloor over the mid-shelf (100 m depth).

3. When wind speeds are weak or moderate, eddy processes inshore of the jet drive mid shelf waters shoreward to the inner shelf.

4. These mid-shelf waters rapidly displace previously-upwelled inner-shelf waters.

5. We are using observations from 2012 and 2013 to examine these eddy processes connecting the mid shelf to the inner shelf

6. We are also examining changes in larval delivery, pH, and dissolved oxygen to the inner shelf due to these eddy processes

 

Acknowledgements

We thank David Salazar, Eduardo Romero, Ian Robbins, Christoph Pierre, Christian Orsini, and Eric Hessell for support in glider and mooring deployments. We also thank the Rutgers Glider team for managing the gliders. Clive Dorman provided valuable advice about weather conditions.

 

Funding was provided by: National Science Foundation, NOAA/IOOS: Southern

California Coastal Ocean Observing System (SCCOOS) & Santa Barbara Coastal Long-Term  Ecological Research Project (SBC-LTER)