Near-Shore Circulation in the California Current

Calif.Mar. Res. Comm., CalCOFZ Rept., 16 : 5i-65, 1972

NEAR-SHORE CIRCULATION IN THE CALIFORNIA CURRENT

RICHARD A. SCHWARTZLOSE and JOSEPH 1. REID Scripps Institution of Oceanography La Jolla, California

A great many of you are very familiar with the Figure 4 shows drift-bottle results for December major current along the California Coast. What we 1969. The Davidson Current was very well developed. wish to do today is to review the currents closer to the There was water moving out of southern California coast, since they are equally pertinent to the problem northward along the coast. One drift bottle went to of pollution along the coast. Alaska. The minimum speed of some of the December Figure 1 is a 16-year mean chart of the surface cur- 1969 drift bottles that moved northward in the David- rents (relative to geostrophic flow) along the Califor- son Current was about .3 knot (15 cm/sec). nia coast in winter (January). The normal offshore We have records from other drift bottles in other flow all during the year is towards the southeast. In years during the winter that indicated minimum the wintertime the Davidson Current nearshore moves speeds in the Davidson Current of .6 knot (30 cm/sec) northward opposite to the current farther offshore. for long periods of time, up to 2 to 3 months. This This northward flow in winter is attributed to the Davidson Current is usually a very strong current seasonal change in the wind patterns across California, compared to the offshore California Current and it Oregon and Washington. The winter winds are pri- extends to the Gulf of Alaska. In January 1958 a marily from the south in the coastal area northward large number of drift bottles from central California from San Francisco. were recovered off northern Oregon, Washington and Question: Can you tell us what the speeds are that British Columbia; this was due to a very strong de- correspond to the California Current ? velopment of the Davidson Current. Schwartzlose: The average speed of the general The Davidson Current is usually between 40 and California Current is about 4 to 4 knot (12.5-25 50 miles wide, and in the northern area the overlying cm/sec) . Direct measurements, using drogues, which wind is from the south. In December of 1969 one of we have made off Monterey, southern California and the returned bottles had been released 80 miles from northern Baja California, indicate that speeds up to shore ; it is rare to see any returns from that distance. a knot (50 cm/sec) can obtain for short periods of Our other drift-bottle charts show that the Davidson time. Primarily they are in the range of .4-.6 knot Current is the narrowest at its southern end off Point (20-30 cm/sec). It appears that the general current Conception, and it slowly widens northward. Pearcy 's is not straight, but includes loops and eddies with a Figure 2 that we saw the other day showing the plume general southeast drift at about 4 knot (12.5 cm/sec). of the Columbia River going northward in the winter- Figure 2 is the mean January geostrophic flow at time is a very good indication of the same type of information that the drift bottles show, and at that 200 meters. The northward current or undercurrent is slightly wider than at the surface. This undercur- location the plume appeared, as you recall, to be some- rent persists throughout the year flowing northward thing like 50-60 miles wide. Another interesting feature seen from the drift- at these depths along the coast. There have been some direct measurements of this current by Wooster and bottle returns is that usually we receive few returns from Baja California because the small, scattered Jones using current meters in the area off Cape Col- of nett in northern Baja California, and various meas- population. The returns are from farther south in winter than in summer. urements with drogues have been made by Schwartz- lose, Reid, Jennings, and Brown. Figure 5 is presented to show that the interpretation of the drift-bottle data is at times not clear. Note Figure 3 is the January 1964 flow charts. The dif- that from the January 19 release of bottles just north ference from the mean charts (Figs. 1 and 2) is not of Point Conception one bottle went northward and very great, though more detail can be seen in Fig. 3. one bottle landed well south, near San Diego. There The Davidson Current and the eddy system in south- is, of course, some diffusion or spreading of the bottles ern California are well developed. At 200 meters we as they drift. If the release location is near the bound- see the undercurrent moving northward along the ary of the southward-flowing portion of the California coast. Current, a few of the bottles may spread into that Question: What is the speed of undercurrents? current. In the spring of the year, primarily in April, some- Xchwartzlose: Very low, .l-.2 knot (5-10 cm/sec) times in May, the winds off northern California turn according to the drogue measurements, but at least southward again and the northward flow of the Cali- one series of direct current measurements by Wooster fornia Current ceases (Figure 6). The storm centers and Jones has shown speeds as high as half at knot are farther north than in winter. The current all (25 cm/sec) in a narrow layer. along the coast is usually moving southward, includ- 58 CALIFORNIA COOPERATIVE OCEANIC FISHERIES INVESTIGATIONS ing the area through the Channel Islands. In April Some direct drogue measurements were made earlier the water flows right through the southern California in the CalCOFI Program from the Catalina Channel borderland, and the normal gyre that exists the re- to west of the north end of San Clemente Island. mainder of the year in the southern California border- These data suggest the presence of a number of small land is not found. gyres (Figure 10). Figure 7 presents the 15-year mean geostrophic We have a little more data from a current meter flow for July. The Davidson Current, of course, is not that was placed about 6 miles west of Scripps for 15 present, but we do find the gyre off southern Califor- days. It was about 25 feet below the surface. The data nia. This is the period of time when the California showed a current moving northward about half of Current is the strongest due to the northwest winds. the time, then shifting rapidly within a few hours and In figure 8, the mean July geostrophic flow at 200 moving southward. These nearshore currents were pri- m shows the undercurrent along the coast flowing very marily between 4 knot (12.5 cm/sec) and 4 knot (25 slowly northward from southern Baja California cm/sec). They were persistent for more than a week through northern California. The width of this north- in one direction and then reversed themselves. ward undercurrent appears to be 40 to 50 miles. There have been a number of studies of currents Figure 9 shows the percent of drift-bottle recoveries around sewer outfalls, but usually these are only for from releases at various distances offshore. As can be several days, not for long periods of time. seen, the recovery from 15 miles inward is very good Question: How much water leaves the southern off southern California, whereas it is poor off Baja California bight? Is there internal circulation California, possibly because there are not so many people on the beaches. Many of the recoveries from Schwartzlose: I do not know. There is internal cir- 70 to 90 miles off southern California are from the culation within the bight, but there is mixing with currents that form the gyre within the southern Cali- new water coming in from the main stream of the fornia bight. California Current. The drift bottles that are dropped within 3-5 miles Sargent: Some act like ball bearings between the of the beach usually go ashore very quickly, not far from the point where they have been released. There others. I think this problem is susceptible of a rough are problems in interpretation of the returns, since solution with a computer, but no one has been inter- there is no way of knowing how long the bottles may ested enough to get down and do it. The question was have lain on the beach before recovery or how many asked about the rate of escape, but no one has ever beaches they have been on. done it. REPORTS VOLUME XVI, 1 JULY 1970 TO 30 JUNE 1971 59

130' 125' 1200 115' I100 I I I 1 I II I I I 1 I ;:. I I I I I I I 1 CALCOFI - JANUARY 1950-65 MEAN

35 5"

30 00

1.060 I I I I I - I 1.04 / 1.02 \ / 1.06 \ - I. 7.080 ./ \ ,k' I 25 *I ' 5" I - 1.06 I .on I / I /

I .02

1.01

!O" 20 - +CURRENT DIRECTION INDICATED BY ARROWS - ICPG 1966 I I I I I 1 I I I I I I I I I I I I I 30° 125O 120" 115" 110' MEAN 0/500 db JANUARY

FIGURE 1. Mean January geostrophic flow (195b-1965) (sea surface relotive to the 5OOdecibor surface, in dynamic meters). 60 CALIFORNIA COOPERATIVE OCEANIC FISHERIES INVESTIGATIONS

130' 125" 1200 IIY I 100 I 1 I I I I I I I I I I I I I I I 1 I I C'ALCOFI JANUARY 1950-65 MEAN GEOSTROPHIC FLOW AT 200m DEPTH

[ TOPOGRAPHY OF THE 200-DECIEAR SURFACE, 4( IN DYNAMIC METERS, RELATIVE TO THE 500- 400 DECIBAR SURFACE ) CONTOUR INTERVAL 0.02 DYNAMIC METERS

.4 2 \ I 3' I I I I I \ 44 \1" - 3: ', .ye * 350 I 4 6 4 I - \ \ \ \ I -412 4 \ \ - \ \ \ \ \ .41 - \ .4a \ \ \

3c 300

42 i ' +I I 25 \ 25' \ .e5 I I

> / - .4 6 Y I .44 -CURRENT DIRECTION INDICATED BY ARROWS + .e10 / = 2c -~~~ A0 !OO ..._...... : APPROXIMATE 200 METER BOTTOM CONTOUR ' - DCPG 1966 I I I I 1 I 1 I I I I 1 I I I I I 1 I 30° 125O 1200 I I5O I IO" MEAN 200/500 db JANUARY

FIGURE 2. Mean January geostrophic flow (1950-1965) (200aecibor rurfoce relative to the 500decibor surface, in dynomic meters). REPORTS VOLUME XVI, 1 JULY 1970 TO 30 JUNE 1971 61

135' 1300 125O 1200 115. 1100

300 -

250 -

350 -

309 -

FIGURE 3. Geostrophic flow in January 1964 at the surface and at 200m depth (topography of the 0- and 200-decibar surfacer relative to the 500decibar surface, in dynamic meters). 62 CALIFORNIA COOPERATIVE OCEANIC FISHERIES INVESTIGATIONS

CalCOFI CRUISE 6912

NUMBERS INDICATE DAYS ELAPSE0 BETWEEN DROP AND RECOVERY DITES

FIGURE 5. Drift-bottle returns for January 1964.

115-

FIGURE 4. Drift-bottle returns for December 1969.

FIGURE 6. Drift-bottle returns for April 1960. REPORTS VOLUME XVI, 1 JULY 1970 TO 30 JUNE 1871 63

40 $00

35 350

3v 30"

25" 2 50

20" !OO DCPG 1966 I I 1 I I I I I 1 I 1 I I I I I I I 1 MEAN 0/500 db JULY

FIGURE 7. Mean July geostrophic low (1950-1964) (sea surface relative to the 5OOdecibar surface, in dynamic meters). 64 CALIFORNIA COOPERATIVE OCEANIC FISHERIES INVESTIGATIONS

-CURRENT DIRECTION lNDlC4TED BY ARROWS 2( APPROXIMATE 200 METER BOTTOM CONTOUR OCPG 1966

MEAN 200/500 db JULY

FIGURE 8. Mean geostrophic flaw (195b-1964) (200decibor surface relative to the 500decibar surface, in dynamic meters). REPORTS VOLUME XVI, 1 JULY 1WO TO 30 JUNE 1871 65 I 20"

These release points are near isla nds

20 40 60 80 100 Miles from Coast

FIGURE 9. Drift-bottle recoveries from releases at variour distances offshore.

119' 118'

340

3 30

Clemente Is

119' 118' FIGURE 10. Drogue measurements off southern California in October 1958.