In section 3, it was noted that the zone of biological production was probably wider than that of coastal upwelling. An attempt is made here to make a better estimate of the width of the zone of production as ratio to the width of the coastal upwelling.
The thirteen-year mean temperatures at 10 m off California and Baja California, from 1950 to 1962, are shown in Figure 4, (Lynn, 1967). Because of the intense sampling in the area as part of the Calcofi programme, the distributions are well established. The physical boundary to the coastal upwelling has been drawn by eye where the isotherms tend to run parallel to the coast. From Baja California to Point Conception, the boundary is about 100 km offshore, but from Point Conception to Cape Mendocino it may be nearer 200 km. Lynn has examined the same data in much more detail. All the temperature data were fitted to a harmonic regression by date. The high correlation of temperature with date shows the stable areas, whereas upwelling is shown by low correlation coefficients.
Figure 4. Mean temperature off California at 10 m for the years 1950-62, Lynn (1967); the heavy broken line was drawn by eye where the isotherms lie parallel to the coast
Figure 5 shows the distribution of correlation coefficients and the line has been drawn by following the poor coefficients (using r <.75 as a criterion) at roughly 100 km offshore. Another way of showing the upwelling areas is given in Figure 6, showing the months of occurrence of minima in temperature on the fitted regression curves, the figure shows the average position of upwelling by months. The areas shown are much closer to the biological boundaries and perhaps they indicate the direction of drift offshore from the coast.
Figure 5. Distribution of correlation coefficients of temperature on date at positions off California fitted to a harmonic regression (Lynn, 1967). The heavy broken line was drawn by eye using the coefficient of determination
Figure 6. Months of occurrence of the minima of the harmonic regression of temperature on date off California (Lynn, 1967)
The width of the production zone can be examined in the zooplankton distributions off California from 1950-1959 (Thrailkill, 1956, 1957, 1959, 1961 and 1963). A typical distribution is given in Figure 7; the broken line is drawn at a median density. The width of the zooplankton zone was tabulated by year and area and averaged for the ten years, ranging from 200-500 km. When the estimates from the temperature distributions are compared with those from the distributions of zooplankton, the ratio - width of productive zone to width of physical upwelling - is 2.5. To treat all upwelling areas in the same way, the width is estimated from surface temperature distributions, like that in Figure 5, and is raised by 2.5.
