Long-period ocean swell in Southern California comes from distant storms. In our summer (May -Oct), waves commonly arrive from the Southern Hemisphere, the location of the most energetic Pacific storms. In our winter, our waves are often generated to the North, in the Gulf of Alaska.
South and North swells are both actually present year-round, with south dominant in summer-fall, and North predominant during winter-spring.
Torrey Pines is sheltered from southern swell by Pt La Jolla, whereas San Onofre is exposed (San Onofre is located between Dana Point and Oceanside in Figure 1). Therefore, during summer (May -Oct) wave heights are larger and average periods longer at San Onofre than at Torrey Pines (Figure 2). The pattern is reversed during winter (Nov-April), and wave heights are slightly larger at Torrey Pines.
Perhaps seasonal changes in sand level are smaller at San Onofre because seasonal variations in wave height are smaller than at Torrey Pines. That is, the beach building that occurs during prolonged summer periods of low waves at Torrey Pines does not occur at San Onofre. Differences in sand types could also be important (San Onofre beach has many cobbles).