Our residence is a town home and only the east end of the attic is open to the outside. The roofline extends to the bottom of the attic floor and there are vents in the soffit. The roof is covered with tile and is in the north/south orientation. We need additional ventilation in the attic, especially in the summer (we're in So. California). What is the best solution to adding air circulation?

You could add ventilation domes near the ridge on the roof, or ene the turbines that are operated by movement of air. A study should be made of the free area of ventilation in the soffits, and the free area of the louver that you have. The total FREE area should be 1/150 of the attic floor area, and the area should be divided nearly equally between the osffit vents and the louver, with more in the upper area, up to 80%. The study will tell you where you need to make changes.

Consider installing a power fan in the gable end of the attic. You said the east end is open to outside. Install near the upper ridge with a thermostat control to automatically come on when it gets hot in the attic. One concern is having adequate vents around the perimeter soffits to allow plenty of intake air.

If you install a power fan, it should be controlled by a humidistat in addition to a thermostat, because in cool damp waether, you want the fan to operate, but the temperature may not be high enough to activate the thermostat.