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reasonable question; there is some ambiguity in the nomenclature since the N or S following the UTM ZONE number can refer to either latitude bands, which alpabetically progress from the south to the north, or just Northern hemisphere and Southern hemisphere, thus telling you which way to measure the northing or southing.
Here is a good Wikipedia reference http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system
Since the origin of all UTM zones is at the central meridian of each zone and on the Equator the latitude band information is redundant once you have a northing or southing for a point. Latitude bands N and S both occur in the northern hemisphere so the S you refer to in California is just that, a latitude band and happens to be north of band N which is just above the equator. Potentially confusing so you should have to state what nomenclature is being used, but in common practice the N and S just refer to North and South.