In the North Pacific, variations in isopycnal depth influence the biogeochemical characteristics of the water column and the aragonite saturation horizon (ZΩa) during interannual events. The objective of this study was to assess the effect of the 2011 La Niña conditions on ZΩa in Todos Santos Bay (Baja California, Mexico) and surrounding waters. The results showed that ZΩa variability was modulated by the intensity of interannual conditions and by the water masses that were present in the region. Subarctic Water predominated in the upper 200 m with anomalous characteristics, such as low temperature and low salinity. Also, isopycnals shoaled toward the coast and ZΩa was thus ~30 min the nearshore area, in contrast with the oceanic region, where ZΩa was ~150 m. Prior to this study, there were no records of ZΩa in Todos Santos Bay, nor were there any records of its shallowness.