First: The evidence so far suggests that the extratropical atmospheric anomalies are coordinated by tropical SST anomalies. The extratropical wind anomalies drive extratropical SST anomalies. Most evidence from modeling says that those extratropical SST anomalies have only a weak feedback on the atmospheric flow, but I think this is a question that could use some further study. Second: This is just speculation, but when the NPM is in full flower, warmer than normal SST's extent to the west and south of California, which is were the atmospheric river events come from. So if the pattern does break down temporarily into a low that brings rain to Califormia, then it might be more than in some other year when the SST upstream is colder.