Wavenumber 5

I like Wavenumber 5 as a better explanation of the the California drought-rain sequence than the Pacific Decadal Oscillation. The PDO has been all over the place the last few years, and just seems too inconsistent. Distinguishing causes from effects with Wavenumber 5 is hard, and that's where the erudition starts:

Scientists at the National Center for Atmospheric Research (NCAR) found in a recent study that the persistent high-pressure ridge off the west coast of North America that blocked storms from coming onshore during the winters of 2013-14 and 2014-15 was associated with the wave pattern, which they call wavenumber-5. Follow-up work showed that wavenumber-5 emerged again this winter but with its high- and low-pressure features in a different position, allowing drenching storms from the Pacific to make landfall.

"This wave pattern is a global dynamic system that sometimes makes droughts or floods in California more likely to occur," said NCAR scientist Haiyan Teng, lead author of the California paper. "As we learn more, this may eventually open a new window to long-term predictability."