ENSO
The one real source of seasonal skill California has — weaker than advertised, and much stronger in the south than the north.
What is being measured
THE OCEANIC NIÑO INDEXEl Niño and La Niña are the warm and cool phases of a coupled ocean–atmosphere oscillation in the tropical Pacific. The official yardstick is the Oceanic Niño Index: the three-month running mean sea-surface temperature anomaly in the Niño 3.4 box, from 5°N to 5°S and 170°W to 120°W. A season at or beyond ±0.5°C is warm or cool; CPC calls a formal episode only when five consecutive overlapping seasons hold that threshold.
The physical link to California is the subtropical jet. A warm eastern Pacific shifts convection east, which strengthens and extends that jet toward the southwestern United States — pointing storms at Southern California in particular. That mechanism has a geography built into it, and the geography is exactly what shows up in the data below.
The ONI is measured against a base period CPC re-centres every five years, deliberately removing the tropical Pacific's own long-term warming so the index keeps measuring the oscillation rather than the trend. That makes it the right tool for a seasonal question and disqualifies it entirely from a century-scale one. Trends live on the warming page; nothing on this page speaks to them.
ENSO has a latitude
CORRELATION WITH WATER-YEAR PRECIPITATION, BY DIVISIONEach division's water-year precipitation correlated against the winter (December–February) ONI falling inside that water year. If "El Niño means a wet California" were a good rule, these would all be high and roughly equal. They are neither.
—
The same winter, two Californias
EVERY WATER YEAR SINCE 1950The two ends of the state, plotted the same way: each dot is one water year, positioned by that winter's ONI and by how much precipitation the division received. If ENSO were driving the outcome the cloud would tilt. On the right it does. On the left it barely moves.
Every division, every phase
MEAN PERCENT OF NORMAL BY ENSO CATEGORYThe composite in full: what each division actually received, on average, in each ENSO state. Read down a column to see how a phase treats the state from north to south; read across a row to see how much a single division cares about ENSO at all.
The strong categories contain four and seven winters respectively. A mean over seven years is a description of those seven years, and the next strong El Niño is under no obligation to resemble them. Every cell in the table below carries its own n for exactly this reason, and the range is shown alongside the mean because on this subject the range is the more useful number.
The episodes themselves
CPC'S OWN LIST, RECOMPUTED FROM THE INDEXEvery El Niño and La Niña episode since 1950, derived here by applying CPC's five-season rule to their published index rather than by copying a table — so the list can be checked against the definition it claims to follow.