We next consider the extent to which Pacific climate modes can provide context and a degree of predictability for the JFM precipitation and runoff captured by the SSS10 time series. For comparison with SSS10, the NINO3.4 and PDO indices are JFM-averaged. The strongest ENSO teleconnections occur in JFM31. Correlations between SSS10 and ENSO and PDO indices are weak (taking into account serial correlation) with r = -0.25 (significant), -0.28 (insignificant), respectively. Despite the low correlation with the PDO, multi-year periods of high SSS10/low rain generally correspond to negative PDO phases (Fig. 2a,c).
Scatter plots confirm that while the linear correlations between SSS10 and indices of the PDO (Fig. 4a) and ENSO (Fig. 4b) are weak, there is a strong tendency for high SSS10/low rain winter seasons during negative values of both indices12,13,14,15. The case for the PDO is slightly less obvious for annual values, but with a 5-year smoothing to emphasize decadal polarity changes, the relationship is tighter (Fig. 4a).
JFM ENSO state provides a stronger constraint on year-to-year drought conditions than the PDO, with only one low SSS10 winter (in 1918) evident when the ENSO index dips below − 0.2, and none during strong La Niña events when the JFM-averaged index falls below − 1.0 (Fig. 4b). Thus, La Niña-like conditions strongly favor below-average rain levels for any given winter14, whereas multi-year drought periods tend to fall within prolonged negative PDO phases.
In contrast, wet winters occur during positive PDO and ENSO phases14, but not consistently so as high SSS10/dry winters are just as likely, and high index values do not ensure low SSS10 levels (Fig. 4a,b). With regard to ENSO, the lowest SSS10 winters tend to occur during weak to moderate index levels, and strong El Niño years (Niño 3.4 > 1) have more weak anomalies than strong. A recent study32 notes that the Tropical Northern Hemisphere pattern33 (TNH) is highly correlated with California precipitation variability, and suggests that a combination of ENSO and TNH indices (as opposed to ENSO alone) provides improved predictability of precipitation. Dry/wet years in southern California tend to occur during TNH positive/negative phases. A strong negative 500-hPa geopotential height anomaly is seen at TNH negative phases over the NE Pacific Ocean, and vice versa for positive phases34. Negative phase TNH conditions support cloud formation and precipitation whereas positive phase TNH conditions reduce the winter storm landfalling probability in California32. A scatter plot of SSS10 versus the NINO3.4 and TNH indices confirms these tendencies, with low SSS10 wet winters clustered primarily in the lower right-hand quadrant of Fig. 4c where El Niño and negative TNH conditions meet.
Composites of JFM-averaged surface wind and precipitable water anomalies illustrate the relationship between climate phase and southern California precipitation. During La Niña and negative PDO phases (Fig. 5a), the Aleutian low pressure system over the northeast Pacific weakens (clockwise wind anomaly) with negative precipitable water anomalies and average wind conditions over the western US. During El Niño and positive PDO phase wet years (Fig. 5c), the Aleutian low strengthens and deepens (anticlockwise wind anomaly) forcing the Northern Hemisphere Jetstream (NHJ) southward, resulting in increased storm landfalls on the US west coast and significant positive precipitable water anomalies35,36. During El Niño and positive PDO phase dry years (Fig. 5b), the southeastern extent of the NHJ is located more northwest when compared to rainy year conditions (Fig. 5c), pushing storm landfalls further north and causing negligible to negative precipitable water anomalies in the southern California region.
Shifts in the meridional position of the NHJ, influenced by both ENSO and PDO conditions, modulate extreme winter storm tracks and resulting precipitation to the southwestern US35,37,38. Previous studies have linked PDO and North Pacific Gyre Oscillation (NPGO) phases, acting in conjunction, to the occurrence/frequency of ARs in California39. The combination of positive PDO and negative NPGO phases tend to shift the North Pacific AR belt southward and result in increased AR frequency in southern California32,38,39. In summary, the co-occurrence of El Niño and negative TNH increases the likelihood for rainfall as a whole while PDO positive and NPGO negative phases together can help dictate where ARs will impact, and all of these nuances could be in part due to NHJ shifts and resulting changes in storm tracks. Future research on NHJ positioning during different combinations of climate modes across all seasons would help to assess the usefulness for predictability.