How are warm and cool years in the California Current related to ENSO?

The tropical El Nino-Southern Oscillation (ENSO) is a dominant mode of interannual variability that impacts climate throughout the Pacific. The California Current System (CCS) in the northeast Pacific warms and cools from year to year, with or without a corresponding tropical El Nino or La Nina event. We update the record of warm and cool events in the CCS for 1950-2016 and use composite sea level pressure (SLP) and surface wind anomalies to explore the atmospheric forcing mechanisms associated with tropical and CCS warm and cold events. CCS warm events are associated with negative SLP anomalies in the NE Pacific-a strong and southeastward displacement of the wintertime Aleutian Low, a weak North Pacific High, and a regional pattern of cyclonic wind anomalies that are poleward over the CCS. We use a first-order autoregressive model to show that regional North Pacific forcing is predominant in SST variations throughout most of the CCS, while remote tropical forcing is more important in the far southern portion of the CCS. In our analysis, cool events in the CCS tend to be more closely associated with tropical La Nina than are warm events in the CCS with tropical El Nino; the forcing of co-occurring cool events is analogous, but nearly opposite, to that of warm events. Plain Language Summary The California Current System in the northeast Pacific warms and cools from year to year, with or without a corresponding tropical El Nino or La Nina event. We update the record of warm and cool events in the California Current for 1950-2016 and use sea level pressure and surface wind data to explore the atmospheric forcing of these events. California Current warm events are associated with a strong and southeastward displacement of the wintertime Aleutian Low, a weak North Pacific High and a regional pattern of poleward coastal wind anomalies. Regional North Pacific forcing is predominant in sea surface temperature variations throughout most of the California Current, while remote tropical forcing is more important in the far southern portion. In our analysis, local cool events tend to be more closely associated with tropical La Nina than are warm events with El Nino; the forcing of co-occurring cool events is analogous, but nearly opposite, to that of warm events. Understanding variations between years in the California Current may help predict and manage changes in fisheries and climate of the region.