A Surprise in the North Pacific: Results From Applying a New Nonlinear Method for Calibrating Ice Cores

A deeper understanding of the behavior of North Pacific extratropical cyclones and anticyclones prior to the instrumental era is needed to advance our understanding of North Pacific climate variability. To help achieve this objective, we develop and use a new nonlinear ice core calibration procedure with the Eclipse (3017 m a.s.l.) and Mt. Logan (5400 m a.s.l.) ice core records from Yukon, Canada to isolate the ranges of ice core values that are consistently associated with North Pacific wintertime sea-level pressure (SLP) anomalies. Over the calibration period (1872-2001), each ice core record is ranked and divided into 10 groups of 13 years. Then for each group, the frequency of positive and negative SLP anomalies at each grid point is contoured and the composite mean SLP anomaly values are shaded. These plots elucidate areas where statistically significant SLP anomalies occur frequently in association with groups of ice core values. This new calibration procedure shows that the lowest and the two highest groups of Mt. Logan annual [Na+] are sensitive to SLP anomalies in the central and eastern Pacific and the second lowest [Na+] group is sensitive to western Pacific SLP anomalies. The highest and lowest Eclipse cold-season accumulation groups are most sensitive to SLP anomalies more distant in the western and central Pacific. This result is surprising in light of stable isotope studies suggesting a more distant moisture source for Mt. Logan. A reconstruction using these calibrated records indicates the Aleutian Low was predominantly weaker than average between 1699-1871. Our results highlight that having these geographically close ice core records is important to developing a deeper understanding of North Pacific climate variability.