Recent and dramatic changes in Pacific storm trajectories recorded in δ 18 O from Bristlecone Pine tree ring cellulose

A 300‐year stratigraphy of annual cellulosic δ 18 O VSMOW is presented for the long‐lived Bristlecone Pine ( Pinus longaeva) from the White Mountains of California. The δ 18 O VSMOW stratigraphy exhibits two distinctive characteristics: a bidecadal oscillation during the 20th century with peak excursions of 4‰ and a dramatic 9‰ shift in mean δ 18 O values in the mid‐19th century. The bidecadal δ 18 O variability during the 20th century does not correlate well with the bidecadal pattern of drought recurrence as reflected in the Palmer Drought Severity Index. Geochemical modeling of the cellulose δ 18 O VSMOW indicates local climate conditions, particularly humidity and rainfall amount, should have been important influences on the cellulose δ 18 O variability, but these influences cannot explain the larger bidecadal excursions or the shift in values during the mid‐19th century. The large isotope variations must reflect sustained shifts in the frequency of subtropical versus polar storms that affect the isotopic composition of rainfall at the site. The mid‐19th century isotopic shift correlates with the major climate shift across the northern hemisphere as documented in a wide range of proxy records. We hypothesize that the large isotopic shift in the 19th century represents the hydrologic response to a regime shift in mean wintertime atmospheric circulation that changed storm trajectories brought about by a more southerly position of the midlatitude jet, increased trade wind strength, and a change in the frequency of the Pacific Decadal Oscillation. The extent to which the hydrologic changes influenced the growth behavior of the high‐altitude Bristlecone Pine in the White Mountains remains an important unanswered question.