TRANS-PACIFIC CLIMATE VARIABILITY OVER THE PAST MILLENNIUM

The relationship between tropical Pacific processes (e.g., El Nino Southern Oscillation; ENSO) and high latitude climate on a variety of timescales in the past remains a fundamental question, particularly with respect to any lead-lag associations during significant climate transitions. Several recent observational and modeling studies have documented changes in ocean/atmosphere dynamics in the tropical Pacific system at the onset of the Little Ice Age (LIA), ca. mid-thirteenth century. This shift appears to represent a change in ENSO statistics, however two hypotheses have emerged to explain it. One scenario involves a shift towards a more persistent El Nino-like state, perhaps with an accompanying southward migration in the Intertropical Convergence Zone, mainly due to decreased solar forcing in the tropics during the LIA. An alternative scenario, based on tropical precipitation reconstructions, posits a more persistent La Nina-like state during the LIA. In either case, theory and models based on modern observational data suggest a symmetrical extra-tropical Pacific response to tropical Pacific perturbations. To test these ideas during the past millennium, specifically whether the 13^th century LIA onset shows a consistent trans-Pacific response to tropical forcing, we examine several existing paleoclimate datasets from the North and South Pacific regions focusing particularly on new ice core timeseries. Composite records of continuous high-resolution (subannual to decadal) ice core, tree ring, and sediment datasets do indeed show a relatively consistent pattern of cooling and/or change in hydroclimatology near 1250 AD. Glacier extent records from New Zealand and coastal Alaska generally corroborate this finding, showing coeval advances beginning in the 13^th century. However, New Zealand glaciers also display advances during the preceding Medieval Climate Anomaly, possibly pointing to atmospheric circulation changes that were not mirrored in the North Pacific. We attempt to place this 13^th century shift into a broader Holocene context using available longer paleoclimate records, as well as resolve the change with current knowledge regarding solar and volcanic forcing over the past millennium.