Simulated changes in extreme temperature and precipitation events at 6 ka

We have employed a high-resolution regional climate model (RCM) to test the sensitivity of extreme climate events to 6 ka orbital forcing, using western North America as a case study. Calculated differences (6 ka − Control) in annual extreme precipitation event frequency were mostly positive throughout the RCM domain, as were differences in extreme event contribution to total annual precipitation. These annual mean differences were driven by changes in the seasonal distribution of extreme precipitation events at 6 ka, which was in turn linked to changes in the daily variability of upper-level zonal wind speeds over the northeast Pacific. Positive differences in the frequency of extreme maximum daily temperature values occurred inland in the RCM domain, with peak differences of 28 days/year over the Great Basin. Likewise, days in which the maximum daily temperature exceeded 32 °C were 24% more frequent in the 6 ka integration than in the control integration and heat waves lasted up to 12 days longer. Mean first and last freeze dates were generally earlier in the 6 ka integration along the Pacific coast and over the southern Great Basin, and later over the northern Great Basin and interior of the Pacific Northwest. We propose that, by exerting controls on paleoclimate archives unique from changes in the mean climate state, such changes in the frequency and duration of extreme climate events could have played a substantial role in shaping the mid-Holocene paleoclimate record of the region, and that extreme climate events may have been similarly important in other regions and during other time periods.