Projections of climate changes over mid-high latitudes of Eurasia during boreal spring: uncertainty due to internal variability

This study examines uncertainties of projected spring surface air temperature (SAT) and precipitation trends during 2006–2060 at regional scales over the mid-high latitudes of Eurasia due to internal variability based on 40 ensemble members projections of CCSM3. The 40 ensemble members are initiated at a slightly different atmospheric conditions but with the same external forcing. Thus, the differences of the projected spring SAT and precipitation trends among the 40 ensemble members are attributed to the internal variability. Results suggest that superposition of internal variability and external forcing leads to a large spread of projected spring SAT and precipitation trends over Eurasia. In comparison, the projected spring precipitation trend has a larger uncertainty than the spring SAT trend. In particular, the signal-to-noise ratios of spring SAT (precipitation) trends are larger than two (one) over most regions of the mid-high latitudes of Eurasia. The internal atmospheric circulation variability is an important source of the uncertainties of the projected spring SAT and precipitation trends. The first mode of the internal atmospheric circulation variability resembles the Arctic Oscillation pattern. The second mode displays feature similar to the North Atlantic Oscillation and anomalous Siberian High patterns. A dynamical adjustment technique is employed to reduce internal atmospheric circulation generated variability in spring SAT and precipitation trends. Result indicates that projected trends of the dynamically adjusted spring SAT and precipitation over the next 55 years over Eurasia are similar across the 40 ensemble members both in the spatial structure and amplitude.