Mechanisms contributing to the warming hole and the consequent U.S. east-west differential of heat extremes

A linear trend calculated for observed annual mean surface air temperatures over the U.S. for the second half of the 20th century shows a slight cooling over the southeastern part of the country, the so-called "warming hole", while temperatures over the rest of the country rose significantly. This east-west gradient of average temperature change has contributed to the observed pattern of changes of record temperatures as given by the ratio of daily record high temperatures to record low temperatures with a comparable east-west gradient. Ensemble averages of 20th century climate simulations in CCSM3 show a slight west-east warming gradient, but no warming hole. A warming hole appears in only several ensemble members in the CMIP3 multi-model dataset, and in one ensemble member of simulated 20th century climate in CCSM3. In this model the warming hole is produced mostly from internal decadal timescale variability originating mainly from the equatorial central Pacific associated with the Interdecadal Pacific Oscillation (IPO). Analyses of a long control run of the coupled model, and specified convective heating anomaly experiments in the atmosphere-only version of the model, trace the forcing of the warming hole to positive convective heating anomalies in the central equatorial Pacific Ocean near the Dateline. Cold-air advection into the southeastern U.S. in winter, and low-level moisture convergence in that region in summer, contribute most to the warming hole in those seasons. Projections show a disappearance of the warming hole, but ongoing greater surface temperature increases in the western U.S. compared to the eastern U.S.