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 pattern of average temperature change has contributed
to the observed pattern of changes of daily record temperatures with a
comparable east-west differential of the ratio of daily record high
temperatures to record low temperatures.
Ensemble averages of 20th century
climate simulations show a slight west-east warming gradient, but no warming
hole, suggesting that it is not the product of external forcing but is
internally generated. A warming hole that appears in one ensemble member of
simulated 20th century climate in CCSM3 is analyzed as a case study to show
that it is a product of 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 an
atmosphere-only model, trace the forcing of the warming hole to positive
SST, precipitation and convective heating anomalies in the central
equatorial Pacific Ocean near the Dateline. Different processes related to
the teleconnections from the tropical western Pacific occur in different
seasons, with cold air advection into the southeastern U.S. in winter, and
low level moisture convergence in that region in summer, contributing most
to the warming hole in those seasons.
Projections show ongoing fluctuations
of this pattern superimposed over warming from increasing greenhouse gases,
implying a future larger increase in the ratio of daily record high
temperatures to record low minimum temperatures in the southeastern U.S.
compared to the past 50 years.