08 October 2015

Disappearance of the Southeast U.S. "Warming Hole" with the Late 1990s Transition of the Interdecadal Pacific Oscillation


Observed surface air temperatures over the contiguous U.S. for the second half of the 20th century showed a slight cooling over the southeastern part of the country, the so-called “warming hole”, while temperatures over the rest of the country warmed.  Since the average of climate model simulations with increasing greenhouse gases showed no warming hole, this phenomenon called into question the processes involved with global warming.  However, this pattern then reversed after about the year 2000.   Climate model simulations show that the disappearance of the warming hole in the early-2000s was associated with the transition of the Interdecadal Pacific Oscillation (IPO) from positive (warmer-than-average tropical Pacific sea surface temperatures, SSTs) to negative (cooler-than-average tropical Pacific SSTs) in the late 1990s.  Analysis of specified convective heating anomaly sensitivity experiments in an atmosphere-only model traces the disappearance of the warming hole to negative sea surface temperature (SST) anomalies and consequent negative precipitation and convective heating anomalies in the central equatorial Pacific Ocean associated with the negative phase of the IPO after 2000.  These internally-generated processes in the tropical Pacific cause large-scale changes in atmospheric circulation that affect surface temperatures over the U.S.  This study highlights the importance of naturally-occurring Pacific decadal climate variability in producing decadal-timescale regional U.S. surface temperature trends.