Last Millennium Climate and Its Variability in CCSM4

• Published in: Journal of climate 2013-02, Vol.26 (4), p.1085-1111
• Main Authors: Landrum, Laura, Otto-Bliesner, Bette L., Wahl, Eugene R., Conley, Andrew, Lawrence, Peter J., Rosenbloom, Nan, Teng, Haiyan
• Format: Article
• Language: English
• Subjects: 20th century; Archives & records; Asian monsoons; Atmospheric forcing; Climate; Climate change; Climate models; Climate proxies; Climate system; Climatology. Bioclimatology. Climate change; Computer simulation; Confidence intervals; Cooling; Earth, ocean, space; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; Exact sciences and technology; External geophysics; General circulation models; Geophysics. Techniques, methods, instrumentation and models; Global cooling; Ice ages; La Nina; Little Ice Age; Marine; Meteorology; Modeling; North Atlantic Oscillation; Northern Hemisphere; Ocean-atmosphere system; Oceans; Oscillations; Pacific Decadal Oscillation; Paleoclimatology; Proxy client servers; Proxy reporting; Proxy statements; Reconstruction; Regions; Sea ice; Simulation; Simulations; Southern Hemisphere; Southern Oscillation; SPECIAL Community Climate System Model CCSM4 COLLECTION; Surface temperature; Temperature; Temperature changes; Variability; Volcanic eruptions
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/jcli-d-11-00326.1

An overview of a simulation referred to as the ‘‘Last Millennium’’ (LM) simulation of the Community Climate System Model, version 4 (CCSM4), is presented. The CCSM4 LM simulation reproduces many large-scale climate patterns suggested by historical and proxy-data records, with Northern Hemisphere (NH) and Southern Hemisphere (SH) surface temperatures cooling to the early 1800s Common Era by ∼0.5°C (NH) and ∼0.3°C (SH), followed by warming to the present. High latitudes of both hemispheres show polar amplification of the cooling from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA) associated with sea ice increases. The LM simulation does not reproduce La Niña–like cooling in the eastern Pacific Ocean during the MCA relative to the LIA, as has been suggested by proxy reconstructions. Still, dry medieval conditions over the southwestern and central United States are simulated in agreement with proxy indicators for these regions. Strong global cooling is associated with large volcanic eruptions, with indications of multidecadal colder climate in response to larger eruptions. The CCSM4’s response to large volcanic eruptions captures some reconstructed patterns of temperature changes over Europe and North America, but not those of precipitation in the Asian monsoon region. The Atlantic multidecadal oscillation (AMO) has higher variance at centennial periods in the LM simulation compared to the 1850 nontransient run, suggesting a long-term Atlantic Ocean response to natural forcings. The North Atlantic Oscillation (NAO), Pacific decadal oscillation (PDO), and El Niño–Southern Oscillation (ENSO) variability modes show little or no change. CCSM4 does not simulate a persistent positive NAO or a prolonged period of negative PDO during the MCA, as suggested by some proxy reconstructions.