1 Hadley Circulation Abstract September 9, 2002 Changes in Hadley circulation strength over the last millennium as implied by coral records of tropical Pacific climate Kim M. Cobb*, H. Cheng † , R.L. Edwards † , and C.D. Charles ‡ * California Institute of Technology, Pasadena, CA † University of Minnesota, Minneapolis, MN ‡ Scripps Institution of Oceanography, University of California-San Diego, La Jolla, CA Tropical sea surface temperatures (SST) play a critical role in the global atmospheric circulation because they drive the transport of heat and moisture poleward via the Hadley cell. The sensitivity of the large-scale atmospheric circulation to changes in tropical SST is demonstrated during El Niño events, when tropical Pacific warming significantly alters temperature and rainfall patterns around the globe. Despite the important role that tropical SST’s play in global climate change, the sparse nature of instrumental and proxy-based records of SST in the tropics make characterizing the relationship between tropical Pacific SST and past global climate changes difficult. We use modern and fossil corals from Palmyra Island (6°N, 162°W), located in the central tropical Pacific, to reconstruct interannual to centennial tropical Pacific SST for five different intervals of the last millennium. The Palmyra coral-based climate proxy records fill an important gap in our knowledge of recent climate change, because extensive networks of extratropical climate proxy records resolve prolonged periods of relatively warm and cold conditions during the last millennium – the Medieval Warm Period (MWP) (~800-1200A.D.) and the Little Ice Age (LIA) (~1500-1800A.D.), respectively. Determining the response of the tropical Pacific climate system to the climate forcings that caused the MWP and the LIA is key to developing a more complete understanding of the tropical Pacific climate system and its role in global climate change. The fidelity of the Palmyra coral proxy records is established through calibrations between the modern coral oxygen isotopic (δ 18 O) record and the instrumental record of climate change. Such calibrations suggest that the Palmyra coral δ 18 O can be used to reconstruct regional-scale tropical Pacific SST variability to an accuracy of ±0.3°C (Figure 1) (Cobb et al., 2001). For the fossil corals, our approach involves applying U/Th-series dating techniques in combination with δ 18 O analysis to generate well-dated coral-based climate proxy records, many of which overlap in time. The overlapping corals allow for a rigorous assessment of coral δ 18 O reproducibility, which is found to be highest for interannual variability and lowest for mean coral δ 18 O. By splicing overlapping sequences together, we produce multi-century, monthly-resolved records of tropical Pacific climate to which we apply a variety of time-series analysis techniques. Taken together, the corals suggest that mean climate conditions were relatively stable (±0.5°C) throughout the last millennium, with the exception of relatively cool conditions during the MWP and increasingly warm conditions during the late 20 th century (Figure 2c). Interannual and decadal-scale variability are superimposed on this low-frequency variability, exhibiting a wide range of characteristics that appear to be uncorrelated to the observed changes in the mean state (Figures 2a,b, and c). When compared to records of extratropical climate variability, the