Western Pacific Fossil Coral Records Reveal Change in Tropical Climate during the Past Millenium

The western tropical Pacific is a key center of action for global climate: ENSO events often initiate in this region, and changes in western Pacific Ocean heat storage can modulate the rate of global temperature rise. However, the behavior of the western Pacific before the 20^th century is not well constrained. For example, during the Little Ice Age (LIA), paleoclimate proxies disagree on whether the western Pacific was warmer and wetter ("La Niña-like") or cooler and drier ("El Niño-like"). Further, the paucity of long, high-resolution records from this region precludes analysis of changes in ENSO variability over the last millennium. We address these limitations with new paleoclimate records from the Republic of the Marshall Islands (7°N, 171°E) in the western tropical Pacific. This study presents six coral records, reflecting local sea surface temperature and salinity variability, which span key periods of the LIA (3 records), MCA (1 record), and 20^th century (2 records). We assess changes in the mean and variance of δ¹⁸O among periods, as well as changes in δ¹³C and coral growth rate (density, extension, and calcification). The oxygen isotope data reveal a significant trend from cooler and drier conditions during the MCA and LIA, to warmer and wetter conditions in the 20^th century. We evaluate possible causes of these observed trends, including expansion/contraction of the Western Pacific Warm Pool, changes in the intensity and location of the Intertropical Convergence Zone, and strengthening/weakening of the Walker Circulation, and explore the implications of these new records for the mean state and ENSO variability over the past millennium.