Multi-decadal forcing of precipitation along the southeastern USA during the late Holocene

Thursday, 18 December 2014
Christopher E Bernhardt1, Andrew Kemp2 and Miriam Jones1, (1)U.S. Geological Survey., Reston, VA, United States, (2)Tufts University, Department of Earth and Ocean Sciences, Medford, MA, United States
During the late Holocene, the North Atlantic Oscillation (NAO), Atlantic Multi-decadal Oscillation (AMO), and El Nino Southern Oscillation (ENSO) all in part influenced precipitation along the southeastern United States (US) coast. While proxy records show a warm/dry Medieval Climate Anomaly (MCA) along the eastern US, Little Ice Age (LIA) records demonstrate a regional heterogeneity with the more northern sites indicating wet conditions and southern sites indicating dry.

We used detailed pollen, microscopic charcoal, and carbon accumulation records from a well-dated core collected in a salt marsh along the Nassau River, Fl and compared it with other regional proxies of climate to help discern the multi-decadal drivers of climate during the late Holocene in the southeastern US. The Nassau River is ideal because it is sensitive to changes in both summer and winter precipitation unlike rivers farther south (summer) and north (winter). Pollen indicative of sustained dry conditions (e.g. Amaranthaceae) and microscopic charcoal increase during MCA-I, which is characterized by strong La Nina conditions and positive NAO and AMO. Carbon accumulation rates are elevated during MCA-I compared to MCA-II. During the LIA, Amaranthaceae pollen and microscopic charcoal drastically decline while fire suppressed pollen taxa, which indicate wetter conditions, increase. LIA carbon accumulation rates are among the lowest in the record. Proxy records suggest this period is dominated by El Nino conditions and a negative NAO and AMO. During the LIA, the AMO includes regional differences in precipitation during any single phase. For instance, during a positive (negative) AMO, the US northeast coast experiences less (greater) precipitation, while the south Florida experiences greater (less) precipitation. Therefore, our pollen and microscopic charcoal data suggest that the phase of the AMO during the LIA was a major driver of spatial heterogeneity in precipitation observed between south Florida and more northerly sites along the eastern United States.