HR: 1340h
AN: H13E-0470    [Abstracts]
TI: North American Droughts of the Mid-late Nineteenth Century: a History and Model Simulation
AU: * Herweijer, C
EM: celineh@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Seager, R C
EM: rich@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AB: Unlike the major droughts of the 20th century that are readily identified in the instrumental record, similar events in the 19th century have be identified using a combination of proxy data, historical accounts and a sparse collection of early instrumental records. In the United States, three distinct periods of widespread and persistent drought stand out in these records for the latter half of the 19th century: 1856-1865, 1870 -1878 and 1890 - 1896. Each of these events is shown to coincide with the existence of an anomalously cool, La Nina -like tropical Pacific. To examine the physical mechanisms behind these droughts two simple experiments were performed: the first forces an atmosphere general circulation model (AGCM) with the observed history of SSTs everywhere from 1856 to 2001 (the GOGA experiment), the second forces the AGCM only with tropical Pacific SSTs, being coupled to a two-layer entraining mixed layer (ML) ocean (the POGA-ML experiment). The AGCM used is the National Center for Atmospheric Research (NCAR) Climate Community Model 3 (CCM3). The extent to which the ensemble mean of the climate model is able to simulate the observed drought intervals is a test of both whether the dry conditions are SST-forced, globally or from the tropical Pacific only, and also whether the model is able to capture this. Due to a sparsity of instrumental precipitation data at this time, proxy evidence from tree rings is used as verification. A comparison of modeled soil moisture with PDSI data, a proxy for soil moisture, from the North American Drought Atlas (Cook et al., 2001) is made. Both the POGA-ML and GOGA ensemble means capture the three multi-year droughts of the mid-late nineteenth century identified by the `drought atlas' PDSI data. In each case the models reproduce the large-scale features of these droughts, albeit with some error, indicating that the droughts were SST forced. The similarity of the POGA-ML and GOGA simulations implies that the component of the each drought signal that is forced by the SST is driven by the SST anomalies from within the tropical Pacific - a La Nina-like tropical Pacific. The global atmosphere-ocean context of the 1856 - 1865 `Civil War drought interval is also examined in more detail to elucidate the dynamical mechanism behind this particular drought - the severest of the century .
DE: 4203 Analytical modeling
DE: 4522 El Ni¤o
DE: 3309 Climatology (1620)
DE: 1812 Drought
DE: 1620 Climate dynamics (3309)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting