North American 1818–1824 drought and 1825–1840 pluvial and their possible relation to the atmospheric circulation

[1] This study aims to test the hypothesis that persistently negative and positive phases of the Pacific-North American (PNA) atmospheric circulation pattern for the December–March period are responsible for explaining the North American 1818–1824 summer drought and the 1825–1840 summer pluvial, respectively. In this view, reduced or enhanced soil moisture in winter and early spring may perpetuate dry or wet summer conditions, respectively. For these sensitivity simulations, an ocean-atmosphere coupled climate model of intermediate complexity is used. Rough reconstructions of stream function at 500 hPa over the North Pacific sector averaged over 1818–1824 and 1825–1840 for December–March are assimilated in this model, using a recently developed technique. This technique ensures that in an averaged sense, the reconstructed pattern is retrieved while leaving atmospheric and climatic variability to develop freely. Tree-ring-based Palmer Drought Severity Index (PDSI) reconstructions for these periods show many similarities with the numerical experiments, indicating that the simulation of the hydrological aspect of climate in the Pacific-North American sector is consistent with the available reconstructions, despite the simplified nature of the model and the approach. The simulations show that as a drought is set up in winter and early spring, evaporation decreases and moisture availability in the soils decrease, leading to heat waves which perpetuate the drought into the summer season. Wet conditions are perpetuated from the winter/early spring into summer by a reorganization of atmospheric circulation over the contiguous United States, increasing northward transport of maritime subtropical air from the Gulf of Mexico into the Great Plains region.

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