Spatial and Temporal Variability of Southern California Precipitation over the Last 400 yr and Relationships to Atmospheric Circulation Patterns

Abstract

Tree-ring chronologies have been used to develop a 400-yr spatial reconstruction of precipitation in coastal and near-interior southern California. This was accomplished by reconstructing the first two precipitation principal components of a 29-station network, which together account for over 83% of the precipitation variance. The first precipitation principal component is representative of a regionwide pattern of above or below normal precipitation and is reconstructed well. Reconstruction of the second precipitation principal component, which represents a north?south contrast in precipitation, is less reliable but still provides useful information. Results show that mean regionwide precipitation during the last 100 yr has been unusually high and less variable compared to other periods in the past. The modern period of the record has also been characterized by a high degree of north?south contrast but not unusually high compared to the record as a whole. Strong contrast during the modern period is due to anomalously wet conditions in the southern part of the region, but this does not characterize all earlier periods. Fluctuations in high-frequency variability have been much larger than fluctuations in mean for both the first and second precipitation principal components. Most notable are the low variance in the early eighteenth century and the high variance in the middle of the nineteenth century. An intriguing hypothesis that arises from this study is that there may be important long-term fluctuations in the spatial pattern of precipitation anomalies. For approximately the first 200 yr of the precipitation reconstruction (1560?1760), precipitation anomalies were more common in the northern part of the study area than the southern. Comparisons of modern precipitation records with sea level pressure data indicate that this is likely to occur during periods of strong meridional flow. This suggests that there may have been a higher frequency of blocking and meridional flow over California and the eastern Pacific during the Little Ice Age. In any case, it appears that the main difference in precipitation was not a change in the regionwide mean, but in the spatial distribution of anomalies.