Anomaly Patterns of Climate over the Western United States, 1700-1930, Derived from Principal Component Analysis of Tree-ring Data

Major anomaly patterns of annual tree-ring growth in “drought-sensitive’’ trees of the Western United States, 1931-1962, seem to reflect corresponding patterns in monthly precipitation amounts, which in turn may be related to circulation anomalies. The three most important anomaly patterns that dominated the 1931-1962 period were also prominent in the tree-growth data during the preceding period 1100-1930. These patterns can thus be expected to maintain their importance during at least the immediate future. Time series of eigenvector amplitudes show how the relative importance of the anomaly patterns changes through time. These can be studied for properties lending possible forecasting value and may provide important insight into the past behavior of the atmospheric circulation over the Western United States.