On the seasonality of climate fluctuations over the contiguous United States

Thesis (Ph. D.)--University of Washington, 1986The dominant spatial patterns of climatic fluctuation over the contiguous United States during the winter and summer seasons (1931-82) are examined using monthly mean temperature and monthly total precipitation anomalies for the 344 climatic divisions. These data are expanded into orthogonal components using rotated principal component analysis.Related patterns of sea-level pressure over the Northern Hemisphere (1931-82) and sea surface temperature over both the North Pacific and the North Atlantic (1950-79) for these dominant patterns are discussed. These results are used to evaluate the seasonal dependence of climate fluctuations.The relationships between sea-level pressure and sea surface temperature and the climate fluctuations over the contiguous United States are weak during summer and strong during winter. The dominant climate fluctuations are related to the intensity and position of the dominant features in the climatological mean sea level pressure field. The results confirm that severe winters over the eastern United States are well correlated with both the Pacific North American Pattern and the North Atlantic Oscillation. Temperature anomalies along the east slope of the Rockies occur in connection with the North Pacific Oscillation. Hot, dry summers tend to be associated with stronger than normal anticyclones over both the Pacific and the Atlantic, and a deeper than normal continental low over the Candian Rockies.The pattern of sea-surface temperature anomalies over the Pacific with east-west contrasts is strongly correlated with climatic fluctuations over the southeastern United States during winter. This relationship provides evidence of remote forcing of climate fluctuations over the United States. In summertime, the sea surface temperature anomaly pattern with east-west contrasts is correlated with temperatures over the Pacific Northwest only. Another important relationship during summer is the link between sea surface temperature anomalies over the Gulf of Mexico and the month to month persistence of temperature over the Gulf States. These results suggest that climate fluctuations are sensitive to local thermal forcing during summer.The Palmer Drought Severity Index, relative humidity and dewpoint anomalies were used to document the relationship between the soil moisture and summertime climate anomalies. The results suggest that some of the summer climate fluctuations over the contiguous United States are forced by soil moisture anomalies