Characteristic anomaly patterns of summer sea-level pressure for the Northern Hemisphere

Abstract

Characteristic type patterns of Northern Hemisphere sea-level pressure anomaly for summer (July-August) were identified by analyzing the pressure departure maps for the 77 summers from 1900 through 1976, using a pattern recognition algorithm based on the use of the correlation coefficient to measure similarity between map patterns. The procedure allows assessment of statistical significance of the anomaly for each data point on each type pattern. Six types were identified, and the first four of these contained over 93% of those anomalies which were significant at the 99% confidence level. Significant anomalies occurred least frequently in middle latitudes, rather than at low latitudes where the standard deviations of the actual departure values are smallest.

The two most prominent type patterns have approximately the same configuration, but with opposite anomaly signs. Observed pressure anomaly patterns resembling the first type tended to occur during the period of relatively high mean annual Northern Hemisphere surface temperature, from about 1925 to 1955. Anomaly patterns resembling the second type occurred mainly before 1920, when the mean annual Northern Hemisphere temperature was lower than it has been since. The third and fourth type patterns suggest modes of covariance between the "monsoon" cirulation in southeast Asia and its less dramatic counterpart in the southwestern United States.