Monthly Variations in the Relationship Between Mid-Tropospheric Flow and Surface Temperatures across the United States.
Mid-tropospheric flow has a major influence on surface temperatures through the differential advection of air masses into different mid-latitude regions. Because of the importance of mid-tropospheric flow on temperatures, long-range temperature forecasts are largely dependent upon forecasts of mid-tropospheric flow. Monthly variations in the strength of the relationship between mid-tropospheric flow and surface temperatures can have a major influence on the accuracy of long range temperature forecasts during different months of the year. In this study, the monthly variations in the association between mid-tropospheric flow and mid-latitude surface temperatures are examined by correlating multiple indices of mean monthly 500-mb flow with mean monthly surface temperatures across the coterminous United States for the period from 1951 to 1980. Specifically, mean monthly 500-mb pressure height differences across 39 transects are used as indices of the strength and type of mid-tropospheric flow across North America and the eastern north Pacific. Twenty-two north-south transects are used to provide measures of the strength of zonal flow across different areas. Seventeen east-west transects are used to provide information on the strength and direction of meridional flow across different locales. Difference of means tests were used to examine the relationship between mid-tropospheric flow and surface temperatures during the extreme warmest versus the extreme coldest months for each month of the year. Correlation coefficients were computed to show the monthly variations in the strength and direction of the relationship between mid-tropospheric flow and surface temperatures. Results indicate that mid-tropospheric flow is most strongly associated with surface temperatures during the transition seasons of spring and autumn and during the month of January. The associations are found to be weakest during the summer and during February. However, overall, although there are major changes in the key areas of flow from one season to the next, the overall strength of the association between the mid-tropospheric circulation and surface temperatures is fairly similar throughout the year.
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- Physical Geography; Physics: Atmospheric Science