Interannual Variations of Summer Precipitation and Temperature in the Central United States

Statistical analysis revealed that some significant variations shown in annual precipitation and average temperature are not observed in the variations of seasonal precipitation and temperature. This indicates the need of evaluation of seasonal precipitation and temperature variations which are valuable for seasonal predictions. Data analysis showed that there are significant interannual variations of summer precipitation in the central United States. The period of these variations is 3-5 years. They have the largest variance in the variations of both summer and annual precipitation in the region. Temperature averaged over summer season in the region also possesses a variation of interannual scale with a period of 2-3 years. The analysis indicated an absence of interannual time scale variations of both precipitation and temperature in the other seasons in the central US.

Relationship of the interannual variations of summer season precipitation in the central US with development of El Nino is investigated. The latter phenomenon has a pronounced periodicity of 3-5 years. Comparisons of composited flow patterns in El Nino years and normal years (defined as the years with neither El Nino and La Nina events) in the region revealed that in El Nino years the low level southerly jet stream shifts westward from its normal position in the eastern Texas and the Gulf coast. Because the shifted low level jet has weaker strength with drier air from the Mexican Plateau there is less moisture supply from the Gulf area to the central US, resulting in less summer precipitation in the region in the peak years of El Nino. The leading EOF pattern of 700hPa geopotential height in La Nina years shows a configuration encouraging moisture convergence from the Gulf area into the central US. However, these remote effects explain only a fraction of the variations of the summer season precipitation and temperature in the region. The difference of the major frequencies in interannual scale variations of summer precipitation and temperature suggest local effects on these variations. Results from analysis of regional effects on the interannual variations of summer precipitation and temperature are presented.