Two precipitation systems in the summertime over North America, the North-American Monsoon (NAM) and the US Midwest summer precipitation have been found to be negatively related to one and another on time scales ranging from seasonal to interannual. Both of them individually influence the economy of the respective local regions considerably. In this paper, we will investigate the atmospheric circulation changes that are possibly linked to this variability of precipitation at the two time scales. Linear and nonlinear baroclinic diagnostic models are used to study these circulation changes.

Our linear and nonlinear models simulate the changes of stationary wave patterns over the US in summer fairly well. The stationary wave response to individual forcings, such as diabatic heating, orography and transients, are examined. The combination of orographic forcing and diabatic heating is capable of reproducing the change in stationary wave pattern indicating that diabatic heating and orographic forcing play an important role in the stationary wave changes in summer. The non-linear interaction between these two forcings is the dominant part of the non-linear interaction term. The role of transient forcing was found to be negligible. The importance of orographic forcing has been further emphasized through the role of changes in the basic state that contribute significantly to the changes in stationary wave maintenance.

An examination of the stationary wave features associated with interannual variations of the wet and dry North American monsoons will also be performed in the near future.