The Role of the North American Topography on the Maintenance of the Great Plains Summer Low-Level Jet*

Summer precipitation over the central United States depends strongly on the strength of the Great Plains low-level jet (LLJ). The Geophysical Fluid Dynamics Laboratory’s new generation of the atmospheric general circulation model (GCM) and the linear and nonlinear stationary wave models are used in this study to examine the role of North American topography in maintaining the Great Plains summer mean LLJ and precipitation. Atmospheric GCM experiments were first performed with and without the North American topography and with prescribed climatological sea surface temperatures. Results show that the Great Plains LLJ disappears completely in the experiment when the North American topography is removed, while the summer seasonal mean LLJ is well simulated in the experiment with full earth topography. In the absence of the North American topography, the summer precipitation is significantly reduced over the central United States and increased along the Gulf States and northeast Mexico. Linear and nonlinear stationary wave models are used to determine the physical mechanisms through which the North American topography maintains the Great Plains time mean LLJ. Possible mechanisms include the physical blocking of the topography and the induced flow over and around the mountains, the thermal effect due to the elevation of the topography, and the transient thermal and vorticity forcing due to the modification of transient eddy activities in the presence of the topography. The linear and nonlinear model results indicate that the dominant mechanism for maintaining the time mean Great Plains LLJ is through the nonlinear effect of the trade wind along the southern flank of the North Atlantic subtropical high encountering the east slope of the Sierra Oriental and causing the flow to turn northward. As the flow turns north along the east slope of the North American topography, it obtains anticyclonic shear vorticity and thus the LLJ. The effect of the thermal forcing is negligible, while the effect of transient forcing is only important in extending the jet farther northward and eastward. The results suggest that variations in the strength of the North Atlantic subtropical anticyclone and the associated trade wind over the Caribbean Sea and the Gulf of Mexico may be important for understanding the interannual variation of the Great Plains LLJ and U.S. precipitation.