The Oregon State University Mars MM5 was used in a comprehensive high-resolution study of northern polar summertime circulations. Three simulations (Ls = 120, Ls = 135, and Ls = 150) characterize the changing circulation. The atmosphere is dry, and model dynamics are hydrostatic. A modified TES thermal inertia map provides a realistic simulation of the polar thermal environment. The highest-resolution nest (18 km) resolves complex flows near the cap; zonal-mean easterlies (~10 m/s) and zonal-mean katabatic winds (~5 m/s) near the surface are relatively steady during this study. Katabatic flows are shallow (~300 m); the easterlies are deeper (~1.5 km). Transient eddies are very important within the first scale height; they are excited by mechanisms, and at locations, that change dramatically during this short study period. At Ls = 120 they form along the residual cap edge with a zonal wave number one structure, producing strong excursion winds (10--15 m/s) that blow consistently across the cap. By Ls = 135, strong eddies are seen to form on the northern slopes of Alba Patera and Tharsis. These eddies are quite suggestive of the large annular cloud structures seen in Hubble Space Telescope and Mars Orbital Camera imagery at this location and season and can traverse the high latitudes to reach the residual cap before dissipating. Eddies in the earlier two simulations appear to be primarily excited by energetic flows near the surface. By Ls = 150 an early fall polar jet causes strong winter-like baroclinic eddies to develop. The transient eddies found in this study are probably important in the water cycle of the northern residual cap.