THE REVERSAL OF STRATOSPHERIC WINDS OVER NORTH AMERICA

The individual stratospheric reversals of 1957, 1958 and 1959, and of the normals are summarized and compared. The erratic spring reversal occurs as the winter vortex is displaced from time to time by a warm high pressure cell moving from mid-latitudes to polar latitudes. Occasionally such intrusions prove temporary and the vortex returns to its polar location at other times, a complicated cellular pattern eventually develops into a stable summer polar anticyclone. Over the western hemisphere it is suggested that easterlies first appear at highest levels near 55 60 degrees N in the early spring, then advance downward and northward in time. As the reversal process is not circumpolarly symmetric, southward progression is to be expected in the opposite hemisphere to the northward progression. The fall reversal appears to be far more regular and very rapid, starting in August near 55 degrees N and propagating upward. By mid-September all the stratosphere north of 55 degrees N has usually changed to westerly flow, only 2 - 3 wk after its start. Author