FIG. 1. Illustration of the two basic stratospheric QBO wind conditions that occur over the tropics at 50 mb and 30 mb during the summer season of both hemispheres. The left diagram shows conditions during the easterly phase of the QBO when easterly winds occur on the equator and winds at 10°N are strongly from the east. The right diagram, by contrast, shows conditions during the westerly phase of the QBO when stratospheric winds on the equator are from the west and winds at 10°N latitude are only weakly from the east. Hurricane activity is suppressed with conditions of the left diagram (east phase) and enhanced for conditions of the right diagrams. Length of arrows is proportional to wind speed.