FIG. 2: August-October climatological mean fields: (a) tropospheric vertical wind shear (200 minus 850 mb flow with a contour interval of 3 m s^-1, with shear values less than 9 m s^-1 shaded); (b) 200 mb streamlines and zonal wind speeds (contour interval of 3 m s^-1, with positive (negative) values solid (dashed) and any easterly component is shaded); (c) 850 mb streamlines and zonal wind speeds (contour interval of 2 m s^-1, with positive (negative) values solid (dashed) and any easterly component greater than 6 m s^-1 is shaded). Climatology is calculated from the 1979-1995 base period monthly means.