Fig. 4. The time-mean velocity potential field (chi) for 15 March-15 April 1996 at (a) the 150-hPa level and (b) the surface. The contour interval is 2 x10⁶ m² s^-1; negative contours are dashed. Divergent wind vectors are superimposed; the arrow scaling is shown at lower right. Shading shows regions where OLR is 230 W m^-2.