Slide 8 of 23
Vertical cross section of a polar trough…along 48S. Notice area of low level convergence (negative values/red) between 65W-57W and below 700 hPa, along and ahead of the trough. Aloft note divergence (positive values/blue) ahead of the trough.
Behind the trough, as expected, upper convergence dominates, with low level divergence evident below 600 hPa.