Figure 4: Objective analysis of geopotential height (thick continuous every 60 gpm) and absolute vorticity advection every 5 × 10-10 s-2 at 300 hpa at 12:00 UTC on 25 March 1992. Negative advection isopleths are dotted, and the zero one is dot-dashed. Positive advection isopleths are thin continuous. L1 and L2 denote the centres of the surface depressions, and T1 and T2 are the corresponding trough lines.