The3-Dspiralstructureresultingfromthebalancebetweenthepressuregradientforce,Coriolisforce,andviscousforceisacommonatmosphericmotionpattern.Ifthenonlinearadvectivetermsareconsidered,thistypicalpatterncanbebifurcated.Itisshownthatthesurfacelowpressurewithconvergentcyclonicvorticityandsurfacehighpressurewithpergentanticyclonicvorticityareallstableundercertainconditions.Theanomalousstructurewithconvergentanticyclonicvorticityisalwaysunstable.Buttheanomalousweakhighpressurestructurewithconvergentcyclonicvorticitycanexist,andthisdenotesthecyclone'sdyingout.
