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Alternatively, a dynamic definition of the tropopause, the ''dynamic tropopause'', is used with the aid of [[potential vorticity]], which is defined as the product of the isentropic [[density]], i.e. the density that arises from using [[potential temperature]] as the vertical coordinate, and the [[absolute vorticity]].<ref>{{cite paper|last1= Hoskins|first1= B. J.|last2= McIntyre|first2= M. E.|last3= Robertson|first3= A. W.|year= 1985|title= On the use and significance of isentropic potential vorticity maps|journal= [[Quarterly Journal of the Royal Meteorological Society]]|volume= 111|pages= 877{{ndash}}946}}</ref> given that this quantity attains quite different values for the troposphere and the stratosphere. Instead of using the vertical temperature gradient as the defining variable, the dynamic tropopause surface is expressed in ''[[potential vorticity unit]]s'' (PVU), with the tropopause layer typically lying within the 1.5–2 PVU surface in the [[Northern Hemisphere]], although no universal values exist. Given that the absolute vorticity is positive (negative) in the Northern ([[Southern Hemisphere|Southern]]) hemisphere, the threshold value should be taken as positive (negative) north (south) of the equator. Theoretically, to define a global tropopause in this way, the two surfaces arising from the positive and negative thresholds need to be matched near the equator using another type of surface such as a constant [[potential temperature]] surface. Nevertheless, the dynamic tropopause is useless at equatorial latitudes because both the absolute vorticity is close to zero —consequently making potential vorticity to vanish— and the isentropes are almost vertical.
It is also possible to define the tropopause in terms of chemical composition. For example, the lower stratosphere has much higher [[ozone]] concentrations than the upper troposphere, but much lower [[water vapor]] concentrations, so appropriate cutoffs can be used.supot
The tropopause is not a "hard" boundary. Vigorous [[thunderstorm]]s, for example, particularly those of tropical origin, will [[convective overshoot|overshoot]] into the lower stratosphere and undergo a brief (hour-order) low-frequency vertical [[oscillation]]. Such oscillation sets up a low-frequency atmospheric [[gravity wave#Atmosphere_dynamics_on_Earth|gravity wave]] capable of affecting both atmospheric and oceanic currents in the region.
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