Symmetric instability

From Glossary of Meteorology

symmetric instability

Similar to inertial instability but caused by the imbalance between pressure gradient and inertial forces for infinitesimal disturbances that meridionally displace fluid along isentropes (in the atmosphere) or isopycnals (in the ocean).

For geostrophic motion in the Northern Hemisphere, symmetric instability may occur only if the potential vorticity is negative.