The universal drift instability and other drift instabilities driven by density and temperature gradients in a toroidal system are investigated in both linear and nonlinear regimes via particle simulation. Runs in toroidal and cylindrical geometry show dramatic differences in plasma behavior, primarily due to the toroidicity‐induced coupling of rational surfaces through the poloidal mode numberm. In the toroidal system studied, the eigenmodes are seen to possess (i) an elongated, nearly global radial extent, (ii) a higher growth rate than in the corresponding cylindrical system, (iii) an eigenfrequency nearly constant with radius, and (iv) a global temperature relaxation and enhancement of thermal heat conduction. Most importantly, the measured &khgr;ishows an increase with radius and an absolute value on the order of that observed in experiment. On the basis of the present observations, it is argued that the increase in &khgr;iwith radius observed in experiment is caused by the global nature of heat convection in the presence of toroidicity‐induced mode coupling.