Large-scale instability in hydrodynamics with stable temperature stratification driven by small-scale helical force

In this work we consider the effect of a small-scale helical driving force on fluid with a stable temperature gradient with Reynolds number Re<<1. At first glance, this system does not appear to have any instability. However, we show that large-scale vortex instability appears in fluid despite its stable stratification. In the non-linear mode, this instability gets saturated and gives a large number of stationary spiral vortex structures. Among these structures there is a stationary helical soliton and a kink of a new type. The theory is built on the rigorous asymptotical method of multi-scale development.