A diffusive Fisher-KPP equation with free boundaries and time-periodic advections

Abstract: We consider a reaction-diffusion-advection equation of the form: $u_t=u_{xx}-\beta(t)u_x+f(t,u)$ for $x\in (g(t),h(t))$, where $\beta(t)$ is a $T$-periodic function representing the intensity of the advection, $f(t,u)$ is a Fisher-KPP type of nonlinearity, $T$-periodic in $t$, $g(t)$ and $h(t)$ are two free boundaries satisfying Stefan conditions. This equation can be used to describe the population dynamics in time-periodic environment with advection. Its homogeneous version (that is, both $\beta$ and $f$ are independent of $t$) was recently studied by Gu, Lou and Zhou \cite{GLZ}. In this paper we consider the time-periodic case and study the long time behavior of the solutions. We show that a vanishing-spreading dichotomy result holds when $\beta$ is small; a vanishing-transition-virtual spreading trichotomy result holds when $\beta$ is a medium-sized function; all solutions vanish when $\beta$ is large. Here the partition of $\beta(t)$ is much more complicated than the case when $\beta$ is a real number, since it depends not only on the "size" $\bar{\beta}:= \frac{1}{T}\int_0^T \beta(t) dt$ of $\beta(t)$ but also on its "shape" $\tilde{\beta}(t) := \beta(t) - \bar{\beta}$.