\section{Supplementary Material}
See \href{https://drive.google.com/file/d/1yvnHXd_6vbp1HvzpY4MiztVpQ6-bhLsh/view?usp=sharing}{supplementary text} for the details of (I) simulation set-up, (II) modal energy evolution equation, and (III) energy transfer rates between the background flow and the fluctuations. A \href{https://drive.google.com/file/d/1SA_0_pPlZNIbLnBS9Fhy9cNdU5SXwVnC/view?usp=sharing}{simulation movie} supplements Fig.~\ref{fig:f5} of this Letter.

\section{Acknowledgements}
\begin{acknowledgments}
The authors thank K.~Burns and {\tt Dedalus} developers for assistance. Thanks also to N.~Hurst for offering helpful discussions. This material is based upon work funded by the Department of Energy [DE-SC0022257] through the NSF/DOE Partnership in Basic Plasma Science and Engineering.
We also gratefully acknowledge the Award no.~DE-FG02-04ER54742, supported by the U.S. Department of Energy, Office of Science, Fusion Energy Sciences. In addition, AEF acknowledges support from NSF Grant Nos.~AST-1814327 and AST-1908338. All simulations were performed using the XSEDE supercomputing resources via Allocation No.~TG-PHY130027. 

The data that support the findings of this study are available from the corresponding author upon reasonable request.
\end{acknowledgments}


