# Fluctuations and large deviations of Reynolds stresses in zonal jet dynamics

## TL;DR

This paper develops a novel method to compute the large deviation rate function for Reynolds stress fluctuations in turbulent flows, revealing highly non-Gaussian behavior and advancing understanding of rare flow transitions.

## Contribution

It introduces a matrix Riccati equation approach to calculate the large deviation rate function for Reynolds stresses in turbulent flows under quasilinear approximation.

## Key findings

- First computation of the large deviation rate function for Reynolds stress.
- Reynolds stress fluctuations are highly non-Gaussian.
- Method applied to barotropic flow on a rotating sphere.

## Abstract

The Reynolds stress, or equivalently the average of the momentum flux, is key to understanding the statistical properties of turbulent flows. Both typical and rare fluctuations of the time averaged momentum flux are needed to fully characterize the slow flow evolution. The fluctuations are described by a large deviation rate function that may be calculated either from numerical simulation, or from theory. We show that, for parameter regimes in which a quasilinear approximation is accurate, the rate function can be found by solving a matrix Riccati equation. Using this tool we compute for the first time the large deviation rate function for the Reynolds stress of a turbulent flow. We study a barotropic flow on a rotating sphere, and show that the fluctuations are highly non-Gaussian. This work opens up new perspectives for the study of rare transitions between attractors in turbulent flows.