Marginal instability and deep cycle turbulence in the eastern equatorial Pacific Ocean

Deep cycle mixing in the cold tongue of the equatorial Pacific Ocean is associated with a mean flow regime in which the gradient Richardson number R i (a ratio of stratification to shear that affects the evolution of turbulence) fluctuates about a critical value near . This is the state of marginal instability (MI), a stable equilibrium between forcing by the trade winds (which works to reduce R i ) and turbulence (which works to increase R i ). Besides providing insight into the physics of deep cycle turbulence, MI is easily recognized in moored records of currents and density, and may therefore provide a valuable indicator of turbulence in historical data where direct turbulence measurements were not made. In this initial study, the seasonal cycle of MI is described. MI is present for 9 months of the year but disappears in March, April, and May, consistent with the recently discovered springtime minimum of equatorial turbulence.