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 Ri (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 Ri) and turbulence (which works to increase Ri). 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.
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