Strain: Observations of the Vertical Gradient of Isopycnal Vertical Displacement

Abstract

In this work a profiling CTD, operated from the research platform FLIP, is used to monitor the fine-scale density field as a function of both depth and time. A sequence of 10 000 CTD profiles from the surface to 560 m is examined. The data were obtained off the Southern California coast in the 1986 PATCHEX experiment. The vertical separation between successive isopycynal surfaces is tracked. The separation is related to the vertical derivative of vertical displacement, and is here referred to as the strain. The purpose of this work is to present a simple picture of the fine scale strain field as it evolves in time as well as depth. When viewed in isopycnal following coordinates, the qualitative nature of the strain field depends on the characteristic vertical scale over which it is estimated. The ?20 m strain? field has a wavelike character, dominated by inertial and semidiurnal tidal motion. Wavelike fluctuations are seen in the 20 m strain field even at subinertial frequencies. This suggests that nonlinear processes are significant even at these relatively large vertical scales. The ?2 m strain? more closely resembles the classic picture of fine structure. Lenses of low density gradient fluid are separated by sheets of higher gradient water. The lenses are seen to persist up to eight hours. They can propagate with respect to the density field over tens of meters. The low gradient regions evolve into regions of high gradient and visa versa. The probability density function (PDF) of isopycnal separation is Gaussian for isopycnal pairs of large (?20 m) mean separation. As mean separation distance is decreased, the skewness of the distribution increases. Since all scalar fields in the sea are strained by the same velocity field, fluctuations in the fine-scale vertical gradients of a variety of quantities are correlated. Averages of the products of fine scale gradients can differ significantly from products of the averages.