Water Movements at a Mid-Central Basin Site: Time and Space Scales, Relation to Wind, and Internal Pressure Gradients

Abstract

Current meter and thermistor string records made at a mid-basin site from May, 1979, through February, 1980, and during August, 1980, are used to determine the time and space scales of horizontal motion in mid lake and to relate these to the major forcing variables. Motions at frequencies larger than 0.125 cycles per hour are horizontally coherent over a few km only, whereas lower frequency motions may cohere significantly over tens of km in the stratified season. Of the four depths sampled, 10 m, 15 m, 20 m, and 21 m, highest current speeds are associated with circularly polarized clockwise rotating motion at the inertial period at 15-m depth. Surveillance cruise data show the mid central basin array to be located in a zone of relatively flat thermocline topography but suggested that internal pressure gradients might at times be large enough to influence bottom flow. Limited small-scale sampling in the vicinity of the mid-basin array reveals the existence of intense but short scale (5 km) internal pressure gradients that are undersampled by the array of thermistor strings (separated by 10 km). Multivariate statistical techniques in the frequency domain indicate that the internal pressure gradients estimated by means of the array of thermistor strings are of marginal dynamical significance (although the smaller scale gradients may in fact contribute substantially to the observed variance). A simple model of locally driven currents appropriate to a region of constant depth is proposed. This model and the statistical analyses together help to interpret the effects of wind stress and Coriolis forces. Many of the observed features of circulation, including seasonal evolution, can be related to the role of stratification in governing the vertical distribution of turbulent mixing.