Wind‐induced water movements in the South Basin of Windermere

SUMMARY. The results of direct current measurements in the South Basin of Windermere are presented and related to wind history and horizontal variations in near‐surface water temperature. Currents were measured by Lagrangian methods using drift‐bottles and depth‐specific drogues. Except when sudden calms followed strong winds, internal seiche movements had little effect on horizontal transport in the epilimnion. The most important factor governing mass water movement was direct wind forcing. Variations in wind speed accounted for 93% of the variation in near‐surface current speed. For wind speeds between 100 and 500 cm s−1 the wind factor (current speed/wind speed) decreased linearly with wind speed. At wind speeds above 500 cm s−1 the wind factor remained relatively constant around 1 %. Coriolis effects deflected near‐surface currents 4–38° to the right of the wind. The degree of deflection was strongly correlated with the relative depth DE/D* (where DE was the depth of the epilimnion and D* the depth of frictional resistance). The influence of the Coriolis force also produced pronounced rotations, with depth, of the wind‐driven current. The circulation pattern within the epilimnion was broadly that of a distorted conveyor belt moving at some angle to the wind axis. The strength of the transverse circulation was greatest at low wind speeds with a deep thermocline. Richardson‐number calculations suggest that the thermocline generally acts as a low‐friction boundary between a turbulent epilimnion and a relatively quiescent hypolimnion. Horizontal variations in water temperature, although ranging from only 0.2 to 1.0°C per km, served as a good indirect ‘tracer’ of the circulation pattern.