Abstract: ADCP observations of internal standing waves waves in a seasonally stratified lake



Authors: Ben Lincoln

An ADCP and thermistor chain have been used to make direct measurements of the small currents (~1-5cm/s) generated by the internal seiche within Lake Bala, North Wales Observations over a seasonally stratified period showed oscillations in the velocity structure with clear phase reversals within the water column.  Cross spectral analysis of the velocity structure was used in conjunction with normal mode analysis to identify several modes of oscillation of the internal seiche.  The first-order vertical mode internal seiche was observed throughout the stratified period, with a period of oscillation ranging from 23 hours to a minimum of 10 hours occurring at the time of peak stratification.   

Seiches of the second-order vertical mode were observed when a three layer structure was observed and the thermocline region was thick relative to the surface layer. The thickening of the thermocline resulted from major mixing events where tilting of the thermocline exposed it to wind mixing.  Early in the season this exposure was due to strong winds and weak stratification. Later in the year resonant forcing of the 12h period first-order vertical mode generated an energetic response exposing the thermocline again.  Resonant forcing may also have been important in the dominance of the second-order modes during summer when the period of oscillation was close to 24h.

The length scale of the internal seiche decreased through the year from ~4000m to ~2500m. This was explained in terms of the deepening of the thermocline and gently sloping bathymetry of the lake.  A strong feedback between the stratification and the internal response of the lake has been observed and could have significant impacts on the ecology of the lake. This study shows that ADCPs can be a useful tool in observing the low energy dynamics of the internal seiche and shows promise in linking physical and biological processes within lakes.