Analysis of wind-induced thermocline oscillations of Lake Tanganyika

Naithani, Jaya [UCL] Deleersnijder, Eric [UCL] Plisnier, PD

An analysis is presented of the wind-induced thermocline oscillations of Lake Tanganyika, East Africa. The region undergoes a four month dry season and the wet season for the rest of the year. The dry season is characterised by nearly constant high southeasterly winds, while for the rest of the year mild wind blows generally from the northeast. Observations show that the dry season high winds cause tilting of the thermocline, being higher/lower than normal at the southern/northern ends of the lake. The thermocline tries to restabilise itself after the cessation of dry season winds and oscillates for the rest of the year. A non-linear reduced-gravity model is used to study the thermocline oscillations of the lake. The numerical simulations satisfactorily represent the oscillations, their period and amplitude. Different forcing conditions (thermocline depth, wind stress and stability) are used in the model and their effect on the period and amplitude of the oscillations are studied. The amplitude of oscillations ranges from 15 to 45 m, while their period varies from 3 to 4 weeks according to the variation in the model parameters. Wavelet transform is used to study the evolution of periods of oscillations with depth in the time series of observations and along the length of the lake using model simulations. Wavelet spectra presents several dominant modes including the semidiurnal, diurnal, synoptic, intraseasonal variability, besides the modes representing the wind-induced thermocline oscillations.