9 - Free and forced thermocline oscillations in Lake Tanganyika

Summary

All year long, the thermocline of Lake Tanganyika (Central Africa) oscillates about two equilibrium states. The thermocline is tilted downward toward the north during the dry season, due to the wind bringing the warm surface water from south to north. The equilibrium position of the thermocline is horizontal during the wet season. The oscillations about these two equilibrium states may be of two types. The free oscillations are due to the seasonal cycle of the wind stress, while the forced oscillations are a direct response to the intraseasonal variability of the surface forcing. It has already been suggested that both have a three- to four- week oscillation period. The Factor Separation method is here used to show that the forced oscillations of the thermocline are about twice as large as the free ones.

Introduction

Lake Tanganyika is located to the east of central Africa, and is shared by four developing countries: Democratic Republic of the Congo, Burundi, Tanzania, and Zambia. It lies between 3° 20′ and 8° 45′ S and 29° 05′ to 31° 15′ E. It is about 650 km long and 50 km wide on average. The mean depth of the lake is about 570 m, with a maximum depth of 1470m (Fig. 9.1). That makes it the second deepest lake in the world, the deepest being Lake Baikal in Russia. Thermal stratification is well marked and present all year long, so that one can identify two distinct layers: the surface and the bottom layers.