Differential cooling drives large‐scale convective circulation in Lake Tanganyika

Abstract

Field data, scaling analysis, and three-dimensional numerical experiments demonstrate that the dominant large-scale circulation pattern during the southeast trade winds in Lake Tanganyika (East Africa) is a downwind flow in the metalimnion, with a returning southward upwind flow in the upper region of the epilimnion. This is in the opposite direction to all prior literature available on the lake, which has assumed that the southeast trade winds must drive a northward surface flow due to momentum induced by the wind. The gradient in wind speed and humidity along the length of the lake and the resulting gradient in heat exchange with the atmosphere result in warm water accumulating at the northern end of the lake. The north basin of Lake Tanganyika, thus, acts as a heat sink while the south basin is a net source of heat to the atmosphere. The resulting variation in buoyancy flux is strong enough to drive a convective flow in the upper layer in the opposite direction to the wind. A large-scale convective circulation results in an annual exchange of 6.2 3 10 12 m 3 of water and an annual net transport of heat (averaging 0.2 terawatt) from the northern to the southern region of the lake. Currents in the upper 100-m layer, with a mean flow of 0.2 Sverdrup and flow speed of 0.08 m s 21 , can be described by the superposition of a parallel convective flow with the internal seiche motion.

A fully consistent and conservative vertically adaptive coordinate system for SLIM 3D v0.4 with an application to the thermocline oscillations of Lake Tanganyika

“…17 are consistent with the results shown in Fig. 8 from Verburg et al (2011), which modelled the 1996 lake dynamics. In May, which corresponds to the beginning of the strong wind season, the 26.5 • C isotherm outcrops at the middle of the lake in both studies, while the 25.5 • C isotherm outcrops only at the southern tip of the lake.…”

“…The southward surface current going in the opposite direction of the wind, as it was suggested by Verburg et al (2011), is also observed in SLIM results, although this current is most likely due to the weakening of the wind stress and pressure gradient pushing the water mass back to its equilibrium position. This comparison with the results of Verburg et al (2011) for the year 1996 motivates to investigate further the lake circulation, for a longer period using weather forcings from the COSMO-CLM 2 model (Davin and Seneviratne, 2012).…”

“…In May, which corresponds to the beginning of the strong wind season, the 26.5 • C isotherm outcrops at the middle of the lake in both studies, while the 25.5 • C isotherm outcrops only at the southern tip of the lake. The agreement between SLIM 3D and Verburg et al (2011) is also good at the end of this wind season, which corresponds to beginning of August in 2003 and July in 1996.…”

“…2, Mortimer, 1961), they proposed a reversed circulation with the deepest water of the epilimnion following the wind and the surface water flowing in the opposite direction. Verburg et al (2011) quantified the conditions for which they proposed a counter-wind surface circulation, driven by the surface heat flux of the lake. They used a 3-D model (Hodges et al, 2000) P. Delandmeter et al: A consistent and conservative vertically adaptive coordinate system for SLIM 3D Figure 1.…”

Understanding the performance of the FLake model over two African Great Lakes

“…During last decades, the African Great Lakes experienced fast changes in ecosystem structure and functioning, and their future evolution is a major concern (O'Reilly et al, 2003; Verburg and Hecky, 2009). To better understand the present lake hydrodynamics and their relation to aquatic chemistry and biology, several comprehensive one-, two-or three-dimensional hydrodynamic models have been developed and applied in standalone mode to lakes in this region (Schmid et al, 2005;Naithani et al, 2007;Gourgue et al, 2011;Verburg et al, 2011). However, to investigate the two-way interactions between climate and lake processes over East Africa, a correct representation of lakes within regional climate models (RCMs) and general circulation models (GCMs) is essential (Stepanenko et al, 2013; see Appendix for a list of all acronyms, variables and simulation names).…”