  Coupled mixed convective and stratified systems are common in natural flows.
To study experimentally the associated dynamics, we use a singular property of
water: its non-linear equation of state is characterised by a maximum density
close to $4^{\circ}$C. By heating the top of a tank at $35^{\circ}$C and
cooling the bottom at $0^{\circ}$C, a two-layer configuration spontaneously
appears. The convective motion in the bottom layer consists mostly of a
large-scale circulation and rising cold plumes. This turbulent flow generates
propagating internal gravity waves in the upper stably-stratified layer. PIV
measurements are performed and spectral characteristics of the convection and
internal gravity waves are presented. An horizontal large-scale reversing flow
in the stratified layer is observed which is viscously driven by a third,
intermediate layer. This buffer layer is located between the convective and
stratified layers and is thermally coupled with the convective one, hence
sustaining a strong horizontal shear. Three dimensional direct numerical
simulations with geometry and physical parameters close to the experimental
ones corroborate our experimental results.
