= Solution
The <density anomaly of water> makes <mass density> increase with temperature up to $T_m$ and decrease above $T_m$. Since temperature increases with depth in the stated layer, the density profile first increases to its maximum $\rho_m$ at the $T=T_m$ isotherm and then decreases toward the warm bottom.
The upper region has denser water below lighter water and hence <stable density stratification>. Below the density maximum, density decreases downward, so lighter water lies beneath denser water and gives <unstable density stratification>. The lower region therefore overturns by <thermal convection>, while the cold upper region remains a stagnant cap. This coexistence is <penetrative convection>.
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