  Double diffusive convection (DDC), which is the buoyancy driven flow with
fluid density depending on two scalar components, is ubiquitous in many natural
and engineering enviroments. Of great interests are scalers transfer rate and
flow structures. Here we systematically investigate DDC flow between two
horizontal plates, driven by an unstable salinity gradient and stabilized by a
temperature gradient. Counterintuitively, when increasing the stabilizing
temperature gradient, the salinity flux first increases, even though the
velocity monotonically decreases, before it finally breaks down to the purely
diffusive value. The enhanced salinity transport is traced back to a transition
in the overall flow pattern, namely from large scale convection rolls to
well-organised vertically-oriented salt fingers. We also show and explain that
the unifying theory of thermal convection originally developed by Grossmann and
Lohse for Rayleigh-B\'{e}nard convection can be directly applied to DDC flow
for a wide range of control parameters (Lewis number and density ratio),
including those which cover the common values relevant for ocean flows.
