  The ocean thermohaline circulation, also called meridional overturning
circulation, is caused by water density contrasts. This circulation has large
capacity of carrying heat around the globe and it thus affects the energy
budget and further affects the climate. We consider a thermohaline circulation
model in the meridional plane under external wind forcing. We show that, when
there is no wind forcing, the stream function and the density fluctuation
(under appropriate metrics) tend to zero exponentially fast as time goes to
infinity. With rapidly oscillating wind forcing, we obtain an averaging
principle for the thermohaline circulation model. This averaging principle
provides convergence results and comparison estimates between the original
thermohaline circulation and the averaged thermohaline circulation, where the
wind forcing is replaced by its time average. This establishes the validity for
using the averaged thermohaline circulation model for numerical simulations at
long time scales.
