  The current configuration of the ocean overturning involves upwelling
predominantly in the Southern Ocean and sinking predominantly in the Atlantic
basin. The reasons for this remain unclear, as both models and paleoclimatic
observations suggest that sinking can sometimes occur in the Pacific. We
present a six-box model of the overturning in which temperature, salinity and
low-latitude pycnocline depths are allowed to vary prognostically in both the
Atlantic and Pacific. The overturning is driven by temperature, winds, and
mixing and modulated by the hydrological cycle. In each basin there are three
possible flow regimes, depending on whether low-latitude water flowing into
northern surface boxes is transformed into dense deep water, somewhat lighter
intermediate water, or light water that is returned at the surface. The
resulting model combines insights from a number of previous studies and allows
for nine possible global flow regimes. For the modern ocean, we find that
although the interbasin atmospheric freshwater flux suppresses Pacific sinking,
the equator-to-pole flux enhances it. When atmospheric temperatures are held
fixed, seven possible flow regimes can be accessed by changing the amplitude
and configuration of the modern hydrological cycle . North Pacific overturning
can strengthen with either increases or decreases in the hydrological cycle, as
well as under reversal of the interbasin freshwater flux. Tipping-point
behavior of both transient and equilibrium states is modulated by parameters
such as the poorly constrained lateral diffusive mixing. If hydrological cycle
amplitude is varied consistently with global temperature, northern polar
amplification is necessary for the Atlantic overturning to collapse
