Aspects of the Global Thermohaline Circulation in the Absence of Wind Forcing

The global ocean’s overturning circulation plays an important role in climate and climate variability through its transport of heat, freshwater and nutrients. As part of this three-dimensional overturning circulation, dense waters sink in narrow regions at high latitudes in the North Atlantic and along the Antarctic coast. To close this circulation, it is generally assumed that either intense interior mixing by winds and internal tides, or wind-driven upwelling is required to bring these water masses back to the surface. Nevertheless, more recent work questions this requirement for winds and tides, arguing that surface buoyancy forcing alone can drive such a circulation through a process known as rotating horizontal convection. In particular, it has been shown that the presence of a re-entrant channel, such as the Southern Ocean, is required for rotating horizontal convection to generate many features of the global ocean’s overturning circulation. Building on previous work in which rotating horizontal convection was forced by only thermal forcing, here we demonstrate, using an idealised eddying ocean model with both thermal and haline surface forcing, that rotating horizontal convection can produce many of the observed features of the global ocean’s overturning circulation. These results therefore suggest that a global “thermohaline circulation” can exist in the ocean in the absence of winds and in the limit of small vertical diffusion.