The article “Upwelling in the Southern Ocean” by Adele Morrison, Thomas Frölicher, and Jorge Sarmiento (Physics Today, January 2015, page 27) describes important developments in our understanding of the role that ocean overturning plays in the climate system. However, there continues to be an essential gap in our knowledge.

The great ocean conveyor—the thermohaline circulation—is an idealized single-cell meridional overturning circulation that was first proposed1 as a catalyst in abrupt climate change by Wallace Broecker in the late 1980s. The complexity of the ocean overturning circulations, including Southern Ocean upwelling that results from Ekman turning, was already recognized, as evidenced in contemporary publications from the Scientific Committee on Antarctic Research.

The energy that overturning circulations require to overcome ocean stratification is widely accepted as being derived from two sources: generation of negative buoyancy under forming sea ice as salt is expelled and thus increases surface water density; and wind stress that, acting on the surface water, generates Ekman turning and regions of convergence (downwelling) and divergence (upwelling).

Generally omitted in discussions is the energy required to warm water as it circulates and is transformed from bottom water, near 0 °C at a depth of 4000–5000 m, to equatorial surface water at 25–30 °C. If, as is generally reported, the circulation time is about 1000 years, then the average heating rate of the ascending water is of the order of 25 W/m2 or 30–40% of the tropical excess of incident solar radiation over emission of thermal radiation to space. The absorbed heat is subsequently transported poleward by the surface currents and radiated to space.

Rarely discussed in the literature is the impact on weather and climate from a variation in rate of ocean overturning, or even a change in circulation mode as proposed by Broecker. A change in the rate of overturning will both alter the tropical ocean surface temperature and vary the partitioning of poleward heat transport between the ocean and atmospheric circulations. The interannual variations in equatorial Pacific Ocean upwelling associated with El Niño events underscore the potential for longer-term weather and climate response. There is an urgent need to quantify the extent to which part of recent global warming might be attributed to a slowing of larger-scale overturning ocean circulations, particularly because a significant component of the overturning is regulated by the nonlinear interaction between high-latitude westerly winds and Ekman turning of the surface ocean currents.

1.
W.
Broecker
,
The Great Ocean Conveyor: Discovering the Trigger for Abrupt Climate Change
,
Princeton U. Press
(
2010
). 978-0691143545