https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2009GL041961
The idea is of activating the ocean metabolism by artificially upwelling nutrients from the deeper layers to the surface, which can be metabolized by photosynthetic organisms. It is similar to that of fertilizing the ocean by dumping iron oxide in the water.
The interesting thing is how sensitive is the Earth holobiont to this kind of manipulation. According to the article, artificial upwelling would "be able to sequester atmospheric CO2 at a rate of about 0.9 PgC/yr," which is about a tenth of the current carbon emissions. In itself, it would not change the trend, but it is still a lot, and if it were continued for decades it would make a difference considering the unavoidable decline of fossil fuel production generated by depletion.
But, of course, things like pipes and flap valves could hardly be deployed in the necessary amounts, considering that humans see it as much more important to use their remaining resources to make war on each other. Yet, it is impressive to think that what the pipes are supposed to do used to be done by whales before they were exterminated (https://www.pnas.org/doi/10.1073/pnas.1502549112). So, humans have already modified the Oceans' system in the opposite direction.
Likely, more whales would cool the planet. And they would produce themselves if just left in peace, no need for huge pipes and pumps!
