Hannah's text on the biology of climate change appears to suggest that forcing due to changes in obliquity can act in opposite ways in Antarctic waters. On one hand,

"Low obliquity brings cool summers to both hemispheres, which favors ice buildup in the north and intensified circumpolar current in the south. The intensification of the circumpolar current reduces upwelling of $\ce{CO2}$-rich water. The reduction in atmospheric $\ce{CO2}$ cools the planet..."

On the other hand,

"high obliquity results in warmer summers in both hemispheres. This begins to melt the continental ice sheets in the north, whereas in the south it intensifies circumpolar currents and winds, pumping $\ce{CO2}$-rich water to the surface and warming."

How can "intensified" ACC both increase and decrease upwelling? How does this "intensification" occur, that is what is the linkage between warmer or cooler Antarctic summers and the intensification? Apparently ocean - atmosphere $\ce{CO2}$ exchange is important here but the fertilization effect of upwelling is not?


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