The Water Mass Transformation in the Upper Limb of the Overturning Circulation in the Southern Hemisphere

The warming and salinification of the northwards flowing water masses from the Southern Ocean to the tropics are studied with Lagrangian trajectories simulated using fields from an Earth System Model. The trajectories are used to trace the geographical distribution of the water mass transformation and connect it with the pathways of the upper limb of the overturning circulation in the Southern Hemisphere. In the Antarctic Circumpolar Current water gains heat just below the mixed layer, mainly when the layer is thin during Austral spring and summer. This gain is therefore suggested to be a consequence of heat flux from the atmosphere and mixing processes at the base of the mixed layer. In the Southern Hemispheric subtropical gyres on the other hand, a large warming and salinification of the northwards flowing water results from internal mixing with other warmer and more saline water masses. Close to the Antarctic shelf waters are getting fresher as a result of ice melting, whereas further north, in the Antarctic Circumpolar current, waters are getting more saline as a result of evaporation. Our results show that it is not only the heat and freshwater fluxes through the sea surface that control the heat and salt changes of the upper limb of the overturning circulation in the Southern Hemisphere. In fact, internal mixing accounts for 25% of the heat change, and 22% of the salinity change.

Automated summary

The study explored the warming and salinification of water masses flowing from the Southern Ocean to the tropics using Lagrangian trajectories simulated with an Earth System Model. Results showed that heat gain in the Antarctic Circumpolar Current is mainly due to heat flux from the atmosphere and mixing processes, while the warming and salinification of water masses in the Southern Hemispheric subtropical gyres result from internal mixing with other warmer and more saline water masses.