| Summary: | A recent hypothesis postulated that paleoclimate changes to the Southern Hemisphere westerlies were characterized by the modulation of the wintertime South Pacific Split Jet. We explore this hypothesis further through simulating changes to the ocean circulation from Split Jet modulation, contrasting them against changes associated with the wintertime Southern Annular Mode (SAM). Three responses distinguish the Split Jet from the SAM impact on ocean circulation. (i) A weaker Split Jet strengthens the South Pacific subtropical gyre, leading to stronger western boundary currents and warming of the sea surface temperatures (SSTs) surrounding New Zealand. (ii) A positive SAM leads to an increase in the Antarctic Circumpolar Current and specifically Drake Passage throughflow. And (iii) a weaker Split Jet leads to increased formation of Subantartic Mode Water, whereas a positive SAM leads to increased Antarctic Intermediate Water. Both a weaker Split Jet and positive SAM lead to increased Southern Ocean meridional overturning circulation, though it is more pronounced for the latter. However, enhanced ventilation of deep water in both cases increases atmospheric pCO2 by only 1–3ppm, because the associated cooling and efficient nutrient utilization in the model effectively negates the venting of deep ocean carbon. Both a weaker Split Jet and positive SAM enhance oxygenation of the deep ocean and intermediate waters but diminish oxygenation of the eastern equatorial Pacific. Our results provide guidance to distinguish SAM-like changes from Split Jet-like changes in paleoceanographic records, and we discuss the case of early deglacial transition to Heinrich 1.
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