Description: An essential element of modern ocean circulation and climate is the Atlantic meridional overturning circulation(AMOC), which includes deep-water formation in the subarctic North Atlantic. However, a comparable overturningcirculation is absent in the Pacific, the world’s largest ocean, where relatively fresh surface waters inhibit North Pacificdeep convection. We present complementary measurement and modeling evidence that the warm, ~400–ppmv(parts per million by volume) CO2world of the Pliocene supported subarctic North Pacific deep-water formationand a Pacific meridional overturning circulation (PMOC) cell. In Pliocene subarctic North Pacific sediments, we reportorbitally paced maxima in calcium carbonate accumulation rate, with accompanying pigment and total organiccarbon measurements supporting deep-ocean ventilation-driven preservation as their cause. Together with highaccumulation rates of biogenic opal, these findings require vigorous bidirectional communication between surfacewaters and interior waters down to ~3 km in the western subarctic North Pacific, implying deep convection. Redox-sensitive trace metal data provide further evidence of higher Pliocene deep-ocean ventilation before the 2.73-Ma(million years) transition. This observational analysis is supported by climate modeling results, demonstratingthat atmospheric moisture transport changes, in response to the reduced meridional sea surface temperaturegradients of the Pliocene, were capable of eroding the halocline, leading to deep-water formation in the westernsubarctic Pacific and a strong PMOC. This second Northern Hemisphere overturning cell has important implica-tions for heat transport, the ocean/atmosphere cycle of carbon, and potentially the equilibrium response of thePacific to global warming.

Description: The mid-Pliocene (∼3 Ma) is one of the most recent warm periods with high CO2 concentrations in the atmosphere and resulting high temperatures, and it is often cited as an analog for near-term future climate change. Here, we apply a moisture budget analysis to investigate the response of the large-scale hydrological cycle at low latitudes within a 13-model ensemble from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The results show that increased atmospheric moisture content within the mid-Pliocene ensemble (due to the thermodynamic effect) results in wetter conditions over the deep tropics, i.e., the Pacific intertropical convergence zone (ITCZ) and the Maritime Continent, and drier conditions over the subtropics. Note that the dynamic effect plays a more important role than the thermodynamic effect in regional precipitation minus evaporation (PmE) changes (i.e., northward ITCZ shift and wetter northern Indian Ocean). The thermodynamic effect is offset to some extent by a dynamic effect involving a northward shift of the Hadley circulation that dries the deep tropics and moistens the subtropics in the Northern Hemisphere (i.e., the subtropical Pacific). From the perspective of Earth's energy budget, the enhanced southward cross-equatorial atmospheric transport (0.22 PW), induced by the hemispheric asymmetries of the atmospheric energy, favors an approximately 1∘ northward shift of the ITCZ. The shift of the ITCZ reorganizes atmospheric circulation, favoring a northward shift of the Hadley circulation. In addition, the Walker circulation consistently shifts westward within PlioMIP2 models, leading to wetter conditions over the northern Indian Ocean. The PlioMIP2 ensemble highlights that an imbalance of interhemispheric atmospheric energy during the mid-Pliocene could have led to changes in the dynamic effect, offsetting the thermodynamic effect and, hence, altering mid-Pliocene hydroclimate.