Complexity in the Southern Ocean meridional overturning circulation and carbon source/sink
The ocean has taken in a significant amount of anthropogenic carbon dioxide(Cant)since the Industrial Revolution,which effectively mitigates the greenhouse effect on the globe's climate.Among others,the Southern Ocean surrounding Antarctica,defined broadly as the oceanic areas south of 35°S,has absorbed approximately 40％of the anthropogenic CO2 through its deep-reaching meridional overturning circulation(MOC).The formation of Antarctic bottom water(AABW)propels the lower cell of the Southern Ocean overturning,carrying Cant to the abyssal ocean and playing an essential role in the global ocean carbon pump.In 2023,studies based on observations indicated a reduction in the volume of the coldest component of the AABW since the 1970s,which was subsequently reported by Science using the title'Earth's carbon pump is slowing'.It is indicated that deep ocean currents are weakening,resulting in a reduced capacity to capture Cant,which will exacerbate climate warming.By emphasizing the essential role of the ocean in climate change,this report highlights the necessity of enhancing the research effort of the deep ocean.However,the sequence of AABW reduction leading to a slowdown in deep-ocean circulation and a weakening of the ocean carbon pump encompasses many intricate oceanic and climatic processes,over which the scientific communities have not reached consensus views.There remain major gaps in the understanding of these issues across observational,modeling,and theoretical studies.To complement the perspective provided by this report and to better inform the decision-makers and the public,the scientific basis should be clarified.The present paper provides a review of four pertinent scientific issues to enhance the understanding of decision-makers and the public.(1)The formation and changes of AABW,(2)Changes in wind-driven Southern Ocean circulation,(3)Climate change impacts on the Southern Ocean,and(4)The relationship between ocean carbon pump and ocean circulation.In addition to an updated review of established understanding,uncertainties in observations and model simulations are also discussed.We demonstrate that while the reduction in AABW production suggests a deceleration of deep-ocean MOC,the wind-driven upper-layer MOC in the Southern Ocean has intensified due to changes in westerly winds.Estimating the total impact of the upper and lower MOCs on the carbon pump of the Southern Ocean and the global ocean presents significant challenges.Furthermore,the responsiveness of the ocean carbon pump to changes in ocean circulation is also subject to temporal variation.The biological pump,governed by biogeochemical processes,plays a crucial role alongside the solubility pump in ocean circulation,significantly influencing the overall dynamics of the ocean carbon pump in a changing climate.Owing to the difficulties in the observation of ocean circulations,our understanding of the changing MOC relies predominantly upon climate models.Yet,realistically simulating the high-latitude ocean circulation and formation of deep-ocean water masses is still a challenging task for the state-of-the-art climate models.Most models cannot resolve fundamental processes in the formation of the AABW,such as gravity currents along the continental slope.In addition,systematic biases have been detected in the externally forced changes of the Southern Ocean climate in climate models.The strengthening of westerly winds is underestimated in models compared to observations,which inevitably affect the simulated changes in ocean circulation and carbon pump.Improving scientific understanding and model simulation is essential for making accurate predictions regarding changes in the MOC and carbon pump.Consequently,it appears premature to assert a slowdown of the carbon pump.
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