No abstract

In recent decades, Antarctic ice sheet/shelf melting has been accelerated, releasing freshwater into the Southern Ocean. It has been suggested that the meltwater flux could lead to cooling in the Southern Hemisphere, which would retard global warming and further induce a northward shift of the Intertropical Convergence Zone (ITCZ). In this study, we use experimental ensemble climate simulations to show that Antarctic meltwater forcing has distinct regional climate impacts over the globe, leading in particular to regional warming in East Asia, which offsets the global cooling effect by the meltwater forcing. It is suggested that Antarctic meltwater forcing leads to a negative precipitation anomaly in the Western North Pacific (WNP) via cooling in the tropics and the northward shift of the ITCZ. This suppressed convection in WNP induces an anticyclonic flow over the North Pacific, which leads to regional warming in East Asia. This hypothesis is supported by analyses of interensemble spread and long-term control simulations. Plain Language Summary In recent decades, greenhouse warming has accelerated the melting of Antarctic glaciers, which discharges freshwater into the Southern Ocean and therefore reduces the surface density. Surface freshening in the Southern Ocean induces cooling and sea ice expansion on the surface, such that it could delay global warming and further lead to a northward shift of the Intertropical Convergence Zone (ITCZ). Here, we examine the distinct regional impacts of Antarctic meltwater forcing over the globe by analyzing experiments with and without meltwater forcing. For example, the Antarctic meltwater forcing induces a global cooling but leads to regional warming in East Asia. We find that Antarctic meltwater forcing leads to reduced convection in the Western North Pacific (WNP) due to the northward shift of the ITCZ and an overall cooling in the tropics. This circulation change in WNP induces regional warming in East Asia via atmospheric teleconnection.

Under the ongoing and potential risks from anthropogenic warming, net negative carbon dioxide (CO2) emissions are inevitable to stabilize or recover the Earth's climate. It is important not only to understand climate irreversibility in response to CO2 removal but also to understand how fast each component of the climate system will recover to its original state. Based on idealized CO2 ramp‐up and ‐down ensemble simulations, here we show that the initial buoyancy states of the Arctic Ocean, such as upper ocean salinity and density, are vital to determining how fast Arctic and global mean temperatures will recover on a centennial time scale. The denser initial Arctic oceanic condition is linked to faster recovery of the Atlantic meridional overturning circulation (AMOC) in the ramp‐down period, which is further accelerated by strong positive AMOC‐salt‐advection feedback. Faster AMOC recovery can delay Arctic temperature recovery by transporting warmer water into the northern subpolar Atlantic during the ramp‐down period. In addition, denser Arctic water enhances vertical mixing, which also results in delayed Arctic cooling under a strong vertical temperature gradient in the subpolar‐to‐polar Atlantic. Our findings suggest that the Arctic's initial states have a centennial memory for the future Arctic and global climate changes.

Convective extreme El Niño (CEE) events, characterized by strong convective events in the eastern Pacific, are known to have a direct link to anomalous climate conditions worldwide, and it has been reported that CEE will occur more frequently under greenhouse warming. Here, using a set of CO 2 ramp-up and ramp-down ensemble experiments, we show that frequency and maximum intensity of CEE events increase further in the ramp-down period from the ramp-up period. These changes in CEE are associated with the southward shift of the intertropical convergence zone and intensified nonlinear rainfall response to sea surface temperature change in the ramp-down period. The increasing frequency of CEE has substantial impacts on regional abnormal events and contributed considerably to regional mean climate changes to the CO 2 forcings.

IIt has been suggested that the freshwater flux due to the recent melting of the Antarctic ice-sheet/shelf will suppress ventilation in the Southern Ocean. In this study, we performed idealized earth system simulations to examine the impacts of Antarctic meltwater on surface phytoplankton biomass in the Antarctic Ocean. The enhanced stratification due to the meltwater leads to a decrease in the surface nitrate concentration but an increase in the surface dissolved iron concentration. These changes are associated with the reduced upwelling of nitrate-rich deep water and the trapped iron exported from the terrestrial sediment. Because of the limited iron availability in the Southern Ocean, the trapped iron in surface water enhances the chlorophyll concentration in the open ocean. However, in the marginal sea along the Antarctic coastline where the iron is relatively sufficient, a nitrate reduction induces a chlorophyll decrease, indicating a regime shift from iron-limited to nitrate-limited conditions.