Marine heatwaves (MHWs), prolonged ocean temperature extremes, have been enhanced by global warming in recent decades. More intense and longer MHWs have increasingly negative impacts on marine organisms that threaten their resilience of marine ecosystems. In this study, we investigated global marine phytoplankton biomass (chlorophyll) estimated by satellite ocean color and its response to MHWs on global and regional scales. We find that MHWs typically decreases chlorophyll concentrations in the tropics and mid-latitudes, with increases at high latitudes. The magnitude of chlorophyll responses to MHWs is increased in response to higher intensity and longer duration of MHWs. We find a change in the response from negative to positive chlorophyll responses to MHWs across the 40◦–50◦ latitude bands in both hemispheres where the strongest meridional gradient in nitrate concentration exists. In these response-changing regions, the latitudinal contrast of the chlorophyll response is more distinctive in the warm season rather than in the cold season because of the shallower climatological mixed layer. The present study highlights the global phytoplankton responses to MHWs and their sensitivity to MHWs properties that imply the importance of upper-ocean interactions between phytoplankton and the mixed-layer.

The El Niño-Southern Oscillation (ENSO) strongly influences phytoplankton in the tropical Pacific, with El Niño conditions suppressing productivity in the equatorial Pacific (EP) and placing nutritional stresses on marine ecosystems. The Geophysical Fluid Dynamics Laboratory's (GFDL) Earth System Model version 4.1 (ESM4.1) captures observed ENSO-chlorophyll patterns (r = 0.57) much better than GFDL's previous ESM2M (r = 0.23). Most notably, the observed post-El Niño “chlorophyll rebound” is substantially improved in ESM4.1 (r = 0.52). We find that an anomalous increase in iron propagation from western
Pacific (WP) subsurface to the cold tongue via the equatorial undercurrent (EUC) and subsequent post-El Niño surfacing, unresolved in ESM2M, is the primary driver of chlorophyll rebound. We also find that this chlorophyll rebound is augmented by high post-El Niño dust-iron deposition anomalies in the eastern EP. This post-El Niño chlorophyll rebound provides a previously unrecognized source of marine ecosystem resilience independent from the La Niña that sometimes follows.

It 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 (SO). In this study, we performed idealized earth-system simulations to examine the impacts of Antarctic meltwater on the biomass of surface phytoplankton in the Antarctic Ocean. The enhanced stratification due to the meltwater leads to a decrease in surface nitrate concentration, but an increase in the surface concentration of dissolved iron. These changes are associated with the reduced upwelling of nitrate-rich deep water and the trapped iron exported from terrestrial sediment. Because of the limited iron availability in the SO, 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.

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.