Sang-Ki Lee

The warm, swift Gulf Stream (GS) plays a key role in shaping weather and climate patterns across the North Atlantic. Using 31‐year (1993–2023) of continuous satellite altimeter observations, we identify a pronounced intensification of the free‐flowing GS after its separation from the coast beginning around 2014 and persisting thereafter, following a prolonged period (2005–2014) of weaker transport. This intensification coincides with a northward displacement and reduced meandering of the GS path, which appears to be linked to a spin‐up of the North Atlantic subtropical gyre. This gyre acceleration is largely driven by an increase in the negative wind stress curl, partially modulated by the two leading modes of atmospheric variability in the region: the North Atlantic Oscillation and the East Atlantic Pattern. Our analysis also suggests that GS intensification may have a local air–sea feedback leading to anomalous wind stress curl.

The historical evolution of the Atlantic Meridional Overturning Circulation (AMOC) remains debated due to limited continuous observations. To bridge this gap, we leverage historical hydrographic data spanning 1955 to 2022 to constrain a high-resolution global ocean model. Here, we show an overall weakening of the AMOC relative to a 1955–1964 baseline, interrupted by a transient recovery during 1995–2004. The regional footprint of this weakening shifts over time. While earlier reductions (~ 7% during 1965–1994) are driven by suppressed dense water formation in the subpolar North Atlantic, steeper recent declines (~ 11% during 2005–2022) stem from combined reductions across the subpolar North Atlantic and Nordic Seas, despite partially compensating increases in the Arctic Ocean. Sensitivity experiments reveal that the AMOC weakening is overwhelmingly driven by a long-term freshening regime, manifested by repeated episodes of high-latitude salinity anomalies—occurring during the 1970s–1990s and again in recent years—that are broadly tied to Arctic export and Greenland ice melt. Finally, while a relatively cold thermal state during 1975–2004 partially buffers the freshwater-driven decline, this thermal compensation is absent during 2005–2022 due to anomalous warming of the Nordic Seas and the Labrador Current, acting alongside the freshwater forcing to drive the steeper modern decline of the AMOC.

We used a high-resolution (1/12°) Modular Ocean Model version 6 implementation for the Northwest Atlantic Ocean (MOM6-NWA12) to dynamically downscale Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1) projections for the 21st century. Simulations were conducted under four different Coupled Model Intercomparison Project Phase 6 emission scenarios. MOM6-NWA12 accurately simulates the spatial patterns of sea surface temperature, salinity, and dynamic sea surface height (SSH) during the historical period. In particular, the Gulf Stream's strength, position, recirculation, and separation from the US East Coast are significantly improved in MOM6-NWA12 compared to the coarse-resolution GFDL-ESM4.1. Projected end-of-century warming varied strongly between scenarios, from ∼4 °C under prior “worst case” emissions scenarios (SSP-585), 2∼3 °C under intermediate scenarios (SSP-245, SSP-370) more consistent with current trajectories, to ∼1 °C under aggressive mitigation (SSP-126). Consistent with a significant weakening of the Atlantic Meridional Overturning Circulation projected by GFDL-ESM4.1, MOM6-NWA12 shows a substantial volume transport reduction in the Western Boundary Current (WBC) system (i.e., Yucatan Current, Florida Current, Antilles Current, and the Deep Western Boundary Current) toward the late 21st century (between 23 % and 38 %, varying by scenario). This projected weakening of the WBC system and the associated reduction in the coastal upwelling of cold, fresh subsurface waters lead to a significant increase in ocean temperature, salinity, and dynamic SSH along the US southeast and northeast Coasts, particularly in the South Atlantic Bight.

We used a high-resolution (1/12°) Modular Ocean Model version 6 implementation for the the Northwest Atlantic Ocean (MOM6-NWA12) to dynamically downscale Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1) projections for the 21st century. Simulations were conducted under four different Coupled Model Intercomparison Project Phase 6 emission scenarios. MOM6-NWA12 accurately simulates the spatial patterns of sea surface temperature, salinity, and dynamic sea surface height (SSH) during the historical period. In particular, the Gulf Stream's strength, position, recirculation, and separation from the U.S. East Coast are significantly improved in MOM6-NWA12 compared to the coarse-resolution GFDL-ESM4.1. Projected end-of-century warming varied strongly between scenarios, from ~ 4 °C under prior "worst case" emissions scenarios (SSP-585), 2~3 °C under intermediate scenarios (SSP-245, SSP-370) more consistent with current trajectories, to ~ 1 °C under aggressive mitigation (SSP-126). Consistent with a significant weakening of the Atlantic Meridional Overturning Circulation projected by GFDL-ESM4.1, MOM6-NWA12 shows a substantial volume transport reduction in the Western Boundary Current (WBC) system (i.e., Yucatan Current, Florida Current, Antilles Current, and the Deep Western Boundary Current) toward the late 21st century (between 23 and 38 %, varying by scenario). This projected weakening of the WBC system and the associated reduction in the coastal upwelling of cold, fresh subsurface waters lead to a significant increase in ocean temperature, salinity, and dynamic SSH along the U.S. southeast and northeast Coasts, particularly in the South Atlantic Bight. These localized changes have significant implications for future sea level rise, marine ecosystems, and fish populations in these highly vulnerable regions.

We used a high-resolution (1/12°) Modular Ocean Model version 6 implementation for the the Northwest Atlantic Ocean (MOM6-NWA12) to dynamically downscale Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1) projections for the 21st century. Simulations were conducted under four different Coupled Model Intercomparison Project Phase 6 emission scenarios. MOM6-NWA12 accurately simulates the spatial patterns of sea surface temperature, salinity, and dynamic sea surface height (SSH) during the historical period. In particular, the Gulf Stream's strength, position, recirculation, and separation from the U.S. East Coast are significantly improved in MOM6-NWA12 compared to the coarse-resolution GFDL-ESM4.1. Projected end-of-century warming varied strongly between scenarios, from ~ 4 °C under prior "worst case" emissions scenarios (SSP-585), 2~3 °C under intermediate scenarios (SSP-245, SSP-370) more consistent with current trajectories, to ~ 1 °C under aggressive mitigation (SSP-126). Consistent with a significant weakening of the Atlantic Meridional Overturning Circulation projected by GFDL-ESM4.1, MOM6-NWA12 shows a substantial volume transport reduction in the Western Boundary Current (WBC) system (i.e., Yucatan Current, Florida Current, Antilles Current, and the Deep Western Boundary Current) toward the late 21st century (between 23 and 38 %, varying by scenario). This projected weakening of the WBC system and the associated reduction in the coastal upwelling of cold, fresh subsurface waters lead to a significant increase in ocean temperature, salinity, and dynamic SSH along the U.S. southeast and northeast Coasts, particularly in the South Atlantic Bight. These localized changes have significant implications for future sea level rise, marine ecosystems, and fish populations in these highly vulnerable regions.

We used a high-resolution (1/12°) Modular Ocean Model version 6 implementation for the the Northwest Atlantic Ocean (MOM6-NWA12) to dynamically downscale Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1) projections for the 21st century. Simulations were conducted under four different Coupled Model Intercomparison Project Phase 6 emission scenarios. MOM6-NWA12 accurately simulates the spatial patterns of sea surface temperature, salinity, and dynamic sea surface height (SSH) during the historical period. In particular, the Gulf Stream's strength, position, recirculation, and separation from the U.S. East Coast are significantly improved in MOM6-NWA12 compared to the coarse-resolution GFDL-ESM4.1. Projected end-of-century warming varied strongly between scenarios, from ~ 4 °C under prior "worst case" emissions scenarios (SSP-585), 2~3 °C under intermediate scenarios (SSP-245, SSP-370) more consistent with current trajectories, to ~ 1 °C under aggressive mitigation (SSP-126). Consistent with a significant weakening of the Atlantic Meridional Overturning Circulation projected by GFDL-ESM4.1, MOM6-NWA12 shows a substantial volume transport reduction in the Western Boundary Current (WBC) system (i.e., Yucatan Current, Florida Current, Antilles Current, and the Deep Western Boundary Current) toward the late 21st century (between 23 and 38 %, varying by scenario). This projected weakening of the WBC system and the associated reduction in the coastal upwelling of cold, fresh subsurface waters lead to a significant increase in ocean temperature, salinity, and dynamic SSH along the U.S. southeast and northeast Coasts, particularly in the South Atlantic Bight. These localized changes have significant implications for future sea level rise, marine ecosystems, and fish populations in these highly vulnerable regions.

We used a high-resolution (1/12°) Modular Ocean Model version 6 implementation for the the Northwest Atlantic Ocean (MOM6-NWA12) to dynamically downscale Geophysical Fluid Dynamics Laboratory Earth System Model version 4.1 (GFDL-ESM4.1) projections for the 21st century. Simulations were conducted under four different Coupled Model Intercomparison Project Phase 6 emission scenarios. MOM6-NWA12 accurately simulates the spatial patterns of sea surface temperature, salinity, and dynamic sea surface height (SSH) during the historical period. In particular, the Gulf Stream's strength, position, recirculation, and separation from the U.S. East Coast are significantly improved in MOM6-NWA12 compared to the coarse-resolution GFDL-ESM4.1. Projected end-of-century warming varied strongly between scenarios, from ~ 4 °C under prior "worst case" emissions scenarios (SSP-585), 2~3 °C under intermediate scenarios (SSP-245, SSP-370) more consistent with current trajectories, to ~ 1 °C under aggressive mitigation (SSP-126). Consistent with a significant weakening of the Atlantic Meridional Overturning Circulation projected by GFDL-ESM4.1, MOM6-NWA12 shows a substantial volume transport reduction in the Western Boundary Current (WBC) system (i.e., Yucatan Current, Florida Current, Antilles Current, and the Deep Western Boundary Current) toward the late 21st century (between 23 and 38 %, varying by scenario). This projected weakening of the WBC system and the associated reduction in the coastal upwelling of cold, fresh subsurface waters lead to a significant increase in ocean temperature, salinity, and dynamic SSH along the U.S. southeast and northeast Coasts, particularly in the South Atlantic Bight. These localized changes have significant implications for future sea level rise, marine ecosystems, and fish populations in these highly vulnerable regions.

In the South Atlantic Bight (SAB), changes in the Gulf Stream (GS), particularly its strength and proximity to the coast, are thought to be primary factors determining the shelf‐break upwelling rate. However, it is still not entirely clear if and to what extent those factors influence cross‐shelf nutrient fluxes and shape the ocean biogeochemistry at interannual and longer timescales. Here, we use a high‐resolution regional ocean‐biogeochemical model and an ocean reanalysis product (1993–2022), along with a satellite‐derived chlorophyll data set (1997–2022), to investigate the interannual ocean‐biogeochemical variability in the SAB. Regional model outputs suggest that year‐to‐year changes in phytoplankton production are indeed largely driven by upwelling of cold and nutrient‐rich water to the shelf‐break. The upwelling variability, reflected in bottom temperature and vertically integrated production patterns, is strongly linked to surface velocity changes in the GS near the shelf break, but weakly related to the depth‐integrated GS transport. The GS's velocity changes, and the temperature and production anomalies, are well correlated to the alongshore wind stress, suggesting that local wind is the leading driver of the shelf‐break upwelling variability at interannual timescales. Those relationships are also supported by circulation patterns from ocean reanalysis and satellite chlorophyll anomalies. Finally, examining the simulated shelf‐slope interchanges in the carbonate system, we find that shelf‐break upwelling significantly increases bottom acidification, a pattern linked to the low carbonate concentration in the slope waters. This study thus provides new insight for understanding and predicting GS and winds impacts on biogeochemical patterns from the SAB.

The Atlantic Meridional Overturning Circulation (AMOC) carries oceanic heat northward to the subpolar North Atlantic (SPNA), where the surface water cools and sinks to the deep ocean. According to a large‐ensemble simulation under a medium‐to‐high emission scenario, the surface cooling and oceanic heat convergence in the SPNA may decrease to ∼20% of their historical levels by 2100. We show here that the projected weakening of the AMOC volume transport alone cannot explain such a large decrease in the heat convergence rate. Our analysis indicates that, due to the suppression of deep‐water formation in the SPNA, the AMOC's lower limb becomes shallower, carrying relatively warm water southward away from the SPNA. This in turn accelerates the decrease in oceanic heat transport to the SPNA per unit AMOC weakening. These results are supported in other multi‐ensemble models analyzed, despite large inter‐model spreads.