An official website of the United States government
Abstract Deep-reaching warming along the boundary of the Antarctic Circumpolar Current and the subtropical gyre is a consistent feature of multidecadal observational estimates and projections of future climate. In the Indian basin, the maximum ocean heat content change is collocated with the powerful Agulhas Return Current (ARC) in the west and Subantarctic Front (SAF) in the east, forming a southeastward band we denote as the ARC–SAF. We find that this jet-confined warming is linked to a poleward shift of these strong currents via the thermal wind relation. Using a suite of idealized ocean-only and partially coupled climate model experiments, we show that strong global buoyancy flux anomalies consistently drive a poleward shift of the ARC–SAF circulation and the associated heat content change maximum. To better understand how buoyancy addition modifies this circulation in the absence of wind stress change, we next apply buoyancy perturbations only to certain regions. Buoyancy addition across the Indian and Pacific Oceans (including the ARC–SAF) gives rise to a strong baroclinic circulation response and modest poleward shift. In contrast, buoyancy addition in the North Atlantic drives a vertically coherent poleward shift of the ARC–SAF, which we suggest is associated with an ocean heat content perturbation communicated to the Southern Ocean via planetary waves and advected eastward along the ARC–SAF. Whereas poleward-shifting circulation and banded warming under climate change have been previously attributed to poleward-shifting winds in the Southern Ocean, we show that buoyancy addition can drive this circulation change in the Indian sector independent of changing wind stress. Significance StatementThis research aims to identify which changes at the atmosphere–ocean interface cause ocean warming localized within strong Southern Ocean currents under climate change. Whereas previous regional studies have emphasized the role of changes in Southern Hemisphere winds, we show that these currents are also sensitive to additional heat and freshwater input into the ocean—even in the faraway North Atlantic. Adding heat and freshwater shifts the currents southward, which is dynamically tied to the “band” of ocean warming seen in both long-term observations and climate change projections. We demonstrate that the warming climate will modify ocean circulation in unexpected ways; the consequences for the ocean’s ability to continue removing anthropogenic heat and carbon from the atmosphere remain poorly understood.
more »
« less
Historical Changes in Wind‐Driven Ocean Circulation Can Accelerate Global Warming
Abstract Mitigation and adaptation strategies for climate change depend on accurate climate projections for the coming decades. While changes in radiative heat fluxes are known to contribute to surface warming, changes to ocean circulation can also impact the rate of surface warming. Previous studies suggest that projected changes to ocean circulation reduce the rate of global warming. However, these studies consider large greenhouse gas forcing scenarios, which induce a significant buoyancy‐driven decline of the Atlantic Meridional Overturning Circulation. Here, we use a climate model to quantify the previously unknown impact of changes to wind‐driven ocean circulation on global surface warming. Wind‐driven ocean circulation changes amplify the externally forced warming rate by 17% from 1979 to 2014. Accurately simulating changes to the atmospheric circulation is key to improving near‐term climate projections.
more »
« less
- Award ID(s):
- 1951713
- PAR ID:
- 10527879
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 4
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Wind-driven and thermohaline circulations, two major components of global large-scale ocean circulations, are intrinsically related. As part of the thermohaline circulation, the Atlantic Meridional Overturning Circulation has been observed and is expected to decline over the twenty-first century, potentially modulating global wind-driven circulation. Here we perform coupled climate model experiments with either a slow or steady Atlantic overturning under anthropogenic warming to segregate its effect on wind-driven circulation. We find that the weakened Atlantic overturning generates anticyclonic surface wind anomalies over the subpolar North Atlantic to decelerate the gyre circulation there. Fingerprints of overturning slowdown are evident on Atlantic western boundary currents, encompassing a weaker northward Gulf Stream and Guiana Current and a stronger southward Brazil Current. Beyond the Atlantic, the weakened Atlantic overturning causes a poleward displacement of Southern Hemisphere surface westerly winds by changing meridional gradients of atmospheric temperature, leading to poleward shifts of the Antarctic Circumpolar Current and Southern Ocean meridional overturning circulations.more » « less
-
Abstract A large part of the variability in the Atlantic meridional overturning circulation (AMOC) and thus uncertainty in its estimates on interannual time scales comes from atmospheric synoptic eddies and mesoscale processes. In this study, a suite of experiments with a 1/12° regional configuration of the MITgcm is performed where low-pass filtering is applied to surface wind forcing to investigate the impact of subsynoptic (<2 days) and synoptic (2–10 days) atmospheric processes on the ocean circulation. Changes in the wind magnitude and hence the wind energy input in the region have a significant effect on the strength of the overturning; once this is accounted for, the magnitude of the overturning in all sensitivity experiments is very similar to that of the control run. Synoptic and subsynoptic variability in atmospheric winds reduce the surface heat loss in the Labrador Sea, resulting in anomalous advection of warm and salty waters into the Irminger Sea and lower upper-ocean densities in the eastern subpolar North Atlantic. Other effects of high-frequency variability in surface winds on the AMOC are associated with changes in Ekman convergence in the midlatitudes. Synoptic and subsynoptic winds also impact the strength of the boundary currents and density structure in the subpolar North Atlantic. In the Labrador Sea, the overturning strength is more sensitive to the changes in density structure, whereas in the eastern subpolar North Atlantic, the role of density is comparable to that of the strength of the East Greenland Current. Significance StatementA key issue in understanding how well the Atlantic meridional overturning circulation is simulated in climate models is determining the impact of synoptic (2–10 days) and subsynoptic (shorter) wind variability on ocean circulation. We find that the greatest impact of wind changes on the strength of the overturning is through changes in energy input from winds to the ocean. Variations in winds have a more modest impact via changes in heat loss over the Labrador Sea, alongside changes in wind-driven surface currents. This study highlights the importance of accurately representing the density in the Labrador Sea, and both the strength and density structure of the East Greenland Current, for the correct representation of overturning circulation in climate models.more » « less
-
Abstract Changes in tropical deep convection with global warming are a leading source of uncertainty for future climate projections. A comparison of the responses of active sensor measurements of cloud ice to interannual variability and next-generation global storm-resolving model (also known ask-scale models) simulations to global warming shows similar changes for events with the highest column-integrated ice. The changes reveal that the ice loading decreases outside the most active convection but increases at a rate of several percent per Kelvin surface warming in the most active convection. Disentangling thermodynamic and vertical velocity changes shows that the ice signal is strongly modulated by structural changes of the vertical wind field towards an intensification of strong convective updrafts with warming, suggesting that changes in ice loading are strongly influenced by changes in convective velocities, as well as a path toward extracting information about convective velocities from observations.more » « less
-
Abstract The evolution of the spatial pattern of ocean surface warming affects global radiative feedback, yet different climate models provide varying estimates of future patterns. Paleoclimate data, especially from past warm periods, can help constrain future equilibrium warming patterns. By analyzing marine temperature records spanning the past 10 million years with a regression‐based technique that removes temporal dimensions, we extract long‐term ocean warming patterns and quantify relative sea surface temperature changes across the global ocean. This analysis revealed a distinct pattern of amplified warming that aligns with equilibrated model simulations under high CO2conditions, yet differs from the transient warming pattern observed over the past 160 years. This paleodata‐model comparison allows us to identify models that better capture fundamental aspects of Earth's warming response, while suggesting how ocean heat uptake and circulation changes modify the development of warming patterns over time. By combining this paleo‐ocean warming pattern with equilibrated model simulations, we characterized the likely evolution of global ocean warming as the climate system approaches equilibrium.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023GL102846
