Fingerprints for Early Detection of Changes in the AMOC

The detection of trends and variations in the Atlantic Meridional Overturning Circulation (AMOC) is an important and at times controversial topic. On average, CMIP6 models project a 1 Sv/decade decrease in the strength of the AMOC in response to anthropogenic climate change. Atlantic subpolar decadal sea surface temperature variations of 0.5º indicate an associated change in AMOC strength of 2 Sv. These are challenging thresholds of signal detection for AMOC observing.   Estimates of the AMOC streamfunction, such as those from the RAPID array, have a number of sources of variability ranging from short term Ekman transport to variations in the strength of North Atlantic Deep Water associated with deep water formation that have a slower timescale. Climate model studies have shown that Ekman transport contributes little to the signal of future AMOC decline.   We look at the nearly 20 years of data from the RAPID array from a signal to noise perspective. Fluctuations associated with Ekman transport are the largest contribution to noise in the AMOC estimates and hold no signal of low frequency change. Deeper layers show more of the low frequency signal. We amplify this low frequency signal by removing the impact of noise derived from the Ekman transport on the deep temperature and salinity. Finally, we show that the best place for detection of low frequency, climatic changes in AMOC is in the deepest North Atlantic Deep Water, with the noise of the wind removed.

A simplified coupled ocean–atmosphere model, where an atmospheric general circulation model (AGCM) is fully coupled to a 2½-layer reduced-gravity ocean model (RGO) over the tropical Atlantic basin, is presented in the context of studying the role of the Atlantic meridional overturning circulation (AMOC) in tropical Atlantic variability (TAV). In the ocean model, the strength of the AMOC is controlled by specifying mass transport at open boundaries. The fidelity of the reduced-physics model in capturing major features of tropical Atlantic variability, as well as its response to the AMOC changes, is demonstrated in a series of model experiments. The results of the experiments reveal the relative importance of oceanic processes and atmospheric processes in AMOC-induced tropical Atlantic variability–change. It is found that the oceanic processes are a primary factor contributing to the warming at and south of the equator and the precipitation increase over the Gulf of Guinea, while atmospheric processes are responsible for the surface cooling of the tropical North Atlantic and southward displacement of ITCZ. A systematic investigation of the coupled system response to changes in AMOC strength indicates that the SST over the cold-tongue region responds nonlinearly to AMOC changes. The sensitivity of the SST response increases rapidly when AMOC strength decreases below a threshold value. Such nonlinear behavior is also found in precipitation response over the Gulf of Guinea. These results suggest that complex and competing atmosphere–ocean processes are involved in TAV response to AMOC changes and the nature of the response can vary from one region to another. This complexity should be taken into consideration in Atlantic abrupt climate studies.

Northward ocean heat transport (OHT) plays a key role in Arctic climate variability and change. Unforced climate model simulations suggest that at decadal and longer timescales, strengthened Atlantic meridional overturning circulation (AMOC) is correlated with increased OHT into the Arctic. Yet greenhouse gas forced simulations predict increased Arctic OHT while AMOC weakens. Here we partition OHT changes into contributions from dynamic circulation changes and thermodynamic temperature advection, as well as meridional overturning and gyre changes. We find that under decadal‐scale internal variability, strengthened AMOC converges heat in the subpolar gyre; anomalous heat is advected into the Arctic by both time mean circulations and strengthened gyre circulations. Under greenhouse gas forcing, weakened AMOC reduces subpolar gyre heat convergence; yet Arctic OHT increases as mean overturning and strengthened gyre circulations advect warmed surface waters. Thus, caution should be exercised when inferring Arctic OHT from AMOC, as the relationship between OHT and AMOC changes depends on whether they are internally generated or externally forced.

Numerous studies have suggested that variations in the strength of the Atlantic meridional overturning circulation (AMOC) may drive predictable variations in North Atlantic sea surface temperature (NASST). However, two recent studies have presented results suggesting that coupled models disagree on both the sign and the phasing of the correlation between AMOC and NASST indices. These studies analyzed linearly detrended output from twentieth-century historical simulations in phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). The present study argues that the apparent disagreement among models arises from a comingling of two processes: 1) a bottom-up effect in which unforced AMOC changes lead to NASST changes of the same sign and 2) a top-down effect in which forced NASST changes lead to AMOC changes of the opposite sign. Linear detrending is not appropriate for separating these two effects because the time scales of forced and unforced variations are not well separated. After forced variations are properly removed, the models come into much closer agreement with each other. This argument is supported by analysis of CMIP5 historical simulations, as well as preindustrial control simulations and a 29-member ensemble of the Community Earth System Model, version 1, covering the period 1920–2005. Additional analysis is presented suggesting that, even after the data are linearly detrended, a significant portion of observed NASST persistence may be externally forced.

[1] The Atlantic meridional overturning circulation (AMOC) is an important component of the climate system because of the associated heat and freshwater transports. Global warming is projected to weaken the AMOC by up to 50% towards the end of the 21st century. Here we show a delayed aspect of climate change, linked to AMOC changes, in an idealised scenario of greenhouse-gas-increase and subsequent mitigation as projected by two comprehensive coupled climate models. Under an imposed increase in CO2, there is a reduction in meridional exchange of ocean waters due to the associated weakening AMOC, and an intensification of the hydrological cycle, which result in a tendency to increase salinity in the subtropics and to freshen the northern latitudes. The AMOC and meridional ocean transports recover during the subsequent mitigation phase. As the reservoir of very warm and saline water previously built up in the subtropics is transported northwards, a consequent massive increase of salinity in the Arctic/subpolar North Atlantic results in large density increases in the deep water formation regions. This drives an overshoot in the strength of the AMOC by 30–100% relative to its pre-industrial strength. This AMOC overshoot gives an extended period of anomalously strong northward heat transport, maintaining warmer northern high latitudes for decades after the atmospheric CO2 concentration returns to preindustrial values. This work demonstrates the important role of coupling between the hydrological cycle and large scale ocean dynamics in future climate change, and that some aspects of currently committed climate change have yet to be revealed.

<p>The early Pliocene, with atmospheric CO<sub>2</sub> concentrations at levels similar to today, is seen as a case study for Earth’s future climate evolution. During this period the progressive closing of the Central American Seaway led to increased poleward heat and salt transport within the Atlantic with North Atlantic Deep Water (NADW) becoming warmer and saltier and resulting in an enhanced Atlantic Meridional Overturning Circulation (AMOC). In order to evaluate how stable the Pliocene AMOC really was, we are producing surface and deep-water records for IODP Site U1313 (41°N, 33°W, 3412m) for the interval from 3.3 to 4.1 Ma. This site is ideally located to monitor past AMOC changes with North Atlantic Drift waters at the surface and NADW, exported by the deep western boundary current, in the deep. Surface water conditions are reconstructed based on the stable isotope data of planktonic foraminifer species <em>Globigerinoides ruber</em> (white) or <em>Globigerinoides extremus</em> with centennial-scale resolution and on sea-surface temperatures (U<sup>k</sup><sub>37' </sub>alkenone thermometer) with an average 4 ky resolution. Stable isotope records of the benthic foraminifer genus <em>Cibicidoides</em> reveal changes in the deep water.</p><p>Besides the interglacial/glacial cycles, higher frequency oscillations are recorded in both the planktonic and benthic foraminifer stable isotope records. Varying surface water conditions, especially during Late Pliocene interglacial periods, are reflected in the <em>Globigerinoides</em> isotope data and appear to be linked to salinity changes since they are not recorded in the sea-surface temperature data. The high-frequency oscillations in the planktonic isotope records are related to precession (insolation) forcing, especially its harmonics in the 5.5 ky and 11 ky ranges. The benthic δ<sup>13</sup>C values indicate nearly continuous NADW presence and confirm a strong AMOC throughout the studied interval, also during most of the glacial periods. Excluding the pronounced M2 glacial, glacial stage Gi 6 had a stronger impact on the AMOC, as revealed by cooler, less ventilated surface waters and a less ventilated NADW, than Gi 2 and Gi 4. Overall, the AMOC was strong throughout, but experienced high frequency oscillations at a level similar to the middle Pleistocene interglacial periods.</p>

Did North Atlantic cooling and freshening from 3.65–3.5 Ma precondition Northern Hemisphere ice sheet growth?

The relationship between Atlantic meridional overturning circulation (AMOC) variability and high‐latitude North Atlantic buoyancy changes is complicated by the latter both driving, and responding to, AMOC changes. A maximum covariance analysis applied to a 1,201‐year preindustrial control simulation reveals two leading modes that separate these two distinct roles of North Atlantic temperature and salinity as related to AMOC variability. A linear combination of the two modes accounts for most of the variation of a widely used AMOC index. The same analysis applied to another control simulation known to possess two distinct regimes of AMOC variability—oscillatory and red‐noise—suggests that the North Atlantic buoyancy‐forced AMOC variability is present in both regimes but is weaker in the latter, and moreover there is pronounced multidecadal/centennial AMOC behavior in the latter regime that is unrelated to North Atlantic buoyancy forcing.

Climate models project an Atlantic Meridional Overturning Circulation slowdown under anthropogenic greenhouse warming. Despite a ~1.5 °C increase in the global mean surface temperature, debate remains on whether and when this circulation has slowed. Here we identify a distinctive temperature fingerprint in the equatorial Atlantic that signals the Atlantic Meridional Overturning Circulation change through numerical ocean model experiments. The equatorial Atlantic is a crucial crossroads for the circulation anomalies to propagate to other oceans. A slowdown drives a mid-depth (1000–2000 m) warming in the equatorial Atlantic in a decade via baroclinic Kelvin waves. Analysis of climate models shows that this mid-depth temperature change is a better indicator for Atlantic Meridional Overturning Circulation change on decadal and longer timescales than other surface proxies. Observations reveal a robust mid-depth warming since 1960 that emerged from natural variability in the early 2000s, suggesting a slowdown that already started in the late 20th century.

The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle1, 2. The AMOC has been shown to be weakening in recent years 1 ; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC1, 3-5. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA-sourced from melting glaciers and thickened sea ice that developed earlier in the LIA-weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet 6 . Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here.

The North Atlantic Ocean has a crucial role in the regional and global climate, with significant socio-economic impacts related to droughts, hurricanes and changes in the weather pattern. The Atlantic Meridional Overturning Circulation (AMOC) is the main component of ocean heat transport to the North Atlantic as part of the Earth’s major circulation system. The AMOC is considered a tipping element of the planet’s climate, and abrupt and strong changes in this global-scale circulation could lead to shifts in European and North American climates and warming in the Southern Ocean and Antarctica. Hence, it is critical to better comprehend the drivers influencing the AMOC state and its variability.   The direct connection between the AMOC and the deep-water formation is being challenged by recent studies, suggesting this relationship cannot explain alone the broad spectrum of AMOC variations. Moreover, the Gulf Stream is responsible for a substantial part of meridional heat transport and is considered part of the AMOC system in the North Atlantic. Recent studies show the latitudinal displacement of the current could affect the AMOC variability. Furthermore, the advection at intermediate level (800-1200m) of the Mediterranean outflow is suggested to be a possible key contributor to the strength and variability of the AMOC, although large uncertainty remains concerning the pathway of these waters into the North Atlantic and its contribution in the precondition phase of the deep-water formation in the Labrador and Greenland Island Norwegian sea. Understanding the influence of each of these drivers is vital.   The scarcity of direct observational records in space and time poses an important challenge to disentangle the AMOC drivers and their variability in the North Atlantic, especially since the North Atlantic Ocean presents unusual variability at decadal and multi-decadal scales. Previous studies have used either short observation records, low-resolution numerical models or indirect (proxy) observations to investigate the AMOC. Long-term and three-dimensional datasets are needed to understand the ocean’s changes at large space and time scales. Therefore, the aim of our work is to investigate the multi-drivers of the AMOC variability using a large ensemble (32 members) of global historical ocean reanalyses covering the period from 1961 to 2022. We focus on the deep-water formation, Gulf Stream shift and Mediterranean Outflow Water (MOW) as multi-drivers of the AMOC in the North Atlantic. It is the first time that a large ensemble of 60-year observation-based ocean dataset is used to investigate multiple drivers responsible for changes in the AMOC and the North Atlantic’s decadal climate. We calculated indexes for each driver and used multivariate regression methods (e.g. LASSO) to explore the role of multi-drivers. Preliminary results show that at the interannual time scale, the variability of the Gulf Stream is well correlated with the AMOC variability and at a longer time scale, the deep-water formation and Mediterranean outflow have a close correlation with the AMOC transport. Further investigation is ongoing to quantify the relative contribution from these drivers to the AMOC variability at different time scales.

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in regulating the global climate. The AMOC change in response to global warming has important environmental and, potentially, societal impacts but remains an issue with large uncertainty. Here we use a series of coupled climate model experiments to reveal the overlooked role of Atlantic subtropical salinification, a robust consequence of an intensified hydrological cycle, in inhibiting AMOC weakening under global warming. Without subtropical salinification, the AMOC weakening more than doubles in response to a doubling of CO2, primarily driven by a reduced zonal salinity gradient that weakens the geostrophic component of AMOC through the thermal wind relation. This larger AMOC weakening reduces surface warming in the Northern Hemisphere by as much as 1–3 K at northern high latitudes when subtropical salinification is inhibited.

Although the Atlantic Meridional Overturning Circulation (AMOC) is considered a critical climate tipping element, its impacts on the terrestrial carbon cycle in Earth system models remain uncertain. Using the Earth system model, CLIMBER-X, we investigate the response of vegetation carbon to idealized AMOC collapse under pre-industrial conditions. We assess the role of carbon-climate feedback by comparing simulations incorporating interactive carbon cycles with experimental results set at atmospheric CO₂ concentrations.The results indicate that AMOC collapse leads to a large-scale change of vegetation carbon, with significant differences in responses between the Northern and Southern Hemispheres. The simulated global vegetation carbon response depends on whether the carbon-climate interaction is considered in the model, highlighting the importance of interactive carbon cycle processes. Our findings indicate the sensitivity of terrestrial vegetation carbon to AMOC changes and suggest that it is important to account for ocean-terrestrial-carbon coupling in Earth system models when assessing potential AMOC tipping events.

Western boundary current in relation to Atlantic Subtropical Gyre dynamics during abrupt glacial climate fluctuations

Abstract. We investigate the impact of the strength of the Atlantic Meridional Overturning Circulation (AMOC) at 26∘ N on the prediction of North Atlantic sea surface temperature anomalies (SSTAs) a season ahead. We test the dependence of sea surface temperate (SST) predictive skill in initialised hindcasts on the phase of the AMOC at 26∘N, invoking a seesaw mechanism driven by AMOC fluctuations, with positive SSTAs north of 26∘ N and negative SSTAs south of 26∘ N after a strong AMOC and vice versa. We use initialised simulations with the MPI-ESM-MR (where MR is mixed resolution) seasonal prediction system. First, we use an assimilation experiment between 1979–2014 to confirm that the AMOC leads a SSTA dipole pattern in the tropical and subtropical North Atlantic, with the strongest AMOC fingerprints after 2–4 months. Going beyond previous studies, we find that the AMOC fingerprint has a seasonal dependence and is sensitive to the length of the observational window used, i.e. stronger over the last decade than for the entire time series back to 1979. We then use a set of ensemble hindcast simulations with 30 members, starting each February, May, August and November between 1982 and 2014. We compare the changes in skill between composites based on the AMOC phase a month prior to each start date to simulations without considering the AMOC phase and find subtle influence of the AMOC mechanism on seasonal SST prediction skill. We find higher subtropical SST hindcast skill at a 2–4-month lead time for June–July–August (JJA) SSTA composites based on the AMOC phase at May start dates than for the full time period. In other regions and seasons, we find a negligible impact of the AMOC seesaw mechanism on seasonal SST predictions due to atmospheric influence, calling for caution when considering such a mechanism. Our method shows that, for May start dates following strong AMOC phases, summer SST hindcast skill over the subtropics increases significantly compared to that of weak AMOC phases. This suggests that in the assessment of SST skill for a season ahead an eye should be kept on the initial AMOC state.