US AMOC Science Team Meeting

Climate models are important tools for investigating how the climate might change in the future, however recent observations have suggested that these models are unable to capture the overturning in subpolar North Atlantic correctly, casting doubt on their projections of the Atlantic Meridional Overturning Circulation (AMOC). Here we compare the overturning and surface water mass transformation in a set of CMIP6 models with observational estimates. There is generally a good agreement, particularly in the recent conclusion from observations that the mean overturning in the east (particularly in the Iceland and Irminger seas) is stronger than that in the Labrador Sea. The overturning in the Labrador Sea is mostly found to be small, but has a strong relationship with salinity: fresh models have weak overturning and saline models have stronger mean overturning and stronger relationships of the Labrador Sea overturning variability with the AMOC further south. We also find that the overturning reconstructed from surface flux driven water mass transformation is a good indicator of the actual overturning, though mixing can damp variability and shift signals to different density classes.

Laura Jackson

Hadley Centre, Met Office

Frederic Castruccio is inviting you to a scheduled Zoom meeting.

Join Zoom Meeting https://ucar-edu.zoom.us/j/92203574653

Meeting ID: 922 0357 4653

Passcode: QeZtt5xH

One tap mobile +19292056099,,92203574653#,,,,*54023371# US (New York) +13017158592,,92203574653#,,,,*54023371# US (Washington DC)

Dial by your location +1 929 205 6099 US (New York) +1 301 715 8592 US (Washington DC) +1 312 626 6799 US (Chicago) +1 669 900 6833 US (San Jose) +1 253 215 8782 US (Tacoma) +1 346 248 7799 US (Houston)

Meeting ID: 922 0357 4653 Passcode: 54023371 Find your local number: https://ucar-edu.zoom.us/u/ac3UObmrUv

Frederic Castruccio

NCAR

Newly available mooring observations from the Overturning in the Subpolar North Atlantic Program (OSNAP) show an abrupt decline in Iceland Scotland Overflow (ISOW) salinity from 2017 to 2018 summer. Previous declines in ISOW salinity of similar magnitude have largely been attributed to changes in convectively formed deep waters in the Nordic Seas on decadal time scales. We show that this rapid decline in salinity was driven by entrainment of a major upper ocean salinity anomaly in the Iceland Basin. This is shown by tracking the propagation of the upper ocean anomaly into ISOW using a combination of mooring and Argo observations, surface drifter trajectories, and numerical model results. A 2-year total transit time from the upper ocean into the ISOW layer was found. The results show that entrainment allows for rapid modification of ISOW, and consequently the lower limb of Atlantic Meridional Overturning Circulation, on subdecadal timescales. The latest mooring observations now show sustained freshening through the ISOW layer despite rebounding salinity in the upper ocean entrained waters.

Manish Devana

University of Miami, RSMAS

Understanding the mechanisms driving variability in the Atlantic Meridional Overturning Circulation (AMOC) on different timescales is essential for better predictions of our evolving climate. The newly updated time series (August 2014 to June 2020) from OSNAP (Overturning in the Subpolar North Atlantic Program) continues to reveal strong intra-annual and interannual variability. However, this six-year record allows us, for the first time, to examine the observation-based seasonal variability of the subpolar overturning circulation. We find that the overturning peaks in late spring from April through June and reaches the minimum in winter for both OSNAP West (a section from the coast of Labrador to West Greenland) and OSNAP East (a section from East Greenland to the Scottish shelf). An analysis of seasonality suggests that the delay between wintertime transformation and the observed overturning peak in late spring is consistent with the advection and export of dense water superimposed with a seasonally varying Ekman transport. The overturning circulation plays a major role in the seasonal variability of meridional heat and freshwater transport across the full OSNAP array.

Yao Fu

Georgia Institute of Technology

Iceland-Scotland Overflow Water (ISOW) is formed from dense Nordic Seas waters that spill over the Iceland-Scotland Ridge and entrain lighter thermocline and intermediate waters as they descend into the Iceland Basin. The combined product waters form into a Deep Western Boundary Current along the eastern flank of the Reykjanes Ridge that has been continuously measured for six years (2014-2020) as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). The new OSNAP observations show a transport of approximately 6 Sv of ISOW along the Reykjanes Ridge, larger than previously estimated, with more than 5 Sv of ISOW ultimately being exported southward from the Iceland Basin. Water mass analysis shows that the original Nordic Seas overflow of ~3 Sv is nearly doubled through entrainment, similar to the behavior of the Denmark Strait Overflow. These new results suggest that a substantial fraction of the Iceland-Scotland Overflow water exported from the Iceland Basin must flow southward in the eastern basin of the Atlantic, rather than passing through the Charlie-Gibbs Fracture Zone and other gaps in the Mid-Atlantic Ridge to join the deep western boundary current system of the western subpolar gyre.

William Johns <bjohns@rsmas.miami.edu>

University of Miami

Since 2009, the Meridional Overturning Circulation (MOC) in the South Atlantic has been observed with an array of in situ moorings on each side of the basin at 34.5°S, the South Atlantic MOC Basin-wide Array (SAMBA). To date, the component of the meridional transport inshore of the shallowest moorings on either side (about 1300 m depth) has been estimated using a time-mean from an ocean model simulation due to lack of better observations. However, because of their position offshore of the shelf break, the transport that is not directly observed by the SAMBA moorings is expected to be significant. There is a need to estimate the unobserved inshore transport to better resolve the variability in the MOC transport at 34.5S. In this study, a new method is developed to estimate the inshore geostrophic transport by combining along-track coastal altimetry data with existing in situ observations. The analysis of available in-situ observations suggests that the currents on each side of the SAMBA array are mostly barotropic or equivalent barotropic. This property allows for the estimation of the meridional geostrophic transport inshore of the SAMBA array using along-track altimetry. It is found that the northward transport inshore of SAMBA on the Eastern side of the array and the southward transport on the Western side tend to compensate each other on average. However, the altimetry-derived variability of the total inshore component (~4 Sv) is not negligible compared to the ~17 Sv mean MOC transport at 34.5°S, demonstrating the importance of monitoring.

Matthieu Le Henaff <matthieu.lehenaff@noaa.gov>

University of Miami/CIMAS - NOAA/AOML

The role of wind forcing on the dynamics of the flow along the West Greenland outer shelf is analyzed using data from a high-resolution mooring array deployed between 2014-2018 near Cape Farewell. The data are part of the Overturning in the Subpolar North Atlantic Program (OSNAP). The flow along the shelf is an important source of freshwater to the subpolar North Atlantic, as it transports Arctic-origin and Greenland meltwaters cyclonically around the Labrador Sea. The ability for this freshwater to penetrate into the interior has important consequences for dense water formation and the lower limb of the Atlantic Meridional Overturning Circulation. However, the processes by which these freshwaters are fluxed offshore remain an open question. In this study, we identify and characterize all of the major upwelling events over the four-year record and diagnose the atmospheric drivers of these events. We find that upwelling events are triggered by developing forward tip jets that occur when atmospheric low pressure systems impinge on the high topography of southern Greenland. The enhanced along-coast northwesterly winds drive a cross-stream Ekman cell, which in turn fluxes freshwater offshore at the surface and warm, salty, Atlantic-origin water onshore at depth. The offshore flux of freshwater into the interior Labrador Sea is shown to inhibit deep convection on the eastern side of the basin. The onshore flux of warm water onto the shelf could have important implications for glacial fjord temperatures and thus the release of increased freshwater into the boundary current system.

Astrid Pacini <apacini@whoi.edu>

Woods Hole Oceanographic Institution

The Atlantic Meridional Overturning Circulation (AMOC) causes a northward Meridional Heat Transport and affects climate and weather patterns, regional sea levels, and ecosystems. Therefore, a continued monitoring of the AMOC variability is crucial for a better understanding of the Earth system dynamics. A suite of efforts are geared towards estimating the AMOC transport at several latitudes using specific methodologies subject to the type of instrumentation used, data availability, and operational constraints. Currently, two observational arrays monitor the AMOC in the South Atlantic at 11°S and 34.5°S, synthetic temperature and salinity profiles are used for estimating the AMOC at 20, 25, 30 and 35°S, and XBT-based AMOC estimates are performed at 35°S. In this work, the AMOC transport at 22°S from 2007 to present is estimated from XBT and ARGO databases. An optimized weighted average method is considered to generate gridded profiles, which are used to estimate the AMOC streamfunction. Two parameters are tested: spatial radius (ΔX) and temporal box (ΔT). The median value of T and S, given all ΔX and ΔT used, is selected for the optimized section. In comparison to the Argo gridded product, our product shows larger dynamic height variability and stronger local features. AMOC estimations are analyzed and compared to different AMOC products (e.g. model, synthetic and in-situ observations). Future steps involve exploring the relationship between AMOC, boundary currents, coastal sea level and nutrient stream.

ivenis.pita@noaa.gov

Ivenis Pita

University of Miami

The eastern sub-polar north Atlantic experienced an unprecedented upper ocean freshening event during 2014-2017. Recent hypotheses attribute this to re-routing of Arctic-origin freshwater anomalies via the North Atlantic Current as a consequence of anomalous wind stress curl in the subpolar region. These Arctic anomalies propagate primarily via the East and West Greenland Currents or the Baffin Island Current into the Labrador Sea. They enter the Labrador Current and subsequently merge with the North Atlantic Current off Newfoundland. They then retroflect and propagate into the eastern sub-polar gyre. Using the ECCO (Estimating the Circulation and Climate of the ocean) and ASTE (Arctic Subpolar Gyre State Estimate) state estimates, we extend the hypothesis that Arctic freshwater re-routed through the Labrador current is responsible for the 2014-2017 freshening event in the Iceland basin and explore the role of the AMOC in setting these salinity anomalies. We investigate the strength of the overturning circulation in the state estimates and the freshwater and heat transports associated with it into the Iceland basin. We find that the gyre circulation dominates the overturning in setting upper ocean salinity anomlies in the eastern subpolar north Atlantic. Based on our results, we hypothesize that the freshening of the eastern sub-polar North Atlantic is primarily a balance between the freshwater export from the Arctic and transport of subtropical salinity anomalies as modulated by the interannual atmospheric forcing in the two basins, with only a minor role of the AMOC.

Ali Siddiqui

Johns Hopkins University

Since 2015, Spray autonomous underwater gliders have routinely sampled across the Gulf Stream between Miami, FL and Cape Cod, MA. Over the course of more than 40 missions, gliders have returned more than 280 high-resolution transects across the Gulf Stream. Results and impacts of this measurement campaign across a range of spatial and temporal scales are reviewed. Among these are: detailing the along-stream evolution of Gulf Stream volume transport; producing three-dimensional estimates of mean and eddy kinetic energy in and near the Gulf Stream; identifying large-amplitude internal waves and thick bottom mixed layers near rough topography; and providing constraints for numerical simulations of the Gulf Stream system.

Robert Todd

Woods Hole Oceanographic Institution

Predictability timescales for wintertime sea surface temperature (SST) in the North Atlantic are estimated from gridded observations and individual ensembles from Community Earth System Model – Large Ensemble (CESM-LES), with and without interactive ocean dynamics. The predictability is estimated by integrating the local autocorrelation function at each grid-point to yield a characteristic predictability timescale. In both observations and fully-coupled version of the CESM, predictability timescales for SST are longest in the subpolar North Atlantic, where values are on the order of 3-5 years for observations and the individual ensemble members. Remarkably, SSTs in slab-ocean version of the CESM-LES have similar magnitudes in predictability in this region, though the spatial distribution of predictability timescale differs between the fully coupled and slab-ocean versions of the model. Model comparisons suggest that interactive ocean dynamics are responsible for the enhanced predictability in the vicinity of subpolar gyre and and Grand Banks regions. Observations estimates of predictability timescale in these two region are notably different from model estimates.

Laurie Trenary <ltrenary@gmu.edu>

George Mason University and Courant Institute, NYU

Results from an eddying (1/12°) Atlantic simulations along with the observations are used to characterize the Atlantic meridional overturning circulation (AMOC) in the subpolar North Atlantic. The model results exhibit a similar vertical and horizontal structure of the mean circulation and a significant part of the transport variability as observed from 2014 to 2018 in OSNAP. We found that 1) Part of the modeled AMOC variability is directly linked to the wind stress variability through Ekman transport near the surface. 2) The seasonal variability of modeled AMOC is largely contributed from the variability in the southward western boundary current, through the up/down movement of the isopycnals. 3) The high-frequency variability of the modeled AMOC is contributed from the variability of the northward flows in the North Atlantic Current that is modulated by the wind-driven subpolar gyre.

Xiaobiao Xu

Center for Ocean-Atmospheric Prediction Studies, Florida State University

Previous work has shown that anthropogenic aerosol (AA) forcing drives a strengthening in the Atlantic Meridional Overturning Circulation (AMOC) in CMIP6 historical simulations over 1850--1985. However, the mechanisms are not fully understood. Across CMIP6 models, it is shown that there is a strong correlation between surface heat loss over the subpolar North Atlantic (SPNA) and the forced change in the AMOC. However, by separating the models into those with a `strong’ and `weak’ response to AA forcing, it is found that the spread in surface heat loss and AMOC changes are not strongly related to downwelling surface shortwave radiation over the SPNA. Rather, it is found that the spread in AMOC response is primarily caused by changes in turbulent heat fluxes. Furthermore, the increased turbulent heat flux cooling is largely a result of a colder and dryer atmosphere in models with strong AA forcing. In particular, we hypothesize that large AA driven cooling anomalies over the continents, and especially over North America, are key for mediating this indirect AMOC response to AA forcing via the advection of cold and dry air over the ocean. The simulated AMOC also appears to feedback on itself positively in two distinct ways by modulating surface temperatures (and hence turbulent heat fluxes) and surface density anomalies due to freshwater transport changes. Finally, a comparison of key indices to observations suggests that the AMOC response in the `strong' models is not likely to be consistent with observations.

Jon Robson

NCAS, University of Reading

The global mean sea level (GMSL) rise caused by ocean warming and terrestrial glacier melting has already increased the frequency of coastal inundations in many low-lying regions. Superimposed on the GMSL are large-scale and low-frequency (interannual-to-decadal) sea level variations driven by ocean and atmosphere dynamics. The magnitude of these dynamic sea level changes is comparable to the magnitude of the GMSL rise over the last two decades. The leading mode of the interannual-to-decadal sea level variability in the North Atlantic exhibits a tripole pattern, with the subtropical band varying out-of-phase with both the tropical and the subpolar bands. Here, we show that changes in oceanic heat content in the subtropical band of the tripole are mainly due to the AMOC-driven divergence of the meridional heat transport, while the impact of surface heat fluxes is smaller. The tripole-related gyre-scale changes in heat content are correlated with coastal sea level variations and, together with the GMSL rise, provide background conditions for tidal and synoptic sea level fluctuations. An increase of the background sea level even by a few centimeters can facilitate the high-frequency fluctuations breaking the flood thresholds. We demonstrate that the tripole-related sea level rise south of Cape Hatteras and in the Gulf of Mexico in 2010-2015 accounted for up to 30-50% increase in the frequency of flooding events. Because the gyre-scale sea level is proportional to the time-integral of the AMOC-driven divergence of heat and freshwater, sustained observations of the AMOC provide a potential for sea level predictability.

Denis Volkov

NOAA-AOML, University of Miami

The sensitivity to resolution of simulated historical AMOC, meridional heat transport (MHT), and related North Atlantic fields is examined using a pair of CMIP6-OMIP2 simulations performed with the CESM2 model: one at low resolution (LR; nominal 1°) and the other at high resolution (HR; nominal 0.1°). Both simulations are initialized from observed climatology and spun up through 4 consecutive forcing cycles using JRA55-do atmospheric state fields and fluxes (1958-2018). The long spin-up allows for an assessment of how resolution impacts model drift and the signal-to-noise characteristics of forced variability in the North Atlantic. Historical AMOC/MHT variability is predominantly decadal and is qualitatively similar in HR and LR. Despite lower signal-to-noise in general in HR, large-scale metrics exhibit robust forced signals that dominate over eddy-driven noise. In particular, the mechanisms underpinning high decadal predictability in the subpolar Atlantic appear robust to model resolution.

Stephen Yeager

National Center for Atmospheric Research

Increased freshwater input to the Subpolar North Atlantic from Greenland ice melt and the Arctic could strengthen stratification in deep convection regions and impact the overturning circulation. However, freshwater pathways from the east Greenland shelf to deep convection regions are not fully understood. We investigate the role of strong wind events at Cape Farewell in driving surface freshwaters from the East Greenland Current to the Irminger Sea.

Using a high-resolution model, we identify strong wind events and investigate their impact on freshwater export. Westerly tip jets are associated with the strongest and deepest freshwater export across the shelfbreak, with a mean of 37.5 mSv of freshwater in the first 100 m (with reference salinity 34.9). These wind events tilt isohalines and extend the front offshore, especially over Eirik Ridge. Moderate westerly events are associated with weaker export across the shelfbreak (mean of 15.9 mSv) but overall contribute to more freshwater export throughout the year, including in summer, when the shelf is particularly fresh. Particle tracking shows that half of the surface waters crossing the shelfbreak during tip jet events are exported away from the shelf, either entering the Irminger Gyre, or being driven over Eirik Ridge. During strong westerly wind events, sea ice detaches from the coast and veers towards the Irminger Sea, but the contribution of sea ice to freshwater export at the shelfbreak is minimal compared to liquid freshwater export due to limited sea ice cover at Cape Farewell.

Elodie Duyck

Department of Ocean Systems, NIOZ, Royal Netherlands Institute for Sea Research, Texel, The Netherlands

Climate models exhibit large differences in the mean state and variability of the Atlantic meridional overturning circulation (AMOC), including in AMOC mean strength and the characteristic amplitude, frequency and other characteristics its variability. Across CMIP5 and CMIP6 models, AMOC long-term variability ranges from decadal to multi-centennial and its magnitude from a fraction to several Sverdrups (Sv). In this study, we conduct ensemble experiments, using the latest coupled model from Institut Pierre Simon Laplace (IPSL-CM6A-LR), to investigate systematically how this variability depends on AMOC mean state. In the control simulations of this model AMOC mean volume transport is about 12Sv, while AMOC variability is dominated by two distinct modes – an interdecadal mode with periodicity between 20-30 years and a centennial mode with periods of 100-200 years. The former mode is weaker and driven by temperature variations, while the latter is stronger and driven by salinity anomalies. To modify the AMOC mean state in the model we use an indirect method that relies on robust atmospheric teleconnections from the tropical Indian ocean (TIO) to the Atlantic as described by recent studies (Hu and Fedorov, 2019; Ferster et al., 2021). Both studies have shown that warming the TIO results in an increased AMOC strength, while cooling the TIO results in a weakened AMOC. To change the Indian ocean temperature in our perturbation experiments we nudge TIO surface temperature and the experiments last for nearly 1000 years, reaching quasi-equilibrium. This approach allows us to go from a nearly collapsed AMOC state below 3Sv to a more realistic mean state of about 16Sv. We find that both modes of AMOC variability persist throughout the experiments while their amplitude increases monotonically with the mean AMOC strength, yielding almost linear relationships between the amplitude of variability of the two modes and AMOC mean strength (0.04 Sv per 1 Sv and +0.07 Sv per 1 Sv, respectively). In the experiments that generate 16Sv of AMOC transport, the average amplitudes of the two modes (standard deviation) reach nearly 0.7 and 1.4Sv. Lastly, we compare the dynamical mechanisms of the two modes and their climate impacts. A corollary of this study is that at least in this coupled climate model, a stronger AMOC leads to stronger climate variability.

Alexey Fedorov

Yale University and LOCEAN/IPSL

Density staircases are ubiquitous in the global ocean. Here we show that density staircases form in a deep western boundary current as it crosses the equator. We propose that the staircases are generated by the excitement of symmetric instability in the flow.

Fraser Goldsworth <frasergocean@gmail.com>

University of Oxford, Department of Atmospheric Oceanic and Planetary Physics

The strength and variability of the Atlantic Meridional Overturning Circulation (AMOC) at subpolar latitudes is dominated by water mass transformation in the eastern Subpolar North Atlantic (SPNA). However, the distribution of this overturning across the individual circulation pathways of the eastern Subpolar Gyre (SPG) remains poorly understood. Here, we introduce a novel Lagrangian measure of the density-space overturning to diagnose and quantify the principal pathways of the overturning circulation within an eddy-permitting ocean model hindcast. By tracing the trajectories of water parcels initialised from the northward inflows across the OSNAP East section, we show that water mass transformation along the pathways of the SPG collectively accounts for 55% of the mean strength of the eastern subpolar AMOC. Two pathways crossing Reykjanes Ridge north of OSNAP East are found to dominate the mean overturning within the eastern SPG. Water parcels following the primary cross-ridge pathway, sourced from the Sub-Arctic Front, form upper North Atlantic Deep Water by circulating horizontally across sloping isopycnals in less than 2 years. A secondary cross-ridge route, entraining overflow waters south of the Iceland-Faroes Ridge, is an important conduit for water masses of subtropical origin to penetrate the deep ocean on subdecadal timescales.

On extending our Lagrangian analysis to explore the seasonality of overturning at OSNAP East, we find a clear distinction between the advection times of pathways that contribute substantially to the mean strength of the AMOC and those responsible for its variability on seasonal timescales. Seasonality in our Lagrangian overturning metric is driven by water parcels which are transformed by seasonal heating/cooling during rapid recirculation (3-6 months) in the upper 250m of the Irminger and Iceland Basins. The recirculation time of water parcels circulating in the upper Irminger Sea also exhibits a strong seasonal cycle: parcels advected northwards in winter typically require an additional 1.5 months to return to OSNAP East compared with those released during summer. The decrease in upper Irminger Sea recirculation times through spring leads to increasing convergence of southward transport within the density classes of the upper limb in the East Greenland Current during autumn, and thus induces a seasonal minimum in the AMOC. On accounting for seasonal changes in the density structure of the Irminger Basin, variations in the southward transport of the East Greenland Current can account for 63% of the seasonal cycle of Eulerian overturning at OSNAP East in our hindcast simulation.

Oliver Tooth <oliver.tooth@seh.ox.ac.uk>

University of Oxford

We investigate the climate system response to observed NAO surface heat flux forcing in the Subpolar North Atlantic (SPNA) using ensemble simulations conducted with three CMIP6-class climate models. Both positive and negative anomalous NAO forcings are applied for 10 years, then the simulations evolve freely for 10-20 additional years. Our key findings are:

- Consistent, but with varying amplitude, anomalous water mass transformation/formation (WMT/WMF) in the western SPNA in response to the forcing

- Anomalous WMT/WMF largely associated with changes in isopycnal outcropping area

- Differences in amplitude between the models believed to be due to different background stratification

- Zonal SSH gradient, generated by anomalous deep water in the western SPNA, inducing meridional geostrophic flow in the upper ocean (upper AMOC limb)

- Slow advection of anomalous deep water delaying the spinup of the upper limb of density-space AMOC, relative to the lower AMOC limb, and associated advection of warm subtropical waters into the SPNA

- Consistent upper ocean warming in the SPNA and climate responses in the surrounding regions

- Overall, these multi-model results largely consistent with thermohaline mechanisms previously identified in both low- and high-resolution CESM models

Who Kim

National Center for Atmospheric Research

Single-forcing, idealised experiments using a state-of-the-art coupled climate model (HadGEM3-GC3.1) were performed to understand the mechanism linking North American and European anthropogenic sulphate aerosol emissions to North Atlantic variability. Medium and (0.25° ocean, ~60km atmosphere) and low-resolution (1° , ~135km) versions of the model were used to assess how model differences may impact on the forced response. We show that the aerosol increases initially cool the North Atlantic SST which induces surface density anomalies and strengthening of the Atlantic Meridional Overturning Circulation (AMOC). The strengthened AMOC leads to a lagged warming of the Subpolar North Atlantic. However, the AMOC response and subsequent warming is much stronger in the medium-resolution model, despite an overall stronger radiative forcing in the low-resolution model. We show evidence that this AMOC difference is consistent with differences in the sea ice response in a key region of the Subpolar North Atlantic. These results indicate that while surface temperature, sea ice and the AMOC are all sensitive to aerosol forcing in the HadGEM3-GC3.1 models, small regional differences between the model climatologies can significantly alter the pattern and magnitude of the large-scale response.

Michael Lai

University of Reading, Department of Meteorology

While most models agree that the AMOC becomes weaker under greenhouse gas emission and is likely to weaken over the 21st century, they disagree on the projected magnitudes of the AMOC weakening. In this work, we use 31 CMIP6 models and report that the models with stronger AMOC strength in mean state climate tend to project stronger AMOC weakening in both 1pctCO2 and abrupt-4×CO2 simulations. We show possible process to interpret the inter-model correlation and verify it through idealized experiments. For models with stronger mean state AMOC, stratification in the upper Labrador Sea is weaker, allowing for stronger mixing of the surface buoyancy flux. In response to CO2 increase, buoyancy forcing penetrates to deeper Labrador Sea in the models with stronger upper ocean mixing. The subsurface warming and the corresponding decrease in density over the Labrador Sea would further lead to AMOC weakening in subtropics due to advective processes from the Deep Western Boundary Current. In our time series analysis, most CMIP6 models agree that the decrease in subsurface Labrador Sea density leads AMOC weakening in subtropics by several years. Also, idealized experiments conducted in an ocean-only model show that the subsurface warming over 500-1500m in Labrador Sea leads to stronger AMOC weakening in several years, while the warming that is too shallow (<500m) or too deep (>1500m) in Labrador Sea causes little AMOC weakening. Our results suggest that a better representation of mean state AMOC is necessary for narrowing the inter-model uncertainty of AMOC weakening to greenhouse gas emission and its corresponding impacts on future warming projections.

Yuan-Jen Lin <yuanjenlin@gmail.com>

Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York; Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan

The relative importance of ocean and atmospheric dynamics in generating Atlantic Multidecadal Variability (AMV) remains an open question. Comparison between climate models with slab and fully-dynamic ocean components are often used to address this question, but such comparison has not been not very informative for understanding contributions from individual upper-ocean processes. We investigate the role of seasonal variation and mixed-layer entrainment in AMV using a hierarchy of stochastic models that solely consider the local upper ocean response to stochastic atmospheric forcing and its impact on the surface heat exchanges. To better understand the difference between pre-industrial control simulations using the Community Earth System Model 1 (CESM1) coupled with fully-dynamic and slab ocean models, we estimate the stochastic model parameters, including heat flux feedback, mixed-layer depth, and stochastic forcing amplitude, from each respective CESM1 simulation. Despite its simplicity, the stochastic model reproduces temporal characteristics of sea surface temperature (SST) variability in the subpolar gyre, including reemergence, seasonal-to-interannual persistence and power spectra. Furthermore, unrealistically persistent SST of the CESM1-slab ocean simulation is reproduced in slab ocean-like configurations when the mixed-layer depth is constant. Our stochastic model also shows the vertical entrainment acts to dampen the SST variability, thus explaining why the slab ocean simulation exhibits larger SST variance than the full ocean case. In terms of the spatial pattern, the stochastic model driven by temporally stochastic, spatially coherent forcing patterns reproduces the canonical AMV pattern. However, the amplitude of low frequency variability is underestimated, suggesting a role for ocean dynamics beyond entrainment.

Glenn Liu

Massachusetts Institute of Technology/Woods Hole Oceanographic Institution

The Atlantic Meridional Overturning Circulation (AMOC) regulates the global transport of heat, freshwater, trace gases and nutrients in the Atlantic sector. Published proxy records and modeling studies, reviewed by the IPCC, are consistent with a weakening AMOC in the warming climate. Here, we examine AMOC changes in the quadrupled CO2 experiments conducted under the CMIP6 program. While the forcing is extreme, the abrupt nature of the transition facilitates diagnosing the relevant forcings. The results suggest that AMOC weakens in response to freshwater input in the subpolar gyre, due primarily to sea ice melt. The resulting freshwater flows south along the eastern coast of North America and then eastward, north of the Gulf Stream. This weakens the density gradient across the North Atlantic Current, decreasing the associated vertical shear and consequently the transport. As such, the inflow to the northern downwelling regions is cut off. This is in contrast with the common perception that freshwater “caps the convection regions”, stifling deep water production. Changes in surface temperature have a weaker effect, and there are no consistent changes in local wind forcing among the models. The results thereby indicate that increases freshwater discharge, primarily from the Labrador Sea, is a precursor to AMOC weakening.

Gaurav Madan <gaurav.madan@geo.uio.no>

Department of Geoscience, University of Oslo

The Mid-Atlantic Ridge influences North Atlantic Ocean circulation and contributes to the topographic coupling between the subpolar gyre to the Atlantic meridional overturning circulation (e.g., Yeager 2015). Many studies both classic and recent have investigated bathymetry’s influence on ocean circulation through the addition of idealized bathymetric ridges and bumps to ocean model experiments. However, to the best of our knowledge, the influence of the MAR on ocean circulation and climate has not been examined using the opposite approach – through its removal. Here, we investigate the North Atlantic Ocean’s response to the removal of the MAR in a preliminary experiment (Flatlantic) that has otherwise realistic boundary conditions. The basin-scale form of the MAR is removed from the quarter-degree resolution GFDL Ocean Model version 4 (OM4) bottom topography via high pass spatial filtering and additional smoothing at the edges. We then force OM4 with a single CORE-cycle of JRA55-do and compare the Flatlantic experiment to the OM4 integration for OMIP2 (Ocean Model Intercomparison Project version 2). We will show the initial adjustment of North Atlantic surface fields and circulation to the removal of the MAR. We anticipate that the MAR may have a climate-relevant impact on ocean circulation.

Elizabeth Maroon

(1) University of Wisconsin-Madison, (2) Hewlett Packard Enterprise

Over Greenland, the Younger Dryas (YD; ~12,900-11,700 years ago) was characterized by rapid shifts into and out of extremely cold conditions near the end of the last deglaciation. YD air temperature changes are generally attributed to changes in Atlantic Meridional Overturning Circulation (AMOC). Past research has suggested that AMOC should have caused larger YD climate changes in south Greenland compared to central Greenland due to differences in proximity to the North Atlantic. To investigate YD climate in south Greenland, we measured the δ18O values of aquatic moss, extracted moss cellulose, and chironomid chitin (insects) from a uniquely old lacustrine record to reconstruct lake water δ18O values (inferring precipitation). δ18O values are then compared directly with ice core records (DYE3 and GISP2) and used to constrain annual air temperature changes. Additionally, we used subfossil chironomid assemblages to reconstruct summer air temperature. The three independent δ18O records from sediments were consistent, with a decrease of 4.7‰ at the YD onset and increases of 5.9-7.7‰ at the YD termination. This potentially reflects an annual temperature decrease of ~14°C and increase of ~18-23°C. Compared to ice core δ18O records, we find that YD climate changes were likely largest in south Greenland and decreased northward, consistent with AMOC as a driver. The chironomid assemblage-based reconstruction suggests YD summers were ~3°C cooler than the preceding Allerød and warmed 6.5°C following the YD. This may suggest AMOC more strongly influences winter temperatures. Overall, our record shows clear climatic shifts during the YD consistent with changes in AMOC.

Pete Puleo <peterpuleo2024@u.northwestern.edu>

(1) Northwestern University, Department of Earth and Planetary Sciences, Evanston, IL, United States, (2) Durham University, Department of Geography, Durham, United Kingdom, (3) Dalhousie University, School for Resource and Environmental Studies, Halifax, NS, Canada

Climate models simulate the AMOC and processes related to the AMOC very differently. Here, we present the diversity across CMIP6 models in pre-industrial control experiments. The focus lies on simulations of the AMOC, NAO-AMOC interactions, and related variables on interannual to decadal timescales. Regarding the NAO-AMOC interaction, there are large differences in the strength of their relationship, in the location (like the latitude of the AMOC), its periodicity and in the time-lag between both variables. Furthermore, we propose hypotheses of the causes for this diversity in the models. Specific processes involved in NAO-AMOC interaction might be of varying relative importance from model to model, for example, NAO-related buoyancy versus wind-forcing affecting the AMOC. Also, mean state difference like in the North Atlantic sea surface temperature might play an important role for causing differences in the variability across models.

Annika Reintges

(1) National Centre for Atmospheric Science, University of Reading, Reading, UK; (2) National Center for Atmospheric Research, Boulder, CO, USA

The mechanisms of wind-forced variability of the zonal overturning circulation (ZOC) are explored using an idealized shallow water numerical model, quasi-geostrophic theory, and simple analytic conceptual models. Two wind-forcing scenarios are considered: mid-latitude variability in the subtropical/subpolar gyres and large-scale variability spanning the equator. It is shown that the mid-latitude ZOC exchanges water with the western boundary current and attains maximum amplitude on the same order of magnitude as the Ekman transport at a forcing period close to the basin-crossing time scale for baroclinic Rossby waves. Near the equator, large-scale wind variations force a ZOC that increases in amplitude with decreasing forcing period such that wind stress variability on annual time scales forces a ZOC of $O(50\rm\ Sv)$. For both mid-latitude and low-latitude variability the ZOC and its related heat transport are comparable to those of the meridional overturning circulation. The underlying physics of the ZOC relies on the influences of the variation of the Coriolis parameter with latitude on both the geostrophic flow and the baroclinic Rossby wave phase speed.

MEETING PASSCODE: 1XiXFN

Michael Spall

Woods Hole Oceanographic Institution

The Global Meridional Overturning Circulation (GMOC) and its interdecadal changes since the mid-1950s are investigated using a data-constrained ocean & sea-ice model. Consistent with recent observations and externally forced climate model simulations for the twenty-first century, the derived GMOC displays the largest change in the upper and lower overturning cells in the Southern Ocean. The former has strengthened and expanded poleward and downward since the mid-1970s, while the latter has weakened and contracted. Further analysis indicates that these changes are driven by the strengthening Southern Hemisphere westerlies and the increasing Antarctic meltwater discharge and transport into the Ross Sea. In response to these changes in the Southern Ocean, a large-scale adjustment of the GMOC is underway in the South Atlantic and Indo-Pacific Oceans during the most recent decade. In the high-latitude North Atlantic, however, the overturning circulation shows no long-term trend and is largely modulated by the North Atlantic Oscillation.

Sang-Ki Lee

NOAA's Atlantic Oceanographic and Meteorological Laboratory

Annalisa Bracco

Georgia Tech

The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system, which keeps the North Atlantic relatively warm. It is predicted to weaken under climate change. The AMOC may have a tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models. We present initial results from the North Atlantic hosing model intercomparison project, where we applied an idealised forcing of a freshwater flux over the North Atlantic in 9 CMIP6 models. The AMOC weakens in all models from the freshening, but once the freshening ceases, the AMOC recovers in some models, and in others it stays in a weakened state. We discuss how differences in feedbacks affect the AMOC response.

Laura Jackson

Hadley Centre, Met Office

Altimeter sea surface height measurements and in situ hydrographic data are combined in this work to derive monthly synthetic temperature and salinity (T/S) profiles along zonal transbasin sections in both the North (26.5°N) and South (20°S, 25°S, 30°S, and 35°S) Atlantic Ocean since 1993, which are then used to estimate the MOC and meridional heat transport (MHT). Consistent with previous studies, the results indicate that the MHT is highly correlated with the MOC at all five latitudes. At 26.5°N the mean MHT from synthetic profiles (1.09±0.21 PW) is slightly lower than that from the RAPID-MOCHA-WBTS array (1.20±0.28 PW), but their interannual variabilities show good agreements. Both the geostrophic and Ekman components contribute significantly to the MHT variability, with the geostrophic component dominating during 1993-2004 and the Ekman component dominating during 2005-2014. In the South Atlantic, the MHT seasonal cycle strengthens southward from 20°S to 34.5°S. On interannual time scale, the MHT at 20°S and 25°S experiences larger variations than that at 30°S and 34.5°S, in part due to the fact that the geostrophic and Ekman components work together to strengthen the MHT changes, while they tend to work against each other at 30°S and 34.5°S. Large MHT anomalies show northward propagation from 34.5°S to 20°S wit a 9-month lag. Results shown here suggest that integrating data from different observing platforms provides better means to estimate the MOC and MHT in near real time.

Shenfu Dong

Atlantic Oceanographic and Meteorological Laboratory, NOAA

The oceans are a vital part of the climate system and presumably played an integral role in climate modulation during Quaternary glacial cycles. For an improved fundamental understanding of this role and of the ocean system it is important to get accurate information about the past sourcing of deep ocean water because it is related to marine carbon storage, heat transport, and ventilation. Yet, to date deep Atlantic water mass sourcing during past glacials is still debated, and interpretations based on different proxies such as carbon isotopes and neodymium isotopes appear to be at odds.

Here, I present the latest results of a mixing model based on pan-Atlantic data compilations of five different proxies from the Last Glacial Maximum and Heinrich Stadial 1. I show that widespread northern sourced deep waters are necessary to explain proxy observations and estimate their extent in the glacial Atlantic. These analyses offer new detailed insights into glacial deep Atlantic water mass sourcing and reconcile findings from studies using individual proxies. They suggest widespread prevalence of northern sourced deep waters during both glacial maximum and stadial conditions, contradicting earlier inferences of suppressed AMOC activity or depth.

Patrick Blaser <patrick.blaser@unil.ch>

University of Lausanne, Switzerland

The Atlantic Meridional Overturning Circulation (AMOC) transports northward-flowing surface, thermocline and intermediate waters with relatively high temperatures, carrying a large amount of heat from southern to northern latitudes. Accurately assessing the strength of the Atlantic Meridional Overturning Circulation with in situ data and models has been a challenge for oceanographers for decades now. Furthermore, no consensus has been reached yet regarding the slowdown of the overturning circulation, and the driving mechanisms are still poorly understood. We have gathered hydrographic data from zonal sections across the Atlantic from three decades using data from WOCE and GOSHIP repeating sections. For each decade, we have built an inverse model depicting the circulation for the whole Atlantic basin, attending to horizontal and vertical transports between sections. The circulation pattern describes two counter-rotating cells in the upper and abyssal ocean, which follows the path of the main water masses of the Atlantic – the northward upper layers of the thermocline and intermediate waters, the southward deep layers of the North Atlantic Deep Water and the abyssal layers flowing northward with Antarctic Bottom Water. Our results show no changes in the AMOC for all sections analysed over the whole Atlantic for the last thirty years. We also find an increased export of freshwater from the South Atlantic associated with an increase in upper salinity.

Verónica Caínzos

University of Las Palmas de Gran Canaria, Instituto de Oceanografía y Cambio Global

GCM studies which abruptly increase atmospheric CO2 concentrations generally evidence a rapid reduction of a full AMOC from its present state of strongly overturning circulation into one characterized by weak overturning and a reduced northward meridional heat transport. This climate tipping point is frequently discussed in the context of present and past global climate changes. Far less discussed and understood, however, is the eventual recovery of the circulation and the establishment of a new equilibrium state, which early studies found to occur many centuries or millennia following the initial collapse. Motivated by this, we use the Community Earth System Model (CESM1) and a range of abrupt CO2 forcing scenarios to better evaluate the timescales and mechanisms involved in the AMOC’s response and its eventual evolution to equilibrium. We focus on changes in key aspects of the AMOC and ocean thermohaline structure, such as the circulation 3D structure, distributions of salinity and temperature, ocean mixed layer depth and deep water formation, as well as changes in sea ice and atmospheric circulation. These results also shed more light on the characteristics of the AMOC in paleoclimate contexts for which atmospheric CO2 concentrations were markedly different to the present day.

Paul Curtis <paul.curtis@yale.edu>

Yale University, Department of Earth and Planetary Sciences

Despite global warming, a long-term cooling of sea surface temperature (SST) is observed over the last century in the subpolar North Atlantic. This cold blob is hypothesized as an indicator of a weakening AMOC (AMOC). Here we revisit the relationship between the AMOC change and the cold blob in the last century by using historical simulations from the latest CMIP6 models. These models disagree on the sign of AMOC changes in the last century, while the three models that well reproduce the observed cold blob do show a decreasing trend of the AMOC. However, the AMOC decline fails to account for a nonnegligible portion of the cooling, primarily over the eastern subpolar gyre and inter-gyre region. Surface heat budget analysis further suggests that the subpolar cooling due to decreased oceanic heat transport convergence is balanced out by changes in surface turbulent heat flux. Interestingly, the cold blob largely results from radiative forcing. Therefore, processes other than AMOC changes could play an important role in determining the magnitude and the spatial coverage of the cold blob. We further argue that the subpolar SST might not be a good indicator of AMOC changes.

Yifei Fan

Penn State, WHOI

A potential future slowdown of the Atlantic Meridional Overturning Circulation (AMOC) would have profound impacts on global and regional climate, but this slowdown is affected by many processes that may speed up or accelerate the changes. In particular, recent studies have shown that AMOC responds to changes in mean tropical Indian ocean (TIO) temperature. However, AMOC response to internal (unforced) TIO temperature variations has remained largely unexplored. Here, we use the ERSST5 and HadISST4 gridded observational products, as well as pre-industrial control (piControl) simulations with coupled climate models, to illustrate unforced changes in TIO sea surface temperature can drive teleconnections that influence internal variations of North Atlantic climate and AMOC. We separate the unforced observed component of TIO temperature from the forced signal following the residuals method presented by Smith et al. (2019). In the absence of direct AMOC observation we estimate AMOC variability from an SST fingerprint index (SSTAMOC), similarly used in past studies (i.e. Latif et al., (2004), Sevellec et al., (2017), and Caesar et al. (2018)). We find a robust observed relationship between unforced TIO and unforced SSTAMOC when TIO leads by ~30 years. This time-lag is in line with a recently described mechanism of anomalous tropical Atlantic rainfall patterns that originate from TIO warming (Hu and Fedorov, 2019; Ferster et al. 2021). Pre-industrial control simulations with the IPSL-CM6A-LR model confirm this relationship, indicating a time lag of ~30 years between TIO and SSTAMOC variations. This work therefore demonstrates that a pathway between TIO temperature and AMOC exists, contributing to AMOC variability.

Brady S. Ferster <brady.ferster@locean.ipsl.fr>

LOCEAN / IPSL

The sea surface temperature (SST) in the subpolar North Atlantic decreased during the past century, a remarkable feature known as the “warming hole”. It is commonly suggested that the warming hole results from the slowdown of the Atlantic meridional overturning circulation (AMOC). However, we show that a climate model without interactive ocean dynamics can produce a warming hole. Here, a 9-member ensemble of the Community Earth System Model (CESM) coupled only to a mixed-layer ocean or “slab” ocean model (SOM) accounts for 40-50% of the observed cooling trend in the subpolar North Atlantic. Combined with reanalysis, we show the local westerlies can lead to a cooling SST trend. The increasing high-level westerly leads to a warming trend due to combined cloud-shortwave-longwave-latent heat feedback, while the increasing surface westerly amplifies the heat loss from ocean via turbulent heat fluxes that overwhelms the former warming trend and results in the warming hole. We further suggest that wind-driven ocean processes could enhance the cooling.

Chengfei He

University of Miami

Evidence for the persistence of Atlantic Multidecadal Variability (AMV) beyond the instrumental period (<1850 CE) is primarily sourced from atmospheric and terrestrial proxy records. Here, we provide a marine-based record of North Atlantic climate variability from physical and geochemical characteristics of Arctica islandica bivalve shells from the southern Barents Sea. A master shell growth chronology was constructed from 39 shells, providing a 564-year (1449-2014 CE), absolutely-dated record of growth increment width. Annual increments from 23 of these shells were sampled and analyzed for oxygen isotopic composition to provide a 476-year (1539-2014) record of variability related to temperature and salinity in the growing environment. The shell growth and oxygen isotope records both contain frequency characteristics similar to AMV (65-80 years), persisting into the 16th century. A temperature reconstruction from the oxygen isotope data suggests an increase of at least 2°C from the mid-18th century to early 21st century. These results enable insight into long-term dynamics of AMV, the extent of its influence in the high latitudes, and the magnitude of temperature change near the gateway to the Arctic. The results highlight the potential to strengthen regional marine climate syntheses using shell-based records.

Madelyn Mette

U.S. Geological Survey St. Petersburg Coastal and Marine Science Center

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate. Recent observations have highlighted the dominant role of the buoyancy forcing in the transformation of surface waters to the AMOC lower limb at subpolar latitudes. The short (4 years) length of the OSNAP timeseries, however, limits conclusions over longer time scales. To investigate a wide range of temporal scales, we use three 100-years long coupled simulations of HadGEM3-GC3.1, at resolutions ranging from ~130 km atmosphere and 1° ocean to 25 km atmosphere and 1/12° ocean. In line with observations, the models show that the mean overturning and buoyancy-induced transformation are concentrated in the eastern subpolar gyre rather than in the Labrador Sea. However, the horizontal resolution of the models impacts the formation of dense water over the subpolar gyre.

In the medium resolution model, the strong overturning at OSNAP East is mainly explained by strong water mass transformation (WMT) over the Irminger and Iceland Basins. However, WMT does not simply increase with resolution over the eastern subpolar gyre, as a weaker WMT is observed over those basins in the high resolution model. On the contrary, both the overturning at OSNAP West and the WMT over the Labrador Sea do increase with resolution. An exploration of the heat fluxes, density at surface and sea ice extent showed that the large dense water formation observed over the Labrador Sea at high resolution is explained by a combination of biases that increase with the horizontal resolution of the model.

Tillys Petit

University of Reading

Observations from satellite altimetry, Argo floats and wind fields are used to construct three dimensional, monthly absolute velocity fields using the method developed by Schmid (2014). These are used to derive time series of the volume and heat transport for 1993 to 2021 at several latitudes. At 26.5N they are combined with observations of the transport in the Florida Current. There, amplitude of the transport variability is about 15 Sv. The overall time series reveal weak or insignificant trends with quite strong variability on short as well as interannual time scales.

Claudia Schmid

NOAA/AOML/PhOD

The convective activity in the Labrador basin undergoes dramatic interannual-to-decadal variability. Variations in Labrador Sea deep convection eventually propagate throughout the North Atlantic and impact the Atlantic Meridional Overturning Circulation (AMOC) by modifying the poleward heat transport. Recent modeling work highlighted that buoyancy forcing over the Labrador Sea is key in controlling AMOC variability and that AMOC inter-annual signals are closely related to the variability of the Labrador Sea convection, in turn linked to the cumulative NAO. OSNAP observations, however, do not support a significant correlation between AMOC and LSW formation, at least at interannual scales. Biases in coupled climate models and their tendency to overestimate the LSW volume have been indicated as possible causes of the discrepancy. This overestimation, however, is not of the same magnitude across models that particpated in the Coupled Model Intercomparison Project Phase 5 (CMIP5). This study explores what may cause these differences separating the oceanic and atmospheric contributions. We focus on 5 CMIP5 Earth System Models (ESMs) and use separately their oceanic or atmospheric fields to force the regional ocean model. Our objective is to establish cause-effect linkages between the (modeled) Labrador Sea circulation, the atmospheric forcing fields, the oceanic boundary conditions and their variability, and model representation.

Filippos Tagklis

Georgia Institute of Technology

The oceans are the major sink for the excess heat resulting from global greenhouse gas emissions. Therefore, in order to better understand the ocean’s role in mediating anthropogenic warming, high-quality oceanic temperature information is essential. While temperature observations and proxy reconstructions of the surface ocean are relatively plentiful, temperature estimates related to deep ocean water masses are far more limited both spatially and temporally. In particular, there is a distinct lack of temperature data from the deep northwest Atlantic, where water masses formed in the subpolar North Atlantic and Nordic Seas flow southwards as a western boundary current transporting anomalous heat originally from the surface into the deep ocean. A single model inversion of Atlantic subsurface temperature anomalies shows a cooling throughout the whole water column coincident with the Little Ice Age (LIA) followed by warming throughout the Industrial Era. Limited proxy data also exhibit similar behaviour over the last 1ka. To improve our understanding of deep ocean temperatures, this study uses two different benthic foraminiferal temperature proxies to reconstruct past deep ocean temperature changes at three intermediate depth (~2km) sites in the northwest Atlantic over the last 1ka. Multi-species benthic-Mg/Ca records resolve no long-term downcore trend and are relatively noisy with multi-decadal scale variability in excess of the equivalent to ±1°C. However, the downcore averages of reconstructed temperature show good intra-species agreement and are consistent with modern observations over the last ~100-years. In comparison, coeval δ18Oc measurements exhibit behaviour consistent with a period of cooling (~0.75°C) and subsequent warming (~0.5°C) coincident with the LIA and Industrial Era respectively. We conclude that (1) benthic-Mg/Ca does not have a signal to noise ratio capable of resolving temperature changes of less than 1°C, and (2) the deep northwest Atlantic experienced LIA cooling and Industrial Era warming consistent with a model inversion and the available proxy data.

Jack Wharton

University College London