OS1.7

Using the climate model MIROC4m, we simulate self-sustained oscillations of millennial-scale periodicity in the climate and Atlantic meridional overturning circulation under glacial conditions. We show two cases of extreme climatic precession and examine the mechanism of these oscillations. When the climatic precession corresponds to strong (weak) boreal seasonality, the period of the oscillation is about 1,500 (3,000) years. During the stadial, hot (cool) summer conditions in the Northern Hemisphere contribute to thin (thick) sea ice, which covers the deep convection sites, triggering early (late) abrupt climate change. During the interstadial, as sea ice is thin (thick), cold deep-water forms and cools the subsurface quickly (slowly), which influences the stratification of the North Atlantic Ocean. We show that the oscillations are explained by the internal feedbacks of the atmosphere-sea ice-ocean system, especially subsurface ocean temperature change and salt advection feedback with a positive feedback between the subpolar gyre and deep convection.

How to cite: Kuniyoshi, Y., Abe-Ouchi, A., Sherriff-Tadano, S., Chan, W.-L., and Saito, F.: Effect of Climatic Precession on Dansgaard-Oeschger-like oscillations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6750, https://doi.org/10.5194/egusphere-egu22-6750, 2022.

Dominant Euro-Atlantic climate modes such as the North Atlantic Oscillation (NAO), the Eastern Atlantic pattern (EA), the Eastern Atlantic Western Russian pattern (EAWR), and the Scandinavian pattern (SCA) significantly affect interannual-to-decadal Euro-Mediterranean climate fluctuations, especially in winter.

In this contribution, we will present and discuss results from a CMIP6 multi-model analysis performed to investigate the robustness of historical and projected state and variability of such modes under the historical and ssp585 future scenario of anthropogenic forcing (fossil-fueled development with 8.5W/m² forcing level) simulations, focusing on the winter season.

Toward this goal, we first search for a reliable box-based index definition for each of the abovementioned observed climate modes and, then, we perform a comparative assessment of the temporal, spectral and distributional properties of the so-defined indices during the historical (1850-2014) and ssp585 future scenario (2015-2099) time periods, with a special focus on the two interdecadal periods 1960-1999 and 2060-2099.

Results show overall good skills of the historical ensemble to reproduce the observed temporal, spectral and distributional properties of all considered modes. At the end of the 21st Century the ssp585 ensemble yields non-significant distributional changes for NAO, EAWR, and SCA indices and a transition to a stronger baroclinic structure for EA, with persistent positive anomalies in the mid-troposphere enhancing globally-driven warming over the Euro-Mediterranean region. The hemispheric spatial correlation patterns with temperature and precipitation significantly change for all modes, that is, we observe a significant modulation of the teleconnections associated with each index.

How to cite: Cusinato, E., Rubino, A., and Zanchettin, D.: Winter Euro-Atlantic Climate Modes: Future Scenarios From a CMIP6 Multi-Model Ensemble, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3596, https://doi.org/10.5194/egusphere-egu22-3596, 2022.

The 20th century “early warming” (1910-1940) and cooling (1940-1970) of the Northern Hemisphere offer an interesting contrast of periods with opposite temperature trends, similar hemispheric temperature anomalies, yet very different temperature anomaly patterns. These contrasts are particularly clear in the North Atlantic sector, which exhibits large climate variability over a range of time scales, from short (weather regimes) to long (Atlantic Multidecadal Variability). In this study, we explore the role of the atmospheric circulation (North Atlantic jet stream) in determining the temperature anomaly patterns over the 20^th century. While different jet configurations are associated with distinct synoptic temperature patterns in the North Atlantic sector, only some are found to contribute substantially to longer term temperature trends. Notably, the southern jet configuration has the strongest temperature anomalies, with a dipole signal that is opposite from the one under the tilted jet configuration. At the same time, these two jet configurations exhibit relatively large decadal variations in frequency (days of occurrence in given winter seasons), with trends that are almost the opposite. In fact, changes in the frequency of southern and tilted jet “days” alone account for much of the North Atlantic and Arctic temperature variability on decadal time scales, including the differences between the early warming and cooling periods (e.g., the flipped warming versus cooling patterns are associated with fewer southern jet days and more tilted jet days). However, the reconstruction skill of the 30-year mean temperature anomaly in the North Atlantic sector using jet frequency exhibits decadal variability, with high skill scores interestingly coinciding with the positive phases of the Atlantic Multidecadal Variability. The lower reconstruction skill especially during the global warming period from the1980s onwards is likely due to the impact from the warming hole in the North Atlantic, which dominates the temperature patterns in the North Atlantic. Overall, the evolution of Northern Hemisphere surface temperature over the 20^th century is found to be influenced by North Atlantic jet variability, with lower frequency ocean effects contributing more in recent decades.

How to cite: Tao, D., Madonna, E., and Li, C.: Using atmospheric variability to understand the wintertime regional warming and cooling patterns in the North Atlantic Sector, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5057, https://doi.org/10.5194/egusphere-egu22-5057, 2022.

Climate model biases in the North Atlantic (NA) low-level tropospheric westerly jet are a major impediment to reliably representing variability of the NA climate system and its wider influence, in particular over western Europe. We highlight an early-winter equatorward jet bias in Coupled Model Inter-comparison Project (CMIP) models and assess whether this bias is reduced in the CMIP6 models in comparison to the CMIP5 models. Historical simulations from the CMIP5 and CMIP6  are further compared against reanalysis data over the period 1862-2005.  

The results show that an equatorward bias remains significant in CMIP6 models in early winter. Almost all CMIP5 and CMIP6 model realizations exhibit equatorward climatological jet latitude biases with ensemble mean biases of 3.0° (November) and 3.0° (December) for CMIP5 and 2.5° and 2.2° for CMIP6. This represents an approximately one-fifth reduction for CMIP6 compared to CMIP5. The equatorward jet latitude bias is mainly associated with a weaker-than-observed frequency of poleward daily-weekly excursions of the jet to its northern position. A potential explanation is provided.  Our results indicate a strong link between NA jet latitude bias and systematically too-weak model-simulated low-level baroclinicity over eastern North America in early-winter.  

Implications for model representation of NA atmosphere-ocean linkages will be presented. In particular CMIP models with larger equatorward jet biases tend to exhibit weaker correlations between temporal variability in jet speed and sea surface conditions over the NA sub-polar gyre (SPG). This has implications for the ability of climate models to represent key aspects of atmospheric variability and predictability that are associated with atmosphere-ocean interactions in the SPG region.  

How to cite: Bracegirdle, T., Lu, H., and Robson, J.: Equatorward North Atlantic jet biases in CMIP models and implications for simulated regional atmosphere-ocean linkages, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6401, https://doi.org/10.5194/egusphere-egu22-6401, 2022.

Record low surface temperatures were observed in the subpolar North Atlantic during 2015, despite the majority of the global ocean experiencing higher than average surface temperatures. We compute mixed layer temperature budgets in the ECCO Version 4 state estimate to further understand the processes responsible for the North Atlantic cold anomaly. We show that surface forcing was the cause of approximately 75% of the initial cooling in the winter of 2013/14, after which the cold anomaly was sequestered beneath the deep winter mixed layer. Re-emergence of the cold anomaly during the summer/autumn of 2014 was primarily driven by a strong temperature gradient across the base of the mixed layer. Vertical diffusion resulted in approximately 70% of the re-emergence, with entrainment of deeper water driving the remaining 30%. In the summer of 2015, surface warming of the mixed layer was then anomalously low, resulting in the most negative temperature anomalies. Spatial patterns in the budgets show that the initial surface cooling was strongest in the south of the region, due to strong westerly winds related to the positive phase of the East Atlantic Pattern. Subsequent anomalies in surface fluxes associated with the North Atlantic Oscillation were stronger in the north, but the impact on the average temperature of the mixed layer was largely masked by anomalously high winter mixed layer depths.

How to cite: Sanders, R., Jones, D., Josey, S., Sinha, B., and Forget, G.: Using mixed layer heat budgets to determine the drivers of the 2015 North Atlantic cold anomaly in ocean state estimates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1682, https://doi.org/10.5194/egusphere-egu22-1682, 2022.

The North Atlantic Jet Stream is well known to leave an imprint on the North Atlantic SST in the form of a tri-polar pattern.  The majority of the existing research has focused on the winter jet stream position or strength of the jet stream.  Here we look at the response of the North Atlantic SSTs to the strength and position of the North Atlantic Jet Stream across all seasons in the CMIP6 piControl simulations.  For the case of both the strength and position of the jet stream the multi-model mean response is a tripolar SST pattern, with the response to the changes in strength showing a slight horseshoe pattern with the northern and southern most anomalies connected on the east and most evident in the summer.  The SST response to winter and spring jet stream changes persist the longest with the northern most imprint on the SSTs lasting up to 2 years.  The response to changes in the jet stream in the summer and fall leave an imprint on the SSTs lasting atmost into the following year.   Furthermore, we investigate at how these responses vary among the CMIP6 models and potential mechanisms leading to the persistence.

How to cite: Mecking, J., Sinha, B., Harvey, B., Robson, J., and Bracegirdle, T.: Seasonal differences in the persistence of SST’s Response to the North Atlantic Jet Stream, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5829, https://doi.org/10.5194/egusphere-egu22-5829, 2022.

How to cite: Hellmich, L., Matei, D., Suarez-Gutierrez, L., and Müller, W. A.: Contribution of the Atlantic Ocean to European Heat Extremes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5694, https://doi.org/10.5194/egusphere-egu22-5694, 2022.

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.  

How to cite: Jackson, L., Alastrue-De-Asenjo, E., Bellomo, K., Danabasoglu, G., Hu, A., Jungclaus, J., Meccia, V., Saenko, O., Shao, A., and Swingedouw, D.: AMOC thresholds in CMIP6 models: NAHosMIP, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2778, https://doi.org/10.5194/egusphere-egu22-2778, 2022.

Global warming since the industrial revolution has led to a series of changes in the atmosphere and ocean. As a key indicator of global ocean circulation, AMOC has shown a weakening in recent decades from both the observed and simulated results. This process which is not only affected by the local variation of the Arctic, but also by the ocean and atmosphere circulation changes in the middle and lower latitudes, might have important implications for future global climate changes. We employ the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR) and a method of perturbing coupled models to quantify and understand the impact of anthropogenic warming on the slowdown of AMOC. Conducted one control (CTRL) experiment and three sensitivity experiments (60N, 60NS, and GLOB) in which CO2 concentration were abruptly quadrupled either regionally (60N-north of 60°N, 60NS-south of 60°N) or globally (GLOB). The goal of our research is to identify the response of AMOC weakening to the quadrupling of CO2 concentration in different regions and provide future insight into ocean circulation changes in the context of climate warming. Our results show that CO2 forcing outside the Arctic dominates the weakening of AMOC. In a warming climate, the poleward heat transport increased due to the extra-Arctic CO2 forcing, which enhanced the upper ocean average stratification within the mixed-layer depth over Nordic Seas and Labrador Sea and thus weakens the AMOC to a large extent. The warming in upper-layer also lead to the dominant role of temperature contribution to stratification. However, in both the deep convection regions, the mechanism resulting in the strengthening of stratification might be quite different.

How to cite: Chen, J., Wang, X., and Wang, X.: The weakening of AMOC highly linked to climate warming outside the Arctic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4440, https://doi.org/10.5194/egusphere-egu22-4440, 2022.

Following the recommendations of CMIP6, some climate models have for the first time started using a resolution of 1/4° for their oceanic component. This is significant, as it means that large eddies are resolved (so-called eddy-permitting models), introducing chaotic variability in oceanic models. Observational studies of the North Atlantic Subtropical Mode Water (STMW) have found that not all of its variability can be explained by atmospheric variability. The STMW is a water mass formed by ventilation over the winter and is the most abundant T,S class of water in the surface North Atlantic. Consequently it plays a key role in air-sea exchanges over the basin. These elements have motivated the present model investigation of the STMW's ocean-driven (intrinsic) chaotic variability using a NEMO-based, 1/4°, 50-member ensemble simulation of the Northern Atlantic ocean. Using this dataset, six STMW-wide integrated variables are defined and analysed: total volume, and averaged potential vorticity, depth, temperature, salinity and density. The model solution is assessed against the ARMOR3D ocean reanalysis, based on in situ data collected from ARGO floats and satellite observations. The water mass' chaotic variability is estimated from the time-averaged ensemble standard deviation, and is compared to the total variability estimated from the ensemble mean of the temporal standard deviations of all members. Initial results show that chaotic variability is significant for STMW properties at interannual timescales, representing almost half of the total variability of its average temperature. A spectral analysis indicates that chaotic variability remains significant at longer timescales. This suggests that as climate models move towards finer spatial resolution in the ocean, oceanic chaos can be expected to introduce more variability at interannual and longer timescales. This study also highlights the necessity of a good parametrisation of this oceanic chaos in non-eddying ocean models.

How to cite: Narinc, O., Thierry, P., Guillaume, M., and Stéphanie, L.: Chaotic variability of the North Atlantic Subtropical Mode Water, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11847, https://doi.org/10.5194/egusphere-egu22-11847, 2022.

A significant fraction of the Eulerian downwelling feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) has been proposed to occur around the subpolar North Atlantic's continental slopes. While this downwelling ultimately takes place in a thin boundary layer where relative vorticity can be dissipated via friction, it is maintained by a large-scale geostrophic balance and an along-shore densification of the boundary current. We here use modern hydrography data (Argo and shipboard hydrography mainly) to map the long-term mean density field along the continental slope via an optimal interpolation method specifically adapted to the length scales of the boundary current. The overall downstream densification of the boundary region implies a Eulerian-mean downwelling of 2.12 ± 0.43 Sv at 1100 m depth between Denmark Strait and Flemish Cap. While seasonal variations appear to be relatively limited, a clear regional pattern emerges with Eulerian-mean downwelling in the Irminger Sea and western Labrador Sea and upwelling along Greenland western continental slope. Comparisons with independent cross-basin estimates confirm that overturning transport across the marginal seas of the subpolar North Atlantic is mainly explained by vertical volume fluxes along the continental slopes, and suggest the usefulness of hydrographic data alone to estimate the regional pattern of the sinking branch of the AMOC. 

How to cite: liu, Y., Desbruyeres, D., Mercier, H., and Spall, M.: Observation-based estimates of Eulerian-mean boundary downwelling in the western subpolar North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1130, https://doi.org/10.5194/egusphere-egu22-1130, 2022.

The characteristics of Mediterranean Outflow Water (MOW) in the Northeast Atlantic were obtained during the 43^rd cruise of the R/V Akademik Nikolaj Strakhov (October 2019) and the 59^th cruise of the R/V Akademik Ioffe (September 2021) using CTD measurements. MOW is transformed Mediterranean Sea Water flowing down the slopes of the Strait of Gibraltar into the Gulf of Cadiz, where it mixes with underlying North Atlantic Central Water. MOW spreads at water depths between 500–1500 m in the eastern North Atlantic and is characterized by higher temperatures and salinities than other ambient water masses. In 2019 and 2021 MOW was located at depths of about 700–1500 m. The temperature in the core of MOW was in the range of 9.5–11.5 °C, while in 2019 both temperature and salinity were higher than in 2021. The salinity in the core was 36.15 psu in 2019 and 36.08 psu in 2021. The comparison of MOW characteristics obtained in 2019 and 2021 with data obtained in cruises in 1993, 2001 and 2005 from the CLIVAR and Carbon Hydrographic Data Office (https://cchdo.ucsd.edu/) showed that the maximum salinity values were observed in September 1993 and reached 36.17 psu. The minimum value of this parameter in the core of MOW was recorded in April 2001 and was 36.03 psu. According to the data of the 1993–2019 expeditions, the maximum salinity was noted at a depth of 1000–1100 m. In 2021, the core of MOW was slightly deeper — about 1150 m. The temperature in the MOW core in all studied years was in the range of 11.1–11.3 °C, with the exception of 2001, when the maximum temperature in the core was about 10.9 °C.

Acknowledgements

The financing of the expedition and the primary processing of the data obtained on the 59^th cruise of the R/V Akademik Ioffe were carried out at the expense of the state assignment of IO RAS № 0128–2021–0012. The analysis and interpretation of the data were supported by the Russian Science Foundation (project no. 21–77–20004).

How to cite: Bocherikova, I., Krechik, V., Kapustina, M., and Dvoeglazova, N.: Mediterranean Outflow Water characteristics in the Northeast Atlantic in 2019 and 2021., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-355, https://doi.org/10.5194/egusphere-egu22-355, 2022.

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. An unrealistically large sea ice extent induces a weak buoyancy-induced transformation over the western subpolar gyre at low resolution, while a bias in surface density produces too dense water at high resolution. These biases are associated with a shift in the location of dense water formation. The transformation is mainly localized in the interior of the Irminger and Labrador seas at low resolution, and over the boundary current at high resolution. The interannual variability of the transformation is thus driven by different mechanisms between the simulations. In contrast with observations, the interannual variance in air-sea fluxes plays a more prominent role in the variance of transformation along the boundary current at high resolution.

How to cite: Petit, T., Robson, J., and Ferreira, D.: Formation of dense water over the North Atlantic subpolar gyre in a hierarchy of climate models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3068, https://doi.org/10.5194/egusphere-egu22-3068, 2022.

The Irminger Current (IC) is known to be an important contributor to the northward volume transport associated with the Atlantic Meridional Overturning Circulation (AMOC). The IC has a two-core structure with surface intensified velocities and transports warm and saline waters originating from the North Atlantic Current further north. The strength of the subpolar AMOC is continuously measured by the Overturning in the Subpolar North Atlantic Program (OSNAP) since 2014. Recent results highlight that most of the overturning in density space occurs in the array east of Greenland, in the Irminger and Iceland Basins. In previous work we looked into the transport variability of the IC on decadal to interannual time scales and could identify long-term trend related to basin-wide density changes which have the potential to impact AMOC variability. However, the impact of mesoscale variability on northward transport variability in the Irminger Sea has not been studied yet.

In this study, we explore the mesoscale variability in the IC and its impact on northward transport variability.

Previous studies showed that the western flank of the Reykjanes Ridge, where the IC is located, is a region of enhanced eddy kinetic energy. We used high resolution mooring data from 2014 – 2020 from the IC mooring array to investigate its transport variability. The mean volume transport obtained for the IC is 10.4 Sv but it strongly varies on time scales from days to months (std. dev. of 4.3 Sv). The mooring data reveals a seasonal cycle in the eddy kinetic energy with the strongest activity in winter. However, this does not coincide with a seasonal cycle in volume transport. We found the strongest EKE in the western core of the IC. In 2019, an exceptional 6-month intensification of the IC led to exceptionally strong volume transport of the IC of 19.9 Sv in August. Using sea level anomaly maps from satellite altimetry, the intensification was attributed to the presence of a mesoscale eddy in the vicinity of the moorings.  At this time, altimetry shows an anticyclone lingering next to a cyclone in the mooring array, which intensified northward velocities within the IC. We thus conclude that mesoscale variability can directly impact both the transport and the variability of the IC.

Considering the potential importance of mesoscale variability along the Reykjanes Ridge, further research will focus on estimating the mean properties of the eddies, their formation region and their faith using a high-resolution model.

How to cite: Fried, N., Katsman, C. A., and de Jong, M. F.: The impact of mesoscale variability on northward volume transport in the Irminger Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3602, https://doi.org/10.5194/egusphere-egu22-3602, 2022.

Accelerated melting of the Greenland Ice Sheet is considered to become a tipping point in the freshwater balance of the subpolar North Atlantic (SPNA). The ramifications of increased freshwater input have been projected to reduce deep convection in neighboring Labrador and Irminger Seas. The East Greenland Current is a primary pathway for transporting Arctic-sourced freshwater and Greenland glacial meltwater into the SPNA. Understanding the variability of the East Greenland (Coastal) Current (EGC/EGCC) is of high importance, as it contains the first imprint of ice melt which flows directly into the current when entering the open ocean. 

We performed a cross sectional analysis of salinity and temperature along the eastern Greenland shelf using output from an eddy-rich (1/20^o) ocean model (VIKING20X), which is forced with time-varying Greenland freshwater fluxes (Bamber et al., 2018), and the observational-based reanalysis product (GLORYS12V1 [1/12^o]) from 1993 to 2019. A time varying mask referenced to a salinity threshold of ≤ 34.8 psu was used to isolate the EGC close to the shelf at five locations for both winter (JFM) and summer (JAS) months. Selected locations are major ocean gateways, glacier outlets/fjords, and observing arrays: Fram Strait, Denmark Strait, just south of 66^oN (Helheim/~Sermilik) and 63.5^oN (Bernstorff), and OSNAP East extending up to the central Irminger Sea. Export of polar water from the Arctic Ocean through Fram Strait sets the initial, low salinity signature in the EGC, which mixes with Atlantic water further downstream and increases in salinity. However, in our simulation, we find lower salinity values again south of Denmark Strait in summer with some notable fresh imprints of extreme meltwater runoff in individual years, such as 2010 and 2012. Furthermore, we observe that for all the cross sections, excluding Fram Strait, there is a negative trend in salinity from 1993 to 2010 followed by a decade in which the salinity trend at Denmark Strait and further south decouples from that in Fram Strait in winter and summer. We explore the reasons for the temporal variations in salinity (and temperature) along the East Greenland Shelf and the potential of different data products to show early imprints of enhanced meltwater runoff into the EGC.

How to cite: Schiller-Weiss, I., Martin, T., Biastoch, A., and Karstensen, J.: Do salinity variations along the East Greenland shelf show imprints of increasing meltwater runoff?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5135, https://doi.org/10.5194/egusphere-egu22-5135, 2022.

The Atlantic Meridional Overturning Circulation redistributes heat across the Atlantic and is therefore a critical element of the climate system. Increased freshwater fluxes to the subpolar north Atlantic from the Greenland ice sheet and from the Arctic could lead to a strengthening of stratification in deep convection regions, and impact deep water formation and the overturning circulation. However, this additional freshwater first enters the boundary current on the Greenland shelf, and freshwater pathways from the shelf to deep convection regions are still unclear. In this study, we investigate the possible role of winds in driving short-lived freshwater export events from the south-east Greenland shelf to the deep convection region of the Irminger Sea.

Along the south-eastern shelf, strong and consistent north-easterly winds tend to restrain fresh surface waters over the shelf. This wind pattern changes at Cape Farewell, where strong westerly winds could lead to across-shelf export. Using a high-resolution model, we identify strong wind events and investigate their impact on freshwater export. The strongest westerly winds, westerly tip jets, are associated with the strongest and deepest freshwater export across the shelfbreak, with a mean of 40.7 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 17 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.

How to cite: Duyck, E., Gelderloos, R., and De Jong, F.: Wind-driven freshwater export at Cape Farewell, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1786, https://doi.org/10.5194/egusphere-egu22-1786, 2022.

Freshwater plays a key role in the Arctic - North Atlantic climate system, linking ice, ocean and atmospheric dynamics. In particular, large freshwater releases into the subpolar region drive extreme cold anomalies, create sharp sea surface temperature fronts, destabilise the overlying atmosphere, and trigger shifts in major ocean currents. Considering the expected increased freshwater fluxes in future due to more melt, it is critical to understand the resulting climate feedbacks.

Combining observations and models, we present evidence that past changes in Arctic freshwater outflow paced transitions between North Atlantic cold and warm anomalies. This circulation-driven freshwater cycle explained over 50% of the sea surface temperature variability in the subpolar North Atlantic and was particularly pronounced on decadal timescales. However, new findings indicate that the recent freshwater input due to more melting has increased the amplitude and frequency of freshwater variations in the North Atlantic, leading to a shift of power in the North Atlantic climate variability from decadal to interannual timescales. In addition, the interference of the circulation-driven freshwater cycle by melting has contributed to the storage of freshwater in the Arctic Ocean, where it now poses the possible risk of rapid climate change if the freshwater were released. In light of newly identified, Arctic feedbacks to melt-driven freshwater events in the North Atlantic, we suggest that an Arctic freshwater release is becoming increasingly likely.

How to cite: Oltmanns, M. and Moat, B.: Arctic pacing of North Atlantic climate variability through freshwater exports, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2821, https://doi.org/10.5194/egusphere-egu22-2821, 2022.

Where earlier generations of ocean models with resolution of 1° or coarser tended to represent wintertime dense water formation in the North Atlantic mainly as a process of open water convection in the Labrador Sea and Nordic Seas, more recent models with higher resolution, in conjunction with observational programmes such as OSNAP, have presented us with a new, more complex, picture. Watermasses are progressively ventilated and lose buoyancy as they propagate cyclonically westward around the gyre, starting with the formation of Subpolar Mode Water close to the eastern boundary, and eventually leading to Labrador Sea Water, which forms part of the lower limb of the Atlantic meridional overturning circulation (AMOC).

We present a set of hindcast integrations of a global 1/4° NEMO ocean configuration from 1958 until nearly the present day, forced with three standard surface forcing datasets. We use the surface-forced streamfunction, estimated from surface buoyancy fluxes, along with the overturning streamfunction, similarly defined in potential density space, to investigate the causal link between surface forcing and decadal variability in the strength of the AMOC. We confirm that surface heat loss from the Irminger Sea is the dominant mechanism for decadal AMOC variability, while that from the Labrador Sea has about half the amplitude. The AMOC variability is shown to be related to that of the North Atlantic Oscillation, primarily through the surface heat flux, itself dominated by the air-sea temperature difference, and we show that a metric based on the surface-forced streamfunction has predictive value for AMOC variability on interannual to decadal time scales.

How to cite: Megann, A., Blaker, A., Josey, S., New, A., and Sinha, B.: Mechanisms for Late 20th and Early 21st Century Decadal AMOC Variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4010, https://doi.org/10.5194/egusphere-egu22-4010, 2022.

Centennial-scale climate variability in the North Atlantic is characterized by the absence of a clear external forcing. Hence, identifying mechanisms of internal variability at these timescales is crucial to understand low-frequency climate variations. For this task, long control simulations with coupled climate models represent a key tool.

Although significant spectral peaks in centennial variability in the Atlantic Meridional Overturning Circulation (AMOC) were found among some state-of-the-art models, CMIP6 models disagree on the amplitude, periodicity and even existence of centennial AMOC variability. This disagreement motivates the use of models of reduced complexity with idealized setups and perturbed physics ensembles to elucidate the mechanisms of AMOC variability at long timescales.

Here, we investigate multi-millennial piControl simulations of PlaSim-LSG, an earth system model intermediate complexity (EMIC). For a range of vertical oceanic diffusion parameters, PlaSim-LSG exhibits strong oscillations of AMOC strength, as well as of salinity and surface temperatures in the North Atlantic, with a period of about 270 years.

Lag correlation analysis shows that a positive feedback involving the interplay of surface salinity, freshwater flux and sea ice concentration in the Norwegian Sea and the Arctic Ocean is the key driver behind these oscillations. In contrast to previous studies with other models, interhemispheric coupling only plays a minor role. We discuss preliminary results of sensitivity experiments for testing the proposed mechanism, and compare our results with previously proposed mechanisms of AMOC oscillations in CMIP6 models.

How to cite: Mehling, O., Angeloni, M., Bellomo, K., and von Hardenberg, J.: Mechanisms of centennial AMOC variability in a climate model of intermediate complexity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7464, https://doi.org/10.5194/egusphere-egu22-7464, 2022.

Observations between 2004 and 2020 at the RAPID array suggest a weakening trend in the Atlantic Meridional Overturning Circulation (AMOC). To assess the significance of this trend, trends that one might expect from natural variabilty in a time series of this length are assessed using CMIP6 pre-industrial simulations. The observed trend is not found to be statistically significant relative to this benchmark. Both the observed trend and the standard 
deviation of short-term model trends are found to decrease in magnitude with time. The rate of decrease, however, is faster for the observed trend, further calling into question its significance.

To clarify how variability in short-term model trends is related to power spectra of modelled AMOC strength, a conceptual model is developed. Essentially, trend variance is represented by a random walk in which there is one step for each frequency bin of the power spectrum (with step size determined by the frequency and variance of the bin in question). Most models are found underestimate interannual variability in AMOC strength; however, it is the variability at somewhat longer time scales that most influences model trends. This variability is represented quite differently between the various CMIP6 models. The conceptual model is also used to illustrate how the detectability threshold for trend detection (i.e., the 2 sigma level in a PDF of short-term model trends) is altered by the addition of noise added to make AMOC variance more in line with observations. 

How to cite: Straub, D., Kelson, R., and Dufour, C.: Using CMIP6 simulations to assess significance of an AMOC trend seen by the RAPID array, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10282, https://doi.org/10.5194/egusphere-egu22-10282, 2022.

As part of the international Overturning in the Subpolar North Atlantic Program (OSNAP), 135 acoustically-tracked deep floats were deployed from 2014 to 2016 to track the spreading pathways of Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW). These water masses, which originate in the Nordic Seas, compose the deepest branch of the Atlantic Meridional Overturning Circulation. The OSNAP floats provide the first directly observed, comprehensive Lagrangian view of ISOW and DSOW spreading pathways throughout the subpolar North Atlantic. Contrary to a decades-long expectation for how these deep water masses move equatorward, the collection of OSNAP float trajectories, complemented by model simulations, conclusively reveals that their pathways are (a) not restricted to western boundary currents, and (b) remarkably different from each other in character. The spread of DSOW from the Irminger Sea is primarily via the swift deep boundary currents of the Irminger and Labrador Seas, whereas the spread of ISOW out of the Iceland Basin is slower, more diffusive, and along multiple export pathways. The characterization of these overflow water pathways has important implications for our understanding of the Atlantic Meridional Overturning Circulation (AMOC) and its variability. Finally, reconstructions of AMOC variability from proxy data, involving either the strength of boundary currents and/or the property variability of deep waters, should account for the myriad pathways of DSOW and ISOW, but particularly so for the latter.

How to cite: Lozier, S., Bower, A., Furey, H., Drouin, K., Xu, X., and Zou, S.: Overflow Water Pathways in the North Atlantic: New Observations from the OSNAP Program, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13223, https://doi.org/10.5194/egusphere-egu22-13223, 2022.

Cold dense waters flowing south from the Nordic Seas and the Arctic Ocean form strong bottom intensified gravity currents at the Denmark Strait, Iceland-Faroe ridge, and Faroe-Scotland channel. Such overflows generate water-masses with specific hydrographic features which form the lower limb of the thermohaline circulation, responsible for a large fraction of the ocean heat transport on the Globe.

Gravity current representation in ocean models is sensitive to the choice of the vertical coordinate system. Typically, global ocean models use geopotential z-level coordinates, representing the bottom topography as a series of step-like structures. However, this choice results in excessive entrainment and mixing when simulating gravity currents, even when the partial steps parametrization is employed. Conversely, terrain-following coordinates offers a natural representation of overflows but introduce errors in the computation of the pressure gradient force, making their use in global configurations challenging.

To improve the representation of Nordic overflows in global models, Colombo (2018) proposed the use of a local-sigma vertical coordinate, where model surfaces are terrain-following only in the proximity of the Greenland-Scotland ridge, whilst standard z-level coordinates (with partial steps) are used everywhere else. However, the development of such a mesh is not trivial, especially when defining the transition zone between the two vertical coordinates.

Similarly, to improve the representation of cross-shelf exchange in regional configurations Harle et al. (2013) developed a hybrid vertical coordinate (SZT) where terrain-following computational surfaces smoothly transition to z-level with partial steps below a user defined depth.

Recently, Bruciaferri et al. (2018) introduced the Multi-Envelope (ME) s-coordinate system, where computational levels are curved and adjusted to multiple arbitrarily defined surfaces (aka envelopes) rather than following geopotential levels or the actual bathymetry. This allows the optimisation of model levels in order to best represent different physical processes within sub-domains of the model.

In order to overcome the complexities of the local-sigma method, we propose combining this approach with the flexibility of the SZT and ME methods to generate localised versions of these vertical coordinates. We test this new methodology in the region of the Nordic Sea overflows in a ¼° global NEMO configuration. At first, a series of idealised numerical experiments is conducted to assess the ability of the local-SZT and local-ME grids to minimise both horizontal pressure gradient errors and spurious entrainment of overflow waters. Finally, the skill of the new local-ME and local-SZT systems in reproducing observed properties of the Nordic overflows is assessed and compared with the traditional approach of employing geopotential coordinates with partial steps.

Bruciaferri, D., Shapiro, G.I. & Wobus, F. A multi-envelope vertical coordinate system for numerical ocean modelling. Ocean Dynamics 68, 1239–1258 (2018). https://doi.org/10.1007/s10236-018-1189-x

Harle, J.D. et al. 2013. Report on role of biophysical interactions on basin-scale C and N budgets. Deliverable 6.5, European Basin-scale Analysis, Synthesis and Integration (EURO-BASIN) Project, http://eurobasin.dtuaqua.dk/eurobasin/documents/deliverables/D6.5%20Report%20on%20role%20of%20biophysical%20interactions%20on%20C%20N%20budget.pdf

Pedro Colombo. Modélisation des écoulements d’eaux denses à travers des seuils topographiques dans les modèles réalistes de circulation océanique: une démonstration du potentiel que représente l’hybridation d’une coordonnée géopotentielle et d’une coordonnée suivant le terrain. Sciences de la Terre. Université Grenoble Alpes, 2018.

How to cite: Bruciaferri, D., Guiavarc'h, C., Hewitt, H., Harle, J., Almansi, M., and Mathiot, P.: Improving nordic overflows representation in global ocean models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4072, https://doi.org/10.5194/egusphere-egu22-4072, 2022.

About 30% of the carbon dioxide derived from human activities (C_ANTH) has been absorbed by the ocean (DeVries, 2014; Gruber et al., 2019; Friedlingstein et al., 2021), with the North Atlantic (NA) being one of the largest C_ANTH sinks per unit area (Khatiwala et al., 2013; Sabine et al., 2004). In the NA, oceanic C_ANTH uptake strongly relies on the meridional overturning circulation and the associated regional winter deep convection. In fact, the formation and deep spreading of Labrador Sea Water stands as a critical C_ANTH gateway to intermediate and abyssal depths. The NA C_ANTH uptake has fluctuated over the years according to changes in the North Atlantic Oscillation. Biennial observation of the marine carbonate system along the Go-Ship A25-OVIDE section has allowed us assessing the decadal and interannual variability of the C_ANTH storage in the subpolar NA from 2002 to 2021. In this study, we investigate 1) the trend of C_ANTH and 2) the relationship between the C_ANTH saturation, the apparent oxygen utilization, and the ventilation of the water masses between the A25-OVIDE section and the Greenland-Iceland-Scotland sills during 2002-2021. We divided the A25-OVIDE section into three main basins (Irminger, Iceland, and Eastern NA). Our results show that the Irminger Basin presents a more homogenous C_ANTH profile and higher C_ANTH saturation values at depth than the other two basins, which is related to the pronounced convective activity in the Irminger Basin. In contrast, the Eastern NA Basin has higher C_ANTH values at the surface due to its higher surface temperature, but its deep water masses show the lowest C_ANTH values since they are the less ventilated in the section. Our analysis also reveals that, overall, the NA C_ANTH storage has increased during 2002-2021, but varied according to the ventilation changes. While the Eastern NA water masses experienced a relatively constant, although shallower, average ventilation, the Irminger and Iceland Basins underwent a less steady C_ANTH uptake pattern characterized by alternating periods of strong and weak C_ANTH storage.

How to cite: López Mozos, M., Velo, A., Fontela, M., de la Paz, M., Carracedo, L., Fajar, N., García-Ibáñez, M. I., Padín, X. A., Desbruyères, D., Mercier, H., Lherminier, P., and Pérez, F. F.: North Atlantic CO2 sink variability revealed by the Go-Ship A25-OVIDE section, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11978, https://doi.org/10.5194/egusphere-egu22-11978, 2022.

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 in the Labrador Sea (OSNAP West) suggests that the delay between wintertime transformation and the observed overturning peak in late spring is consistent with the advection and export of dense Labrador Sea Water along the western boundary. Further analysis is required to understand the mechanism driving seasonal overturning across OSNAP East.

How to cite: Fu, Y. and Lozier, M. S. and the OSNAP Team: Seasonal cycle of the overturning circulation in the subpolar North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13044, https://doi.org/10.5194/egusphere-egu22-13044, 2022.

Marine Heatwaves (MHWs) are ocean extreme events, characterized by anomalously high temperatures, which can have drastic ecological impacts. The Northeast U.S. continental shelf is of great economical importance being home to a highly productive ecosystem. Local warming rates exceed the global average and the region experienced multiple MHWs in the last decade with severe consequences for regional fisheries. Due to the lack of subsurface observations, the depth-extent of MHWs is not well known, which however hampers assessing impacts on pelagic and benthic ecosystems. This study utilizes a global ocean circulation model with a high-resolution (1/20°) nest in the Atlantic to investigate the depth structure of MHWs and associated drivers on the Northeast U.S. continental shelf. It is shown that MHWs exhibit varying spatial extents, with some only appearing at depth. Highest intensities are found around 100m depth with temperatures exceeding the climatological mean by up to 7°C, while surface intensities are typically smaller around 3°C. Distinct vertical structures are associated with different spatial patterns and drivers. Investigation of the co-variability of temperature and salinity revealed that over 80% of MHWs at depth (>50m) coincide with extreme salinity anomalies. Two case studies provide insight into opposing MHW patterns at the surface and at depth, being forced by anomalous air-sea heat fluxes and Gulf Stream warm core ring interaction, respectively, the latter hinting at the importance of local ocean dynamics. The results highlight the relevance of subsurface/deep MHWs, underlining the need of continuous subsurface measurements. Working towards a more quantitative assessment of WCRs, their interaction with the shelf break and impact on the shelf's hydrography, an eddy-tracking algorithm will be applied on the model output. This will also allow to further investigate the model's skill in representing mesoscale features in the Gulf Stream region.

How to cite: Grosselindemann, H., Ryan, S., Ummenhofer, C., Martin, T., and Biastoch, A.: Marine Heatwaves and their Depth Structures on the Northeast US Continental Shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1015, https://doi.org/10.5194/egusphere-egu22-1015, 2022.

The Northwest Atlantic (NWA) plays a critical role in the global ocean circulation and regulates the global climate system through meridional transport in western boundary currents as well as through deep convection. Global climate change is projected to significantly impact ocean circulation, vertical mixing, and sea ice dynamics in the NWA, with important implications for the area’s biological productivity and carbon export. These physical and biological features and their variabilities are challenging to numerical ocean models and often poorly represented in global climate models. This creates a difficulty in projecting future changes in nutrient dynamics, production, and carbon export. To address these challenges we have developed an advanced coupled circulation-sea ice-biogeochemistry modelling system for the NWA. This modelling system is based on the Regional Ocean Modeling System (ROMS), the Community Sea Ice Model (CICE), and a biogeochemical model including oxygen dynamics and carbon chemistry. The model domain spans the area from Cape Hatteras to Baffin Bay and from the east coast of North America to the central North Atlantic, with the horizontal grid resolution ranging from ~8 km in the south to ~2 km in the north. The circulation and sea ice models are forced by atmospheric and oceanic reanalysis data at the surface and lateral boundaries, respectively. The circulation model is additionally forced by tides, river discharge, and continental runoff. Preliminary model results are presented and compared to various types of observations, with a focus over coastal waters and the deep convection region of the Labrador Sea.

How to cite: Ohashi, K., Laurent, A., Renkl, C., Dilmahamod, F., Yang, S., Fennel, K., Oliver, E., and Sheng, J.: An advanced coupled modelling system to study interactions among the circulation, sea ice, and biogeochemistry in the Northwest Atlantic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10294, https://doi.org/10.5194/egusphere-egu22-10294, 2022.

Hydrological and hydrochemical characteristics of the Discovery Gap bottom water (deeper than 4000 m) were obtained during the 59^th cruise of the R/V Akademik Ioffe (October 2021). Discovery Gap is the narrow gap of 150 km long, 10–50 km wide, oriented from southwest to northeast in Azores–Gibraltar Fracture Zone (Northeast Atlantic). Water with a potential temperature of less than 2 °C (modified Antarctic Bottom Water (AABW)) and high silicon concentrations was detected in the Discovery Gap. The terminal point of propagation of modified AABW in the exit sill of the Gap (depth more than 4700 m). There was cyclonic circulation in the Discovery Gap Narrows (the narrowest point of the Discovery Gap, 10 km wide, located in the northeastern part of it): in the northeast direction of more than 14 cm/s speed and with the high phosphate concentrations (1.46-1.54 μmol/l), in the southwest direction of 6-8 cm/s speed and with a low phosphate content (1.39-1.40 μmol/l). The localization of the extrema of phosphorus concentrations correlates with the maximum flow velocities, which may be associated with advective processes.

Acknowledgments:

The expedition and the hydrochemical processing of the data received during the 59^th cruise of the R/V Akademik Ioffe was carried out with a support of the state assignment of the IO RAS (No. 0128-2021-0012), the hydrophysical measurements were supported by the Russian Science Foundation (project no. 21-77-20004).

We thank the crew of the R/V Akademik Ioffe for assistance, B.V. Chubarenko for valuable comments and E.I. Gmyrya for preparing the map.

How to cite: Dvoeglazova, N., Kapustina, M., Krechik, V., and Bocherikova, I.: Hydrological and hydrochemical haracteristics of the modified AABW in the Discovery Gap (Northeast Atlantic) in 2021., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-478, https://doi.org/10.5194/egusphere-egu22-478, 2022.