The southern entrance to the Rockall Trough is subject to a complex set of dynamic processes, influenced by Atlantic gyre interactions, the North Atlantic Current, slope boundary currents, variable wind stress forcing, mesoscale activity, and a changing supply of modified water masses formed elsewhere in the Atlantic. These processes drive large temporal and spatial variations, and mixing of surface and intermediate water mass properties that advect through the Trough and drive variations in the deeper waters circulating around it. Here, we investigate variability across the southern and central Rockall Trough from standard hydrographic sections (2006–2022) and deepwater moored subsurface measurements, to better understand changes in water column characteristics and water mass modification during advection through the Rockall Trough and track the aftermath of recent freshening events. Rapid and longer-term physical changes are assessed along with spatial variability and watermass interaction. Interannual variability is large across intermediate depths, deeper circulations are regenerated and a salinity core associated with the eastern boundary current is detailed. Establishing, maintaining, monitoring and analysis of observational ocean time-series datasets are a fundamental requirement for managing and conserving crucial biological resources and are key to understanding oceanic and earth system change.

<p>It has been suggested previously that the long-term warming hole in the subpolar North Atlantic, that is the relative cooling in this region compared to the rest of the globe, is an indicator of a slowdown of the Atlantic Meridional Overturning Circulation (Caesar et al., 2018; Drijfhout et al., 2012; Rahmstorf et al., 2015), yet other drivers like aerosols or a change in the local atmospheric forcing (e.g., the wind stress curl) have been proposed (Li et al., 2021; Piecuch et al., 2017). The still not fully answered question of the driver(s) of the warming hole also raises the question of whether or not ocean temperatures in the subpolar North Atlantic can be used as an indicator for AMOC strength. While several studies suggest that AMOC strength and temperatures in the subpolar North Atlantic are dynamically linked through the AMOC’s northward heart transport (Dima et al., 2022; Latif et al., 2022; Zhang, 2008), a recent model-based study suggests that the correlation between temperature-based AMOC index (Caesar et al., 2018) and AMOC strength depends largely on the subtraction of the global warming signal (Little et al., 2020).</p> <p>Based on the knowledge that the AMOC transports both heat into and freshwater out of the North Atlantic, we apply a lead-lag correlation analysis to both the North Atlantic’s heat and freshwater content to identify the region and the time lag that give the strongest correlation with the strength of the AMOC (to make use of the available observational data we consider the AMOC strength at 26˚N). We find that an AMOC weakening (strengthening) leads to cooling (warming) and simultaneous freshening (salinification) in the eastern subpolar North Atlantic with the upper ocean (200-1000m) contents showing a higher correlation with AMOC strength than the surface (0-200m) contents. The temporal evolution of heat and freshwater content in the eastern subpolar gyre region are furthermore strongly anticorrelated, with a correlation value of -0.82 (for the annual values) as expected for an AMOC (or otherwise advective) driven signal. On longer time scales this anticorrelation decreases unless the heat content is corrected for a large scale warming signal. This could suggest that it is indeed necessary to look at the relative not the absolute temperature evolution in the subpolar North Atlantic to extract the AMOC signal.</p> <p>Both the absolute freshening in the eastern subpolar North Atlantic as well as the relative (compared to the rest of the North Atlantic) cooling in this region suggest a linear AMOC trend of about -2 Sv from 1957-2013.</p> <p>References</p> <p>Caesar, L., et al. (2018).  https://doi.org/10.1038/s41586-018-0006-5</p> <p>Dima, M., et al. (2022).  https://doi.org/10.1007/s00382-022-06156-w</p> <p>Drijfhout, S., et al.  (2012). https://doi.org/10.1175/jcli-d-12-00490.1</p> <p>Latif, M., et al.  (2022). https://doi.org/10.1038/s41558-022-01342-4</p> <p>Li, L., et al.  (2021). https://doi.org/10.1007/s00382-021-06003-4</p> <p>Little, et al.  (2020).  https://doi.org/10.1029/2020gl090888</p> <p>Piecuch, C. G., et al.  (2017).  https://doi.org/https://doi.org/10.1002/2017JC012845</p> <p>Rahmstorf, S., et al.  (2015). https://doi.org/10.1038/nclimate2554</p> <p>Zhang, R. (2008). <span>https://doi.org/10.1029/2008GL035463</span></p>

The Atlantic Meridional Overturning Circulation (AMOC), a crucial element of the Earth's climate system, is projected to weaken over the course of the twenty-first century which could have far reaching consequences for the occurrence of extreme weather events, regional sea level rise, monsoon regions and the marine ecosystem. The latest IPCC report puts the likelihood of such a weakening as 'very likely'. As our confidence in future climate projections depends largely on the ability to model the past climate, we take an in-depth look at the difference in the twentieth century evolution of the AMOC based on observational data (including direct observations and various proxy data) and model data from climate model ensembles. We show that both the magnitude of the trend in the AMOC over different time periods and often even the sign of the trend differs between observations and climate model ensemble mean, with the magnitude of the trend difference becoming even greater when looking at the CMIP6 ensemble compared to CMIP5. We discuss possible reasons for this observation-model discrepancy and question what it means to have higher confidence in future projections than historical reproductions. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

Over the past years the North Atlantic has been the main scene of three interesting phenomena: a long-term warming hole (i.e. Drijfhout et al., 2012), a reoccurring cold blob (i.e. Duchez et al., 2016) and an unusual freshening in recent years (Holliday et al., 2020).All three have been linked to either changes in ocean circulation causing i.e., anomalous heat transports, atmospheric circulation changes that, i.e., lead to enhanced surface heat loss or changes in precipitation patterns, – or a combination of the two. While it appears that the main drivers of these phenomena have been identified, the relative importance of them as well as the connections between the three are still unclear.To assess this, we study the correlation of the main atmospheric and oceanic drivers in the North Atlantic region and the upper ocean heat (OHC) and freshwater content (FWC). By looking at OHC and FWC we remove some of the noise visible in the sea surface data, and it further enables us to remove the direct influence of the atmosphere by subtracting the heat and freshwater air-sea fluxes from the data.The results indicate that long-term changes in the western subpolar North Atlantic are caused by the direct effects of changes in the atmosphere, while the eastern subpolar North Atlantic is more strongly influenced by changes in the ocean circulation causing a simultaneous cooling/freshening or warming/salinification, respectively. This has e.g., implications for the definition of temperature or salinity based AMOC indices (as used in e.g., Boers, 2021; Caesar et al., 2018) that often average quantities over the whole or even just the western subpolar North Atlantic. These should be redefined focusing on the eastern part.ReferencesBoers, N. (2021). Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nature Climate Change, 11(8), 680-688. https://doi.org/10.1038/s41558-021-01097-4Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700), 191-196. https://doi.org/10.1038/s41586-018-0006-5Drijfhout, S., van Oldenborgh, G. J., & Cimatoribus, A. (2012). Is a Decline of AMOC Causing the Warming Hole above the North Atlantic in Observed and Modeled Warming Patterns? Journal of Climate, 25(24), 8373-8379. https://doi.org/10.1175/jcli-d-12-00490.1Duchez, A., Frajka-Williams, E., Josey, S. A., Evans, D. G., Grist, J. P., Marsh, R., . . . Hirschi, J. J. M. (2016). Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave. Environmental Research Letters, 11(7), 074004. https://doi.org/10.1088/1748-9326/11/7/074004Holliday, N. P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-López, C., . . . Yashayaev, I. (2020). Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Communications, 11(1), 585. https://doi.org/10.1038/s41467-020-14474-y

1Irish Climate Analysis and Research Units (ICARUS), Department of Geography, Maynooth University, Maynooth, Ireland. 2Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, Potsdam, Germany. 3Department of Geography, University College London, London, UK. 4Department of Mathematics and Statistics, Maynooth University, Kildare, Ireland. 5Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany. ✉e-mail: levke.caesar@mu.ie We thank the authors for their comment and welcome the opportunity to respond. Kilbourne et al.1 make the argument that if more proxies from the North Atlantic had been considered, the conclusion reached in Caesar et al.2 would have been different. Although it is certainly true that a comprehensive set of palaeoceanographic proxy data available in the North Atlantic region shows a complex picture of the evolution of the North Atlantic over the past two millennia3, most of these proxies are not strongly linked to the strength of the Atlantic Meridional Overturning Circulation (AMOC). The main objective of Caesar et al.2 was to compare a range of previously published reconstructions of features that have been linked to the AMOC, which, due to the different proxies used, represent different subsystems and/or processes related to the AMOC. Our goal was to check the consistency between these reconstructions and to assess the statistical significance of the main changes found. Kilbourne et al.1 argue that the AMOC proxies shown in Caesar et al.2 have substantial caveats that were not discussed and that not all the available proxy records potentially related to AMOC variability were considered. Indeed, the fact that each individual AMOC proxy has influencing factors other than the AMOC (as is the case with all proxy data) was the main motivation for this compilation of multiple proxies. The advantage is that, as the proxies come from different locations and with different sources of uncertainties, they have different AMOC-unrelated influencing factors and any common behaviour is therefore probably AMOC related. Kilbourne et al. list a number of possible influences on each proxy in addition to the AMOC1, which include the general global warming signal, changes in atmospheric conditions, the global nitrogen cycle and shifts in the location of the AMOC’s deep currents (Fig. 1). Like Kilbourne et al.1, we see those factors as a source of uncertainty for the individual proxy, but we argue that the diverse nature of these additional influencing factors is evidence that the common evolution found in the different proxies is not driven by these factors (as it is unlikely that they all changed contemporaneously), but by the driver that is common to all proxies: the AMOC. A more detailed look at the individual proxies supports our conclusion that the combination of these proxies is, indeed, suitable in reconstructing the long-term evolution of the AMOC. For example, the temperature AMOC index by Rahmstorf et al.4, which is based on land data, shows the same signal as the marine proxy by Thornalley et al.5, which is based on the analysis of planktic foraminifera. The advantage of foraminiferal-based subpolar temperature proxies is that they typically record subsurface temperatures, Reply to: Atlantic circulation change still uncertain

The recent IPCC AR6 report highlighted that, in contrast to ocean variables such as sea level and ocean heat content, where predicted and simulated rises due to anthropogenic climate change are being borne out by observations, the Atlantic Meridional Overturning Circulation (AMOC) has not conclusively shown a predicted decline and that, in fact, contradictions remain between observations and simulations through the 20th century. The AMOC is at its weakest in 1000 years based on a compilation of paleo and instrumental proxies (Caesar et al. 2021). However, a reconstruction based on in-situ hydrographic profiles and informed by AMOC variability derived from the RAPID array shows no decline in the past 30 years (Worthington et al. 2021). Here, we show that there is no contradiction between these two results: when taken with the appropriate lag, the in-situ reconstruction matches with sea surface temperature (SST) reconstructions and the pattern of paleo proxies. Convergence is evident in observations and reconstructions of the AMOC since the 1990s but what of prior to this? Instrumental reconstructions based on SSTs show a decline in the AMOC in the mid-20th century. The impact of the AMOC on SSTs is significant, especially on long timescales, but is not the only factor impacting SSTs. Alternative explanations for the mid 20th century cooling of Atlantic SSTs are that the cooling is linked with sulphate aerosol emission (Menary et al. 2020). This surface cooling may have led to a strengthening AMOC—the converse relationship to SST-based AMOC proxies. We conclude by considering the challenges of instrumental-based reconstructions of the AMOC and potential avenues for reconciliation of outstanding contradictions to settle a baseline from which to observe the future AMOC slowdown that is near-universally predicted by climate models. Caesar, L., G. D. McCarthy, D. J. R. R. Thornalley, N. Cahill, and S. Rahmstorf, 2021: Current Atlantic Meridional Overturning Circulation weakest in last millennium. Nat. Geosci., 14, 118–120, doi:10.1038/s41561-021-00699-z. https://doi.org/10.1038/s41561-021-00699-z (Accessed May 14, 2021). Menary, M. B., and Coauthors, 2020: Aerosol-Forced AMOC Changes in CMIP6 Historical Simulations. Geophys. Res. Lett., 47, e2020GL088166, doi:10.1029/2020GL088166. https://doi. (Accessed May 14, 2021). Worthington, E. L., B. I. Moat, D. A. Smeed, J. V. Mecking, R. Marsh, and G. D. McCarthy, 2021: A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline. Ocean Sci., 17, 285–299, doi:10.5194/os-17-285-2021.

While there is increasing paleoclimatic evidence that the Atlantic Meridional Overturning Circulation (AMOC) has weakened over the last one to two hundred years (Caesar et al., 2018; Thornalley et al., 2018), this is not confirmed by climate model simulations. Instead, the new simulations from the 6th Coupled Model Intercomparison Project (CMIP6) show a slight strengthening of the multimodel mean AMOC from 1850 until about 1985 (Menary et al., 2020), attributed to anthropogenic aerosol forcing. Arguing for a recent weakening of the AMOC, some studies attribute the emergence of the North Atlantic warming hole as a sign of the reduced meridional heat transport associated with a weaker AMOC (e.g. Caesar et al., 2018), yet this cold anomaly has also been interpreted as being aerosol-forced (Booth et al., 2012) and therefore not necessarily a sign of a weakening AMOC but rather a possible driver of a strengthening of the AMOC.Looking beyond temperature, a fresh anomaly has recently emerged in the subpolar North Atlantic (Holliday et al., 2020). While a strengthening AMOC has been linked with an increase in salinity in the subpolar gyre region (Menary et al., 2013), an AMOC weakening would, due to the salt-advection feedback, likely lead to a reduction in salinity in the North Atlantic region. To shed some light on the question of whether the cold anomaly is internally (AMOC) or externally (aerosol-forced) driven we consider the co-variability of salinity and temperature in the North Atlantic in respect of changes in surface fluxes or alternate drivers.ReferencesBooth, B.B.B., Dunstone, N.J., Halloran, P.R., Andrews, T. and Bellouin, N., 2012. Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature, 484(7393): 228–232.Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G. and Saba, V., 2018. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700): 191-196.Holliday, N.P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-López, C., Hátún, H., Johns, W., Josey, S.A., Larsen, K.M.H., Mulet, S., Oltmanns, M., Reverdin, G., Rossby, T., Thierry, V., Valdimarsson, H. and Yashayaev, I., 2020. Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Communications, 11(1): 585.Menary, M.B., Roberts, C.D., Palmer, M.D., Halloran, P.R., Jackson, L., Wood, R.A., Müller, W.A., Matei, D. and Lee, S.-K., 2013. Mechanisms of aerosol-forced AMOC variability in a state of the art climate model. Journal of Geophysical Research: Oceans, 118(4): 2087-2096.Menary, M.B., Robson, J., Allan, R.P., Booth, B.B.B., Cassou, C., Gastineau, G., Gregory, J., Hodson, D., Jones, C., Mignot, J., Ringer, M., Sutton, R., Wilcox, L. and Zhang, R., 2020. Aerosol-Forced AMOC Changes in CMIP6 Historical Simulations. Geophysical Research Letters, 47(14): e2020GL088166.Thornalley, D.J.R., Oppo, D.W., Ortega, P., Robson, J.I., Brierley, C.M., Davis, R., Hall, I.R., Moffa-Sanchez, P., Rose, N.L., Spooner, P.T., Yashayaev, I. and Keigwin, L.D., 2018. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature, 556(7700): 227-230.

North Atlantic climate variability is dominated by two important subsystems, the Atlantic Meridional Overturning Circulation (AMOC) and the Sub-Polar Gyre (SPG). While the AMOC is responsible for the transport of mass and heat into higher latitudes, SPG has been linked with large-scale changes in the subpolar marine environment. The changes in strength, intensity and positions of the constituent currents of the SPG impose variabilities in the distribution of heat and salt in the North Atlantic Ocean. Consequently, the predictability on decadal scales of the two subsystems is of huge importance for the understanding of variability in the North Atlantic. Our contribution investigates the decadal and multi-decadal predictability of these subsystems within the Max Planck Institute for Meteorology Earth System Model (MPI-ESM). We analyse the model’s capability to predict these subsystems as well as the dependence of the two subsystems on each other. These investigations open new opportunities for a better understanding of the impact of the North Atlantic onto important marine ecosystems and its changes in the upcoming decade.

The Atlantic Meridional Overturning Circulation (AMOC)—one of Earth’s major ocean circulation systems—redistributes heat on our planet and has a major impact on climate. Here, we compare a variety of published proxy records to reconstruct the evolution of the AMOC since about ad 400. A fairly consistent picture of the AMOC emerges: after a long and relatively stable period, there was an initial weakening starting in the nineteenth century, followed by a second, more rapid, decline in the mid-twentieth century, leading to the weakest state of the AMOC occurring in recent decades.

Global ocean physical and chemical trends are reviewed and updated using seven key ocean climate change indicators: (i) Sea Surface Temperature, (ii) Ocean Heat Content, (iii) Ocean pH, (iv) Dissolved Oxygen concentration (v) Arctic Sea Ice extent, thickness, and volume (vi) Sea Level and (vii) the strength of the Atlantic Meridional Overturning Circulation (AMOC). The globally averaged ocean surface temperature shows a mean warming trend of 0.062 ± 0.013°C per decade over the last 120 years (1900–2019). During the last decade (2010–2019) the rate of ocean surface warming has accelerated to 0.280 ± 0.068°C per decade, 4.5 times higher than the long term mean. Ocean Heat Content in the upper 2,000 m shows a linear warming rate of 0.35 ± 0.08 Wm –2 in the period 1955–2019 (65 years). The warming rate during the last decade (2010–2019) is twice (0.70 ± 0.07 Wm –2 ) the warming rate of the long term record. Each of the last six decades have been warmer than the previous one. Global surface ocean pH has declined on average by approximately 0.1 pH units (from 8.2 to 8.1) since the industrial revolution (1770). By the end of this century (2100) ocean pH is projected to decline additionally by 0.1–0.4 pH units depending on the RCP (Representative Concentration Pathway) and SSP (Shared Socioeconomic Pathways) future scenario. The time of emergence of the pH climate change signal varies from 8 to 15 years for open ocean sites, and 16–41 years for coastal sites. Global dissolved oxygen levels have decreased by 4.8 petamoles or 2% in the last 5 decades, with profound impacts on local and basin scale habitats. Regional trends are varying due to multiple processes impacting dissolved oxygen: solubility change, respiration changes, ocean circulation changes and multidecadal variability. Arctic sea ice extent has been declining by −13.1% per decade in summer (September) and by −2.6% per decade in winter (March) during the last 4 decades (1979–2020). The combined trends of sea ice extent and sea ice thickness indicate that the volume of non-seasonal Arctic Sea Ice has decreased by 75% since 1979. Global mean sea level has increased in the period 1993–2019 (the altimetry era) at a mean rate of 3.15 ± 0.3 mm year –1 and is experiencing an acceleration of ∼ 0.084 (0.06–0.10) mm year –2 . During the last century (1900–2015; 115y) global mean sea level (GMSL) has rised 19 cm, and near 40% of that GMSL rise has taken place since 1993 (22y). Independent proxies of the evolution of the Atlantic Meridional Overturning Circulation (AMOC) indicate that AMOC is at its weakest for several hundreds of years and has been slowing down during the last century. A final visual summary of key ocean climate change indicators during the recent decades is provided.

Accepted Manuscript is “the version of the article accepted for publication including all changes made as a result of the peer review process, and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an ‘Accepted Manuscript’ watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors” This Accepted Manuscript is © 2020 The Author(s). Published by IOP Publishing Ltd.

According to established understanding, deep-water formation in the North Atlantic and Southern Ocean keeps the deep ocean cold, counter-acting the downward mixing of heat from the warmer surface waters in the bulk of the world ocean. Therefore, periods of strong Atlantic meridional overturning circulation (AMOC) are expected to coincide with cooling of the deep ocean and warming of the surface waters. It has recently been proposed that this relation may have reversed due to global warming, and that during the past decades a strong AMOC coincides with warming of the deep ocean and relative cooling of the surface, by transporting increasingly warmer waters downward. Here we present multiple lines of evidence, including a statistical evaluation of the observed global mean temperature, ocean heat content, and different AMOC proxies, that lead to the opposite conclusion: even during the current ongoing global temperature rise a strong AMOC warms the surface. The observed weakening of the AMOC has therefore delayed global surface warming rather than enhancing it. Social Media Abstract : The overturning circulation in the Atlantic Ocean has weakened in response to global warming, as predicted by climate models. Since it plays an important role in transporting heat, nutrients and carbon, a slowdown will affect global climate processes and the global mean temperature. Scientists have questioned whether this slowdown has worked to cool or warm global surface temperatures. This study analyses the overturning strength and global mean temperature evolution of the past decades and shows that a slowdown acts to reduce the global mean temperature. This is because a slower overturning means less water sinks into the deep ocean in the subpolar North Atlantic. As the surface waters are cold there, the sinking normally cools the deep ocean and thereby indirectly warms the surface, thus less sinking implies less surface warming and has a cooling effect. For the foreseeable future, this means that the slowing of the overturning will likely continue to slightly reduce the effect of the general warming due to increasing greenhouse gas concentrations.

The North Atlantic has a major influence on the climate of Europe. In the past, decadal prediction systems have shown consistent prediction skill in the North Atlantic for initialised models, indicating the potential to exploit this skill for better predictions on the continent. One prime area of potential for this approach is Ireland, due to its proximity to the Atlantic.Until now, the prediction skill for the island of Ireland is limited, leading to the conclusion that dynamical models alone are not able to transfer the prediction skill from the North Atlantic to the surrounding land masses. Therefore, the project Aigéin, Aeráid, agus athrú Atlantaigh (A4) aims to establish new physical and statistical approaches to enhance the skill. This includes a better understanding on the oceanographic processes leading to the prediction skill in the North Atlantic as well as usage of statistical-dynamical predictions.This contribution will give an overview of the approaches and a first look on the factors we anticipate to use in our analysis. Special attention will be given to the statistical approaches for the statistical dynamical prediction as well new verification procedures to evaluate them.

The Atlantic Meridional Overturning Circulation (AMOC) is a major mechanism for northward heat transport on our planet and the prime reason why the Northern Hemisphere is warmer than the Southern Hemisphere (Feulner et al. 2013). The AMOC is a sensitive non-linear system dependent on subtle thermohaline density differences in ocean water, and major AMOC transitions have been implicated e.g. in millennial climate events during the last glacial (Rahmstorf 2002). There is evidence that the AMOC is slowing down in response to modern global warming, as predicted by climate models (Caesar et al. 2018). We will review and compile proxy evidence for AMOC changes during the past 1-2 millennia, including e.g. Sherwood et al. 2011, Thibodeau et al. 2018, Thornalley et al. 2018, Rahmstorf et al. 2015, Zanna et al. 2019. We conclude that there now is substantial and consistent evidence from multiple independent sources for a modern AMOC slowdown that is unprecedented in at least a millennium. References Caesar, L., S. Rahmstorf, A. Robinson, G. Feulner, and V. Saba. 2018. Nature, 556: 191-96. Feulner, G, S Rahmstorf, A Levermann, and S Volkwardt. 2013. Journal of Climate, 26: 7136-50. Rahmstorf, S. 2002. Nature, 419: 207-14. Rahmstorf, S., Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford, and Erik J. Schaffernicht. 2015. Nature Climate Change, 5: 475-80. Sherwood, O. A., M. F. Lehmann, C. J. Schubert, D. B. Scott, and M. D. McCarthy. 2011. Proc Natl Acad Sci U S A, 108: 1011-5. Thibodeau, Benoit, Christelle Not, Jiang Hu, Andreas Schmittner, David Noone, Clay Tabor, Jiaxu Zhang, and Zhengyu Liu. 2018. Geophysical Research Letters, 45: 12,376-12,85. Thornalley, D. J. R., D. W. Oppo, P. Ortega, J. I. Robson, C. M. Brierley, R. Davis, I. R. Hall, P. Moffa-Sanchez, N. L. Rose, P. T. Spooner, I. Yashayaev, and L. D. Keigwin. 2018. Nature, 556: 227-30. Zanna, L., S. Khatiwala, J. M. Gregory, J. Ison, and P. Heimbach. 2019. Proc Natl Acad Sci U S A, 116: 1126-31.

A better understanding of precipitation dynamics in the Indian subcontinent is required since India's society depends heavily on reliable monsoon forecasts. We introduce a non-linear, multiscale approach, based on wavelets and event synchronization, for unravelling teleconnection influences on precipitation. We consider those climate patterns with the highest relevance for Indian precipitation. Our results suggest significant influences which are not well captured by only the wavelet coherence analysis, the state-of-the-art method in understanding linkages at multiple timescales. We find substantial variation across India and across timescales. In particular, El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) mainly influence precipitation in the south-east at interannual and decadal scales, respectively, whereas the North Atlantic Oscillation (NAO) has a strong connection to precipitation, particularly in the northern regions. The effect of the Pacific Decadal Oscillation (PDO) stretches across the whole country, whereas the Atlantic Multidecadal Oscillation (AMO) influences precipitation particularly in the central arid and semi-arid regions. The proposed method provides a powerful approach for capturing the dynamics of precipitation and, hence, helps improve precipitation forecasting.

Sea surface temperature (SST) patterns can – as surface climate forcing – affect weather and climate at large distances. One example is El Niño-Southern Oscillation (ENSO) that causes climate anomalies around the globe via teleconnections. Although several studies identified and characterized these teleconnections, our understanding of climate processes remains incomplete, since interactions and feedbacks are typically exhibited at unique or multiple temporal and spatial scales. This study characterizes the interactions between the cells of a global SST data set at different temporal and spatial scales using climate networks. These networks are constructed using wavelet multi-scale correlation that investigate the correlation between the SST time series at a range of scales allowing instantaneously deeper insights into the correlation patterns compared to traditional methods like empirical orthogonal functions or classical correlation analysis. This allows us to identify and visualise regions of – at a certain timescale – similarly evolving SSTs and distinguish them from those with long-range teleconnections to other ocean regions. Our findings re-confirm accepted knowledge about known highly linked SST patterns like ENSO and the Pacific Decadal Oscillation, but also suggest new insights into the characteristics and origins of long-range teleconnections like the connection between ENSO and Indian Ocean Dipole.

Atlantic Meridional Overturning Circulation (AMOC) projections are uncertain due to both model errors, as well as internal climate variability. An AMOC slowdown projected by many climate models is likely to have considerable effects on many aspects of global and North Atlantic climate. Previous studies to make probabilistic AMOC projections have broken new ground. However, they do not drift-correct or cross-validate the projections, and do not fully account for internal variability. Furthermore, they consider a limited subset of models, and ignore the skill of models at representing the temporal North Atlantic dynamics. We improve on previous work by applying Bayesian Model Averaging to weight 13 Coupled Model Intercomparison Project phase 5 models by their skill at modeling the AMOC strength, and its temporal dynamics, as approximated by the northern North-Atlantic temperature-based AMOC Index. We make drift-corrected projections accounting for structural model errors, and for the internal variability. Cross-validation experiments give approximately correct empirical coverage probabilities, which validates our method. Our results present more evidence that AMOC likely already started slowing down. While weighting considerably moderates and sharpens our projections, our results are at low end of previously published estimates. We project mean AMOC changes between periods 1960–1999 and 2060–2099 of −4.0 Sv and −6.8 Sv for RCP4.5 and RCP8.5 emissions scenarios respectively. The corresponding average 90% credible intervals for our weighted experiments are [−7.2, −1.2] and [−10.5, −3.7] Sv respectively for the two scenarios.

The Atlantic meridional overturning circulation (AMOC)—a system of ocean currents in the North Atlantic—has a major impact on climate, yet its evolution during the industrial era is poorly known owing to a lack of direct current measurements. Here we provide evidence for a weakening of the AMOC by about 3 ± 1 sverdrups (around 15 per cent) since the mid-twentieth century. This weakening is revealed by a characteristic spatial and seasonal sea-surface temperature ‘fingerprint’—consisting of a pattern of cooling in the subpolar Atlantic Ocean and warming in the Gulf Stream region—and is calibrated through an ensemble of model simulations from the CMIP5 project. We find this fingerprint both in a high-resolution climate model in response to increasing atmospheric carbon dioxide concentrations, and in the temperature trends observed since the late nineteenth century. The pattern can be explained by a slowdown in the AMOC and reduced northward heat transport, as well as an associated northward shift of the Gulf Stream. Comparisons with recent direct measurements from the RAPID project and several other studies provide a consistent depiction of record-low AMOC values in recent years. A characteristic ‘fingerprint’ of sea-surface temperatures suggests that the Atlantic overturning circulation has slowed substantially since the mid-twentieth century, as predicted by climate models in response to increasing carbon dioxide emissions.