A new concept for the paleoceanographic evolution of Heinrich event 1 in the North Atlantic

New records of planktonic foraminiferal δ¹⁸O and lithic and foraminiferal counts from Eirik Drift are combined with published data from the Nordic Seas and the “Ice Rafted Debris (IRD) belt”, to portray a sequence of events through Heinrich event 1 (H1). These events progressed from an onset of meltwater release at ～19 ka BP, through the ‘conventional’ H1 IRD deposition phase in the IRD belt starting from ～17.5 ka BP, to a final phase between 16.5 and ～15 ka BP that was characterised by a pooling of freshwater in the Nordic Seas, which we suggest was hyperpycnally injected into that basin. After ～15 ka BP, this freshwater was purged from the Nordic Seas into the North Atlantic, which preconditioned the Nordic Seas for convective deep-water formation. This allowed an abrupt re-start of North Atlantic Deep Water (NADW) formation in the Nordic Seas at the Bølling warming (14.6 ka BP). In contrast to previous estimates for the duration of H1 (i.e., 1000 years to only a century or two), the total, combined composite H1 signal presented here had a duration of over 4000 yrs (～19–14.6 ka BP), which spanned the entire period of NADW collapse. It appears that deep-water formation and climate are not simply controlled by the magnitude or rate of meltwater addition. Instead the location of meltwater injections may be more important, with NADW formation being particularly sensitive to surface freshening in the Arctic/Nordic Seas.     

The GIN Sea—A synthesis of its physical oceanography and literature review 1972–1985

A synthesis and review of the physical oceanography of the GIN Sea (Greenland, Iceland and Norwegian Seas) is presented. An accompanying bibliography is included from 1972 through 1985. Some 1986 works are also noted. Emphasis is placed on describing the GIN Sea as a major semi-enclosed basin that plays a role unique among the world's oceans: by providing a strong two-way advective exchange between the ice-covered Polar Sea and the North Atlantic Ocean and at the same time acting as the primary site for Northern Hemisphere bottom water formation. The water masses for the GIN Sea are defined, in a manner as consistent as possible with the literature. The large-scale oceanographic and meteorological factors influencing the water mass formation and circulation are described. A sample calculation of the various horizontal and vertical thermohaline-driven exchanges is given.     

Meltwater pulses in the northern North Atlantic: retrodiction and forecast by numerical modelling

Changes in sea surface salinity, especially by sudden meltwater pulses, are the most effective process to modify the circulation in the Greenland–Iceland–Norwegian (GIN) seas. With “Sensitivity and Circulation of the Northern North Atlantic” (SCINNA), a three-dimensional ocean general circulation model, several experiments addressing the possible effects of meltwater inputs of different intensities were carried out. The experiments used (a) the last glacial maximum (LGM) reconstruction based on oxygen isotopes data from sediment cores and (b) the modern conditions of the GIN seas for their initial states. Meltwater inputs from Europe as recorded during the last deglaciation succeeding the LGM change the circulation pattern drastically. These pulses can push the high-salinity inflow from the northeast Atlantic away from Europe over to the southern coast of Iceland, thus allowing the low-salinity meltwater to spread all over the GIN seas. As a result, the deepwater formation in this region can be turned off and the circulation system shifts from the normal cyclonal-antiestuarine into an anticyclonal-estuarine mode. On the contrary, meltwater pulses originating from Greenland due to global warming mainly intensify the East Greenland Current without altering the overall circulation and temperature/salinity patterns significantly because they chiefly enhance the salinity minimum off the Greenland coast.     

北欧海深层水的研究进展简

As a connection region between North Atlantic and Arctic Oceans, the Nordic Sea plays a critical role in global climate system. In the Nordic Seas, surface water converts into intermediate water and deep water after cooling and other effects. These waters transport southward, and enter into North Atlantic as a form of overflow, therefore, they are the main source of the North Atlantic Deep Water(NADW), which play a key role in global ocean conveyor. The causes and processes of the deep water formation in the Nordic Seas are still uncertain. Based on a review of current and historical research results of the deep water in the Nordic Seas, the most important process for deep water formation convection is addressed. Factors and physical processes that may have impact on deep water formation are summarized. The transport of deep water in the Nordic Seas is summed up. Multi year variation of the deep water is described with the aim of giving some instructions and research directions to the readers.     

Rapid climatic changes during the Greenland stadial 1 (Younger Dryas) to early Holocene transition on the Norwegian Barents Sea coast

A pollen-based quantitative climate reconstruction from a lake-sediment core on the Norwegian Barents Sea coast provides insights about climatic change over the Greenland stadial 1 (GS-1) to early-Holocene transition. GS-1 was characterized by low July mean temperatures ( c . 6.0°C) and dry conditions probably resembling modern arctic deserts. The increase in July mean temperatures to the Holocene level (10.0-12.0°C) took place in a two-step pattern interrupted by a short cool period with July mean temperatures of c . 8.0°C during the early Preboreal at c . 11450-11200 cal. yr BP. The reconstruction also suggests two other early-Holocene coolings of c . 1.5°C, dating to 10900-10800 cal. yr BP and 10400-10200 cal. yr BP, synchronously with short-term decreases in δ18 O values in the Greenland ice cores. These results reflect the highly unstable nature of the early-Holocene climate in northernmost Fennoscandia. Apart from the cooling at 10900-10800 cal. yr BP, the reconstructed cold events correlate with fluxes of fresh water to the North Atlantic and related reductions of North Atlantic deep-water formation, suggesting that the rapid climate changes resulted from the dynamics of the North Atlantic thermohaline circulation and oceanic energy transport during the GS-1 to early-Holocene transition.     

Millennial-scale climate changes on South Georgia, Southern Ocean

The location of South Georgia (54°S, 36°W) makes it a suitable site for the study of the climatic connections between temperate and polar environments in the Southern Hemisphere. Because the mass balance of the small cirque glaciers on South Georgia primarily responds to changes in summer temperature they can provide records of changes in the South Atlantic Ocean and atmospheric circulation. We use grey scale density, weight-loss-on-ignition, and grain size analyses to show that the proportion of glacially eroded sediments to organic sediments in Block Lake was highly variable during the last 7400 cal yr B.P. We expect that the glacial signal is clearly detectable above noise originating from nonglacial processes and assume that an increase in glacigenic sediment deposition in Block Lake has followed Holocene glacier advances. We interpret proglacial lake sediment sequences in terms of summer climate warming and cooling events. Prominent millennial-scale features include cooling events between 7200 and 7000, 5200 and 4400, and 2400 and 1600 cal yr B.P. and after 1000 cal yr B.P. Comparison with other terrestrial and marine records reveals that the South Georgian record captures all the important changes in Southern Hemisphere Holocene climate. Our results reveal a tentative coupling between climate changes in the South Atlantic and North Atlantic because the documented temperature changes on South Georgia are anti-phased to those in the North Atlantic.     

Seasonally ice free glacial nordic seas without deep water ventilation

At present the Nordic Seas are a key region of North Atlantic Deep Water (NADW) formation. Two alternative scenarios have been suggested by some authors for the Last Glacial Maximum: (i) the Nordic Seas were permanently covered by sea ice, preventing the formation of NADW, or (ii) that they were seasonally free of ice and that deep water formation did occur. A modified scenario is presented here based on parallel ocean circulation modelling results from the GFDL primitive equation model and a planetary geostrophic model. It is suggested that the glacial Nordic Seas were at least seasonally ice free, but it is observed that there was never deep water formation from the surface; rather it occurred only in the North Atlantic south of 40°–50°N. North of 40°N, the weaker LGM northward flowing thermohaline conveyor is subducted below a reverse conveyor which occurred to a depth of over 1000 m. Various modelling experiments presented here indicate that the reversed conveyor was primarily caused by the colder conditions of the glacial North Atlantic that led to far stronger zonality of glacial analogue of the North Atlantic Current.     

Modelling tempo-spatial signatures of Heinrich Events: influence of the climatic background state

Different sea surface temperature (SST) reconstructions for the Last Glacial Maximum are applied to a hybrid-coupled climate model. The resulting oceanic states are perturbed by North Atlantic meltwater inputs in order to simulate the effect of Heinrich Events on the Atlantic thermohaline circulation (THC) and SST. The experiments show that both the Atlantic SST signature of the meltwater event and the time span of THC recovery strongly depend on the climatic background state. Data-model comparison reveals that the overall spatial signature of SST anomalies is captured much better in the glacial meltwater experiments than in an analogous experiment under present-day conditions. In particular, a breakdown of the modern THC would induce a much stronger temperature drop in high northern latitudes than did Heinrich Events during the ice age. Moreover, our results suggest that the present-day circulation can settle into a stable ‘off’ mode, whereas the glacial THC was mono-stable. Mono-stability may serve as an explanation for the recovery of the THC after Heinrich Event shutdowns during the Last Glaciation.     

Freshwater pulses in the eastern Arctic Ocean during Saalian and Early Weichselian ice-sheet collapse

Improved multiparameter records from the northern Barents Sea margin show two prominent freshwater pulses into the Arctic Ocean during MIS 5 that significantly disturbed the regional oceanic regime and probably affected global climate. Both pulses are associated with major iceberg-rafted debris (IRD) events, revealing intensive iceberg/sea ice melting. The older meltwater pulse occurred near the MIS 5/6 boundary (∼131,000 yr ago); its ∼2000 year duration and high IRD input accompanied by high illite content suggest a collapse of large-scale Saalian Glaciation in the Arctic Ocean. Movement of this meltwater with the Transpolar Drift current into the Fram Strait probably promoted freshening of Nordic Seas surface water, which may have increased sea-ice formation and significantly reduced deep-water formation. A second pulse of freshwater occurred within MIS 5a (∼77,000 yr ago); its high smectite content and relatively short duration is possibly consistent with sudden discharge of Early Weichselian ice-dammed lakes in northern Siberia as suggested by terrestrial glacial geologic data. The influence of this MIS 5a meltwater pulse has been observed at a number of sites along the Transpolar Drift, through Fram Strait, and into the Nordic Seas; it may well have been a trigger for the North Atlantic cooling event C20.     

Variability of the Arctic front during the last climatic cycle: application of a novel molecular proxy

In the Nordic Seas, the Arctic front (AF) marks the boundary between the waters of the North Atlantic Drift/Norwegian Current and those of the Arctic domain. Long- or short-term shifts in the position of the AF may affect climate conditions in the northern hemisphere. Arctic water masses are also the loci of modern open ocean convection; hence, defining these areas in the past is important for reconstructing and modelling ocean circulation and its variability. C₃₇ alkenones are biomarkers for some algae of the Class Prymnesiophyceae (e.g. coccolitho-phorids such as Emiliania huxleyi). These alga occur in most parts of the oceans, in ice-free conditions, and are found nowadays throughout the Nordic Seas. We have related the sedimentary abundance of the tetraunsaturated C₃₇ alkenone (C_37:4) to two types of water masses in the Nordic seas. In locations affected by Atlantic water masses percentages of C_37:4 are less than 5%, whereas in Arctic type water masses these increase to more than 5%. We propose that this observation can be used as a modern analogue to reconstruct the position of the AF in North Atlantic Quaternary sediments. Using this novel molecular proxy we can infer that the southward migration of the AF in the NE Atlantic reached ≈ 50 °N during the last glacial maximum (LGM), but perhaps only 60 °N during the Younger Dryas, and that ocean conditions free of sea ice prevailed throughout the Northern North Atlantic in summer.     

Non‐synchronous deposition of North Atlantic Ash Zone II in Greenland ice cores,and North Atlantic and Norwegian Sea sediments: an example of complex glacial‐stage tephra transport

Tephra provides regional chronostratigraphical marker horizons that can link different climate archives with highly needed accuracy and precision. The results presented in this work exemplify, however, that the intermittent storage of tephra in ice sheets and during its subsequent iceberg transport, especially during glacial stages, constitutes a potential source of serious error for the application of tephrochronology to Nordic Seas and North Atlantic sediment archives. The peak shard concentration of the rhyolitic component of the North Atlantic Ash Zone II (NAAZ‐II) tephra complex, often used to correlate marine and ice core records in Marine Isotope Stage (MIS) 3, is shown to lag the eruption event by ca. 100–400 years in some North Atlantic and Norwegian Sea cores. While still allowing for a correlation of archives on millennial timescales, this time delay in deposition is a major obstacle when addressing the lead–lag relationship on short timescales (years to centuries). A precise and accurate determination of lead–lag relationships between archives recording different parts of the climate system is crucial in order to test hypotheses about the processes leading to abrupt climate change and to evaluate results from climate models. Copyright © 2011 John Wiley & Sons, Ltd.     

千年尺度气候变率的研究

2 0世纪后期 ,千年 (ka)尺度气候变率的研究取得了重要的进展 ,这表现在以下几个方面 :(1)格陵兰冰芯及深海沉积证明 ,在末次冰期中普遍存在平均周期为 1 5ka的循环 ,有人认为全新世也存在这种循环 ,小冰期就是最近一个循环的冷期。 (2 )每个循环由 1个相对暖期 (间冰阶 )及 1个冷期(冰阶 )组成 ,称为Dansgaard/Oeshger循环 (D/O循环 )。 (3)连续几个D/O循环的冰阶气温愈来愈低 ,海因里希事件 (H事件 )就发生在最冷的冰阶之后。 (4 )自 15kaBP到 6 8kaBP共确定出 6次H事件 ,分别称为H1…H6,有的作者认为新仙女木事件 (YD)与H事件形成机制近似 ,可以称为H0 。 (5 )D/O循环与H事件的成因 ,目前尚无定论。但热盐环流 (THC)变化的学说得到了较多作者的承认。这个学说认为 :北大西洋北部的大量融冰使海面为冷的淡水控制 ,影响了大传送带中海水的下沉 ,从而削弱了深水的形成。北大西洋THC减弱 ,使向北输送的热量减少 ,使北大西洋气候更寒冷。一旦深水形成再次增加 ,完成一个D/O循环。H事件形成的机制与之类似 ,不过过程变化更为激烈。 (6 )这样 ,THC有 3种模态 :现代模 (北大西洋有两个泵 )、冰阶模 (一个泵 ) ,及H事件 (无泵 )。 (7)海洋环流模式已经对THC的变化及模态之间的转换进行了模拟 ,至少在一定程度?     

Thermosteresis and Transportation of Atlantic Water Along Eurasian Basin of the Arctic Ocean

It is summarized based on previous studies that warm and salty Atlantic Water (AW) brings huge amount of heat into Arctic Ocean and influences oceanic heat distribution and climate. Both heat transportation and heat release of AW are key factors affecting the thermal process in Eurasian Basin. The Arctic circumpolar boundary current is the carrier of AW, whose flow velocity varies to influence the efficiency of the warm advection. Because the depth of AW in Eurasian Basin is much shallower than that in Canadian Basin, the upward heat release of AW is an important heat source to supply sea ice melting. Turbulent mixing, winter convention and double-diffusion convention constitute the main physical mechanism for AW upward heat release, which results in the decrease of the Atlantic water core temperature during its spreading along the boundary current. St. Anna Trough, a relatively narrow and long trough in northern continental shelf of Kara Sea, plays a key role in remodeling temperature and salinity characteristics of AW, in which the AW from Fram Strait enters the trough and mixes with the AW from Barents Sea. Since the 21^st Century, AW in the Arctic Ocean has experienced obvious warming and had the influence on the physical processes in downstream Canada Basin, which is attributed to the anomalous warming events of AW inflowing from the Fram Strait. It is inferred that the warming AW is dominated by a long-term warming trend superimposed on low frequency oscillation occurring in the Nordic Seas and North Atlantic Ocean. As the Arctic Ocean is experiencing sea ice decline and Arctic amplification, the role of AW heat release in response to the rapid change needs further investigation.     

Rapid bottom-water circulation changes during the last glacial cycle in the coastal low-latitude NE Atlantic

Previous paleoceanographic studies along the NW African margin focused on the dynamics of surface and intermediate waters, whereas little attention has been devoted to deep-water masses. Currently, these deep waters consist mainly of North Atlantic Deep Waters as part of the Atlantic Meridional Overturning Circulation (AMOC). However, this configuration was altered during periods of AMOC collapse. We present a high-resolution reconstruction of bottom-water ventilation and current evolution off Mauritania from the last glacial maximum into the early Holocene. Applying redox proxies (Mo, U and Mn) measured on sediments from off Mauritania, we describe changes in deep-water oxygenation and we infer the evolution of deep-water conditions during millennial-scale climate/oceanographic events in the area. The second half of Heinrich Event 1 and the Younger Dryas were recognized as periods of reduced ventilation, coinciding with events of AMOC reduction. We propose that these weakening circulation events induced deficient deep-water oxygenation in the Mauritanian upwelling region, which together with increased productivity promoted reducing conditions and enhanced organic-matter preservation. This is the first time the effect of AMOC collapse in the area is described at high resolution, broadening the knowledge on basin-wide oceanographic changes associated with rapid climate variability during the last deglaciation.     

Variations in temperature and extent of Atlantic Water in the northern North Atlantic during the Holocene

We compare six high-resolution Holocene, sediment cores along a S–N transect on the Norwegian–Svalbard continental margin from ca 60°N to 77.4°N, northern North Atlantic. Planktonic foraminifera in the cores were investigated to show the changes in upper surface and subsurface water mass distribution and properties, including summer sea-surface temperatures (SST). The cores are located below the axis of the Norwegian Current and the West Spitsbergen Current, which today transport warm Atlantic Water to the Arctic. Sediment accumulation rates are generally high at all the core sites, allowing for a temporal resolution of 10–10² years. SST is reconstructed using different types of transfer functions, resulting in very similar SST trends, with deviations of no more than ±1.0/1.5 °C. A transfer function based on the maximum likelihood statistical approach is found to be most relevant. The reconstruction documents an abrupt change in planktonic foraminiferal faunal composition and an associated warming at the Younger Dryas–Preboreal transition. The earliest part of the Holocene was characterized by large temperature variability, including the Preboreal Oscillations and the 8.2 k event. In general, the early Holocene was characterized by SSTs similar to those of today in the south and warmer than today in the north, and a smaller S–N temperature gradient (0.23 °C/°N) compared to the present temperature gradient (0.46 °C/°N). The southern proxy records (60–69°N) were more strongly influenced by slightly cooler subsurface water probably due to the seasonality of the orbital forcing and increased stratification due to freshening. The northern records (72–77.4°N) display a millennial-scale change associated with reduced insolation and a gradual weakening of the North Atlantic thermohaline circulation (THC). The observed northwards amplification of the early Holocene warming is comparable to the pattern of recent global warming and future climate modelling, which predicts greater warming at higher latitudes. The overall trend during mid and late Holocene was a cooling in the north, stable or weak warming in the south, and a maximum S–N SST gradient of ca 0.7 °C/°N at 5000 cal. years BP. Superimposed on this trend were several abrupt temperature shifts. Four of these shifts, dated to 9000–8000, 5500–3000 and 1000 and 400 cal. years BP, appear to be global, as they correlate with periods of global climate change. In general, there is a good correlation between the northern North Atlantic temperature records and climate records from Norway and Svalbard.     

Late Quaternary changes in surface water and deep water masses of the Nordic Seas and north-eastern North Atlantic: a review

Quantitative and semiquantitative proxy data based on more than 200 core-top samples and 100 deep-sea cores lead to important new insights about late Quaternary changes in paleo-oceanography, climate and microfaunal habitats in the north-eastern North Atlantic and Nordic Seas, insights resulting from a detailed investigation by the Kiel research project SFB 313/132 summarized in this paper. Planktonic foraminifera species provide reliable tracers of past sea surface temperatures and currents. The genus Beella in particular was found to trace subtropical water masses up to the far north. Benthic foraminifera species served as sensors of bottom currents and local flux rates of organic matter. New orders of time resolution are reached via stable isotope stratigraphy and accelerator mass spectrometry carbon-14 dating, allowing the identification of meltwater events lasting a few hundred years and shorter, a time range where, however, the yet unquantified role of bioturbation presents a growing problem. Based on this high-resolution stratigraphy a number of time slices (synoptic time intervals) are defined to reconstruct the incursion of Atlantic water masses, to map paleocurrent patterns within the Nordic Seas and the north-eastern North Atlantic and to test alternative circulation models — for example, for the last glacial maximum (LGM) and various meltwater episodes. These are clearly coeval with Dansgaard-Oeschger events found in Greenland ice cores, with the actual cause of the flickering climate as yet unknown. Likewise, there is ongoing controversy about the extent of past sea-ice cover and about possible changes from the present anti-estuarine to estuarine mode of deep water exchange between the North Atlantic and the Nordic Seas during the LGM. South of Iceland, however, the history of deep water renewal over the last glacial cycle covering the last 30000 years was largely deciphered.     

Simulation of Holocene cooling events in a coupled climate model

Three potential mechanisms behind centennial-scale Holocene cooling events are studied in simulations performed with the coupled climate model ECBilt–CLIO: (1) internal variability, (2) solar forcing, and (3) freshwater forcing. In experiments with constant preindustrial forcings, three centennial-scale cooling events occur spontaneously in 15,000 years. These rare events represent an unstable internal mode of variability that is characterised by a weaker thermohaline circulation, a more southward location of the main site of deep-water formation, expanded sea-ice cover and cooling of 10 °C over the Nordic Seas. This mode is visited more frequently when the climate is cooled by abruptly reducing the solar constant by 5 or 3 Wm⁻². Prescribing a solar forcing of the same magnitude, but following a sinusoidal function with a period of 100 or 1000 years, does not result in any centennial-scale cooling events. The latter forcing does however result in more frequent individual cold years in the North Atlantic region that are related to local weakening of the deep convection and sea-ice expansion. Adding realistic freshwater pulses to the Labrador Sea is also able to trigger centennial-scale cooling events with temperature anomalies resembling proxy evidence for the cooling event at 8.2 kyr BP, suggesting that freshwater forcing is a valid explanation for early Holocene cooling events.     

Spatial and temporal variability of Holocene temperature in the North Atlantic region

The early Holocene climate of the North Atlantic region was influenced by two boundary conditions that were fundamentally different from the present: the presence of the decaying Laurentide Ice Sheet (LIS) and higher than present summer solar insolation. In order to assess spatial and temporal patterns of Holocene climate evolution across this region, we collated quantitative paleotemperature records at sub-millennial resolution and synthesized their temporal variability using principal components analysis (PCA). The analysis reveals considerable spatial variability, most notably in the time-transgressive expression of the Holocene thermal maximum (HTM). Most of the region, but especially areas peripheral to the Labrador Sea and hence closest to the locus of LIS disintegration, experienced maximum Holocene temperatures that lagged peak summer insolation by 1000-3000 years. Many sites from the northeastern North Atlantic sector, including the Nordic Seas and Scandinavia, either warmed in phase with maximum summer insolation (11,000-9000 years ago) or were less strongly lagged than the Baffin Bay-Labrador Sea region. These spatially complex patterns of Holocene climate development, which are defined by the PCA, resulted from the interplay between final decay of the LIS and solar insolation forcing.     

北大西洋涛动变率研究进展

介绍了近年来北大西洋涛动研究的最新进展。 NAO指数序列的建立取得了很多成果 ,包括一些观测气象记录的序列以及利用树木年轮、冰芯等代用资料建立的近 30 0多年的序列 ,这些长的序列显示 NAO不仅有突出的年际变率 ,也有显著的年代际变率。总结了 NAO对地面温度、降水、北大西洋飓风和北半球臭氧等影响的一些研究成果。NAO的低频变率可能与气候系统内部的相互作用以及外部强迫有关。许多模拟研究发现 NAO与温盐环流有密切的联系 ,但是这种关系还有待观测资料的证实。全球气候变暖也可能是影响 NAO变率的一个不可忽视的因素。     

Blueprints for Medieval hydroclimate

According to tree ring and other records, a series of severe droughts that lasted for decades afflicted western North America during the Medieval period resulting in a more arid climate than in subsequent centuries. A review of proxy evidence from around the world indicates that North American megadroughts were part of a global pattern of Medieval hydroclimate that was distinct from that of today. In particular, the Medieval hydroclimate was wet in northern South America, dry in mid-latitude South America, dry in eastern Africa but with strong Nile River floods and a strong Indian monsoon. This pattern is similar to that accompanying persistent North American droughts in the instrumental era. This pattern is compared to that associated with familiar climate phenomena. The best fit comes from a persistently La Niña-like tropical Pacific and the warm phase of the so-called Atlantic Multidecadal Oscillation. A positive North Atlantic Oscillation (NAO) also helps to explain the Medieval hydroclimate pattern. Limited sea surface temperature reconstructions support the contention that the tropical Pacific was cold and the subtropical North Atlantic was warm, ideal conditions for North American drought. Tentative modeling results indicate that a multi-century La Niña-like state could have arisen as a coupled atmosphere–ocean response to high irradiance and weak volcanism during the Medieval period and that this could in turn have induced a persistently positive NAO state. A La Niña-like state could also induce a strengthening of the North Atlantic meridional overturning circulation, and hence warming of the North Atlantic Ocean, by (i) the ocean response to the positive NAO and by shifting the southern mid-latitude westerlies poleward which (ii) will increase the salt flux from the Indian Ocean into the South Atlantic and (iii) drive stronger Southern Ocean upwelling.