North Atlantic Thermohaline Circulation Research Articles

Links between salinity variation in the Caribbean and North Atlantic thermohaline circulation.

Variations in the strength of the North Atlantic Ocean thermohaline circulation have been linked to rapid climate changes during the last glacial cycle through oscillations in North Atlantic Deep Water formation and northward oceanic heat flux. The strength of the thermohaline circulation depends on the supply of warm, salty water to the North Atlantic, which, after losing heat to the atmosphere, produces the dense water masses that sink to great depths and circulate back south. Here we analyse two Caribbean Sea sediment cores, combining Mg/Ca palaeothermometry with measurements of oxygen isotopes in foraminiferal calcite in order to reconstruct tropical Atlantic surface salinity during the last glacial cycle. We find that Caribbean salinity oscillated between saltier conditions during the cold oxygen isotope stages 2, 4 and 6, and lower salinities during the warm stages 3 and 5, covarying with the strength of North Atlantic Deep Water formation. At the initiation of the Bølling/Allerød warm interval, Caribbean surface salinity decreased abruptly, suggesting that the advection of salty tropical waters into the North Atlantic amplified thermohaline circulation and contributed to high-latitude warming.

New experiment deploys observing array in N. Atlantic to investigate rapid climate change

Studies of past climate using paleo‐data suggest that large and rapid climate changes have occurred throughout history, and that changes in the thermohaline circulation of the ocean are a major factor in many of these cases. Some of these past rapid changes occurred over a period as brief as a decade, whereas we tend to think of significant climate change as occurring over centuries or millennia. The third assessment report of the Intergovernmental Panel on Climate Change OPCQ, released in 2001, discussed the potential slowdown of the North Atlantic thermohaline circulation under a global warming scenario (http://www.ipcc.ch/). In addition, a recent US. National Research Council report highlighted the concerns over the possibility of rapid changes in climate and their potential socioeconomic impacts (http://www.nap. edu/catalog/10136.html). The new U.K.Natural Environment Research Council (NERQ Rapid Climate Change Programme (RAPID) is, in part, a response to such concerns.

Unstable Younger Dryas climate in the northeast North Atlantic

The Younger Dryas stadial (Greenland Stadial 1) has long been recognized in proxy records throughout the North Atlantic region as a stable, cool period that started and ended abruptly. However, modeling experiments in which the Younger Dryas cooling was forced by meltwater predict much more unstable climatic conditions compared to the proxy records. Here we present evidence of a climatically unstable Younger Dryas based on a decadal-resolution marine proxy record from the Norwegian continental shelf. This record shows that the North Atlantic was characterized in the early Younger Dryas by Arctic Water and an increasing accumulation of fresh surface water that peaked at 12,500 calendar yr B.P. For ∼300 yr during the late Younger Dryas, the sea-surface temperatures fluctuated between glacial values (∼2 °C) and interglacial values (∼10 °C) on a subdecadal to multidecadal time scale. This period of extreme instability preceded the more stable and warm Holocene, starting ca. 11,500 calendar yr B.P. These results are consistent with modeling experiments that link the Younger Dryas climate changes to fluctuations of the North Atlantic thermohaline circulation.

Holocene annual mean temperature changes in Estonia and their relationship to solar insolation and atmospheric circulation patterns

We reconstructed annual mean temperature ( T ann) trends from three radiocarbon-dated Holocene pollen stratigraphies from lake sediments in Estonia, northern Europe. The reconstructions were carried out using a North-European pollen-climate calibration model based on weighted averaging partial least-squares regression. The cross-validated prediction error of the model is 0.89°C and the coefficient of determination between observed modern T ann values and those predicted by the model is 0.88. In the reconstruction, the Holocene thermal maximum (HTM) is distinguishable at 8000–4500 cal yr B.P., with the expansion of thermophilous tree species and T ann on average 2.5°C higher than at present. The pollen-stratigraphical data reflect progressively warmer and drier summers during the HTM. Analogously with the modern decadal-scale climatic variability in North Europe, we interpret this as an indication of increasing climatic continentality due to the intensification of anticyclonic circulation and meridional air flow. Post-HTM cooling started abruptly at around 4500 cal yr B.P. All three reconstructions show a transient (ca. 300 years) cooling of 1.5–2.0°C at 8600–8000 cal yr B.P. We tentatively correlate this cold event with the North-Atlantic “8.2 ka event” at 8400–8000 cal yr B.P. Provided that the 8.2 ka event was caused by freshening of the North-Atlantic surface water, our data provide evidence of the climatic and vegetational responsiveness of the boundary of the temperate and boreal zones to the weakening of the North-Atlantic thermohaline circulation and the zonal energy transport over Europe. No other cold events of comparable magnitude are indicated during the last 8000 years.

Water Mass Transformation in the North Atlantic and Its Impact on the Meridional Circulation: Insights from an Ocean Model Forced by NCEP–NCAR Reanalysis Surface Fluxes

Decadal-scale climate variability in the North Atlantic thermohaline circulation is simulated using a sigma-coordinate primitive equation model, forced by NCEP–NCAR reanalysis surface forcing fields for the period from 1958 to 1997. Surface heat and freshwater flux are expressed in terms of surface thermal and haline density inputs, diagnosed by the model. Variability in surface density fluxes is closely correlated with the North Atlantic Oscillation and demonstrates differences with the original surface heat and freshwater fluxes. Leading modes of surface water mass transformation are considered in the T–S plane. They identify decadal-scale variability associated with the transformation of the Labrador Sea Waters and Subtropical Mode Waters. Analysis of the model responses to the surface forcing shows an immediate reaction of meridional heat transport to the wind stress curl, resulting in a decrease of meridional heat transport at 48°N and an increase in the subtropics. Delayed baroclinic responses to the surface heat forcing are identified at time lags of 3 and 7 yr. The 3-yr response is represented by an increase in the total meridional heat transport in subpolar latitudes and its simultaneous increase in the Tropics and midlatitudes. The 7-yr delayed response to the surface heat forcing is associated with the strengthening of meridional heat transport at all latitudes. However, 7-yr responses may be influenced by the self-correlation in the meridional heat transport and forcing function. Meridional overturning is largely responsible for the variability observed, demonstrating high correlation with meridional heat transport.

Implications of changes in freshwater flux from the Greenland ice sheet for the climate of the 21st century

Two simulations of the 21st century climate have been carried out using, on the one hand, a coarse resolution climate general circulation model and, on the other hand, the same model coupled to a comprehensive model of the Greenland ice sheet. Both simulations display a gradual global warming up to 2080. In the experiment that includes an interactive ice sheet component, a strong and abrupt weakening of the North Atlantic thermohaline circulation occurs at the end of the 21st century. This feature is triggered by an enhanced freshwater input arising mainly from a partial melting of the Greenland ice sheet. As a consequence of the circulation decline, a marked cooling takes place over eastern Greenland and the northern North Atlantic. This result underlines the potential role of the Greenland ice sheet in the evolution of climate over the 21st century.

Formation and propagation of great salinity anomalies

North Atlantic/Arctic ocean and sea ice variability for the period 1948–2001 is studied using a global Ocean General Circulation Model coupled to a dynamic/thermodynamic sea ice model forced by daily NCEP/NCAR reanalysis data [Kalnay et al., 1996]. Variability of Arctic sea ice properties is analysed, in particular the formation and propagation of sea ice thickness anomalies that are communicated via Fram Strait into the North Atlantic. These export events led to the Great Salinity Anomalies (GSA) of the 1970s, 1980s and 1990s in the Labrador Sea (LS). All GSAs were found to be remotely excited in the Arctic, rather than by local atmospheric forcing over the LS. Sea ice and fresh water exports through the Canadian Archipelago (CAA) are found to be only of minor importance, except for the 1990s GSA. Part of the anomalies are tracked to the Newfoundland Basin, where they enter the North Atlantic Current. The experiments indicate only a minor impact of a single GSA event on the strength of the North Atlantic Thermohaline Circulation (THC).

Southern Ocean sea‐ice control of the glacial North Atlantic thermohaline circulation

A coupled model simulates a shallower and weaker North Atlantic Deep Water circulation at the Last Glacial Maximum, compared to the modern, with an enhanced intrusion of Antarctic Bottom Water into the North Atlantic. These circulation changes are caused by the enhanced Antarctic Bottom Water formation, which is triggered by the enhanced equatorward sea‐ice transport, ultimately by increased westerlies, in the Southern Ocean at the Last Glacial Maximum.

Medieval Warm Period, Little Ice Age and 20th century temperature variability from Chesapeake Bay

We present paleoclimate evidence for rapid (<100 years) shifts of ∼2–4 °C in Chesapeake Bay (CB) temperature ∼2100, 1600, 950, 650, 400 and 150 years before present (years BP) reconstructed from magnesium/calcium (Mg/Ca) paleothermometry. These include large temperature excursions during the Little Ice Age (∼1400–1900 AD) and the Medieval Warm Period (∼800–1300 AD) possibly related to changes in the strength of North Atlantic thermohaline circulation (THC). Evidence is presented for a long period of sustained regional and North Atlantic-wide warmth with low-amplitude temperature variability between ∼450 and 1000 AD. In addition to centennial-scale temperature shifts, the existence of numerous temperature maxima between 2200 and 250 years BP (average ∼70 years) suggests that multi-decadal processes typical of the North Atlantic Oscillation (NAO) are an inherent feature of late Holocene climate. However, late 19th and 20th century temperature extremes in Chesapeake Bay associated with NAO climate variability exceeded those of the prior 2000 years, including the interval 450–1000 AD, by 2–3 °C, suggesting anomalous recent behavior of the climate system.

Millennial-scale glacial variability versus Holocene stability: changes in planktic and benthic foraminifera faunas and ocean circulation in the North Atlantic during the last 60 000 years

Two piston cores, DS97-2P from the Reykjanes Ridge in the central North Atlantic Ocean (1685 m water depth) and ENAM33 from southwest of the Faeroe Islands in the NE Atlantic (1217 m water depth), have been investigated for their planktic and benthic foraminiferal content. DS97-2P is situated near the Subarctic Front and productivity measured by accumulation rates of benthic and planktic foraminifera has been generally high during the Holocene. The productivity shows a clear decrease from an early Holocene maximum to a late Holocene minimum. Coeval changes in the benthic faunas indicate that the food supply changed from large, irregular pulses during the early Holocene to a more sustained flux during the late Holocene. Presumably in concert with decreasing bottom current activity oxygen conditions in the bottom water became poorer. Another feature of the late Holocene is an increasing instability of the North Atlantic thermohaline circulation regime. Nevertheless, the changes in faunal composition and productivity during the Holocene were gradual as compared to the discontinuous distribution patterns and abrupt productivity shifts during the glacial. The glacial shifts were on a millennial time scale and correlate with the interstadial–stadial phases of the Dansgaard–Oeschger cycles in the Greenland ice cores. The faunas of the warm interstadial phases resembled the Holocene faunas, and both surface and bottom productivity was high. The faunas suggest that the interstadial circulation pattern was very similar to the modern system with convection in the Nordic seas and generation of North Atlantic Deep Water. The planktic faunas during the cold stadials and Heinrich events were completely dominated by the polar species Neogloboquadrina pachyderma s, and surface conditions were cold and the productivity low. The benthic faunas were dominated by species that presently thrive in areas with a low amount of food and reduced oxygen content. The water column was probably stratified with low saline, cold surface water overlying poorly aerated, intermediate water masses.

Relative timing of deglacial climate events in Antarctica and Greenland.

The last deglaciation was marked by large, hemispheric, millennial-scale climate variations: the Bølling-Allerød and Younger Dryas periods in the north, and the Antarctic Cold Reversal in the south. A chronology from the high-accumulation Law Dome East Antarctic ice core constrains the relative timing of these two events and provides strong evidence that the cooling at the start of the Antarctic Cold Reversal did not follow the abrupt warming during the northern Bølling transition around 14,500 years ago. This result suggests that southern changes are not a direct response to abrupt changes in North Atlantic thermohaline circulation, as is assumed in the conventional picture of a hemispheric temperature seesaw.

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 δ18O 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.

Sensitivity of the thermohaline circulation to tropical and high latitude freshwater forcing during the last glacial‐interglacial cycle

Recent studies indicate that the tropical freshwater budget of the Atlantic is modulated by changes in El Niño‐Southern Oscillation (ENSO). If anomalies of the Atlantic freshwater balance persist on the order of decades, these might have a large influence on the North Atlantic thermohaline circulation (THC). Here we present a sensitivity study in which we use a model scenario for ENSO behavior during the last 120 kyr to force a simplified model of the THC with corresponding freshwater exchange between the tropical Atlantic and Pacific. If the steady state strength of the North Atlantic THC is similar to the present‐day, its response to the forcing is of the order of 1–3 Sv (1 Sv = 106 m3 s−1). No mode changes of the THC are simulated for reasonable values of the coupling constant between freshwater exchange and ENSO. If, on the other hand, the steady state overturning is significantly weaker, a collapse of the THC occurs due to forcing from the tropics. The modeled THC variation due to tropical freshwater perturbations are compared to those resulting from middle‐ and high‐latitude freshwater forcing due to long‐term growth and decay of Northern Hemisphere ice sheets. It is found that both forcings are of similar amplitude except during the deglaciation when high latitude forcing dominates. A possible out‐of‐phase relationship between deep water formation in the North Pacific and North Atlantic is also explored.

Forcing of the deep ocean circulation in simulations of the Last Glacial Maximum

From the interpretation of different proxy data it is widely believed that the North Atlantic thermohaline circulation during the maximum of the last ice age ∼21,000 years ago was considerably weaker than today. Recent equilibrium simulations with a coupled ocean‐atmosphere‐sea ice model successfully simulated a reduction in North Atlantic Deep Water (NADW) formation consistent with reconstructions. Here we examine the influence of different air‐sea fluxes on simulated changes in the deep ocean circulation between the Last Glacial Maximum and present day. We find that changes in the oceanic surface freshwater fluxes are the dominant forcing mechanism for the reduced Atlantic overturning. Diminished export of freshwater out of the Atlantic drainage basin through the atmosphere decreases surface salinities in the North Atlantic, leading to less NADW formation in the colder climate. Changes in heat fluxes, which lead to increased sea surface densities in the North Atlantic and therefore to an enhanced overturning, are of secondary importance. Wind stress variations seem to play a negligible role. The degree to which the Atlantic freshwater export and hence the NADW formation are reduced depends on the formulation of the atmospheric hydrological cycle and on the strength of the overturning in the present‐day simulation. Simulated changes in sea surface properties for a large variety of overturning strengths are compared with different reconstruction data sets. The results depend strongly on the data set used. Sea surface temperature reconstructions from Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP) and earlier salinity reconstructions based on planktonic foraminifera are most consistent with a significant reduction of the circulation, while recent reconstructions using dinocyst assemblages allow no unequivocal conclusion.

Localization of abrupt change in the North Atlantic thermohaline circulation

Recent climate model experiments, as well as paleoclimate records, suggest that the meridional overturning circulation or “thermohaline circulation” in the Atlantic Ocean could change abruptly as a result of global warming, and that this could have a significant impact on European climate. We use a reduced‐gravity model to investigate the response of the Atlantic overturning circulation to changes in forcing. We find that variability at decadal and higher frequencies is confined to a single hemisphere. This implies that (a) overturning variability resulting from high frequency changes in buoyancy forcing in the Labrador and Greenland Seas will be limited to the North Atlantic, and (b) any observed decadal and higher frequency fluctuations in North Atlantic overturning can only result from changes in the surface fluxes within the North Atlantic basin itself. These results suggest that Southern Ocean wind forcing is not important for North Atlantic overturning on decadal and shorter timescales.

Comparison between instrumental, observational and high resolution proxy sedimentary records of Late Holocene climatic change—a discussion of possibilities

Variability of the North Atlantic region is a key factor in the workings of global climate. Changes related to orbital factors, and to the thermohaline circulation (THC), occur on timescales ranging from 1000 to 100,000 years, whereas subdecadal, decadal and multidecadal trends are associated with changes in atmospheric circulation, sea surface temperature (SST), and salinity. Long-term changes are known only from proxies, whereas shorter periodicities are identified in instrumental and other observational data. Between lies a range of less well known centennial periodicities, too long to be recorded observationally, and too short for all but the highest resolution proxy records, such as tree rings, atmospheric Δ 14C, and varved lake sediments. Numerous investigations of such periodicities involve comparisons between the proxy and the observational record. However, results depend on the length of the datasets being analysed, the relative strength of the periodicities present, whether data are stationary or non-stationary, the degree to which they have been smoothed in order to remove particular fluctuations, and the methods of analysis employed. Ideally, comparisons should be between datasets of equal length and of similar properties, but these criteria are not always easy to fulfill. A search for periodicities in a range of well known multidecadal datasets reveals that whilst some frequencies previously identified in the Central England Temperature (CET) record, and in the Koch sea ice Index, are statistically significant, those found by several authors in the Index of the North Atlantic Oscillation (NAO) are not, as the index approximates to ‘white noise’. Instead, decadal periodicity of the NAO described by several authors appears to be result of smoothing of the raw data. Periodicities of 6.1, 33.9 and 213 years identified in a proxy record from laminated sediments from Loch Ness, Scotland appear, respectively to be related to (a) subdecadal variations in SST and salinity in the North Atlantic ocean, (b) multidecadal interactions between the NAO and the North Atlantic THC, and (c) a well known centennial solar periodicity.

Mediterranean Sea surface radiocarbon reservoir age changes since the last glacial maximum.

Sea surface reservoir ages must be known to establish a common chronological framework for marine, continental, and cryospheric paleoproxies, and are crucial for understanding ocean-continent climatic relationships and the paleoventilation of the ocean. Radiocarbon dates of planktonic foraminifera and tephra contemporaneously deposited over Mediterranean marine and terrestrial regions reveal that the reservoir ages were similar to the modern one (approximately 400 years) during most of the past 18,000 carbon-14 years. However, reservoir ages increased by a factor of 2 at the beginning of the last deglaciation. This is attributed to changes of the North Atlantic thermohaline circulation during the massive ice discharge event Heinrich 1.

Integrated assessment of abrupt climatic changes

One of the most controversial conclusions to emerge from many of the first generation of integrated assessment models (IAMs) of climate policy was the perceived economic optimality of negligible near-term abatement of greenhouse gases. Typically, such studies were conducted using smoothly varying climate change scenarios or impact responses. Abrupt changes observed in the climatic record and documented in current models could substantially alter the stringency of economically optimal IAM policies. Such abrupt climatic changes — or consequent impacts — would be less foreseeable and provide less time to adapt, and thus would have far greater economic or environmental impacts than gradual warming. We extend conventional, smooth IAM analysis by coupling a climate model capable of one type of abrupt change to a well-established energy–economy model (DICE). We compare the DICE optimal policy using the standard climate sub-model to our version that allows for abrupt change — and consequent enhanced climate damage — through changes in the strength (and possible collapse) of the North Atlantic thermohaline circulation (THC). We confirm the potential significance of abrupt climate change to economically optimal IAM policies, thus calling into question all previous work neglecting such possibilities — at the least for the wide ranges of relevant social and climate system parameters we consider. In addition, we obtain an emergent property of our coupled social–natural system model: “optimal policies” that do consider abrupt changes may, under relatively low discount rates, calculate emission control levels sufficient to avoid significant abrupt change, whereas “optimal policies” disregarding abrupt change would not prevent this non-linear event. However, there is a threshold in discount rate above which the present value of future damages is so low that even very large enhanced damages in the 22nd century, when a significant abrupt change such as a THC collapse would be most likely to occur, do not increase optimal control levels sufficiently to prevent such a collapse. Thus, any models not accounting for potential abrupt non-linear behavior and its interaction with the discounting formulation are likely to miss an important set of possibilities relevant to the climate policy debate.

Mechanisms Determining the Atlantic Thermohaline Circulation Response to Greenhouse Gas Forcing in a Non-Flux-Adjusted Coupled Climate Model

Abstract Models of the North Atlantic thermohaline circulation (THC) show a range of responses to the high-latitude warming and freshening characteristic of global warming scenarios. Most simulate a weakening of the THC, with some suggesting possible interruption of the circulation, but others exhibit little change. The mechanisms of the THC response to climate change using the HadCM3 coupled ocean–atmosphere general circulation model, which gives a good simulation of the present-day THC and does not require flux adjustment, were studied. In a range of climate change simulations, the strength of the THC in HadCM3 is proportional to the meridional gradient of steric height (equivalent to column-integrated density) between 30°S and 60°N. During an integration in which CO2 increases at 2% per year for 70 yr, the THC weakens by about 20%, and it stabilizes at this level if the CO2 is subsequently held constant. Changes in surface heat and water fluxes are the cause of the reduction in the steric height gradie...

Abrupt and sudden climatic transitions and fluctuations: a review

AbstractThis review paper summarizes recent research work on abrupt climatic changes and oscillations. The climatic system is viewed as a dissipative, highly non‐linear system, under non‐equilibrium conditions, and, as such, should be expected to have some unusual properties. These unusual properties include bifurcation points with marked instability just before the point, magnification of semi‐periodic oscillations around bifurcation points, and variations in the strength of teleconnections with distance from equilibrium. These properties are discussed and illustrated for the climatic system using both the historical, Holocene and glacial climatic records. It is found that there are abrupt climatic changes and oscillations on all time‐scales. The amplitudes and frequencies of climate variability and teleconnections are found to vary between different time periods. A number of persistent oscillations exist, particularly one about 1500 years, but their amplitudes vary considerably between time periods. The Holocene appears to be no more climatically benign than the similar period in the Eemian. The importance of the North Atlantic thermohaline circulation for generating abrupt climatic changes in Europe, particularly in association with sudden pulses of fresh water, is illustrated. The concept of antiphase temperature changes between the North and South Atlantic is discussed. Externally generated abrupt climatic deteriorations owing to explosive volcanic eruptions and variations in solar irradiance are also discussed. Copyright © 2001 Royal Meteorological Society