North Atlantic Thermohaline Circulation Research Articles

Modelled response of Arctic and North Atlantic thermohaline structure and circulation to the prolonged unidirectional atmospheric forcing over the Arctic Ocean

Modelled response of Arctic and North Atlantic thermohaline structure and circulation to the prolonged unidirectional atmospheric forcing over the Arctic Ocean

Causes and course of climate change and its hydrological consequences in the Greater Poland region in 1951-2020

The paper presents effects of changes in climatic elements in the Greater Poland region (Poland), their causes and consequences for shaping the water balance of this area, copying with the most severe water deficit in Poland. The study period covers 70 years (1951–2020). The research identified an abrupt and significant change in the climate of Greater Poland, which started between 1987 and 1989, concerning not only air temperature but also a wider spectrum of climatic elements. The change in the state of the climate, which covers the entire Atlantic-Eurasian circulation sector, re-sults from a sudden change in the macro-circulation conditions in the middle troposphere (500 hPa). The reason for the change in the mid-tropospheric circulation is an equally abrupt and simultaneous change in the intensity of the ocean heat transport by the North Atlantic thermohaline circulation (NA THC). Climate change observed in Greater Poland is manifested in an increase in sunshine duration (SD) and air temperature, a decrease in relative humidity, a change in the cloud structure, and an increase in the degree of sky coverage. The main, physical reason for an increase in air tempera-ture is a rapid and strong increase in SD in the warm half-years, which began after 1988, and a significant increase in the frequency of positive North Atlantic Oscillation (NAO) phases in winters. The ongoing climate change entails various effects, among which the most important is considered to be hydrological consequences. The water balance of Greater Poland is becoming increasingly unfavourable, mainly as a result of a rapid increase in field evaporation.

Contributions of Climate Changes in Temperature and Salinity to the Formation of North Atlantic Thermohaline Circulation Trends in 1951–2017

Contributions of Climate Changes in Temperature and Salinity to the Formation of North Atlantic Thermohaline Circulation Trends in 1951–2017

Variability of the North Atlantic Thermohaline Circulation in Different Phases of the Atlantic Multidecadal Oscillation from Ocean Objective Analyses and Reanalyses

Variability of the North Atlantic Thermohaline Circulation in Different Phases of the Atlantic Multidecadal Oscillation from Ocean Objective Analyses and Reanalyses

Effect of the North Atlantic Thermohaline Circulation on Changes in Climatic Conditions and River Flow in Poland

The purpose of this study is to find connections between the North Atlantic Thermohaline Circulation (NA THC), climate elements, such as cloud cover, precipitation, air temperature, sunshine duration, and relative humidity, and flow of rivers in Poland. The intensity of NA THC was characterized by the DG3L index, which was established to assess changes in the amount of heat transported by NA THC along with the transport of water to the Arctic. The paper explains and discusses the mechanism of impact of the NA THC changeability on the elements of the catchment water balance variability. The positive and negative phases of the DG3L index are strongly correlated with the heat anomalies in the upper layer of the North Atlantic waters. The obtained results show that changes of NA THC have significant impact on weather conditions and selected climate elements in Poland. Statistically significant positive correlations were found between the DG3L index and average annual air temperatures, particularly in April, July, and August, while negative between the DG3L index and the total cloud cover. Consequently, in the years with the positive values of the DG3L index, there are favorable conditions for the strong increase in evaporation and evapotranspiration from the ground surface. This has impact on flow of rivers in Poland, which shows considerable regional differences.

Late Pleistocene and mid-Holocene climate change derived from a Florida speleothem

A stalagmite collected from Brooksville Ridge Cave in west-central Florida was deposited from ∼30 to 20 kyr BP, encompassing Heinrich event 2 (H2), and from ∼5 to 4 kyr BP, the later part of the mid-Holocene. The H2 event in the speleothem occurs at ∼24 kyr BP which is temporally similar to its timing in other paleoclimate reconstructions. However, the oxygen and carbon isotope values indicate the climate in Florida was relatively warm and wet for a glacial period rather than extremely cool and dry, as seen in other regions. The higher temperatures and increased precipitation, compared to the preceding climate state, produced a shift towards enhanced vegetation growth (dense forest) during this period. One possible cause is the faltering of the North Atlantic Thermohaline Circulation, due to increased glacial meltwater input, thereby preventing heat transfer via the Gulf Stream from the tropics to the northerly latitudes. This change in the circulation would lead to warmer Gulf of Mexico and subtropical Atlantic sea surface temperatures (SSTs). These higher SSTs would promote an increase in convective thunderstorm activity due to higher evaporation rates. However, to give this shift some paleoclimate perspective, the average mid-Holocene speleothem oxygen isotope values were more depleted by ∼1‰ indicating even warmer temperatures and higher precipitation amounts compared to the glacial period. Additionally, the carbon isotopes show a ∼3‰ shift towards more negative values, indicating a more heavily forested west-central Florida. The variability within both the mid-Holocene and Late-Pleistocene data support the possibility of atmospheric teleconnections between the tropics/subtropics and northerly latitudes contributing to shorter-term shifts in precipitation amount superimposed on the larger-scale glacial-interglacial isotopic composition of the speleothem.

Thermohaline circulation in the North Atlantic and its simulation with a box model

Features of the North Atlantic thermohaline circulation response to periodic, stochastic, and instantaneous forcing are studied using a four-box model. The present-day circulation is shown to be characterized by a stable quasi-periodic oscillatory mode that manifests itself as the Atlantic Multidecadal Oscillation. The thermohaline catastrophe is unlikely in the modern climate epoch.

Quaternary evolution of the northern North Sea margin through glacigenic debris‐flow and contourite deposition

ABSTRACTThe Norwegian Channel Ice Stream of the Scandinavian Ice Sheet extended across the northern North Sea margin during the mid‐ to late Quaternary, eroding older sediment from the continental shelf. Consequently, little is known about early Quaternary sedimentation on this margin. We use two‐ and three‐dimensional seismic‐reflection data to investigate changing sediment volumes and sources in the northern North Sea through the Quaternary. The northern North Sea Basin was infilled during the early Quaternary by intercalated glacigenic debris‐flows and contourites, which provide a record of the delivery of glacigenic sediment to the slope and the intensity of North Atlantic thermohaline circulation during early Quaternary glacial–interglacial cycles. The infilling of the basin reduced accommodation and led to the deflection of mid‐ to late Quaternary sediments into the Norwegian Sea, forming the North Sea Fan. Close to the onset of the mid‐Quaternary, the south‐western Scandinavian Ice Sheet margin was drained by an ice stream located beneath Måløy Plateau, 60 km east of the Last Glacial Maximum Norwegian Channel Ice Stream. The southward‐flowing Norwegian Sea Bottom Water current was directed into the partially filled northern North Sea Basin during the early Quaternary, and deflected progressively northwards as the basin became infilled.

How northern freshwater input can stabilise thermohaline circulation

The North Atlantic thermohaline circulation (THC) carries heat and salt towards the Arctic. This circulation is partly sustained by buoyancy loss and is generally believed to be inhibited by northern freshwater input as indicated by the ‘box-model’ of Stommel (1961). The inferred freshwater-sensitivity of the THC, however, varies considerably between studies, both quantitatively and qualitatively. The northernmost branch of the Atlantic THC, which forms a double estuarine circulation in the Arctic Mediterranean, is one example where both buoyancy loss and buoyancy gain facilitate circulation. We have built on Stommel's original concept to examine the freshwater-sensitivity of a double estuarine circulation. The net inflow into the double estuary is found to be more sensitive to a change in the distribution of freshwater than to a change in the total freshwater input. A double estuarine circulation is more stable than a single overturning, requiring a larger amount and more localised freshwater input into regions of buoyancy loss to induce a thermohaline ‘collapse’. For the Arctic Mediterranean, these findings imply that the Atlantic inflow may be relatively insensitive to increased freshwater input. Complementing Stommel's thermal and haline flow regimes, the double estuarine circulation allows for a third: the throughflow regime. In this regime, a THC with warm poleward surface flow can be sustained without production of dense water; a decrease in high-latitude dense water formation does therefore not necessarily affect regional surface conditions as strongly as generally thought.

Early Holocene multi-centennial moisture change reconstructed from lithology, grain-size and chemical composition data in the eastern Mu Us desert and potential driving forces

The sequence of paleo-aeolian sands and sandy paleosols from the Shenmu section in the eastern Mu Us desert was used to reconstruct the history of moisture change during the early Holocene. Analyses of lithologies, grain-size distributions and elemental compositions revealed that the early Holocene climate in the eastern Mu Us desert experienced at least seven multi-centennial oscillations between humid and arid episodes. Four humid episodes occurred at 10,800 to 10,200cal. (calendar) yr BP, 9800 to 9400cal.yrBP, 8900 to 8300cal.yrBP and 8000 to 7700cal.yrBP. Three arid episodes occurred at 10,200 to 9800cal.yrBP, 9400 to 8900cal.yrBP, and 8300 to 8000cal.yrBP. These humid-arid oscillations were representative of multi-centennial fluctuations in the history of waxing and waning of the East Asian summer monsoon (EASM) and the East Asian winter monsoon (EAWM) in the eastern margin of the Mu Us desert. Comparisons with other climate records based on pollen-based annual precipitation (PANN) reconstructed from Gonghai Lake and the stalagmite δ18O records from Dongge cave, southern China, supported certain influences of the Indian summer monsoon on moisture variations in the eastern margin of the Mu Us desert. Our results also revealed a possible climatic connection between the variability of the East Asian winter monsoons and air temperature changes at high latitudes in the northern hemisphere and North Atlantic thermohaline circulation during the early Holocene. The similarity between early Holocene climate evolution in the eastern Mu Us desert and several proxies (∆14C residuals, sunspot numbers, spectral analysis of grain-size and elemental compositions) supports the hypothesis that variations in solar insolation are a major external force responsible for East Asian Monsoon variations at the multi-centennial scale during the early Holocene.

Extreme storms during the last 6500 years from lagoonal sedimentary archives in the Mar Menor (SE Spain)

Abstract. Storms and tsunamis, which may seriously endanger human society, are amongst the most devastating marine catastrophes that can occur in coastal areas. Many such events are known and have been reported for the Mediterranean, a region where high-frequency occurrences of these extreme events coincides with some of the most densely populated coastal areas in the world. In a sediment core from the Mar Menor (SE Spain), we discovered eight coarse-grained layers which document marine incursions during periods of intense storm activity or tsunami events. Based on radiocarbon dating, these extreme events occurred around 5250, 4000, 3600, 3010, 2300, 1350, 650, and 80 years cal BP. No comparable events have been observed during the 20th and 21st centuries. The results indicate little likelihood of a tsunami origin for these coarse-grained layers, although historical tsunami events are recorded in this region. These periods of surge events seem to coincide with the coldest periods in Europe during the late Holocene, suggesting a control by a climatic mechanism for periods of increased storm activity. Spectral analyses performed on the sand percentage revealed four major periodicities of 1228 ± 327, 732 ± 80, 562 ± 58, and 319 ± 16 years. Amongst the well-known proxies that have revealed a millennial-scale climate variability during the Holocene, the ice-rafted debris (IRD) indices in the North Atlantic developed by Bond et al. (1997, 2001) present a cyclicity of 1470 ± 500 years, which matches the 1228 ± 327-year periodicity evidenced in the Mar Menor, considering the respective uncertainties in the periodicities. Thus, an in-phase storm activity in the western Mediterranean is found with the coldest periods in Europe and with the North Atlantic thermohaline circulation. However, further investigations, such as additional coring and high-resolution coastal imagery, are needed to better constrain the main cause of these multiple events.

Contribution of ENSO variability to the East Asian summer monsoon in the late Holocene

The Asian monsoon (AM) is an important atmosphere circulation system of the global climate system, acting as a bridge across high-northern latitude and tropic climates. However, dynamical origins of AM variations during the Holocene remain unclear. Here we present stable isotope records (δ18O and δ13C) of a Holocene stalagmite from Niu Cave, central China, to address the nature and causes of East Asian summer monsoon (EASM) changes. Our δ18O record shows a gradual weakening of the EASM intensity from the middle to the late Holocene, corresponding with a decrease in Northern Hemisphere summer insolation and an enhancement of ENSO activity. The δ13C profile exhibits similar variability to δ18O record, suggesting that regional hydrological changes were primarily controlled by the large-scale summer monsoon circulation. Power spectrum analyses of δ18O and δ13C records show that the EASM intensity varies at a dominant periodicity of ~830yr, probably correlated to an ENSO-like cycle. Furthermore, millennial-scale shifts in our records exhibit a strong correlation with that in ENSO proxies over the past 3000years. The EASM-ENSO relationship may be related to the shifts in the mean position of Walker circulation and western North Pacific subtropical high. Our finding suggests that millennial-scale monsoon changes in the late Holocene are primarily modulated by changes in tropical ocean–atmosphere circulation, rather than variations in the North Atlantic thermohaline circulation.

Early–Middle Pleistocene transitions: Linking terrestrial and marine realms

Marked by a progressive increase in the amplitude of climate oscillations, an evolving waveform, and a shift towards a quasi-100 ky frequency, the Early–Middle Pleistocene transition (EMPT), previously known as the Mid-Pleistocene Transition (or Mid-Pleistocene Revolution) (1.4–0.4 Ma), represents a fundamental transformation in the Earth's climate state. The EMPT began with a substantial change in climate dynamics and ended with the Mid-Brunhes Event, signaling the establishment of a new steady state. The reasons for the EMPT while uncertain appear to involve a non-linear response of the Earth climate system. The physical and biotic responses to this transition, amplified by the growth of Northern Hemisphere ice sheets, have been profound. Two important chronostratigraphic markers characterize the EMPT, the Jaramillo Subchron (1.070–0.988 Ma) and the Matuyama–Brunhes Chron boundary (∼773 ka). The latter has been chosen as the primary guide for the Lower–Middle Pleistocene Subseries boundary, as it lies at the approximate midpoint of the EMPT and aids in global recognition both in marine and terrestrial deposits. The Jaramillo Subchron has received less attention, but the late Early Pleistocene is important in Europe because it saw the progressive transition from the Villafranchian to Galerian mammal faunas, and expansion of hominins into western and northern Europe. The Jaramillo Subchron is represented by Marine Isotope Stages (MIS) 31 to 28, with MIS 30 already showing the asymmetrical (sawtooth) pattern characteristic of the Middle Pleistocene. Indeed, while variation in the 40-ky band (obliquity) remains strong throughout the EMPT, low frequency variability begins at around 1250–1200 ky, which coincides with a progressive increase in global ice volume. Against a backdrop of increasingly severe glacial cycles, notably during MIS 36, 34, 24–22 (the so-called “0.9 Ma event”), 16 and 12, pronounced phases of warming are also documented globally, including the “super-interglacial” MIS 31.The early phase of the EMPT is characterized by important glaciations beginning with MIS 36 and continuing to MIS 24–22, a major intensification of the East Asian monsoon system, intensification of loess deposition in northern Europe, development of open landscapes in western Siberia, increased fluvial incision, higher amplitude sea-level change, and spread of large mammals across northern Eurasia, and a strong reduction in the North Atlantic thermohaline circulation. In Europe, the loss of thermophilous plant taxa during the EMPT and indeed throughout the Quaternary is a reminder of the progressive cooling that took place here and elsewhere.

Transport of volume, heat, and salt towards the Arctic in the Faroe Current 1993–2013

Abstract. The flow of warm and saline water from the Atlantic Ocean, across the Greenland–Scotland Ridge, into the Nordic Seas – the Atlantic inflow – is split into three separate branches. The most intense of these branches is the inflow between Iceland and the Faroe Islands (Faroes), which is focused into the Faroe Current, north of the Faroes. The Atlantic inflow is an integral part of the North Atlantic thermohaline circulation (THC), which is projected to weaken during the 21st century and might conceivably reduce the oceanic heat and salt transports towards the Arctic. Since the mid-1990s, hydrographic properties and current velocities of the Faroe Current have been monitored along a section extending north from the Faroe shelf. From these in situ observations, time series of volume, heat, and salt transport have previously been reported, but the high variability of the transport has made it difficult to establish whether there are trends. Here, we present results from a new analysis of the Faroe Current where the in situ observations have been combined with satellite altimetry. For the period 1993 to 2013, we find the average volume transport of Atlantic water in the Faroe Current to be 3.8 ± 0.5 Sv (1 Sv = 106 m3 s−1) with a heat transport relative to 0 °C of 124 ± 15 TW (1 TW = 1012 W). Consistent with other results for the Northeast Atlantic component of the THC, we find no indication of weakening. The transports of the Faroe Current, on the contrary, increased. The overall increase over the 2 decades of observation was 9 ± 8 % for volume transport and 18 ± 9 % for heat transport (95 % confidence intervals). During the same period, the salt transport relative to the salinity of the deep Faroe Bank Channel overflow (34.93) more than doubled, potentially strengthening the feedback on thermohaline intensity. The increased heat and salt transports are partly caused by the increased volume transport and partly by increased temperatures and salinities of the Atlantic inflow, which have been claimed mainly to be caused by the weakened subpolar gyre.

Three-Dimensional Structure of Optimal Nonlinear Excitation for Decadal Variability of the Thermohaline Circulation

AbstractThe decadal variability of the North Atlantic thermohaline circulation (THC) is investigated within a three-dimensional ocean circulation model using the conditional nonlinear optimal perturbation method. The results show that the optimal initial perturbations of temperature and salinity exciting the strongest decadal THC variations have similar structures: the perturbations are mainly in the northwestern basin at a depth ranging from −1500 to −3000 m. These temperature and salinity perturbations act as the optimal precursors for future modifications of the THC, highlighting the importance of observations in the northwestern basin to monitor the variations of temperature and salinity at depth. The decadal THC variation in the nonlinear model initialized by the optimal salinity perturbations is much stronger than that caused by the optimal temperature perturbations, indicating that salinity variations might play a relatively important role in exciting the decadal THC variability. Moreover, the deca...

Об устойчивости термохалинной циркуляции Северной Атлантики

Об устойчивости термохалинной циркуляции Северной Атлантики

Signatures of the late Holocene Neoglacial cold event and their marine–terrestrial linkage in the northwestern Pacific margin

Marine microfossil assemblages in core sediments from the northern East China Sea (ECS) were investigated to understand late Holocene paleoclimatic changes in the northwestern Pacific margin. We find a pronounced alternation of ocean condition during the late Holocene characterized by an abrupt decrease in dinoflagellate cysts and Kuroshio water species of planktonic foraminifera centered at ca. 4000–2500 14C yr BP. Compilation and merger of new and previously published data show that this oceanic event corresponds with terrestrial cooling and dry episodes in the northern China. The synchronicity between marine and terrestrial records is considered to be linked to a weakened Kuroshio influence that is in coupled with intensified winter monsoon, highlighting a significance of oceanic-atmospheric dynamics in determining moisture and heat distribution over both oceanic and terrestrial domains. Superimposed on the late Holocene, the synchronicity between this particular climatic shift in the northwestern Pacific and the Neoglacial cold events in the northern high-latitude regions is tentatively indicative of a global climate signal, possibly associated with dynamics of the North Pacific gyre system and the high latitude North Atlantic thermohaline circulation, and therefore positions of the mean latitude of the Kuroshio extension.

Seasonality in the biplot of Northern Hemisphere temperature anomalies

Northern Hemisphere mean monthly temperature anomalies for the 1890–2010 period are examined for seasonality. The statistical method of biplotting visually synthesizes the major features of the time series for the 12 calendar months into a single plot. The common upward trend in all months and the winter–summer temperature contrast capture more than 80% of the total data variance. A temperature seasonalization is established: winter (January–March), summer (May–October) and transitional months (November, December and April). Two uncorrelated factors underlie this seasonality. The first is the North Atlantic thermohaline circulation, which is indicated by the Atlantic Multidecadal Oscillation and which statistically determines the summer temperature anomalies. The second is the cold ocean–warm land pattern, which is indicated by the hemispheric land–ocean temperature contrast and which statistically determines the winter anomalies. The interannual effect of the El Niño–Southern Oscillation, volcanism and also the Arctic Oscillation/North Atlantic Oscillation mode compete with the background trend to produce a few extreme and outlier years.

Effect of early Pliocene uplift on late Pliocene cooling in the Arctic–Atlantic gateway

Despite the undisputed role of the Arctic Ocean in the modern and Pliocene climate system, the Arctic has only recently attracted public awareness that ongoing, fundamental change in the Arctic cryosphere could be a response to global warming. Clarification of the Arcticʼs role in global climate during the Pliocene is, however, largely hampered by equivocal stratigraphic constraints. From a well-dated Pliocene sequence from the Yermak Plateau, off NW Spitsbergen, we present sedimentological and geochemical data indicating that 4 million years ago terrigenous sediment supply and sources changed abruptly in response to a regional tectonic uplift event. We argue that this event together with contemporary uplift and tilting along the northwestern European continental margin preconditioned the landmasses for glacial ice build-up during intensification of the Northern Hemisphere Glaciation (INHG). Our data further suggest that the final deepening/widening of the Arctic–Atlantic gateway, the Fram Strait, between 6.5 and 5 Ma gradually caused increased deep-water mass exchange which, in turn, likely contributed to the intensification of the North Atlantic thermohaline circulation. Coupled to the North Atlantic warm pool as a regional moisture source, declining atmospheric CO2 levels and other feedback mechanisms during the Pliocene, the regional tectonic activities in the high northern latitudes caused decreased summer ablation and thus allowed the initial build-up of glacial ice both in Scandinavia, and the sub-aerially exposed Svalbard/Barents Sea, culminating in the first large-scale coastline-shelf edge glaciations at ∼2.75Ma ago.

Optimal nonlinear excitation of decadal variability of the North Atlantic thermohaline circulation

Nonlinear development of salinity perturbations in the Atlantic thermohaline circulation (THC) is investigated with a three-dimensional ocean circulation model, using the conditional nonlinear optimal perturbation method. The results show two types of optimal initial perturbations of sea surface salinity, one associated with freshwater and the other with salinity. Both types of perturbations excite decadal variability of the THC. Under the same amplitude of initial perturbation, the decadal variation induced by the freshwater perturbation is much stronger than that by the salinity perturbation, suggesting that the THC is more sensitive to freshwater than salinity perturbation. As the amplitude of initial perturbation increases, the decadal variations become stronger for both perturbations. For salinity perturbations, recovery time of the THC to return to steady state gradually saturates with increasing amplitude, whereas this recovery time increases remarkably for freshwater perturbations. A nonlinear (advective) feedback between density and velocity anomalies is proposed to explain these characteristics of decadal variability excitation. The results are consistent with previous ones from simple box models, and highlight the importance of nonlinear feedback in decadal THC variability.