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.

Atlantic Thermohaline Circulation: from Lorenz model of atmospheric convection toward Ocean Dynamics

Atlantic Thermohaline Circulation: from Lorenz model of atmospheric convection toward Ocean Dynamics

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

Influences on Asian summer monsoon during Dansgaard-Oeschger events 19 to 25 (70–115 kyr B.P.)

The climate variability during Dansgaard-Oeschger (D-O) events 19 to 25 (first recognized in the Greenland ice cores) was recorded in the Asian monsoon region, but the climatological dynamic mechanism is still poorly understood. Here, we present a new 230 Th absolute chronology and high-resolution record of the Asian summer monsoon (ASM) from 117 to 69 kyr B.P. based on 25 230 Th ages, 803 oxygen and carbon isotopes , as well as 493 trace element ratios of the YYZ1 stalagmite from Yangzi Cave in Fengdu County, 120 km from Chongqing City, China. The δ 18 O record supports a strong ASM during marine isotope stages (MIS) 5a and 5c and a weak ASM during MIS 4, 5b, and 5d. Millennial-scale oscillations of D-O 19–25 were also recorded in our speleothem δ 18 O. Multiple environmental proxies, δ 13 C and trace element records of YYZ1 stalagmite can also reflect local climate and environmental changes. A drought event was clearly identified between 93.0 and 91.5 kyr B.P., referred to as the 92 kyr event, which exhibited a rapid and abrupt fluctuation (V-shaped) based on our multi-proxy results. We found that D-O 25 event was not obvious in our record and in other Chinese speleothem records. The D-O 25 event occurred within a period of decreasing North Hemisphere summer insolation (NHSI), which led to different climate backgrounds, e.g., the increase of ice volume and the corresponding weakening of the Northern Atlantic thermohaline circulation . Conversely, the D-O events within the high NHSI period or the warm stages (MIS5a, MIS5c) exhibited more obvious fluctuations than those in the low NHSI period or the cool stages (MIS5b, MIS5d). Thus, we suggest that the differences between D-O events are mainly related to the change of the ASM in response to variable NHSI. Furthermore, the ASM during D-O events was also influenced by climate variations in the Southern Hemisphere (SH) and low latitudes, which should not be ignored. • Variations in Asian summer monsoon (ASM) during MIS 5 were reconstructed in multi-proxy speleothem records. • A drought event was identified in 92 ka. • Most stalagmites records in China showed smaller trends in δ 18 O changes of DO 25 event. • Weak DO events were related to environmental changes at lower latitudes.

MOW strengthening and contourite development over two analog climate cycles (MIS 12–11 and MIS 2–1) in the Gulf of Cadíz: An impact on North Atlantic climate during deglaciation V and MIS 11?

High-resolution records from International Ocean Discovery Program (IODP) sites U1386 and U1387 drilled during IODP Expedition 339 into the Faro drift, made it possible to assess the impact of intensifications of the upper core (MOWU) of the Mediterranean Outflow Water (MOW) and of changes in sediment supply on the sedimentation in the northern Gulf of Cadíz since the Middle Pleistocene. This work focuses on the comparison of records covering Marine Isotope Stage (MIS) 2–1 and MIS 12–11, in order to investigate the behaviour and circulation regime of the MOWU over two climatic cycles of similar astronomical configurations and their associated deglaciation. The analysis of facies established on the basis of grain size, XRF core-scanning, and carbonate content revealed contourite beds formed by the MOWU during MIS 11 and MIS 1 and deglaciations (deglaciation V and I). Contourite sequences show that MOWU velocity at the seabed was higher during MIS 2–1 than during MIS 12–11, and that sediment supply was different between these two climatic cycles. While overall low during MIS 12–11, MOWU intensity increased during deglaciation V and MIS 11 and preceded large ice rafted events and cooling in the North Atlantic Ocean. As a major element of the MOW, MOWU strengthening during deglaciation V likely contributed to higher heat and moisture transport towards the high latitudes inducing a slight increase of calving and size of boreal ice sheets. The MOW-derived injection of heat and salt in the North Atlantic Ocean during deglaciation V might have contributed, through reactivation of the upper AMOC, to the switch of the Atlantic thermohaline circulation from a glacial to an interglacial mode.

Deglacial variability of South China hydroclimate heavily contributed by autumn rainfall

The deglacial hydroclimate in South China remains a long-standing topic of debate due to the lack of reliable moisture proxies and inconsistent model simulations. A recent hydroclimate proxy suggests that South China became wet in cold stadials during the last deglaciation, with the intensification proposed to be contributed mostly by the East Asian summer monsoon (EASM). Here, based on a deglacial simulation in a state-of-the-art climate model that well reproduces the evolution of EASM, winter monsoon (EAWM) and the associated water isotopes in East Asia, we propose that the intensified hydroclimate in South China is also contributed heavily by the rainfall in autumn, during the transition between EASM and EAWM. The excessive rainfall in autumn results from the convergence between anomalous northerly wind due to amplified land-sea thermal contrast and anomalous southerly wind associated with the anticyclone over Western North Pacific, both of which are, in turn, forced by the slowdown of the Atlantic thermohaline circulation. Regardless the rainfall change, however, the modeled δ18Op remains largely unchanged in autumn. Our results provide new insights to East Asia monsoon associated with climate change in the North Atlantic.

Feedback processes modulating the sensitivity of Atlantic thermohaline circulation to freshwater forcing timescales

AbstractPaleo proxy records indicate that abrupt changes in thermohaline circulation (THC) were induced by rapid meltwater discharge from retreating ice sheets. Such abrupt changes in the THC have been understood as a hysteresis behavior of nonlinear system. Previous studies, however, primarily focused on a near-static hysteresis under fixed or slowly varying freshwater forcing (FWF), reflecting the equilibrated response of the THC. This study aims to improve the current understanding of transient THC responses under rapidly varying forcing and its dependency on forcing timescales. The results simulated by an Earth system model suggest that the bifurcation is delayed as the forcing timescale is shorter, causing the Atlantic meridional overturning circulation collapse (recovery) to occur at higher (lower) FWF values. The delayed shutdown/recovery occurs because bifurcation is determined not by the FWF value at the time but by the total amount of freshwater remaining over the THC convection region. The remaining freshwater amount is primarily determined by the forcing accumulation (i.e., time-integrated FWF), which is modulated by the freshwater/salt advection by ocean circulations and freshwater flux by the atmospheric hydrological cycle. In general, the latter is overwhelmed by the former. When the forced freshwater amount is the same, the modulation effect is stronger under slowly varying forcing because more time is provided for the feedback processes.

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

Portraying the Impact of the Tibetan Plateau on Global Climate

AbstractAs the most extensive highland in the world, the Tibetan Plateau (TP) plays an important role in shaping the global climate. Quantifying the effect of the TP on global climate is the first step for a full understanding of the TP’s standing on planet Earth. Through coupled model sensitivity experiments, we draw a panorama of the TP’s global impact in this paper. Our model results show that the absence of the TP would result in a 4°C colder and 10% drier climate in the Northern Hemisphere (NH). The TP has a striking remote effect on the North Atlantic. Removing the TP would enhance the westerlies in the mid- to high latitudes of the NH and weaken the easterlies over the tropical Pacific. More moisture would be relocated from the tropical Pacific to the North Atlantic, shutting down the Atlantic thermohaline circulation, which would eventually result in more than 15°C colder and 20% drier climate over the North Atlantic. Our model results suggest that the presence of the TP may have contributed greatly to the hospitable modern climate in the NH, by promoting the establishment of the thermohaline circulation in the Atlantic, and therefore enhancing the northward ocean heat transport and atmosphere moisture transport across the equator.

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.

State estimation comparison for a high-dimensional nonlinear system by particle-based filtering methods

The sequential filtering scheme provides a suitable framework for estimating and tracking geophysical states of systems as new data become available online. Mathematical foundations of sequential Bayesian filtering are reviewed with emphasis on practical issues for both particle filters and Kalman-based filters. In this study, we further investigate the study of Kim (2005) such that the sequential Importance resampling method (SIR), Ensemble Kalman Filter (EnKF), and the Maximum Entropy Filter (MEF) are tested in a relatively high dimensional ocean model that conceptually represents the Atlantic thermohaline circulation. The model exhibits large-amplitude transitions between strong (thermo-dominated) and weak (salinity-dominated) circulations that represent climate states between ice-age and normal climate.The performance of the particle-based schemes is compared with the convergent results from SIR based on measurement errors, observation locations, and particle sizes in various sets of twin experiments. The sensitivity analysis shows strength and weakness of each filtering method when applied to multimodal non-linear systems. As the number of particles is increased, SIR achieves the convergent results that are mathematically optimal solutions. EnKF shows suboptimal results regardless of sample sizes, and MEF achieves the optimal solution even with a small sample size. Both EnKF, and MEF produces robust results with a relatively small sample size or increased measurement locations. Small measurement errors or short intervals of observations (or, more frequent observations) significantly improve the performances of SIR and EnKF, and MEF still show robust results even with a relatively small sample size or sparse measurement locations when the system experiences the transition between one region to the other region.

An Oldest Dryas glacier expansion on Mount Pelister (Former Yugoslavian Republic of Macedonia) according to 10 Be cosmogenic dating

We provide a geomorphological analysis of a glacial valley on Mount Pelister, in Macedonia. Three boulders from a frontal moraine were dated with the cosmogenic nuclide isotope 10 Be. The results demonstrate that the boulders have been exposed since 15.24 ± 0.85 ka. This age constrains the formation of the frontal moraine to the Oldest Dryas cold event. This age fits with that of other glacier deposits dated to the Older Dryas in the Alps, Balkans, Carpathians and Turkish mountains. The Pelister palaeoglacier has been reconstructed and its equilibrium line altitude extracted, returning a value of 2250 m a.s.l. This is in good agreement with the equilibrium line altitudes of most other reconstructed glaciers of the same age in the circum-Mediterranean mountains, demonstrating a comparable response to the Oldest Dryas event. Other palaeoenvironmental records near the Pelister mountain indicate that the Older Dryas was here characterized by a cold and remarkably dry event. The temporal relationship between Older Dryas glacier advances in the Balkan region and recorded changes in the Atlantic thermohaline circulation during the Laurentide Ice Sheet massive ice discharge (H1 event) confirms the strong climatic link between the pan-Mediterranean regions and the North Atlantic Ocean. Supplementary material: Geochemical laboratory results are available at http://doi.org/10.6084/m9.figshare.c.3830254

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.

Water mass circulation and weathering inputs in the Labrador Sea based on coupled Hf–Nd isotope compositions and rare earth element distributions

The Labrador Sea is one of the key areas for deep water formation driving the Atlantic thermohaline circulation and thus plays an important role in Northern Hemisphere climatic fluctuations. In order to better constrain the overturning processes and the origins of the distinct water masses, combined dissolved Hf–Nd isotopic compositions and rare earth element (REE) distribution patterns were obtained from four water depth profiles along a section across the Labrador Sea. These were complemented by one surface sample off the southern tip of Greenland, three shallow water samples off the coast of Newfoundland, and two deep water samples off Nova Scotia.Although light REEs are markedly enriched in the surface waters off the coast of Newfoundland compared to north Atlantic waters, the REE concentration profiles are essentially invariant throughout the water column across the Labrador Sea. The hafnium concentrations of surface waters exhibit a narrow range between 0.6 and 1pmol/kg but are not significantly higher than at depth.Neodymium isotope signatures (ɛNd) vary from unradiogenic values between −16.8 and −14.9 at the surface to more radiogenic values near −11.0 at the bottom of the Labrador Sea mainly reflecting the advection of the Denmark Strait Overflow Water and North East Atlantic Deep Water, the signatures of which are influenced by weathering contributions from Icelandic basalts. Unlike Nd, water column radiogenic Hf isotope signatures (ɛHf) are more variable representing diverse weathering inputs from the surrounding landmasses. The least radiogenic seawater ɛHf signatures (up to −11.7) are found in surface waters close to Greenland and near the Canadian margin. This reflects the influence of recirculating Irminger Current Waters, which are affected by highly unradiogenic inputs from Greenland. A three to four ɛHf unit difference is observed between Denmark Strait Overflow Water (ɛHf∼−4) and North East Atlantic Deep Water (ɛHf∼−0.1), although their source waters have essentially the same ɛNd signature. This most likely reflects different weathering signals of hafnium delivered to Denmark Strait Overflow Water and North East Atlantic Deep Water (incongruent weathering of old rocks from Greenland versus basaltic rocks from Iceland). In addition, the ɛHf data resolve two layers within the main body of Labrador Sea Water not visible in the ɛNd distribution, which are shallow Labrador Sea Water (ɛHf∼−2) and deep Labrador Sea Water (ɛHf∼−4.5). The latter layer was formed between the late 1980’s and mid 1990’s during the last cold state of the Labrador Sea and underwent substantial modification since its formation through the admixture of Irminger Water, Iceland Slope Water and North East Atlantic Deep Water, which is reflected in its less radiogenic ɛHf signature. The overall behavior of Hf in the water column suggests its higher sensitivity to local changes in weathering inputs on annual to decadal timescales. Although application of Hf isotopes as a tracer for global water mass mixing is complicated by their susceptibility to incongruent weathering inputs they are a promising tracer of local processes in restricted basins such as the Labrador Sea.

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.