Variability Of Thermohaline Circulation Research Articles

Phosphorus and nitrogen trajectories in the Mediterranean Sea (1950–2030): Diagnosing basin-wide anthropogenic nutrient enrichment

Human activities have significantly modified the inputs of land-derived phosphorus (P) and nitrogen (N) to the Mediterranean Sea (MS). Here, we reconstruct the external inputs of reactive P and N to the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS) over the period 1950–2030. We estimate that during this period the land derived P and N loads increased by factors of 3 and 2 to the WMS and EMS, respectively, with reactive P inputs peaking in the 1980s but reactive N inputs increasing continuously from 1950 to 2030. The temporal variations in reactive P and N inputs are imposed in a coupled P and N mass balance model of the MS to simulate the accompanying changes in water column nutrient distributions and primary production with time. The key question we address is whether these changes are large enough to be distinguishable from variations caused by confounding factors, specifically the relatively large inter-annual variability in thermohaline circulation (THC) of the MS. Our analysis indicates that for the intermediate and deep water masses of the MS the magnitudes of changes in reactive P concentrations due to changes in anthropogenic inputs are relatively small and likely difficult to diagnose because of the noise created by the natural circulation variability. Anthropogenic N enrichment should be more readily detectable in time series concentration data for dissolved organic N (DON) after the 1970s, and for nitrate (NO3) after the 1990s. The DON concentrations in the EMS are predicted to exhibit the largest anthropogenic enrichment signature. Temporal variations in annual primary production over the 1950–2030 period are dominated by variations in deep-water formation rates, followed by changes in riverine P inputs for the WMS and atmospheric P deposition for the EMS. Overall, our analysis indicates that the detection of basin-wide anthropogenic nutrient concentration trends in the MS is rendered difficult due to: (1) the Atlantic Ocean contributing the largest reactive P and N inputs to the MS, hence diluting the anthropogenic nutrient signatures, (2) the anti-estuarine circulation removing at least 45% of the anthropogenic nutrients inputs added to both basins of the MS between 1950 and 2030, and (3) variations in intermediate and deep water formation rates that add high natural noise to the P and N concentration trajectories.

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...

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.

Using data to attribute episodes of warming and cooling in instrumental records

The observed global-warming rate has been nonuniform, and the cause of each episode of slowing in the expected warming rate is the subject of intense debate. To explain this, nonrecurrent events have commonly been invoked for each episode separately. After reviewing evidence in both the latest global data (HadCRUT4) and the longest instrumental record, Central England Temperature, a revised picture is emerging that gives a consistent attribution for each multidecadal episode of warming and cooling in recent history, and suggests that the anthropogenic global warming trends might have been overestimated by a factor of two in the second half of the 20th century. A recurrent multidecadal oscillation is found to extend to the preindustrial era in the 353-y Central England Temperature and is likely an internal variability related to the Atlantic Multidecadal Oscillation (AMO), possibly caused by the thermohaline circulation variability. The perspective of a long record helps in quantifying the contribution from internal variability, especially one with a period so long that it is often confused with secular trends in shorter records. Solar contribution is found to be minimal for the second half of the 20th century and less than 10% for the first half. The underlying net anthropogenic warming rate in the industrial era is found to have been steady since 1910 at 0.07-0.08 °C/decade, with superimposed AMO-related ups and downs that included the early 20th century warming, the cooling of the 1960s and 1970s, the accelerated warming of the 1980s and 1990s, and the recent slowing of the warming rates. Quantitatively, the recurrent multidecadal internal variability, often underestimated in attribution studies, accounts for 40% of the observed recent 50-y warming trend.

20th Century variability of Atlantic Meridional overturning circulation: Planetary wave influences on world ocean surface phosphate utilization and synchrony of small pelagic fisheries

The Atlantic Meridional Overturning Circulation (AMOC), part of the global Thermohaline Circulation (THC), is variable. In the present analysis, an Atlantic Dipole Phosphate Utilization (ADPU) index, related to the existing Atlantic Dipole Sea Surface Temperature Anomaly (ADSA) index, is used to represent 20th century changes in AMOC strength that are applied to global ecosystem variability. ADPU index cycles set the timing for the calculation of six 2° latitude–longitude resolution world ocean maps depicting higher surface phosphate utilization (SPU) in some regions when AMOC is weaker and in other regions when AMOC is stronger. The average of these six maps yields a summary map with a pattern of alternating latitudinal SPU regions differentiated by AMOC strength that exhibits relationships with ocean bathymetry and wind-driven currents through a consideration of the THC deep and shallow limbs. The latitudinal pattern of SPU regions exhibits conceptual associations with sardine (S) and anchovy (A) population ranges off Japan (J), California (C), Peru (P) and South Africa (B). These sardine and anchovy populations have exhibited apparently synchronous fluctuations on decadal scales through at least part of the 20th century that is summarized in a Regime Indicator Series (RIS=(JS+CS+PS+BA)–(JA+CA+PA+BS)) index. In the present analysis based on Food & Agriculture Organization (FAO) catch data, a revised Regime Indicator Series index formulation (RIS3=(JS+CA+PS+BA)–(JA+CS+PA+BS)), in which CS and CA catches reverse positions, is defined. AMOC variability represented in ADPU is significantly correlated with the RIS3 index (no lag but a significant range of 14 years) and four of eight small pelagic fisheries (JS, PS, BA, and JA). The post-1950 RIS3 index is significantly correlated with seven of eight small pelagic fisheries but not CS. When the regional small pelagic fisheries are considered as normalized species differences (S-A), ADPU has significant positive correlations with Japan and Peru, and negative tendencies with California and South Africa, while the RIS3 index has significant positive correlations with Japan and Peru and significant negative correlations with California and South Africa. An extended RIS3 index, with pre-1950 catch data based only on Japan and California, continues a coherent, significantly correlated trend with the APDU index through 1920 (no lag but a significant range of 14 years). Though the mechanisms for multidecadal global synchrony are speculative, the global pattern of cyclical AMOC-related, alternating latitudinal SPU regions through the 20th century and the correlation of the ADPU index with the RIS3 and the extended RIS3 indices suggest a link between varying AMOC strength, ocean fertility and global marine ecosystem response. Signals from AMOC variability due to changes in both deep and shallow limb flow intensities propagate as Rossby and Kelvin waves through the Atlantic Ocean and possibly into the Indo-Pacific Ocean. Similar Rossby and Kelvin wave generation outside the Atlantic Ocean as a result of concurrent but complex global THC variability could reinforce AMOC-related planetary waves and contribute to the multidecadal global synchrony of ocean state and of responsive ecosystems. If present patterns continue into the future decades, a weaker AMOC associated with global warming would favor sardine off Japan and Peru and anchovy off California and South Africa.

Debates on the Causes of Global Warming

The controversy between the IPCC and Non-governmental IPCC (NIPCC) on the attribution of global warming are reviewed. IPCC holds that today’s global warming is mainly due to anthropogenic activities rather than natural variability, which is emphasized by NIPCC. The surface temperature observations since the mid-20th century support the hypothesis of anthropogenic impact, but for the last one hundred years or so, natural forcings such as solar activity, volcanic eruptions and thermohaline circulation variations also have had great influences on the Earth’s climate, especially on inter-decadal timescales. In addition, evidence suggests that the Medieval Warm Period (MWP) and Little Ice Age (LIA) are closely associated with the solar activity over the past 1 thousand years. Over the past 10 thousand years, the North Atlantic cold events and solar activity are closely correlated. Nevertheless, the physical mechanisms of the solar-climate variability and interrelation are not well understood, yet. Notably, a prevailing view recently indicates that galactic cosmic rays may result in climatic cooling through modulating global low cloud cover. However, its process and mechanism need to be further investigated. CitationHuang, J.-B., S.-W. Wang, Y. Luo, et al., 2012: Debates on the causes of global warming.Adv. Clim. Change Res., 3(1), doi: 10.3724/SP.J.1248.2012.00038.

Inter-annual productivity variability in the North Aegean Sea: Influence of thermohaline circulation during the Eastern Mediterranean Transient

The North Aegean inter-annual productivity variability was investigated, by means of a coupled hydrodynamic/biogeochemical model simulation, over a multi-year (1985–2001) period covering the Eastern Mediterranean Transient (EMT). The EMT was a historically significant period (1987–1994), characterized by the massive formation of dense waters in the Aegean and their subsequent southward spreading to the Eastern Mediterranean. Our findings suggest that during the dense water formation events, deep nutrient rich water is exported to the South, affecting the N. Aegean nutrient pool. We show that the open sea productivity variability, driven by the deep nutrient pool entrainment to the euphotic zone, is related to both vertical mixing and nutrient fluxes, induced by thermohaline circulation variability. A negative correlation appears between the two processes, as stronger vertical mixing triggers a higher nutrient export. Process oriented experiments adopting maximum and minimum nutrient fluxes to the Southern Aegean revealed a significant impact on the nutrient pool, counterbalancing the effect of different vertical mixing on primary production. The model results presented good correlation with available satellite SST and Chl-a data. The impact of precipitation and BSW inflow variability on the Aegean salinity and dense water formation was investigated, showing a significant contribution by both preconditioning factors.

Impact of Multidecadal Fluctuations in the Atlantic Thermohaline Circulation on Indo-Pacific Climate Variability in a Coupled GCM

Abstract Using a multicentury integration of the third climate configuration of the Met Office Unified Model (HadCM3), the authors show that naturally occurring fluctuations in the Atlantic’s thermohaline circulation (THC) drive small but statistically significant changes in surface air temperature, sea level pressure, and precipitation over the Indo-Pacific region. The surface temperature component of these variations may be described as an interhemispheric seesaw (consistent with earlier studies), with changes in the Southern Hemisphere smaller than those in the Northern Hemisphere. Links between THC variability and variability related to the interdecadal Pacific oscillation (IPO) are evident: when the THC is strong (weak) the IPO variance decreases (increases) considerably, and cold (La Niña–like) IPO events tend to be stronger and more frequent when the THC is in a weak phase. This highlights the possibility that a small part of Indo-Pacific climate variability at multidecadal time scales, including some of the variability linked to the IPO, may be predictable.

Optimal Surface Excitation of the Thermohaline Circulation

Abstract The amplification of thermohaline circulation (THC) anomalies resulting from heat and freshwater forcing at the ocean surface is investigated in a zonally averaged coupled ocean–atmosphere model. Optimal initial conditions of surface temperature and salinity leading to the largest THC growth are computed, and so are the structures of stochastic surface temperature and salinity forcing that excite maximum THC variance (stochastic optimals). When the THC amplitude is defined as its sum of squares (equivalent to using the standard L2 norm), the nonnormal linearized dynamics lead to an amplification with a time scale on the order of 100 yr. The optimal initial conditions have a vanishing THC anomaly, and the complex amplification mechanism involves the advection of both temperature and salinity anomalies by the mean flow and of the mean temperature and salinity by the anomaly flow. The L2 characterization of THC anomalies leads to physically interesting results, yet to a mathematically singular problem. A novel alternative characterizing the THC amplitude by its maximum value, as often done in general circulation model studies, is therefore introduced. This complementary method is shown to be equivalent to using the L-infinity norm, and the needed mathematical approach is developed and applied to the THC problem. Under this norm, an amplification occurs within 10 yr explained by the classic salinity advective feedback mechanism. The analysis of the stochastic optimals shows that the character of the THC variability may be very sensitive to the spatial pattern of the surface forcing. In particular, a maximum THC variance and long-time-scale variability are excited by a basin-scale surface forcing pattern, while a significantly higher frequency and to some extent a weaker variability are induced by a smooth and large-scale, yet mostly concentrated in polar areas, surface forcing pattern. Overall, the results suggest that a large THC variability can be efficiently excited by atmospheric surface forcing, and the simple model used here makes several predictions that would be interesting to test using more complex models.

Impact of Great Salinity Anomalies on the Low-Frequency Variability of the North Atlantic Climate

Abstract In this paper, it is shown that coherent large-scale low-frequency variabilities in the North Atlantic Ocean—that is, the variations of thermohaline circulation, deep western boundary current, northern recirculation gyre, and Gulf Stream path—are associated with high-latitude oceanic Great Salinity Anomaly events. In particular, a dipolar sea surface temperature anomaly (warming off the U.S. east coast and cooling south of Greenland) can be triggered by the Great Salinity Anomaly events several years in advance, thus providing a degree of long-term predictability to the system. Diagnosed phase relationships among an observed proxy for Great Salinity Anomaly events, the Labrador Sea sea surface temperature anomaly, and the North Atlantic Oscillation are also discussed.

Mechanisms Affecting the Overturning Response in Global Warming Simulations

Abstract Climate models used to produce global warming scenarios exhibit widely diverging responses of the thermohaline circulation (THC). To investigate the mechanisms responsible for this variability, a regional Atlantic Ocean model driven with forcing diagnosed from two coupled greenhouse gas simulations has been employed. One of the coupled models (MPI) shows an almost constant THC, the other (GFDL) shows a declining THC in the twenty-first century. The THC evolution in the regional model corresponds rather closely to that of the respective coupled simulation, that is, it remains constant when driven with the forcing from the MPI model, and declines when driven with the GFDL forcing. These findings indicate that a detailed representation of ocean processes in the region covered by the Atlantic model may not be critical for the simulation of the overall THC changes in a global warming scenario, and specifically that the coupled model’s rather coarse representation of water mass formation processes in the subpolar North Atlantic is unlikely to be the primary cause for the large differences in the THC evolution. Sensitivity experiments have confirmed that a main parameter governing the THC response to global warming is the density of the intermediate waters in the Greenland–Iceland–Norwegian Seas, which in turn influences the density of the North Atlantic Deep Water, whereas changes in the air–sea heat and freshwater fluxes over the subpolar North Atlantic are only of moderate importance, and mainly influence the interannual–decadal variability of THC. Finally, as a consequence of changing surface fluxes, the Labrador Sea convection ceases by about 2030 under both forcings (i.e., even in a situation where the overall THC is stable) indicating that the eventual breakdown of the convection is likely but need not coincide with substantial THC changes.

Sea Ice Amplification

GEOLOGY Numerous, millennial-scale warming episodes, called Dansgaard-Oeschger (D-O) events, punctuated the last glacial period. These events, first discovered in deep ice cores from Greenland, are visible in climate records extending from pole to pole, and in Pacific as well as Atlantic marine sediments. One popular hypothesis about the cause of these abrupt climate warmings invokes changes in the strength of the ocean's thermohaline circulation, which affect ocean heat transport. Such a model, however, cannot explain the size of the temperature swings in Greenland, which were as large as 5° to10°C. Li et al. use an atmospheric general circulation model to show that warming and cooling of the magnitude observed in Greenland can be caused by only small changes in the amount of sea ice around it. Furthermore, the sea ice changes that they suggest would also account for variations in snow accumulation and oxygen isotope composition similar to those measured in ice cores from Greenland. Finally, the amount of sea ice retreat proposed is consistent with forcing either by ocean thermohaline circulation variations, or by changes in surface wind stress in the North Atlantic. Thus, sea ice can provide a positive feedback strong enough to cause warming like that which occurred during D-O events. — HJS Geophys. Res. Lett. 32 , L19702 (2005).

Atlantic Thermohaline Circulation in a Coupled General Circulation Model: Unforced Variations versus Forced Changes

Abstract A 1200-yr unforced control run and future climate change simulations using the Parallel Climate Model (PCM), a coupled atmosphere–ocean–land–sea ice global model with no flux adjustments and relatively high resolution (∼2.8° for the atmosphere and 2/3° for the oceans) are analyzed for changes in Atlantic Ocean circulations. For the forced simulations, historical greenhouse gas and sulfate forcing of the twentieth century and projected forcing for the next two centuries are used. The Atlantic thermohaline circulation (THC) shows large multidecadal (15–40 yr) variations with mean-peak amplitudes of 1.5–3.0 Sv (1 Sv ≡ 106 m3 s−1) and a sharp peak of power around a 24-yr period in the control run. Associated with the THC oscillations, there are large variations in North Atlantic Ocean heat transport, sea surface temperature (SST) and salinity (SSS), sea ice fraction, and net surface water and energy fluxes, which all lag the variations in THC strength by 2–3 yr. However, the net effect of the SST and SSS variations on upper-ocean density in the midlatitude North Atlantic leads the THC variations by about 6 yr, which results in the 24-yr period. The simulated SST and sea ice spatial patterns associated with the THC oscillations resemble those in observed SST and sea ice concentrations that are associated with the North Atlantic Oscillation (NAO). The results suggest a dominant role of the advective mechanism and strong coupling between the THC and the NAO, whose index also shows a sharp peak around the 24-yr time scale in the control run. In the forced simulations, the THC weakens by ∼12% in the twenty-first century and continues to weaken by an additional ∼10% in the twenty-second century if CO2 keeps rising, but the THC stabilizes if CO2 levels off. The THC weakening results from stabilizing temperature increases that are larger in the upper and northern Atlantic Ocean than in the deep and southern parts of the basin. In both the control and forced simulations, as the THC gains (loses) strength and depth, the separated Gulf Stream (GS) moves southward (northward) while the subpolar gyre centered at the Labrador Sea contracts from (expands to) the east with the North Atlantic Current (NAC) being shifted westward (eastward). These horizontal circulation changes, which are dynamically linked to the THC changes, induce large temperature and salinity variations around the GS and NAC paths.

Passive mechanism of decadal variation of thermohaline circulation

The decadal variation of thermohaline circulation (THC) is investigated in a simple coupled ocean‐atmospheric two‐box model with the new approach of conditional nonlinear optimal perturbation (CNOP). First, the nonlinear optimal initial perturbations with different constrains are found by this approach. There are two different types of perturbations in the nonlinear regime. One is the freshwater flux perturbation, which is the CNOP of THC and has stronger amplification. The other is salinity flux perturbation, whose amplification is weaker. Freshwater (salt) perturbations weaken (enhance) the mean circulation and hence weaken (enhance) the stability of THC. Second, the passive variabilities of THC are investigated by superposing the initial perturbations to the thermohaline circulation. The passive variabilities found in this model are due to nonnormal and nonlinear growth of initial perturbations. These variabilities, measured as recovering time of perturbations, can cause decadal variability of THC. The results of this paper suggest that CNOP approach is applicable to the investigation of the dependence of the THC sensitivity on the background climate state and is of potential application to the interpretation of past climate change linked to THC variations in nonlinear regime.

Mechanism of Interdecadal Thermohaline Circulation Variability in a Coupled Ocean–Atmosphere GCM

Abstract Interdecadal variability of the Atlantic thermohaline circulation (THC) is studied in the third version of the Hadley Centre global coupled atmosphere–ocean sea-ice general circulation model (HadCM3). A diagnostic approach is used to elucidate the mechanism that governs the variability and its impacts on climate. An irregular and heavily damped THC oscillation with a period around 25 yr is identified. The oscillation appears to be forced by the atmosphere but the ocean is responsible for setting the time scale. Following a minimum in the THC, the mechanism for phase reversal involves the accumulation of cold water in the subpolar gyre, leading to an acceleration of the gyre circulation and the North Atlantic Current. This acceleration increases the transport of saline waters into the regions of active deep convection, raising the upper-ocean density and leading, after adjustment, to acceleration of the THC. The atmosphere stimulates this THC variability in two ways: 1) by forcing the subpolar gyre through (North Atlantic Oscillation) NAO-related wind stress curl and heat flux anomalies; and 2) by direct forcing of the region of active deep convection, also through wind stress curl and heat flux anomalies. The latter is not closely related to the NAO. The mechanism for phase reversal has many similarities to that found in a previous study with a much lower resolution coupled model, suggesting that this mechanism may be quite robust. However the time scale, and details of the atmospheric forcing, differ. The THC variability in HadCM3 has significant impacts on the atmosphere not just in the Atlantic region but also more widely, throughout the global Tropics. The mechanism involves modulation by the THC of the cross-equator SST gradient in the tropical Atlantic. The SST anomalies induce a displacement of the ITCZ in the Atlantic basin with knock-on effects over the other ocean basins. These findings highlight the potential importance of the Atlantic THC as a cause of interdecadal climate variability on a global scale.

Low-Latitude Freshwater Influence on Centennial Variability of the Atlantic Thermohaline Circulation

Abstract Variability of the thermohaline circulation (THC) has been analyzed in a long control simulation by the Met Office's Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It is shown that internal THC variability in the coupled climate system is concentrated at interannual and centennial time scales, with the centennial mode being dominant. Centennial oscillations of the THC can impact surface climate via an interhemispheric SST contrast of 0.1°C in the Tropics and more than 0.5°C in mid- and high latitudes. A mechanism is proposed based on detailed process analysis involving large-scale air–sea interaction on multidecadal time scales. Anomalous northward ocean heat transport associated with a strong phase of the Atlantic THC generates a cross-equatorial SST gradient. This causes the ITCZ to move to a more northerly position with increased strength. The extra rainfall resulting from the anomalous ITCZ imposes a freshwater flux and produces a salinity anomaly in the tropical North Atlantic. Such sustained salinity anomalies slowly propagate toward the subpolar North Atlantic at a lag of 5–6 decades. The accumulated low-salinity water lowers upper-ocean density, which causes the THC to slow down. The oscillation then enters the opposite phase.

A “triple sea‐ice state” mechanism for the abrupt warming and synchronous ice sheet collapses during Heinrich events

Abrupt, switch‐like, changes in sea ice cover are proposed as a mechanism for the large‐amplitude abrupt warming that seemed to have occurred after each Heinrich event. Sea ice changes are also used to explain the colder‐than‐ambient glacial conditions around the time of the glacier discharge. The abrupt warming events occur in this mechanism, owing to rapid sea ice melting which warmed the atmosphere via the strong sea ice albedo and insulating feedbacks. Such abrupt sea ice changes can also account for the warming observed during Dansgaard‐Oeschger events. The sea ice changes are caused by a weak (order of 5 Sv) response of the thermohaline circulation (THC) to glacier discharges. The main point of this work is therefore that sea ice may be thought of as a very effective amplifier of a weak THC variability, explaining the abrupt temperature changes over Greenland. Synchronous ice sheet collapses from different ice sheets around the North Atlantic, indicated by some proxy records, are shown to be possible via the weak coupling between the different ice sheets by the atmospheric temperature changes caused by the sea ice changes. This weak coupling can lead to a “nonlinear phase locking” of the different ice sheets which therefore discharge synchronously. It is shown that the phase locking may also lead to “precursor” glacier discharge events from smaller ice sheets before the Laurentide Ice Sheet discharges. The precursor events in this mechanism are the result rather than the cause of the major glacier discharges from the Laurentide Ice Sheet.

Detecting thermohaline circulation changes from ocean properties in a coupled model

[1] Significant changes of the thermohaline circulation (THC) are likely to cause abrupt climate change. Here we intend to find a simple measure to detect changes in THC through examining several factors proposed to control the THC variations using a coupled climate model. These factors are equatorial-South Atlantic upper ocean temperature, Southern Ocean freshening, inter-basin sea surface salinity contrast, and meridional steric height gradient. Three experiments are analyzed – a present-day control run, a freshwater hosing run and a 1% CO2 run. Results show that if freshwater flux is the primary cause, all examined factors can predict the THC changes. If both thermal and haline forcings are involved, only the Atlantic meridional steric height gradient gives a consistent measure of the THC variations. A new result presented here is that the inter-basin sea surface temperature contrast between North Atlantic and North Pacific is found to be an indicator of THC changes.

Multi-decadal thermohaline variability in an ocean–atmosphere general circulation model

A century scale integration of a near-global atmosphere–ocean model is used to study the multi-decadal variability of the thermohaline circulation (THC) in the Atlantic. The differences between the coupled and two supplementary ocean-only experiments suggest that a significant component of this variability is controlled by either a collective behavior of the ocean and the atmosphere, particularly in the form of air-sea heat exchange, or sub-monthly random noise present in the coupled system. Possible physical mechanisms giving rise to the mode of this THC variability are discussed. The SST anomaly associated with the THC variability resembles an interdecadal SST pattern extracted from observational data, as well as a pattern associated with the 50–60 year THC variability in the GFDL coupled model. In each case, a warming throughout the subpolar North Atlantic but concentrated along the Gulf Stream and its extension is indicated when the THC is strong. Concomitantly, surface air temperature has positive anomalies over the warmer ocean, with the strongest signal located downwind of the warmest SST anomalies and intruding into the western Eurasian Continent. In addition to the thermal response, there are also changes in the atmospheric flow pattern. More specifically, an anomalous northerly wind develops over the Labrador Sea when the THC is stronger than normal, suggesting a local primacy of the atmospheric forcing in the thermohaline perturbation structure.

Oceanic and atmospheric variations during the last deglaciation in the Fram Strait (Arctic Ocean): a coupled high-resolution organic-geochemical and sedimentological study

From a high sedimentation rate core, obtained from the Yermak slope (Arctic Ocean), we deduced short-term climatic fluctuations of the last deglaciation lasting from 17,000 to 9000 years BP. By using a coupled organic-geochemical and sedimentological approach, we reconstructed near-bottom current conditions and assessed their significance in relation with comparable global thermohaline circulation patterns and surface water conditions. Highest concentrations and fluctuations of specific marine phytoplankton biomarkers were obtained in the Bølling-Allerød, when the major deglaciation of the Svalbard-Barents-Sea-Ice-Sheet (SBIS) occurred. The marine biomarker record and the sedimentological record are consistent with climate reconstructions obtained from Greenland ice cores and thermohaline circulation variations. However, beginning with the Younger Dryas (YD) episode, bottom currents decreased during this time period, but surface waters remained very productive. The atmospheric conditions of the western Fram Strait (reconstructed from biomarker data) and thermohaline circulation signals (reconstructed from sedimentological data) were decoupled in the YD. Maximum insolation values of solar radiation and “Föhn”-like winds enabled open water conditions and increased productivity values in the northern Fram Strait, at least in summer.