Strength Of Thermohaline Circulation Research Articles

Understanding Bjerknes Compensation in Atmosphere and Ocean Heat Transports Using a Coupled Box Model

AbstractA coupled box model is used to study the compensation between atmosphere and ocean heat transports. An analytical solution to the Bjerknes compensation (BJC) rate, defined as the ratio of anomalous atmosphere heat transport (AHT) to anomalous ocean heat transport (OHT), is obtained. The BJC rate is determined by local feedback between surface temperature and net heat flux at the top of atmosphere (TOA) and the AHT efficiency. In a stable climate that ensures global energy conservation, the changes between AHT and OHT tend to be always out of phase, and the BJC is always valid. This can be demonstrated when the climate is perturbed by freshwater flux. The BJC in this case exhibits three different behaviors: the anomalous AHT can undercompensate, overcompensate, or perfectly compensate the anomalous OHT, depending on the local feedback. Stronger negative local feedback will result in a lower BJC rate. Stronger positive local feedback will result in a larger overcompensation. If zero climate feedback occurs in the system, the AHT will compensate the OHT perfectly. However, the BJC will fail if the climate system is perturbed by heat flux. In this case, the changes in AHT and OHT will be in phase, and their ratio will be closely related to the mean AHT and OHT. In a more realistic situation when the climate is perturbed by both heat and freshwater fluxes, whether the BJC will occur depends largely on the interplay among meridional temperature and salinity gradients and the thermohaline circulation strength. This work explicitly shows that the energy conservation is the intrinsic mechanism of BJC and establishes a specific link between radiative feedback and the degree of compensation. It also implies a close relationship between the energy balance at the TOA and the ocean thermohaline dynamics.

Wind effect on the Atlantic meridional overturning circulation via sea ice and vertical diffusion

Effects of wind and fresh water on the Atlantic meridional overturning circulation (AMOC) are investigated using a fully coupled climate model. The AMOC can change significantly when perturbed by either wind stress or freshwater flux in the North Atlantic. This study focuses on wind stress effect. Our model results show that the wind forcing is crucial in maintaining the AMOC. Reducing wind forcing over the ocean can cause immediately weakening of the vertical salinity diffusion and convection in the mid-high latitudes Atlantic, resulting in an enhancement of vertical salinity stratification that restrains the deep water formation there, triggering a slowdown of the thermohaline circulation. As the thermohaline circulation weakens, the sea ice expands southward and melts, providing the upper ocean with fresh water that weakens the thermohaline circulation further. The wind perturbation experiments suggest a positive feedback between sea-ice and thermohaline circulation strength, which can eventually result in a complete shutdown of the AMOC. This study also suggests that sea-ice variability may be also important to the natural AMOC variability on decadal and longer timescales.

The Role of Northern Sea Ice Cover for the Weakening of the Thermohaline Circulation under Global Warming

Abstract An increase in atmospheric CO2 concentration and the resulting global warming are typically associated with a weakening of the thermohaline circulation (THC) in model scenarios. For the models participating in the Coupled Model Intercomparison Project (CMIP), this weakening shows a significant (r = 0.62) dependence on the initial THC strength; it is stronger for initially strong overturning. The authors propose a physical mechanism for this phenomenon based on an analysis of additional simulations with the coupled climate models CLIMBER-2 and CLIMBER-3α. The mechanism is based on the fact that sea ice cover greatly reduces heat loss from the ocean. The extent of sea ice is strongly influenced by the near-surface atmospheric temperature (SAT) in the North Atlantic but also by the strength of the THC itself, which transports heat to the convection sites. Consequently, sea ice tends to extend farther south for weaker THC. Initially larger sea ice cover responds more strongly to atmospheric warming; thus, sea ice retreats more strongly for an initially weaker THC. This sea ice retreat tends to strengthen (i.e., stabilize) the THC because the sea ice retreat allows more oceanic heat loss. This stabilizing effect is stronger for runs with weak initial THC and extensive sea ice cover. Therefore, an initially weak THC weakens less under global warming. In contrast to preindustrial climate, sea ice melting presently plays the role of an external forcing with respect to THC stability.

Changes in Atlantic Thermohaline Circulation under Different Atmospheric CO 2 Scenarios in a Climate Model

The changes in the thermohaline circulation (THC) because of the increased CO2 in the atmosphere play an important role in future climate regimes. In this article, a new climate model developed at the Max-Planck Institute for Meteorology is used to study the variation in THC strength, the changes of North Atlantic deep-water (NADW) formation, and the regional responses of the THC in the North Atlantic to increasing atmospheric CO2. From 2000 to 2100, under increased CO2 scenarios (B1, A1B, and A2), the strength of THC decreases by 4 Sv (106 m3/s), 5.1 Sv, and 5. 2 Sv, respectively, equivalent to a reduction of 20%, 25%, and 25. 1% of the present THC strength. The analyses show that the oceanic deep convective activity significantly strengthens in the Greenland-Iceland-Norway (GIN) Seas owing to saltier (denser) upper oceans, whereas weakens in the Labrador Sea and in the south of the Denmark Strait region (SDSR) because of surface warming and freshening due to global warming. The saltiness of the GIN Seas is mainly caused by the increase of the saline North Atlantic inflow through the Faro-Bank (FB) Channel. Under the scenario A1B, the deep-water formation rate in the North Atlantic decreases from 16.2 Sv to 12.9 Sv with increasing CO2.

Analyses of the predicted changes of the global oceans under the increased greenhouse gases scenarios

A new climate model (ECHAM5/MPI-OM1) developed for the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC) at Max-Planck Institute for Meteorology is used to study the climate changes under the different increased CO2 scenarios (B1, A1B and A2). Based on the corresponding model results, the sea surface temperature and salinity structure, the variations of the thermohaline circulation (THC) and the changes of sea ice in the northern hemisphere are analyzed. It is concluded that from the year of 2000 to 2100, under the B1, A1B and A2 scenarios, the global mean sea surface temperatures (SST) would increase by 2.5°C, 3.5°C and 4.0°C respectively, especially in the region of the Arctic, the increase of SST would be even above 10.0°C; the maximal negative value of the variation of the fresh water flux is located in the subtropical oceans, while the precipitation in the eastern tropical Pacific increases. The strength of THC decreases under the B1, A1B and A2 scenarios, and the reductions would be about 20%, 25% and 25.1% of the present THC strength respectively. In the northern hemisphere, the area of the sea ice cover would decrease by about 50% under the A1B scenario.

Sensitivity of the thermohaline circulation to Arctic Ocean runoff

Arctic Ocean river runoff increases over the 20th century raise concerns of the potential impact it may have on the thermohaline circulation (THC) and thus global climate. This study investigates how changes in Arctic river discharge may control THC by a series of experiments with an intermediate complexity global climate model. The experiments show an inverse relationship between THC strength and changes to riverine freshwater discharge, similar to the response of THC to surface freshening of the North Atlantic. Arctic Ocean freshwater export and volume were more sensitive to river runoff than sea ice export. A strong linear relationship between the THC strength and the steric height gradient (depth integrated density anomaly and an important driver for the western boundary current) suggests that the Arctic freshwater pools and fluxes are very effective in translating changes in runoff to THC strength by regulating the ocean water density in the North Atlantic.

A signature of persistent natural thermohaline circulation cycles in observed climate

Analyses of global climate from measurements dating back to the nineteenth century show an ‘Atlantic Multidecadal Oscillation’ (AMO) as a leading large‐scale pattern of multidecadal variability in surface temperature. Yet it is not possible to determine whether these fluctuations are genuinely oscillatory from the relatively short observational record alone. Using a 1400 year climate model calculation, we are able to simulate the observed pattern and amplitude of the AMO. The results imply the AMO is a genuine quasi‐periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation (THC). This relationship suggests we can attempt to reconstruct past THC changes, and we infer an increase in THC strength over the last 25 years. Potential predictability associated with the mode implies natural THC and AMO decreases over the next few decades independent of anthropogenic climate change.

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.

Dynamic sea level changes following changes in the thermohaline circulation

Using the coupled climate model CLIMBER-3α, we investigate changes in sea surface elevation due to a weakening of the thermohaline circulation (THC). In addition to a global sea level rise due to a warming of the deep sea, this leads to a regional dynamic sea level change which follows quasi-instantaneously any change in the ocean circulation. We show that the magnitude of this dynamic effect can locally reach up to ~1 m, depending on the initial THC strength. In some regions the rate of change can be up to 20–25 mm/yr. The emerging patterns are discussed with respect to the oceanic circulation changes. Most prominent is a south-north gradient reflecting the changes in geostrophic surface currents. Our results suggest that an analysis of observed sea level change patterns could be useful for monitoring the THC strength.

海洋大循環と気候変動

The oceans play important roles in the formation of the earth's climate, and in climatic variations and change. One of these important roles is meridional heat transport due to oceanic general circulation, especially three-dimensional thermohaline circulation through the entire water column. The oceans transport a huge amount of heat-as much as that of the atmosphere. Accordingly, the north-south gradient of surface air temperature is greatly mitigated. Corresponding to global warming due to increases of greenhouse gasses, the oceanic heat content hasincreased sharply. Global warming might cause a weakening of the strength of thermohaline circulation.

Bistability of the thermohaline circulation identified through comprehensive 2-parameter sweeps of an efficient climate model

The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic–Pacific moisture flux (ΔF a ) and atmospheric moisture diffusivity (K q ). In a large ensemble of spinup experiments, the values of ΔF a and K q are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor “On” and “Off” states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of ΔF a and K q , reveal the co-existence of two stable THC states (Conveyor “On” and “Off”)—i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a “Conveyor On” state, irreversibly collapses to a “Conveyor Off” state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.

A Theory for the Surface Atlantic Response to Thermohaline Variability

Abstract The response of the upper, warm limb of the thermohaline circulation in the North Atlantic to a rapid change in deep-water formation at high latitudes is investigated using a reduced-gravity ocean model. Changes in deep-water formation rate initiate Kelvin waves that propagate along the western boundary to the equator on a timescale of months. The response in the North Atlantic is therefore rapid. The Southern Hemisphere response is much slower, limited by a mechanism here termed the &ldquo=uatorial buffer.” Since to leading order the flow is in geostrophic balance, the pressure anomaly decreases in magnitude as the Kelvin wave moves equatorward, where the Coriolis parameter is lower. Together with the lack of sustained pressure gradients along the eastern boundary, this limits the size of the pressure field response in the Southern Hemisphere. Interior adjustment is by the westward propagation of Rossby waves, but only a small fraction of the change in thermohaline circulation strength is commun...

Fluxes of 230Th and 231Pa to the deep sea: implications for the interpretation of excess 230Th and 231Pa/ 230Th profiles in sediments

Analysis of samples obtained with deep-sea moored sediment traps deployed at 15 sites representing a wide range of oceanic conditions confirms that the flux of 230Th scavenged to the seafloor remains close to its production rate from the decay of 234U in the overlying water column, and generally validates the use of 230Th as a normalizing tool for paleoflux reconstruction. After correction for trapping efficiency, the flux of 230Th measured in the low flux regions amounts to 90±6% of the production rate, with the notable exceptions of one site near the Arabian Sea upwelling and one site in the Weddell Sea. A 230Th flux equivalent to 120% of the production rate was found in Panama Basin. Similar or more extensive scavenging of 230Th may be occurring at a Pacific margin site off California and south of the Polar Front, but these estimates are obscured by large errors on our trapping-efficiency estimates. In contrast, the flux of 231Pa and the 231Pa/ 230Th ratio can vary strongly with particle flux, following distinct trends in different oceanic basins. In the Atlantic Ocean, 231Pa fluxes and 231Pa/ 230Th are low and not sensitive to particle flux. This is because of the short residence time of deep water in this basin resulting from thermohaline circulation, which prevents the full development of lateral concentration gradients and full expression of boundary scavenging. In the Pacific Ocean, the sensitivity of 231Pa/ 230Th to particle flux is highest, reflecting the longer residence time of deep water. In the southern ocean, 231Pa/ 230Th ratios are invariably high, even when particle fluxes are low, reflecting the predominance of opal, which fractionates minimally between the two radionuclides. Interpretation of 231Pa/ 230Th recorded in sediments is thus complex. In the Atlantic, this ratio is better suited for recording past changes in the strength of thermohaline circulation. In the Pacific, it has the best potential for providing synoptic maps of past changes in particle flux. In the southern ocean, retrieval of information from 231Pa/ 230Th is more difficult and requires a more quantitative understanding of the influence of particle composition.

Ocean thermohaline circulation and sedimentary 231Pa/230Th ratio

Holocene sediments from the Atlantic are characterized by 231Pa/230Th ratios below the production ratio of the two radionuclides in the water column (0.093), whereas Holocene sediments from the Southern Ocean have 231Pa/230Th > 0.093. This pattern of 231Pa deficit and excess was ascribed to southward 231Pa export from the Atlantic by the Atlantic thermohaline circulation (THC) as Pa is scavenged less efficiently by marine particles and more effectively transported by the THC than Th. The same pattern is observed in deposits of the Last Glacial Maximum (LGM), which led to a previous contention that the THC strength did not vary markedly through the last glacial termination. Here we embed a description of trace metal scavenging into a zonally averaged, circulation‐biogeochemistry ocean model to explore the sensitivity of 231Pa/230Th in Atlantic and Southern Ocean sediments to THC changes. Our results show that the production of biogenic opal (which, unlike other marine particles, poorly fractionates Th and Pa) in the Southern Ocean water column determines the spatial pattern of the sensitivity. Also, 231Pa/230Th increases in the North Atlantic but changes little in the South Atlantic and decreases in the Southern Ocean as THC is reduced. The mean 231Pa/230Th of the whole Atlantic is therefore less sensitive to THC changes than the mean 231Pa/230Th of the North Atlantic. The current uncertainties in Atlantic mean 231Pa/230Th are too large to rule out a twofold reduction of the THC at the LGM. However, the increase in North Atlantic mean 231Pa/230Th simulated in response to a twofold THC reduction is larger than the observed change in the North Atlantic mean 231Pa/230Th from the LGM to Holocene. Comparing this change with the modeled sensitivity of North Atlantic 231Pa/230Th to THC variations indicates that the THC at the LGM could not have been reduced by >30% of its present strength. Experiments of transient THC changes indicate that high‐resolution 231Pa/230Th records from North Atlantic sediments could also document thermohaline oscillations on century‐to‐millennial timescales.

The influence of transient surface fluxes on North Atlantic overturning in a coupled GCM Climate Change Experiment

The mechanism by which the model‐simulated North Atlantic thermohaline circulation (THC) weakens in response to increasing greenhouse gas (GHG) forcing is investigated through the use of a set of five multi‐century experiments. Using a coarse resolution version of the GFDL coupled climate model, the role of various surface fluxes in weakening the THC is assessed. Changes in net surface freshwater fluxes (precipitation, evaporation, and runoff from land) are found to be the dominant cause for the model's THC weakening. Surface heat flux changes brought about by rising GHG levels also contribute to THC weakening, but are of secondary importance. Wind stress variations have negligible impact on the THC's strength in the transient GHG experiment.