Atmospheric Water Vapor Transport Research Articles

Ocean Gyres and Abrupt Change in the Thermohaline Circulation: A Conceptual Analysis

Abstract The implications are investigated of representing ocean gyre circulations by a diffusion term in the Stommel and Rooth box models of the thermohaline circulation (THC) in one and two hemispheres, respectively. The approach includes mostly analytical solution and study of the bifurcation structure, but also numerical integration and feedback analysis. Sufficient diffusion (gyre strength) eliminates multiple equilibria from either model, highlighting the need for accurate gyre circulation strength in general circulation models (GCMs) when considering the potential for abrupt climate change associated with THC shutdown. With diffusion, steady-state flow strength in the Rooth model depends on freshwater forcing (i.e., implied atmospheric water vapor transport) in both hemispheres, not only on that in the upwelling hemisphere, as in the nondiffusive case. With asymmetric freshwater forcing, two solutions (strong stable and weak unstable) are found with sinking in the hemisphere with stronger forcing and one solution with sinking in the other hemisphere. Under increased freshwater forcing the two solutions in the hemisphere with stronger forcing meet in a saddle-node bifurcation (if diffusion is sufficiently strong to prevent a subcritical Hopf bifurcation first), followed by flow reversal. Thus, the bifurcation structure with respect to freshwater forcing of the diffusive Rooth model of two-hemisphere THC is similar to that of the Stommel model of single-hemisphere THC, albeit with a very different dynamical interpretation. Gyre circulations stabilize high-latitude sinking in the Stommel model. In the Rooth model, gyre circulations only stabilize high-latitude sinking if the freshwater forcing is weaker in the sinking hemisphere than in the upwelling hemisphere, by an amount that increases with diffusion. The values of diffusion and freshwater forcing at which qualitative change in behavior occurs correspond to the range of the values used in and obtained with GCMs, suggesting that this analysis can provide a conceptual foundation for analyzing the stability of the interhemispheric THC, and also for the potential of the Atlantic THC to undergo abrupt change.

Atmospheric water vapor transport associated with typical anomalous summer rainfall patterns in China

This paper attempts to reveal the atmospheric water vapor transports associated with typical anomalous summer rainfall patterns in China. The results show that origins of water vapor supply related to anomalous rainfall patterns are different from those related to the normal monsoon rainfall. Anomalous pattern 1, with a heavier rainbelt along the middle and lower reaches of the Yangtze River valley, follows from a convergence of the tropical southwest water vapor transport with the midlatitude northeast water vapor transport; the tropical water vapor transport comes directly from the Bay of Bengal and the South China Sea but originally from the Philippine Sea. The anomalous water vapor transport is associated with a southwestward extension of the western Pacific subtropical high and a southward shift of the upper East Asian jet stream. Anomalous pattern 2, with a main rainbelt along the Huaihe River valley, is supported by the convergence of the subtropical southwest water vapor with the midlatitude water vapor transport. The subtropical branch comes directly from the South China Sea but originally from the East China Sea and the adjacent subtropical Pacific to the further east along 20–25°N. The background large‐scale circulation change includes a northwestward extension of the western Pacific subtropical high and an eastward shift of the upper jet stream. Although the cross‐equator flows including the Somali jet supply abundant water vapor for the normal condition of June, July, and August rainfall over China, the tropical water vapor transports related to typical anomalous rainfall anomalies originate from the tropical western Pacific Ocean. The northward transport of anomalous warm water vapor occurs mainly in the lower troposphere, while the transport of midlatitude cold water vapor occurs briefly in the upper troposphere.

Hydrogen and oxygen isotopes in precipitation in the northern part of the North China Plain: climatology and inter‐storm variability

AbstractTo clarify the processes of atmospheric water vapour transport to the northern part of the North China Plain (NCP), isotopic variations in precipitation are investigated. Pattern of seasonal variation in the isotopic composition of precipitation over China can be classified into three types: winter depression, summer depression and intermediate. The northern NCP is located at the northern margin of the region where isotopic composition decreases in summertime, associated closely with the Asian summer monsoon. Inter‐storm variability of isotopic composition over the northern NCP during the monsoon season has an inverse relationship with atmospheric moisture content, although it cannot be accounted for by the amount effect. Thus, the isotopic composition is inferred to be primarily controlled by large‐scale monsoon activity, which supplies water vapour into the northern NCP, rather than by local precipitation processes. From an analysis based on a relationship between the deuterium excess and humidity conditions at possible oceanic source areas, it is revealed that precipitating water in the early monsoon season originates in the South China Sea, and its source area tends to shift to the East China Sea and/or Yellow Sea in the later period of the season. Copyright © 2004 John Wiley & Sons, Ltd.

The westerly fluctuation and water vapor transport over the Qilian-Heihe valley

The westerly fluctuation and the atmospheric water vapor transport over the Qilian-Heihe valley are analyzed and the results show that, in the water vapor transport stream field from Jun to September, this valley is in the westerly stream and the water vapor comes from westerlies water transport via the Black Sea and the Caspian Sea. The net water vapor transport is less net import and different from most areas of the northwest China. The interannual changes in water vapor transport over the valley arise from the westerly fluctuation, and have a positive relationship to the interannual changes in westerly wind speed. The cold air actions from the Mongol low pressure are the primary system that controls the westerly water vapor transport. Its action chain is that, the Mongol low pressure is strengthened → the circulation meridionality will be increased → the cold air will move southwards → the westerly will be stronger → the wind convergence of direction and speed will be stronger → the water vapor convergence transport will be increased → the local water vapor content will be increased. The interannual changes in atmospheric water vapor transport over the valley rely mainly on the convergence transport, but the effect of advection transport is less. The interannual changes of strong or weak westerly affect mainly the convergence transport, and then make the atmospheric water vapor net transport increase or decrease over the Qilian-Heihe valley.

Spatial inhomogeneity of the martian subsurface water distribution: implication from a global water cycle model

In an effort to test and to understand the global hydrogen distribution in the shallow subsurface of Mars retrieved by the Mars Odyssey gamma-ray spectrometer, the present state and movement of water are investigated by a coupled global subsurface–atmosphere water cycle model. It was found that the observed global subsurface hydrogen distribution is largely consistent with the modeled global water cycle, so a large fraction of hydrogen is likely to exist as water, at low and mid latitudes in the form of adsorbed water. Under the present climate the water content in the shallow subsurface becomes higher in the northern hemisphere than in the southern hemisphere as a result of global water cycle, regardless of the initial water distribution in the soil or adsorptive capacity. The higher annual maximum soil temperature in the south, stronger net northward transport of atmospheric water vapor, and the emission of vapor from the northern residual polar cap in northern summer contribute to this hemispheric asymmetry. The generally higher adsorptive capacity of clay minerals in the northern plains may further increase this bias. The longitudinal inhomogeneity is caused by several factors, such as thermal inertia, adsorptive capacity, and atmospheric surface pressure. The water abundance is locally high in low thermal inertia regions (e.g., Arabia Terra) and at deep places where the surface pressure is high (e.g., Hellas); it is low in soil with a low adsorptive capacity (e.g., Tharsis) and high thermal inertia regions (e.g., Solis Planum). Most of the soil humidity near the surface at low and mid latitudes may originate from the atmosphere. The model implies that the upper soil layer should be largely ice-free because otherwise an excessive sublimation and vapor emission into the atmosphere in warm seasons would violate the observational constraints. Moreover, the more uniform latitudinal variation of the observed hydrogen abundance near the surface compared to that of deeper layers is indicative of the presence of adsorbed water instead of ground ice because the adsorbed water content does not as steeply depend on latitude as the ground ice stability. Concerning the regolith mineralogy, montmorillonite can much better account for the observed water cycle than palagonite. While the presence of permanent ground ice appears likely in the polar region below a thin layer, large seasonal cycle of phase change between pore ice and adsorbed water may be possible. Regolith adsorption/desorption is neither negligible nor crucial for the seasonal atmospheric water cycle, but the surface–atmosphere coupling is a major prerequisite for the long-term evolution of subsurface water distribution.

Space and time variation of δ18O and δD in Antarctic precipitation revisited

To better understand the variations over time of precipitation water isotopes measured in polar ice cores, we have developed an intermediate complexity model (ICM) of atmospheric water vapor transport and associated isotopic distillation. The model builds directly from earlier work by D. Fisher and M. Hendricks and is calibrated against the measured modern spatial distribution of δD and deuterium excess (d). Model improvements include a correction to the equation governing advective transport, which solves a major puzzle arising from the earlier work. The new model shows isotopic data to be consistent with dominance of eddy diffusive transport at high latitudes. Model experiments are used to show that a wide variety of climate changes in subtropical source regions can affect Antarctic d and that such d changes are consistently anticorrelated with changes of δD. Magnitudes of ΔδD/Δd are ∼−1 to −5 and are much higher at the ice sheet margin than in the interior of the continent. Changes in the relative dominance of diffusive and advective transport are also shown to cause anticorrelated changes of similar magnitude. Isotopic sensitivities to temperature change (ΔδD/ΔT) are shown to also vary spatially, with low values in marginal zones and higher values inland. Effects of ocean surface relative humidity on Antarctic d are shown to be nearly uniform across the continent (in contrast to earlier results given by Petit et al. [1991]) but quantitatively small. The method used by Vimeux et al. [2001] for folding humidity effects into source temperature effects is supported. We draw attention to a pervasive misapplication of marine composition corrections to Antarctic δD records. Arguments are made in support of simple interpretations of calibrated ice core isotopic time series in terms of temperature changes despite the potential complexities in the system.

Mean moist circulation for PEM‐Tropics missions

Features of atmospheric water vapor and water vapor transport fields during the Pacific Exploratory Mission (PEM) in the Tropical Pacific were studied using the data from the National Centers for Environmental Prediction (NCEP) reanalyses, the Global Precipitation Climatology Project, the Climate Analysis Center, and the European Centre for Medium‐Range Weather Forecasts, with a focus on PEM‐Tropics B. Vertically integrated values of the moisture quantities such as the precipitable water and the zonal and meridional water vapor fluxes were examined. Moisture transports across boundaries of major sectors over the tropical Pacific and their budgets were estimated. The differences of the precipitable water between PEM‐Tropics B and PEM‐Tropics A were highlighted owing to its importance for atmospheric chemistry. During PEM‐Tropics B, the NCEP reanalyses showed that there existed clearly a moist Walker circulation over the northern tropical Pacific during the whole period and a moist Hadley circulation over the eastern Pacific during the first half of the mission. The southwest sector where the South Pacific Convergence Zone (SPCZ) was located had the largest areal mean precipitation and the northeast sector had the smallest, whereas the estimated evaporation rates for both regions were comparable. A more vigorous moist circulation during the second half period led to enhanced precipitation for all the regions except the southeast sector. An interesting and potentially very important meteorological phenomenon during PEM‐Tropics B was the double Intertropical Convergence Zone (ITCZ) structure over the eastern tropical Pacific. We suggest that this structure, along with the similar double convective zones over the western tropical Pacific (i.e., ITCZ and SPCZ), may act as double barriers for the interhemispheric exchanges for this particular season with implications for photochemistry in both hemispheres.

Short-Term and Seasonal Variability of the Atmospheric Water Vapor Transport through the Mackenzie River Basin

Abstract The transport of water vapor through the Mackenzie River basin, a typical high-latitude river basin, is examined for the years 1979–93, with the European Centre for Medium-Range Weather Forecasts reanalysis dataset (ERA). It is shown that the transport of water vapor through the Mackenzie basin is highly variable in space and time. This transport has two distinct modes. During the autumn, winter, and spring, moisture is transported into the basin from the southwest by extratropical cyclones. The source of this moisture is argued to be the subtropical and midlatitude central Pacific Ocean. During the summer, moisture enters the basin from the northwest, with the source region being the Arctic Ocean. The values of monthly water vapor budgets obtained with the objectively analyzed fields are compared with those obtained from interpolated radiosonde data and with other known quantitative information about the water budget of the basin. It is found that the ERA data seriously overestimate the values o...

Realistic representation of the surface freshwater flux in an ice–ocean general circulation model

Ocean general circulation models usually use an equivalent salt-flux in order to represent the freshwater surface inflow/outflow. This unphysical approach has numerous shortcomings, especially for climate studies. A more physical representation has been originally proposed by R.X. Huang [Journal of Physical Oceanography 23 (1993) 2428–2446] for ocean models. It consists in taking into account the vertical velocity at the sea surface. Here this formulation is introduced in a coupled ice–ocean general circulation model designed for climate studies. The treatment of the ice–ocean exchanges needs special care in order to conserve salt and freshwater masses, and to correctly represent the physics involved. This formulation allows to simulate the Goldsbrough–Stommel circulation and the meridional pathway of the freshwater at the ocean surface. Furthermore, the meridional freshwater transport diagnosed using such an approach is more directly comparable to the atmospheric water-vapor transport. Nevertheless, it produces only small changes in the ocean general circulation.

A case study on middle atmospheric water vapor transport during the February 1998 stratospheric warming

We present the first analysis of water vapor measurements made during a stratospheric warming using ground‐based microwave spectroscopy. The measurements were made from the ALOMAR observatory (69°N, 16°E) between January 31 and February 10, 1998. The data show an increased mixing ratio between ≈ 45 and 70 km throughout the warming. The analysis uses temperature data obtained by lidar measurements from the same location. A comparison with results obtained using standard temperature data reveals the importance of ‘good’ temperature measurements for water vapor retrieval from remote sensing data during such events in order to separate temperature effects from atmospheric dynamics.

The role of the individual air-sea flux components in CO 2 -induced changes of the ocean's circulation and climate

In this study we investigate the role of heat, freshwater and momentum fluxes in changing the oceanic climate and thermohaline circulation as a consequence of increasing atmospheric CO2 concentration. Two baseline integrations with a fully coupled ocean atmosphere general circulation model with either fixed or increasing atmospheric CO2 concentrations have been performed. In a set of sensitivity experiments either freshwater (precipitation, evaporation and runoff from the continents) and/or momentum fluxes were no longer simulated, but prescribed according to one of the fully coupled baseline experiments. This approach gives a direct estimate of the contribution from the individual flux components. The direct effect of surface warming and the associated feedbacks in ocean circulation are the dominant processes in weakening the Atlantic thermohaline circulation in our model. The relative contribution of momentum and freshwater fluxes to the total response turned out to be less than 25%, each. Changes in atmospheric water vapour transport lead to enhanced freshwater input into middle and high latitudes, which weakens the overturning. A stronger export of freshwater from the Atlantic drainage basin to the Indian and Pacific ocean, on the other hand, intensifies the Atlantic overturning circulation. In total the modified freshwater fluxes slightly weaken the Atlantic thermohaline circulation. The contribution of the modified momentum fluxes has a similar magnitude, but enhances the formation of North Atlantic deep water. Salinity anomalies in the Atlantic as a consequence of greenhouse warming stem in almost equal parts from changes in net freshwater fluxes and from changes in ocean circulation caused by the surface warming due to atmospheric heat fluxes. Important effects of the momentum fluxes are a poleward shift of the front between Northern Hemisphere subtropical and subpolar gyres and a southward shift in the position of the Antarctic circumpolar current, with a clear signal in sea level.

On the hydrological cycle under paleoclimatic conditions as derived from AGCM simulations

The atmospheric hydrological cycle is compared for different time slices of the late Quaternary. Simulations have been conducted with an atmospheric circulation model at T42 resolution, and we have performed a global evaluation of the atmospheric water vapor transport. The water export from the Atlantic catchment area, important for driving the large‐scale thermohaline ocean circulation, is analyzed in detail. For the Last Glacial Maximum (LGM), we examine the model's sensitivity with respect to tropical cooling relative to the CLIMAP reconstruction which is motivated by recent data. We find that the LGM experiment with tropical cooling is in better agreement with proxy data available. Our experiments indicate that the water vapor transport is strongly affected by three mechanisms: continental drying, eddy moisture transport, and changes in the tropical circulation. Except for the continental drying and the blocking effect of the Laurentide ice sheet, the hydrological cycle is substancially different for both LGM experiments. We find that the hydrological system is rather sensitive to tropical temperature change which is important to understand paleoclimate and future climate changes.

The Atmospheric Water Vapor Cycle in South America and theTropospheric Circulation

The main characteristics of the atmospheric water vapor cycle over the South American continent and the adjacent oceans are investigated using the 22-yr period, from 1976 to 1997, of the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) 40-Year Reanalysis Project database. Precipitation rate and water vapor content fields obtained from this dataset are compared over the region with newly available observed datasets, which combine ground-based and satellite-derived observations. The temporal variation and spatial distribution of the atmospheric water vapor balance equation terms (precipitation rate, evaporation rate, and water vapor flux convergence) are examined with regard to their consistency and relative importance. The net effect of the atmospheric water vapor transport, represented in the last term of the balance equation, is decomposed into the horizontal and vertical convergence terms. The analysis of the latter highlights those regions where the topographic uplift makes a substantial contribution to the total precipitation rate. The former term is further decomposed into the stationary and transient water vapor flux contributions. The comparison of these terms with relevant characteristics of the large-scale tropospheric circulation provides a better understanding of the different precipitation regimes in South America. The mean annual balance satisfactorily closes over most of the oceanic regions. However, important imbalances found in the vicinity of high topographic features, such as in the central Andes, are attributed to large errors in the local computation of the atmospheric water vapor flux. The current results corroborate previous findings on the role of the stationary water vapor flux convergence in the spatial distribution and seasonal variation of the rainfall rate in tropical and subtropical latitudes and extend over the less-investigated continental midlatitudes. The magnitude of the transient water vapor flux convergence is, in general, lower than that of the stationary flux. Nonetheless, in some oceanic and continental regions, they are comparable and seem to be dynamically linked. This interaction, which can be explained by means of a simple transport-gradient model of the transient water vapor flux, could help to clarify the observed seasonal and interannual variability of the rainfall rate in the humid-to-dry transition zone in the southern part of the continent.

Global water balance and atmospheric water vapour transport at last glacial maximum: climate simulations with the Canadian Climate Centre for Modelling and Analysis atmospheric general circulation model

A series of new simulations of the climate state at last glacial maximum has been performed using the Canadian second-generation atmospheric general circulation model and are described herein. The primary goal has been to assess the dynamic changes in the global water balance and water vapour transport that were characteristic of the climate state during this epoch of Earth's history. We pay special attention to comparisons of the atmospheric model simulations of last glacial maximum climate with those produced with a much simpler coupled energy balance-ice-sheet model, which has been designed to simulate the late Pleistocene cycle of glacial-interglacial ice volume variations. Our analyses, using the atmospheric model, demonstrate that the vigour of the hydrological cycle was markedly decreased under last glacial maximum conditions, as would be expected on the simplest thermodynamic grounds. The primary components of the hydrological cycle in the atmospheric model, namely precipitation and evaporation, constitute essential mechanisms that control ice-sheet mass balance. We also investigate changes in the Northern Hemisphere stationary wave patterns, as well as changes in the total and eddy moisture transport by the global circulation at last glacial maximum to illustrate the role played by the dynamics of the atmosphere in the maintenance of the Northern Hemisphere ice sheets. In particular, we find that the enhancement of the stationary wave pattern along with the convergence in atmospheric water vapour transport produces increased cooling and snow accumulation at last glacial maximum over the southeastern lobes of the Laurentide Ice Sheet. This suggests an explanation for the previously unexplained extension of these lobes deep into the New England states.

Global water balance and atmospheric water vapour transport at last glacial maximum: climate simulations with the Canadian Climate Centre for Modelling and Analysis atmospheric general circulation model

Global water balance and atmospheric water vapour transport at last glacial maximum: climate simulations with the Canadian Climate Centre for Modelling and Analysis atmospheric general circulation model

Trends of upper-air circulation and water vapour over equatorial South America and adjacent oceans

A novel 40-year upper-air dataset along with satellite measurements have been analyzed to explore the vertical circulation and water vapour transport and their long-term evolution over equatorial South America and the adjacent oceans. Lower-tropospheric convergence, mid-tropospheric ascending motion, upper-tropospheric divergence, as well as precipitable water, are concentrated in the realm of the Pacific and Atlantic Inter-tropical Convergence Zone (ITCZ) throughout the year and the Amazon basin during austral summer. Water vapour is imported into the Amazon basin from the equatorial and tropical South Atlantic and during austral summer also from the tropical North Atlantic. The recognition of long-term developments in the tropical climate system hinges on the homogeneity of the upper-air dataset. With this qualification, statistically significant increasing trends of lower-tropospheric convergence, upward motion, upper-tropospheric divergence, convergence of atmospheric water vapour transport, and precipitable water are found over the Amazon basin throughout the year. In all seasons, the pattern of atmospheric water vapour transport shows a statistically significant trend towards the development of a clockwise turning vortex over eastern Brazil, concordant with a centre of falling tendency in 1000 mb height and associated vortex in the wind field. The trends towards reduced/enhanced water vapour import at the northern/southern side of the vortex largely offset each other, so that the trend towards greater moisture convergence for the basin as a whole results from the combination of increasing vapour import through the northern and decreasing export through the western and southern boundaries of the Amazon watershed. Copyright © 1999 Royal Meteorological Society

Trends of upper‐air circulation and water vapour over equatorial South America and adjacent oceans

A novel 40-year upper-air dataset along with satellite measurements have been analyzed to explore the vertical circulation and water vapour transport and their long-term evolution over equatorial South America and the adjacent oceans. Lower-tropospheric convergence, mid-tropospheric ascending motion, upper-tropospheric divergence, as well as precipitable water, are concentrated in the realm of the Pacific and Atlantic Inter-tropical Convergence Zone (ITCZ) throughout the year and the Amazon basin during austral summer. Water vapour is imported into the Amazon basin from the equatorial and tropical South Atlantic and during austral summer also from the tropical North Atlantic. The recognition of long-term developments in the tropical climate system hinges on the homogeneity of the upper-air dataset. With this qualification, statistically significant increasing trends of lower-tropospheric convergence, upward motion, upper-tropospheric divergence, convergence of atmospheric water vapour transport, and precipitable water are found over the Amazon basin throughout the year. In all seasons, the pattern of atmospheric water vapour transport shows a statistically significant trend towards the development of a clockwise turning vortex over eastern Brazil, concordant with a centre of falling tendency in 1000 mb height and associated vortex in the wind field. The trends towards reduced/enhanced water vapour import at the northern/southern side of the vortex largely offset each other, so that the trend towards greater moisture convergence for the basin as a whole results from the combination of increasing vapour import through the northern and decreasing export through the western and southern boundaries of the Amazon watershed. Copyright © 1999 Royal Meteorological Society

Spatial and Temporal Structure of Atmospheric Water Vapor Transport in the Mackenzie River Basin

Abstract The transport of water vapor through the Mackenzie River basin, a typical high-latitude river basin, is examined for the period from August to October 1994. The spatial and temporal variability in the transport is considered with both objectively analyzed fields and radiosonde data. Previous studies of the high-latitude water vapor have made use of radiosonde data and have been able to document some features of annual cycle of water vapor transport. These studies have left unresolved many important aspects of moisture transport processes. In particular, detailed information as to the spatial and temporal variation of the transport has not been fully documented or understood. In order to address these important issues, the authors make use of the objectively analyzed fields from the European Centre for Medium-Range Weather Forecasts to study the high-latitude transport of water vapor. This paper presents findings regarding the transport of water vapor over northern parts of North America. It is sh...

Atmospheric Water Vapor Transport in NCEP-NCAR Reanalyses: Comparison with River Discharge in the Central United States

The authors extract the water transport produced by the National Centers for Environmental Prediction reanalysis for a 10-yr period, 1984–93, and compare its convergence into two river basins with an independent dataset, river discharge (streamflow). Analysis focuses on two basins in the United States, the Upper Mississippi and the Ohio–Tennessee Basins, where the relatively high density of routine upper-air observations might be expected to give the reanalysis its closest rendition of the actual water transport. Over periods of several years, water input by the atmosphere should match water output from these basins in streamflow. However, in both basins an imbalance between the two with biases with respect to streamflow approaching 40% is found. The accuracy attributed to river discharge measurements averaged over several years and the apparent lack of significant multiyear storage in the basins lead us to conclude that the bias is largely an inaccuracy in the atmospheric transport. Temporal variability of atmospheric input and streamflow output shows somewhat better correspondence, with statistically significant correlations occurring for both basins on interannual and several-day timescales. The overall behavior suggests that the temporal variability of water transport depicted by the reanalysis can be used to gain insight into the actual variability of atmospheric transport, at least for well-observed regions such as the United States.

The stability of the North Atlantic thermohaline circulation in a coupled ocean-atmosphere general circulation model

The stability of the Atlantic thermohaline circulation against meltwater input is investigated in a coupled ocean-atmosphere general circulation model. The meltwater input to the Labrador Sea is increased linearly for 250 years to a maximum input of 0.625 Sv and then reduced again to 0 (both instantaneously and linearly decreasing over 250 years). The resulting freshening forces a shutdown of the formation of North Atlantic deepwater and a subsequent reversal of the thermohaline circulation of the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic reduction of the Atlantic northward heat transport, resulting in a strong cooling with maximum amplitude over the northern North Atlantic and a southward shift of the sea-ice margin in the Atlantic. Due to the increased meridional temperature gradient, the intertropical convergence zone over the Atlantic is displaced southward and the westerlies in the Northern Hemisphere gain strength. We identify four main feedbacks affecting the stability of the thermohaline circulation: the change in the overturning circulation of the Atlantic leads to longer residence times of the surface water in high-northern latitudes, which allows them to accumulate more precipitation and runoff from the continents. As a consequence the stratification in the North Atlantic becomes more stable. This effect is further amplified by an enhanced northward atmospheric water vapour transport, which increases the freshwater input into the North Atlantic. The reduced northward oceanic heat transport leads to colder sea-surface temperatures and an intensification of the atmospheric cyclonic circulation over the Norwegian Sea. The associated Ekman transports cause increased upwelling and increased freshwater export with the East Greenland Current. Both the cooling and the wind-driven circulation changes largely compensate for the effects of the first two feedbacks. The wind-stress feedback destabilizes modes without deep water formation in the North Atlantic, but has been neglected in almost all studies so far. After the meltwater input stops, the North Atlantic deepwater formation resumed in all experiments and the meridional overturning returned within 200 years to a conveyor belt pattern. This happened although the formation of North Atlantic deep water was suppressed in one experiment for more than 300 years and the Atlantic overturning had settled into a circulation pattern with Antarctic bottom water as the only source of deep water. It is a clear indication that cooling and wind-stress feedback are more effective, at least in our model, than advection feedback and increased atmospheric water vapour transport. We conclude that the conveyor belt-type thermohaline circulation seems to be much more stable than hitherto assumed from experiments with simpler models.