Atlantic watermass and circulation response to persistent freshwater forcing in two coupled general circulation models

The sensitivity of the Atlantic circulation and watermasses to biases in the convergence of moisture into the basin is examined in this study using two different general circulation models. For a persistent positive moisture flux into the tropical Atlantic, the average salinity and temperature in the basin is reduced, mainly below mid-depths and in high latitudes. A transient reduction in the Atlantic overturning strength occurs in this case, with a recovery timescale of 1–2 centuries. In contrast, a similar amount of freshwater directed into the Subpolar North Atlantic results in a persistent reduction in overturning and an increase in basin heat and salt content. In the unperturbed pre-industrial simulations, the Atlantic is unambiguously warmer and saltier than historical observations below mid-depths and in the Nordic Seas. The models’ tropical freshwater flux sensitivities project strongly onto the spatial pattern of this bias, suggesting a common atmospheric deficiency. The integrated Atlantic plus Arctic surface freshwater flux in these models is between ?0.5 and ?0.6 Sv, compared with an observational estimate of ?0.28 Sv. Our results suggest that shortcomings in the models’ ability to reproduce realistic bulk watermass properties are due to an overestimation of the inter-basin moisture export from the tropical Atlantic.     

Response of the Asian summer monsoon to weakening of Atlantic thermohaline circulation

Various paleoclimate records have shown that the Asian monsoon was punctuated by numerous suborbital time-scale events, and these events were coeval with those that happened in the North Atlantic. This study investigates the Asian summer monsoon responses to the Atlantic Ocean forcing by applying an additional freshwater flux into the North Atlantic. The simulated results indicate that the cold North Atlantic and warm South Atlantic induced by the weakened Atlantic thermohaline circulation （THC） due to the freshwater flux lead to significantly suppressed Asian summer monsoon. The authors analyzed the detailed processes of the Atlantic Ocean forcing on the Asian summer monsoon, and found that the atmospheric teleconnection in the eastern and central North Pacific and the atmosphere-ocean interaction in the tropical North Pacific play the most crucial role. Enhanced precipitation in the subtropical North Pacific extends the effects of Atlantic Ocean forcing from the eastern Pacific into the western Pacific, and the atmosphere-ocean interaction in the tropical Pacific and Indian Ocean intensifies the circulation and precipitation anomalies in the Pacific and East Asia.     

Naturally forced multidecadal variability of the Atlantic meridional overturning circulation

The mechanisms by which natural forcing factors alone could drive simulated multidecadal variability in the Atlantic meridional overturning circulation (AMOC) are assessed in an ensemble of climate model simulations. It is shown for a new state-of-the-art general circulation model, HadGEM2-ES, that the most important of these natural forcings, in terms of the multidecadal response of the AMOC, is solar rather than volcanic forcing. AMOC strengthening occurs through a densification of the North Atlantic, driven by anomalous surface freshwater fluxes due to increased evaporation. These are related to persistent North Atlantic atmospheric circulation anomalies, driven by forced changes in the stratosphere, associated with anomalously weak solar irradiance during the late nineteenth and early twentieth centuries. Within a period of approximately 100 years the 11-year smoothed ensemble mean AMOC strengthens by 1.5 Sv and subsequently weakens by 1.9 Sv, representing respectively approximately 3 and 4 standard deviations of the 11-year smoothed control simulation. The solar-induced variability of the AMOC has various relevant climate impacts, such as a northward shift of the intertropical convergence zone, anomalous Amazonian rainfall, and a sustained increase in European temperatures. While this model has only a partial representation of the atmospheric response to solar variability, these results demonstrate the potential for solar variability to have a multidecadal impact on North Atlantic climate.     

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. Received 31 January 1996/Accepted 22 August 1996     

A transient climate change simulation with greenhouse gas and aerosol forcing: projected climate to the twenty-first century

 The potential climatic consequences of increasing atmospheric greenhouse gas (GHG) concentration and sulfate aerosol loading are investigated for the years 1900 to 2100 based on five simulations with the CCCma coupled climate model. The five simulations comprise a control experiment without change in GHG or aerosol amount, three independent simulations with increasing GHG and aerosol forcing, and a simulation with increasing GHG forcing only. Climate warming accelerates from the present with global mean temperatures simulated to increase by 1.7 °C to the year 2050 and by a further 2.7 °C by the year 2100. The warming is non-uniform as to hemisphere, season, and underlying surface. Changes in interannual variability of temperature show considerable structure and seasonal dependence. The effect of the comparatively localized negative radiative forcing associated with the aerosol is to retard and reduce the warming by about 0.9 °C at 2050 and 1.2 °C at 2100. Its primary effect on temperature is to counteract the global pattern of GHG-induced warming and only secondarily to affect local temperatures suggesting that the first order transient climate response of the system is determined by feedback processes and only secondarily by the local pattern of radiative forcing. The warming is accompanied by a more active hydrological cycle with increases in precipitation and evaporation rates that are delayed by comparison with temperature increases. There is an “El Nino-like” shift in precipitation and an overall increase in the interannual variability of precipitation. The effect of the aerosol forcing is again primarily to delay and counteract the GHG-induced increase. Decreases in soil moisture are common but regionally dependent and interannual variability changes show considerable structure. Snow cover and sea-ice retreat. A PNA-like anomaly in mean sea-level pressure with an enhanced Aleutian low in northern winter is associated with the tropical shift in precipitation regime. The interannual variability of mean sea-level pressure generally decreases with largest decreases in the tropical Indian ocean region. Changes to the ocean thermal structure are associated with a spin-down of the Atlantic thermohaline circulation together with a decrease in its variability. The effect of aerosol forcing, although modest, differs from that for most other quantities in that it does not act primarily to counteract the GHG forcing effect. The barotropic stream function in the ocean exhibits modest change in the north Pacific but accelerating changes in much of the Southern Ocean and particularly in the north Atlantic where the gyre spins down in conjunction with the decrease in the thermohaline circulation. The results differ in non-trivial ways from earlier equilibrium 2 × CO₂ results with the CCCma model as a consequence of the coupling to a fully three-dimensional ocean model and the evolving nature of the forcing. Received: 24 September 1998 / Accepted: 8 October 1999     

Atmospheric water vapor flux,bifurcation of the thermohaline circulation,and climate change

Latitudinal heat transport in the ocean and atmosphere represents a fundamental process of the Earth's climate system. The ocean component of heat transport is effected by the thermohaline circulation. Changes in this circulation, and hence latitudinal heat transport, would have a significant effect on global climate. Paleoclimate evidence from the Greenland ice cores and deep sea sediment cores suggests that during much of glacial time the climate system oscillated between two different states. Bimodal equilibrium states of the thermohaline circulation have been demonstrated in climate models. We address the question of the role of the atmospheric hydrological cycle on the global thermohaline circulation and the feedback to the climate system through changes in the ocean's latitudinal heat transport, with a simple coupled ocean-atmosphere energy-salt balance model. Two components of the atmospheric hydrological cycle, i.e., latitudinal water vapor transport and the net flux of water vapor from the Atlantic to the Pacific Ocean appear to play separate roles. If the inter-basin transport is sufficiently large, small changes in water vapor transport over the North Atlantic can effect bifurcation or a rapid transition between two different equilibria in the global thermohaline circulation; maximum difference between the modes occurs in the North Atlantic. If the inter-basin transport is from the Pacific to the Atlantic and sufficiently large, latitudinal vapor transport in the North Pacific controls the bifurcations, with maximum changes occurring in the North Pacific. For intermediate values of inter-basin transport, no rapid transitions occur in either basin. In the regime with vapor flux from the Atlantic to the Pacific, the on mode has strong production of deep water in the North Atlantic and a large flux of heat to the atmosphere from the high latitude North Atlantic. The off mode has strong deep water production in the Southern Ocean and weak production in the North Pacific. Heat transport into the high latitude North Atlantic by the ocean is reduced to about 20% of the on mode value. For estimated values of water vapor transport for the present climate the model asserts that while water vapor transport from the Atlantic to the Pacific Ocean is sufficiently large to make the North Atlantic the dominant region for deep water production, latitudinal water vapor transport is sufficiently low that the thermohaline circulation appears stable, i.e., far from a bifurcation point. This conclusion is supported to some extent by the fact that the high latitude temperature of the atmosphere as recorded in the Greenland ice cores has changed little over the last 9000 years.     

Local and remote impacts of a tropical Atlantic salinity anomaly

The climatic impacts of an enhanced evaporation prescribed during 50 years in the tropical Atlantic are investigated in a coupled ocean–atmosphere general circulation model. Locally, the salinity increase leads to a rapid deepening and cooling of the surface mixed layer. This induces a deepening of the equatorial undercurrent and an intensification of the south equatorial current. A remote atmospheric response to the tropical Atlantic perturbation is detected in the North Atlantic sector after ten years. It has the form of a robust wave-like tropospheric perturbation seemingly excited by the weakening of atmospheric deep convection over the Amazonian basin. Meanwhile, the salt anomaly is carried northward by the mean oceanic circulation. It is traced up to the convection sites and then on its return path at depth towards lower latitudes. Consistent with the density increase, deep convection is enhanced after the arrival of the salt anomaly and the Atlantic meridional overturning circulation (AMOC) intensifies about 20 years after the beginning of the perturbation. The adjustment of the tropical Atlantic to the AMOC intensification then modifies its initial response to the freshwater forcing, leading to a weaker cooling in the northern tropical Atlantic than in the southern tropical Atlantic, a slight northward shift of the tropical Atlantic precipitation pattern and an intensification of the North Brazil current. On the other hand, no significant anomalous precipitations are found in the Pacific. The initial remote atmospheric response is also modulated, by an NAO-like response to the AMOC intensification.     

Response of the North Atlantic subpolar gyre to persistent North Atlantic oscillation like forcing

The response of the North Atlantic subpolar gyre (SPG) to a persistent positive (or negative) phase of the North Atlantic oscillation (NAO) is investigated using an ocean general circulation model forced with idealized atmospheric reanalysis fields. The integrations are analyzed with reference to a base-line integration for which the model is forced with idealized fields representing a neutral state of the NAO. In the positive NAO case, the results suggest that the well-known cooling and strengthening of the SPG are, after about 10 years, replaced by a warming and subsequent weakening of the SPG. The latter changes are caused by the advection of warm water from the subtropical gyre (STG) region, driven by a spin-up of the Atlantic meridional overturning circulation (AMOC) and the effect of an anomalous wind stress curl in the northeastern North Atlantic, which counteracts the local buoyancy forcing of the SPG. In the negative NAO case, however, the SPG response does not involve a sign reversal, but rather shows a gradual weakening throughout the integration. The asymmetric SPG-response to the sign of persistent NAO-like forcing and the different time scales involved demonstrate strong non-linearity in the North Atlantic Ocean circulation response to atmospheric forcing. The latter finding indicates that analysis based on the arithmetic difference between the two NAO-states, e.g. NAO+ minus NAO?, may hide important aspects of the ocean response to atmospheric forcing.     

Influence of the Andes Mountains on South American moisture transport, convection, and precipitation

Mountain ranges are known to have a first-order control on mid-latitude climate, but previous studies have shown that the Andes have little effect on the large-scale circulation over South America. We use a limited-domain general circulation model (RegCM3) to evaluate the effect of the Andes on regional-scale atmospheric dynamics and precipitation. We present experiments in which Andean heights are specified at 250 m, and 25, 50, 75, and 100% of their modern values. Our experiments indicate that the Andes have a significant influence on moisture transport between the Amazon Basin and the central Andes, deep convective processes, and precipitation over much of South America through mechanical forcing of the South American low-level jet (LLJ) and topographic blocking of westerly flow from the Pacific Ocean. When the Andes are absent, the LLJ is absent and moisture transport over the central Andes is mainly northeastward. As a result, deep convection is suppressed and precipitation is low along the Andes. Above 50% of the modern elevation, a southward flowing LLJ develops along the eastern Andean flanks and transports moisture from the tropics to the subtropics. Moisture drawn from the Amazon Basin provides the latent energy required to drive convection and precipitation along the Andean front. Large northerly moisture flux and reduced low-level convergence over the Amazon Basin leads to a reduction in precipitation over much of the basin. Our model results are largely consistent with proxy evidence of Andean climate change, and have implications for the timing and rate of Andean surface uplift.     

On the freshwater forcing and transport of the Atlantic thermohaline circulation

The 'conveyor belt' circulation of the Atlantic Ocean transports large amounts of heat northward, acting as a heating system for the northern North Atlantic region. It is widely thought that this circulation is driven by atmospheric freshwater export from the Atlantic catchment region, and that it transports freshwater northward to balance the loss to the atmosphere. Using results from a simple conceptual model and a global circulation model, it is argued here that the freshwater loss to the atmosphere arises mainly in the subtropical South Atlantic and is balanced by northward freshwater transport in the wind-driven subtropical gyre, while the thermohaline circulation transports freshwater southward. It is further argued that the direction of freshwater transport is closely linked to the dynamical regime and stability of the 'conveyor belt': if its freshwater transport is indeed southward, then its flow is purely thermally driven and inhibited by the freshwater forcing. In this case the circulation is not far from Stommel's saddle-node bifurcation, and a circulation state without NADW formation would also be stable. Received: 10 February 1996 / Accepted: 30 May 1996     

THE INTERBASIN TRANSPORT OF ATMOSPHERIC MOISTURE EVALUATED FROM NCEP/NCAR REANALYSIS DATA*

With the objective of providing a relatively accurate and complete diagram,the global scale interbasin transport of atmospheric moisture on the basis of the NCEP/NCAR reanalysis data for the period 1980 to 1994 is evaluated.The results show that the net zonal vapor flux for the Pacific,the Atlantic and the Indian Oceans is 0.25 Sv,-0.68 Sv and-0.29 Sv respectively.The marking differences in the zonal moisture budget among individual basins are speculated as the reason that dominates the differences in the salinity between the Pacific and the Atlantic Oceans.Though current evaluation on the net zonal moisture flux for the Atlantic basin is generally in qualitative agreement with the previous estimate,quantitative discrepancy is found to exist.According to current statistics,the tropical easterlies carry water vapor of 0.43 Sv from the Atlantic basin across Central America into the Pacific,and the northern westerliesal low water vapor of 0.25 Sv to escape from the Pacific.Quantitative analyses also reveal that the seasonal variation of net zonal vapor flux for the Pacific and the Indian Oceans is stronger than that for the Atlantic,which may be favorable for the maintenance of high salinity feature of the Atlantic Ocean.     

The role of the individual air-sea flux components in CO2-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. Received: 3 May 1999 / Accepted: 11 December 1999     

Response of the South Atlantic circulation to an abrupt collapse of the Atlantic meridional overturning circulation

The South Atlantic response to a collapse of the North Atlantic meridional overturning circulation (AMOC) is investigated in the ECHAM5/MPI-OM climate model. A reduced Agulhas leakage (about 3.1?Sv; 1?Sv?=?10⁶?m³?s^?1) is found to be associated with a weaker Southern Hemisphere (SH) supergyre and Indonesian throughflow. These changes are due to reduced wind stress curl over the SH supergyre, associated with a weaker Hadley circulation and a weaker SH subtropical jet. The northward cross-equatorial transport of thermocline and intermediate waters is much more strongly reduced than Agulhas leakage in relation with an AMOC collapse. A cross-equatorial gyre develops due to an anomalous wind stress curl over the tropics that results from the anomalous sea surface temperature gradient associated with reduced ocean heat transport. This cross-equatorial gyre completely blocks the transport of thermocline waters from the South to the North Atlantic. The waters originating from Agulhas leakage flow somewhat deeper and most of it recirculates in the South Atlantic subtropical gyre, leading to a gyre intensification. This intensification is consistent with the anomalous surface cooling over the South Atlantic. Most changes in South Atlantic circulation due to global warming, featuring a reduced AMOC, are qualitatively similar to the response to an AMOC collapse, but smaller in amplitude. However, the increased northward cross-equatorial transport of intermediate water relative to thermocline water is a strong fingerprint of an AMOC collapse.     

North Atlantic warming: patterns of long-term trend and multidecadal variability

Climate fluctuations in the North Atlantic Ocean have wide-spread implications for Europe, Africa, and the Americas. This study assesses the relative contribution of the long-term trend and variability of North Atlantic warming using EOF analysis of deep-ocean and near-surface observations. Our analysis demonstrates that the recent warming over the North Atlantic is linked to both long-term (including anthropogenic and natural) climate change and multidecadal variability (MDV, ~50–80 years). Our results suggest a general warming trend of 0.031 ± 0.006°C/decade in the upper 2,000 m North Atlantic over the last 80 years of the twentieth century, although during this time there are periods in which short-term trends were strongly amplified by MDV. For example, MDV accounts for ~60% of North Atlantic warming since 1970. The single-sign basin-scale pattern of MDV with prolonged periods of warming (cooling) in the upper ocean layer and opposite tendency in the lower layer is evident from observations. This pattern is associated with a slowdown (enhancement) of the North Atlantic thermohaline overturning circulation during negative (positive) MDV phases. In contrast, the long-term trend exhibits warming in tropical and mid-latitude North Atlantic and a pattern of cooling in regions associated with major northward heat transports, consistent with a slowdown of the North Atlantic circulation as evident from observations and confirmed by selected modeling results. This localized cooling has been masked in recent decades by warming during the positive phase of MDV. Finally, since the North Atlantic Ocean plays a crucial role in establishing and regulating the global thermohaline circulation, the multidecadal fluctuations discussed here should be considered when assessing long-term climate change and variability, both in the North Atlantic and at global scales.     

Natural forcing of climate during the last millennium: fingerprint of solar variability

The variability of the climate during the last millennium is partly forced by changes in total solar irradiance (TSI). Nevertheless, the amplitude of these TSI changes is very small so that recent reconstruction data suggest that low frequency variations in the North Atlantic Oscillation (NAO) and in the thermohaline circulation may have amplified, in the North Atlantic sector and mostly in winter, the radiative changes due to TSI variations. In this study we use a state-of-the-art climate model to simulate the last millennium. We find that modelled variations of surface temperature in the Northern Hemisphere are coherent with existing reconstructions. Moreover, in the model, the low frequency variability of this mean hemispheric temperature is found to be correlated at 0.74 with the solar forcing for the period 1001?C1860. Then, we focus on the regional climatic fingerprint of solar forcing in winter and find a significant relationship between the low frequency TSI forcing and the NAO with a time lag of more than 40?years for the response of the NAO. Such a lag is larger than the around 20-year lag suggested in other studies. We argue that this lag is due, in the model, to a northward shift of the tropical atmospheric convection in the Pacific Ocean, which is maximum more than four decades after the solar forcing increase. This shift then forces a positive NAO through an atmospheric wave connection related to the jet-stream wave guide. The shift of the tropical convection is due to the persistence of anomalous warm SST forcing the anomalous precipitation, associated with the advection of warm SST by the North Pacific subtropical gyre in a few decades. Finally, we analyse the response of the Atlantic meridional overturning circulation to solar forcing and find that the former is weakened when the latter increases. Changes in wind stress, notably due to the NAO, modify the barotropic streamfunction in the Atlantic 50?years after solar variations. This implies a wind-driven modification of the oceanic circulation in the Atlantic sector in response to changes in solar forcing, in addition to the variations of the thermohaline circulation.     

Atmospheric circulation anomalies due to 115 kyr BP climate forcing dominated by changes in the North Pacific Ocean

Climate at the time of inception of the Laurentide Ice Sheet (LIS) at ~115 kyr BP is simulated with the fully coupled NCAR Community Climate System Model (CCSM3) and compared to a simulated preindustrial climate (circa 1870) in order to better understand land surface and atmospheric responses to orbital and greenhouse cooling at inception. The interaction between obliquity and eccentricity produces maximum decrease in TOA insolation in JJA over the Arctic but increases occur over the tropics in DJF. The land surface response is dominated by widespread summer cooling in the Northern Hemisphere (NH), increases in snowfall, and decreases in melt rates and total precipitation. CCSM3 responds to the climate forcing at 115 kyr BP by producing incipient glaciation in the areas of LIS nucleation. We find that the inception of the LIS could have occurred with atmospheric circulation patterns that differ little from the present. The location of the troughs/ridges, mean flow over the Canadian Arctic and dominant modes of the atmospheric circulation are all very similar to the present. Larger changes in mean sea level pressure occur upstream of the inception region in the North Pacific Ocean and downstream in Western Europe. In the North Pacific region, the 115 kyr BP anomalies weaken both the Pacific high and Aleutian low making NH summers look more like the PREIND winters and vice versa. The occurrence of cold JJA anomalies at 115 kyr BP favors outbreaks of cold air not in the winter as in contemporary climates but during the summer instead and reinforces the cooling from orbital and GHG reductions. Increased poleward eddy transport of heat and moisture characterizes the atmospheric response in addition to reduced total cloud cover in the Arctic.     

Assessing the role of North Atlantic freshwater forcing in millennial scale climate variability: a tropical Atlantic perspective

This study analyzes a three-member ensemble of experiments, in which 0.1 Sv of freshwater was applied to the North Atlantic for 100 years in order to address the potential for large freshwater inputs in the North Atlantic to drive abrupt climate change. The model used is the GFDL R30 coupled ocean–atmosphere general circulation model. We focus in particular on the effects of this forcing on the tropical Atlantic region, which has been studied extensively by paleoclimatologists. In response to the freshwater forcing, North Atlantic meridional overturning circulation is reduced to roughly 40% by the end of the 100 year freshwater pulse. Consequently, the North Atlantic region cools by up to 8°C. The extreme cooling of the North Atlantic increases the pole-to-equator temperature gradient and requires more heat be provided to the high latitude Atlantic from the tropical Atlantic. To accommodate the increased heat requirement, the ITCZ shifts southward to allow for greater heat transport across the equator. Accompanying this southward ITCZ shift, the Northeast trade winds strengthen and precipitation patterns throughout the tropical Atlantic are altered. Specifically, precipitation in Northeast Brazil increases, and precipitation in Africa decreases slightly. In addition, we find that surface air temperatures warm over the tropical Atlantic and over Africa, but cool over northern South America. Sea-surface temperatures in the tropical Atlantic warm slightly with larger warm anomalies developing in the thermocline. These responses are robust for each member of the ensemble, and have now been identified by a number of freshwater forcing studies using coupled OAGCMs. The model responses to freshwater forcing are generally smaller in magnitude, but have the same direction, as paleoclimate data from the Younger Dryas suggest. In certain cases, however, the model responses and the paleoclimate data directly contradict one another. Discrepancies between the model simulations and the paleoclimate data could be due to a number of factors, including inaccuracies in the freshwater forcing, inappropriate boundary conditions, and uncertainties in the interpretation of the paleoclimate data. Despite these discrepancies, it is clear from our results that abrupt climate changes in the high latitude North Atlantic have the potential to significantly impact tropical climate. This warrants further model experimentation into the role of freshwater forcing in driving climate change.     

Glacial Thermohaline Circulation and Climate：Forcing from the North or South？

Based on the evidence available from both observations and model simulations, the author proposes a view that may provide a unified interpretation of the North Atlantic thermohaline variability. Because of the slow response time of the Southern Ocean （millennia） and the relatively faster response time of the North Atlantic （centuries）, the North Atlantic thermohaline circulation is controlled predominantly by the climate forcing over the Southern Ocean at the long glacial cycle timescales, but by the North Atlantic climate forcing at the short millennial timescaies.     

Long-term effects of anthropogenic CO<Subscript>2</Subscript> emissions simulated with a complex earth system model

A new complex earth system model consisting of an atmospheric general circulation model, an ocean general circulation model, a three-dimensional ice sheet model, a marine biogeochemistry model, and a dynamic vegetation model was used to study the long-term response to anthropogenic carbon emissions. The prescribed emissions follow estimates of past emissions for the period 1751–2000 and standard IPCC emission scenarios up to the year 2100. After 2100, an exponential decrease of the emissions was assumed. For each of the scenarios, a small ensemble of simulations was carried out. The North Atlantic overturning collapsed in the high emission scenario (A2) simulations. In the low emission scenario (B1), only a temporary weakening of the deep water formation in the North Atlantic is predicted. The moderate emission scenario (A1B) brings the system close to its bifurcation point, with three out of five runs leading to a collapsed North Atlantic overturning circulation. The atmospheric moisture transport predominantly contributes to the collapse of the deep water formation. In the simulations with collapsed deep water formation in the North Atlantic a substantial cooling over parts of the North Atlantic is simulated. Anthropogenic climate change substantially reduces the ability of land and ocean to sequester anthropogenic carbon. The simulated effect of a collapse of the deep water formation in the North Atlantic on the atmospheric CO₂ concentration turned out to be relatively small. The volume of the Greenland ice sheet is reduced, but its contribution to global mean sea level is almost counterbalanced by the growth of the Antarctic ice sheet due to enhanced snowfall. The modifications of the high latitude freshwater input due to the simulated changes in mass balance of the ice sheet are one order of magnitude smaller than the changes due to atmospheric moisture transport. After the year 3000, the global mean surface temperature is predicted to be almost constant due to the compensating effects of decreasing atmospheric CO₂ concentrations due to oceanic uptake and delayed response to increasing atmospheric CO₂ concentrations before.     

Convection induced long term freshening of the subpolar North Atlantic Ocean

This paper is mainly concerned with the understanding and attribution of the recent observed freshening trend in the subpolar North Atlantic Ocean. From previous coupled model studies and an analysis of the long HadCM3 control simulation, it seems unlikely that this freshening trend is a direct consequence of anthropogenically forced climate change. It is shown in this paper that the subpolar North Atlantic can be freshened to the observed degree without invoking substantial large-scale surface freshwater flux changes. The source of freshening can come from a freshwater redistribution within the Arctic/subpolar North Atlantic. The redistribution (involving both liquid water and sea ice) is carried by a perturbed ocean circulation change in the subpolar seas and triggered by deep convection in the Labrador Sea. The freshening can be widespread but mainly in the north and northwest of the subpolar North Atlantic. A sustained 30–40 years freshening trend can be easily identified in specific locations such as the Labrador Sea or in the basin wide integral of freshwater storage. At the peak, the model subpolar North Atlantic can hold around 10,000 km³ of extra freshwater. An analysis of 1,400 years HadCM3 control simulation also reveals a good correlation between freshwater content anomalies and gyre transport in the subpolar North Atlantic on decadal timescales. A general mechanism involving circulation regime changes and freshwater redistribution between the subpolar North Atlantic and the Arctic/Nordic Seas is proposed, which can resolve a number of seemingly contradictory observed changes in the North Atlantic and contributes to the longer term goal of a full understanding of recent North Atlantic fresh water changes.