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.     

European winter temperature variability in a long coupled model simulation: the contribution of ocean dynamics

The interannual variability of the European winter air temperature is partially caused by anomalous atmospheric circulation and the associated advection of air masses, mainly linked to the North Atlantic Oscillation (NAO). However, a considerable part of the temperature variability is not linearly described by atmospheric circulation anomalies. Here, a long control simulation with a coupled atmosphere-ocean climate model is analyzed, with the goal of decomposing the European temperature (ET) anomalies in a part linked to the anomalous atmospheric circulation and a residual. The amount of interannual variability of each contribution is roughly 50%, although at subdecadal (overdecadal) time scales the variability of the residuals is dominant. These residuals are found to be linked to temperature anomalies of the same sign in the whole North Atlantic and Greenland, in contrast to the well-known temperature zonal seesaw associated with the NAO. The association between the residuals and other processes in the North Atlantic has been also analyzed. The thermohaline circulation, closely connected in the model to the intensity of the Gulf Stream, lags the evolution of the temperature residuals by several years and thus is not able to control their evolution. The variability of the oceanic convection in the Northern North Atlantic, on the other hand, correlates with the temperature residual at lags close to zero. It is hypothesized that oceanic convection produces a sea-surface temperature fingerprint that leads to the ET residuals. The implications of these results for multi-year predictability and for empirical climate reconstructions are discussed.     

The North Atlantic Oscillation and oceanic precipitation variability

Global North Atlantic Oscillation (NAO) oceanic precipitation features in the latter half of the twentieth century are documented based on the intercomparison of multiple state-of-the-art precipitation datasets and the analysis of the NAO atmospheric circulation and SST anomalies. Most prominent precipitation anomalies occur over the ocean in the North Atlantic, where in winter a “quadrupole-like” pattern is found with centers in the western tropical Atlantic, sub-tropical Atlantic, high-latitude eastern Atlantic and over the Labrador Sea. The extent of the sub-tropical and high-latitude center and the amount of explained variance (over 50%) are quite remarkable. However, the tropical Atlantic center is probably the most intriguing feature of this pattern apparently linking the NAO with ITCZ variability. In summer, the pattern is “tripole-like” with centers in the eastern Mediterranean Sea, the North Sea/Baltic Sea and in the sub-polar Atlantic. In the eastern Indian Ocean, the correlation is positive in winter and negative in summer, with some link to ENSO variability. The sensitivity of these patterns to the choice of the NAO index is minor in winter while quite important in summer. Interannual NAO precipitation anomalies have driven similar fresh water variations in these “key” regions. In the sub-tropical and high-latitude Atlantic in winter precipitation anomalies have been roughly 15 and 10% of climatology per unit change of the NAO, respectively. Decadal changes of the NAO during the last 50 years have also influenced precipitation and fresh water flux at these time-scales, with values lower (higher) than usual in the high-latitude eastern North Atlantic (Labrador Sea) in the 1960s and the late 1970s, and an opposite situation since the early 1980s; in summer the North Sea/Baltic region has been drier than usual during the period 1965–1975 when the NAO was generally positive.     

Observed and simulated multidecadal variability in the Northern Hemisphere

 Analyses of proxy based reconstructions of surface temperatures during the past 330 years show the existence of a distinct oscillatory mode of variability with an approximate time scale of 70 years. This variability is also seen in instrumental records, although the oscillatory nature of the variability is difficult to assess due to the short length of the instrumental record. The spatial pattern of this variability is hemispheric or perhaps even global in scale, but with particular emphasis on the Atlantic region. Independent analyses of multicentury integrations of two versions of the GFDL coupled atmosphere-ocean model also show the existence of distinct multidecadal variability in the North Atlantic region which resembles the observed pattern. The model variability involves fluctuations in the intensity of the thermohaline circulation in the North Atlantic. It is our intent here to provide a direct comparison of the observed variability to that simulated in a coupled ocean-atmosphere model, making use of both existing instrumental analyses and newly available proxy based multi-century surface temperature estimates. The analyses demonstrate a substantial agreement between the simulated and observed patterns of multidecadal variability in sea surface temperature (SST) over the North Atlantic. There is much less agreement between the model and observations for sea level pressure. Seasonal analyses of the variability demonstrate that for both the model and observations SST appears to be the primary carrier of the multidecadal signal. Received: 8 June 1999 / Accepted: 11 February 2000     

Simulation of the Atlantic meridional overturning circulation in an atmosphere–ocean global coupled model. Part I: a mechanism governing the variability of ocean convection in a preindustrial experiment

A preindustrial climate experiment was conducted with the third version of the CNRM global atmosphere–ocean–sea ice coupled model (CNRM-CM3) for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4). This experiment is used to investigate the main physical processes involved in the variability of the North Atlantic ocean convection and the induced variability of the Atlantic meridional overturning circulation (MOC). Three ocean convection sites are simulated, in the Labrador, Irminger and Greenland–Iceland–Norwegian (GIN) Seas in agreement with observations. A mechanism linking the variability of the Arctic sea ice cover and convection in the GIN Seas is highlighted. Contrary to previous suggested mechanisms, in CNRM-CM3 the latter is not modulated by the variability of freshwater export through Fram Strait. Instead, the variability of convection is mainly driven by the variability of the sea ice edge position in the Greenland Sea. In this area, the surface freshwater balance is dominated by the freshwater input due to the melting of sea ice. The ice edge position is modulated either by northwestward geostrophic current anomalies or by an intensification of northerly winds. In the model, stronger than average northerly winds force simultaneous intense convective events in the Irminger and GIN Seas. Convection interacts with the thermohaline circulation on timescales of 5–10 years, which translates into MOC anomalies propagating southward from the convection sites.     

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     

Effect of seasonal forcing on global circulation in a world ocean general circulation model

 Effects of the seasonal variation in thermohaline and wind forcing on the abyssal circulation are investigated by using an ocean general circulation model. To isolate effects of the seasonality in the thermohaline forcing from those in the wind forcing, we carry out three experiments with (1) annual-mean wind forcing and perpetual-winter thermohaline forcing, (2) annual-mean wind forcing and seasonal thermohaline forcing, and (3) seasonal wind forcing and seasonal thermohaline forcing. The deep water under the seasonal thermohaline forcing becomes warmer than under the perpetual-winter thermohaline forcing. Although the perpetual-winter thermohaline forcing is widely used and believed to reproduce the deep water better than the annual-mean forcing, the difference between the results of the perpetual-winter and the seasonal thermohaline forcing is significant. The seasonal variation of the Ekman convergence and divergence produces meridional overturning cells extending to the bottom because the period of seasonal cycle is shorter than the adjustment timescale by baroclinic Rossby waves. The heat transport owing to those Ekman flows and temperature anomalies makes the upper water (0–200 m) colder at low to mid-latitudes (40S–40N) and warmer at high latitudes. Also the deep water becomes warmer owing to the warming of the northern North Atlantic, the main source region of North Atlantic Deep Water. The model is also synchronously (i.e., without acceleration) integrated with seasonal forcing for 5400 y. A past study suggested that under seasonal forcing, a sufficient equilibrium state can be achieved after only decades of synchronous integration following more than 10 000 y of accelerated integration. Here, the result so obtained is compared with that of the 5400-y synchronous integration. The difference in the global average temperature is as small as 0.12 °C, and most of the difference is confined to the Southern Ocean. Received: 1 May 1998 / Accepted: 5 January 1999     

Seasonal to interannual climate predictability in mid and high northern latitudes in a global coupled model

The upper limit of climate predictability in mid and high northern latitudes on seasonal to interannual time scales is investigated by performing two perfect ensemble experiments with the global coupled atmosphere–ocean–sea ice model ECHAM5/MPI-OM. The ensembles consist of six members and are initialized in January and July from different years of the model’s 300-year control integration. The potential prognostic predictability is analyzed for a set of oceanic and atmospheric climate parameters. The predictability of the atmospheric circulation is small except for southeastern Europe, parts of North America and the North Pacific, where significant predictability occurs with a lead time of up to half a year. The predictability of 2 m air temperature shows a large land–sea contrast with highest predictabilities over the sub polar North Atlantic and North Pacific. A combination of relatively high persistence and advection of sea surface temperature anomalies into these areas leads to large predictability. Air temperature over Europe, parts of North America and Asia shows significant predictability of up to half a year in advance. Over the ice-covered Arctic, air temperature is not predictable at time scales exceeding 2 months. Sea ice thickness is highly predictable in the central Arctic mainly due to persistence and in the Labrador Sea due to dynamics. Surface salinity is highly predictable in the Arctic Ocean, northern North Atlantic and North Pacific for several years in advance. We compare the results to the predictability due to persistence and show the importance of dynamical processes for the predictability.     

Seawater density variations in the North Atlantic and the Atlantic meridional overturning circulation

Seawater property changes in the North Atlantic Ocean affect the Atlantic meridional overturning circulation (AMOC), which transports warm water northward from the upper ocean and contributes to the temperate climate of Europe, as well as influences climate globally. Previous observational studies have focused on salinity and freshwater variability in the sinking region of the North Atlantic, since it is believed that a freshening North Atlantic basin can slow down or halt the flow of the AMOC. Here we use available data to show the importance of how density patterns over the upper ocean of the North Atlantic affect the strength of the AMOC. For the long-term trend, the upper ocean of the subpolar North Atlantic is becoming cooler and fresher, whereas the subtropical North Atlantic is becoming warmer and saltier. On a multidecadal timescale, the upper ocean of the North Atlantic has generally been warmer and saltier since 1995. The heat and salt content in the subpolar North Atlantic lags that in the subtropical North Atlantic by about 8–9 years, suggesting a lower latitude origin for the temperature and salinity anomalies. Because of the opposite effects of temperature and salinity on density for both long-term trend and multidecadal timescales, these variations do not result in a density reduction in the subpolar North Atlantic for slowing down the AMOC. Indeed, the variations in the meridional density gradient between the subpolar and subtropical North Atlantic Ocean suggest that the AMOC has become stronger over the past five decades. These observed results are supported by and consistent with some oceanic reanalysis products.     

GCM simulations of the Last Glacial Maximum surface climate of Greenland and Antarctica

 The LMDz variable grid GCM was used to simulate the Last Glacial Maximum (LGM, 21 ky Bp.) climate of Greenland and Antarctica at a spatial resolution of about 100 km.The high spatial resolution allows to investigate the spatial variability of surface climate change signals, and thus to address the question whether the sparse ice core data can be viewed as representative for the regional scale climate change. This study addresses primarily surface climate parameters because these can be checked against the, limited, ice core record. The changes are generally stronger for Greenland than for Antarctica, as the imposed changes of the forcing boundary conditions (e.g., sea surface temperatures) are more important in the vicinity of Greenland. Over Greenland, and to a limited extent also in Antarctica, the climate shows stronger changes in winter than in summer. The model suggests that the linear relationship between the surface temperature and inversion strength is modified during the LGM. The temperature dependency of the moisture holding capacity of the atmosphere alone cannot explain the strong reduction in snowfall over central Greenland; atmospheric circulation changes also play a crucial role. Changes in the high frequency variability of snowfall, atmospheric pressure and temperature are investigated and possible consequences for the interpretation of ice core records are discussed. Using an objective cyclone tracking scheme, the importance of changes of the atmospheric dynamics off the coasts of the ice sheets, especially for the high frequency variability of surface climate parameters, is illustrated. The importance of the choice of the LGM ice sheet topography is illustrated for Greenland, where two different topographies have been used, yielding results that differ quite strongly in certain nontrivial respects. This means that the paleo-topography is a significant source of uncertainty for the modelled paleoclimate. The sensitivity of the Greenland LGM climate to the prescribed sea surface conditions is examined by using two different LGM North Atlantic data sets. Received: 23 October 1997 / Accepted: 17 March 1998     

The cold event 8200 years ago documented in oxygen isotope records of precipitation in Europe and Greenland

 Stable oxygen isotope ratios of ostracod valves in Late Glacial and Holocene sediments of core AS 92-5 from deep lake Ammersee (southern Germany) reflect variations of mean oxygen isotope ratios in past atmospheric precipitation. The record reconfirms the strong similarity of climate evolution in Europe and Greenland during the last deglaciation. For the first time in Europe, we find a 200-year-long negative δ¹⁸O-excursion, which is contemporaneous with the strongest negative δ¹⁸O-excursion in the Greenland ice around 8.2 ky before present. The 8.2 ky isotopic event on both sides of the North Atlantic ocean is interpreted as a cold period, most probably induced by a perturbation of the North Atlantic thermohaline circulation. We discuss two possible triggering mechanisms: (1) weak forcing (as proposed by Alley et al.), and (2) forcing by a strong and sudden freshwater pulse from the collapse of the Hudson Ice Dome. Received: 27 May 1997 / Accepted: 21 July 1997     

An intimate coupling of ocean–atmospheric interaction over the extratropical North Atlantic and Pacific

The inter-basin teleconnection between the North Atlantic and the North Pacific ocean–atmosphere interaction is studied using a coupled ocean–atmosphere general circulation model. In the model, an idealized oceanic temperature anomaly is initiated over the Kuroshio and the Gulf Stream extension region to track the coupled evolution of ocean and atmosphere interaction, respectively. The experiments explicitly demonstrate that both the North Pacific and the North Atlantic ocean–atmosphere interactions are intimately coupled through an inter-basin atmospheric teleconnection. This fast inter-basin communication can transmit oceanic variability between the North Atlantic and the North Pacific through local ocean-to-atmosphere feedbacks. The leading mode of the extratropical atmospheric internal variability plays a dominant role in shaping the hemispheric-scale response forced by oceanic variability over the North Atlantic and Pacific. Modeling results also suggest that a century (two centuries) long observations are necessary for the detection of Pacific response to Atlantic forcings (Atlantic response to Pacific forcing).     

Exciting natural modes of variability by solar and volcanic forcing: idealized and realistic experiments

Using multi-millenium simulations performed with the three-dimensional climate model ECBILT-CLIO, we analyze how variations in the external forcing can excite low-frequency modes of climate variability. We find that prescribing an idealized, abrupt decrease in solar irradiance can trigger a large perturbation of the oceanic thermohaline circulation (THC) associated with a cooling of more than 5 °C in the North Atlantic over decades to centuries. Using more realistic scenarios that include the variations of solar irradiance and the influence of volcanic eruptions, such large perturbations of the THC are not triggered. Nevertheless, modifications of the forcing can strongly modify the probability of very cold years in the North Atlantic. During those cold years, sea-ice covers a large part of the Nordic Seas and the inflow of warm Atlantic waters at high latitudes is strongly reduced. Those processes induce a temporarily, strong local amplification of the forcing and generate modifications of the atmospheric conditions. Simulations of the last millenium climate using realistic forcing reveal that the probability to have such very cold years in the model is higher during the period AD 1300–1850 than during the first centuries of the second millenium or during the twentieth century. This might explain the higher variability observed during this period in some climate records in the Nordic Seas.     

A 20-year coupled ocean-sea ice-atmosphere variability mode in the North Atlantic in an AOGCM

In order to understand potential predictability of the ocean and climate at the decadal time scales, it is crucial to improve our understanding of internal variability at this time scale. Here, we describe a 20-year mode of variability found in the North Atlantic in a 1,000-year pre-industrial simulation of the IPSL-CM5A-LR climate model. This mode involves the propagation of near-surface temperature and salinity anomalies along the southern branch of the subpolar gyre, leading to anomalous sea-ice melting in the Nordic Seas, which then forces sea-level pressure anomalies through anomalous surface atmospheric temperatures. The wind stress associated to this atmospheric structure influences the strength of the East Greenland Current across the Denmark Strait, which, in turn, induces near-surface temperature and salinity anomalies of opposite sign at the entrance of the Labrador Sea. This starts the second half of the cycle after approximatively 10 years. The time scale of the cycle is thus essentially set by advection of tracers along the southern branch of the subpolar gyre, and by the time needed for anomalous East Greenland Current to accumulate heat and freshwater anomalies at the entrance of the Labrador Sea. The Atlantic meridional overturning circulation (AMOC) does not play a dominant role in the mode that is confined in the subpolar North Atlantic, but it also has a 20-year preferred timescale. This is due to the influence of the propagating salinity anomalies on the oceanic deep convection. The existence of this preferred timescale has important implications in terms of potential predictability of the North Atlantic climate in the model, although its realism remains questionable and is discussed.     

Two climatic states and feedbacks on thermohaline circulation in an Earth system model of intermediate complexity

The McGill Paleoclimate Model-2 (MPM-2) is employed to study climate–thermohaline circulation (THC) interactions in a pre -industrial climate, with a special focus on the feedbacks on the THC from other climate system components. The MPM-2, a new version of the MPM, has an extended model domain from 90S to 90N, active winds and no oceanic heat and freshwater flux adjustments. In the MPM-2, there are mainly two stable modes for the Atlantic meridional overturning circulation (MOC) under the ‘present-day’ forcing (present-day solar forcing and the pre-industrial atmospheric CO₂ level of 280 ppm). The ‘on’ mode has an active North Atlantic deep water formation, while the ‘off’ mode has no such deep water formation. By comparing the ‘off’ mode climate state with its ‘on’ mode analogue, we find that there exist many large differences between the two climate states, which originate from large changes in the oceanic meridional heat transports. By suppressing or isolating each process associated with a continental ice sheet over North America, sea ice, the atmospheric hydrological cycle and vegetation, feedbacks from these components on the Atlantic MOC are investigated. Sensitivity studies investigating the role of varying continental ice growth and sea ice meridional transport in the resumption of the Atlantic MOC are also carried out. The results show that a fast ice sheet growth and an enhanced southward sea ice transport significantly favor the resumption of the Atlantic MOC in the MPM-2. In contrast to this, the feedback from the atmospheric hydrological cycle is a weak positive one. The vegetation-albedo feedback could enhance continental ice sheet growth and thus could also favor the resumption of the Atlantic MOC. However, before the shut-down of the Atlantic MOC, feedbacks from these components on the Atlantic MOC are very weak.     

Closure of the Panama Seaway during the Pliocene: implications for climate and Northern Hemisphere glaciation

The “Panama Hypothesis” states that the gradual closure of the Panama Seaway, between 13 million years ago (13 Ma) and 2.6 Ma, led to decreased mixing of Atlantic and Pacific water Masses, the formation of North Atlantic Deep water and strengthening of the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere (NH) high latitudes, culminating in the intensification of Northern Hemisphere Glaciation (NHG) during the Pliocene, 3.2–2.7 Ma. Here we test this hypothesis using a fully coupled, fully dynamic ocean-atmosphere general circulation model (GCM) with boundary conditions specific to the Pliocene, and a high resolution dynamic ice sheet model. We carry out two GCM simulations with “closed” and “open” Panama Seaways, and use the simulated climatologies to force the ice sheet model. We find that the models support the “Panama Hypothesis” in as much as the closure of the seaway results in a more intense Atlantic thermohaline circulation, enhanced precipitation over Greenland and North America, and ultimately larger ice sheets. However, the volume difference between the ice sheets in the “closed” and “open” configurations is small, equivalent to about 5 cm of sea level. We conclude that although the closure of the Panama Seaway may have slightly enhanced or advanced the onset of NHG, it was not a major forcing mechanism. Future work must fully couple the ice sheet model and GCM, and investigate the role of orbital and CO₂ effects in controlling NHG.     

North African climate variability. Part 1: Tropical thermocline coupling

Summary Tropical ocean thermocline variability is studied using gridded data assimilated by an ocean model in the period 1950–2000. The dominant patterns and variability are identified using EOF analysis applied to E–W depth slices of sea temperatures averaged over the tropics. After removing the annual cycle, an east–west ‘see-saw’ with an interannual to decadal rhythm is the leading mode in each of the tropical basins. In the case of the leading mode in the Pacific, the thermocline oscillation forms a dipole structure, but in the (east) Atlantic and (southwest) Indian Ocean there is a single center of action. The interaction of the ocean thermocline and atmospheric Walker circulations is studied through cross-modulus analysis of wavelet-filtered EOF time scores. Our study demonstrates how tropical ocean thermocline variability contributes to zonal circulation anomalies in the atmosphere. The equatorial Pacific thermocline oscillation explains 62 and 53% of the variability of the Pacific and Atlantic zonal overturning circulations, the latter driving convective polarity between North Africa and South America. The Pacific sea-saw leads the Atlantic zonal circulation by a few months.     

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.     

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.     

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.