Potential role of Atlantic Warm Pool-induced freshwater forcing in the Atlantic Meridional Overturning Circulation: ocean–sea ice model simulations

Recent studies have indicated that the multidecadal variations of the Atlantic Warm Pool (AWP) can induce a significant freshwater change in the tropical North Atlantic Ocean. In this paper, the potential effect of the AWP-induced freshwater flux on the Atlantic Meridional Overturning Circulation (AMOC) is studied by performing a series of ocean–sea ice model experiments. Our model experiments demonstrate that ocean response to the anomalous AWP-induced freshwater flux is primarily dominated by the basin-scale gyre circulation adjustments with a time scale of about two decades. The positive (negative) freshwater anomaly leads to an anticyclonic (cyclonic) circulation overlapping the subtropical gyre. This strengthens (weakens) the Gulf Stream and the recirculation in the interior ocean, thus increases warm (cold) water advection to the north and decreases cold (warm) water advection to the south, producing an upper ocean temperature dipole in the midlatitude. As the freshwater (salty water) is advected to the North Atlantic deep convection region, the AMOC and its associated northward heat transport gradually decreases (increases), which in turn lead to an inter-hemispheric SST seesaw. In the equilibrium state, a comma-shaped SST anomaly pattern develops in the extratropical region, with the largest amplitude over the subpolar region and an extension along the east side of the basin and into the subtropical North Atlantic. Based on our model experiments, we argue that the multidecadal AWP-induced freshwater flux can affect the AMOC, which plays a negative feedback role that acts to recover the AMOC after it is weakened or strengthened. The sensitivity of AMOC response to the AWP-induced freshwater forcing amplitude is also examined and discussed.     

Decadal predictions of the cooling and freshening of the North Atlantic in the 1960s and the role of ocean circulation

In the 1960s North Atlantic sea surface temperatures (SST) cooled rapidly. The magnitude of the cooling was largest in the North Atlantic subpolar gyre (SPG), and was coincident with a rapid freshening of the SPG. Here we analyze hindcasts of the 1960s North Atlantic cooling made with the UK Met Office’s decadal prediction system (DePreSys), which is initialised using observations. It is shown that DePreSys captures—with a lead time of several years—the observed cooling and freshening of the North Atlantic SPG. DePreSys also captures changes in SST over the wider North Atlantic and surface climate impacts over the wider region, such as changes in atmospheric circulation in winter and sea ice extent. We show that initialisation of an anomalously weak Atlantic Meridional Overturning Circulation (AMOC), and hence weak northward heat transport, is crucial for DePreSys to predict the magnitude of the observed cooling. Such an anomalously weak AMOC is not captured when ocean observations are not assimilated (i.e. it is not a forced response in this model). The freshening of the SPG is also dominated by ocean salt transport changes in DePreSys; in particular, the simulation of advective freshwater anomalies analogous to the Great Salinity Anomaly were key. Therefore, DePreSys suggests that ocean dynamics played an important role in the cooling of the North Atlantic in the 1960s, and that this event was predictable.     

Stochastically-driven multidecadal variability of the Atlantic meridional overturning circulation in CCSM3

The Atlantic meridional overturning circulation (AMOC) in the last 250?years of the 700-year-long present-day control integration of the Community Climate System Model version 3 with T85 atmospheric resolution exhibits a red noise-like irregular multi-decadal variability with a persistence longer than 10?years, which markedly contrasts with the preceding ~300 years of very regular and stronger AMOC variability with ~20?year periodicity. The red noise-like multi-decadal AMOC variability is primarily forced by the surface fluxes associated with stochastic changes in the North Atlantic Oscillation (NAO) that intensify and shift northward the deep convection in the Labrador Sea. However, the persistence of the AMOC and the associated oceanic anomalies that are directly forced by the NAO forcing does not exceed about 5?years. The additional persistence originates from anomalous horizontal advection and vertical mixing, which generate density anomalies on the continental shelf along the eastern boundary of the subpolar gyre. These anomalies are subsequently advected by the mean boundary current into the northern part of the Labrador Sea convection region, reinforcing the density changes directly forced by the NAO. As no evidence was found of a clear two-way coupling with the atmosphere, the multi-decadal AMOC variability in the last 250?years of the integration is an ocean-only response to stochastic NAO forcing with a delayed positive feedback caused by the changes in the horizontal ocean circulation.     

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.     

Assessing the sensitivity of the North Atlantic Ocean circulation to freshwater perturbation in various glacial climate states

A striking characteristic of glacial climate in the North Atlantic region is the recurrence of abrupt shifts between cold stadials and mild interstadials. These shifts have been associated with abrupt changes in Atlantic Meridional Overturning Circulation (AMOC) mode, possibly in response to glacial meltwater perturbations. However, it is poorly understood why they were more clearly expressed during Marine Isotope Stage 3 (MIS3, ~60?C27?ka BP) than during Termination 1 (T1, ~18?C10?ka BP) and especially around the Last Glacial Maximum (LGM, ~23?C19?ka BP). One clue may reside in varying climate forcings, making MIS3 and T1 generally milder than LGM. To investigate this idea, we evaluate in a climate model how ice sheet size, atmospheric greenhouse gas concentration and orbital insolation changes between 56?ka BP (=56k), 21k and 12.5k affect the glacial AMOC response to additional freshwater forcing. We have performed three ensemble simulations with the earth system model LOVECLIM using those forcings. We find that the AMOC mode in the mild glacial climate type (56k and 12.5k), with deep convection in the Labrador Sea and the Nordic Seas, is more sensitive to a constant 0.15?Sv freshwater forcing than in the cold type (21k), with deep convection mainly south of Greenland and Iceland. The initial AMOC weakening in response to freshwater forcing is larger in the mild type due to an early shutdown of Labrador Sea deep convection, which is completely absent in the 21k simulation. This causes a larger fraction of the freshwater anomaly to remain at surface in the mild type compared to the cold type. After 200?years, a weak AMOC is established in both climate types, as further freshening is compensated by an anomalous salt advection from the (sub-)tropical North Atlantic. However, the slightly fresher sea surface in the mild type facilitates further weakening of the AMOC, which occurs when a surface buoyancy threshold (?0.6?kg?m^?3 surface density anomaly to the 56k reference state) is stochastically crossed in the Nordic Seas. While described details are model-specific, our results imply that a more northern location of deep convection sites during milder glacial times may have amplified frequency and amplitude of abrupt climate shifts.     

Atmospheric response to the North Atlantic Ocean variability on seasonal to decadal time scales

The NCEP twentieth century reanalyis and a 500-year control simulation with the IPSL-CM5 climate model are used to assess the influence of ocean-atmosphere coupling in the North Atlantic region at seasonal to decadal time scales. At the seasonal scale, the air-sea interaction patterns are similar in the model and observations. In both, a statistically significant summer sea surface temperature (SST) anomaly with a horseshoe shape leads an atmospheric signal that resembles the North Atlantic Oscillation (NAO) during the winter. The air-sea interactions in the model thus seem realistic, although the amplitude of the atmospheric signal is half that observed, and it is detected throughout the cold season, while it is significant only in late fall and early winter in the observations. In both model and observations, the North Atlantic horseshoe SST anomaly pattern is in part generated by the spring and summer internal atmospheric variability. In the model, the influence of the ocean dynamics can be assessed and is found to contribute to the SST anomaly, in particular at the decadal scale. Indeed, the North Atlantic SST anomalies that follow an intensification of the Atlantic meridional overturning circulation (AMOC) by about 9 years, or an intensification of a clockwise intergyre gyre in the Atlantic Ocean by 6 years, resemble the horseshoe pattern, and are also similar to the model Atlantic Multidecadal Oscillation (AMO). As the AMOC is shown to have a significant impact on the winter NAO, most strongly when it leads by 9 years, the decadal interactions in the model are consistent with the seasonal analysis. In the observations, there is also a strong correlation between the AMO and the SST horseshoe pattern that influences the NAO. The analogy with the coupled model suggests that the natural variability of the AMOC and the gyre circulation might influence the climate of the North Atlantic region at the decadal scale.     

Cold-season atmospheric response to the natural variability of the Atlantic meridional overturning circulation

The influence of the natural variability of the Atlantic meridional overturning circulation (AMOC) on the atmosphere is studied in multi-centennial simulations of six global climate models, using Maximum Covariance Analysis (MCA). In all models, a significant but weak influence of the AMOC changes is found during the Northern Hemisphere cold-season, when the ocean leads the atmosphere by a few years. Although the oceanic pattern slightly varies, an intensification of the AMOC is followed in all models by a weak sea level pressure response that resembles a negative phase of the North Atlantic Oscillation (NAO). The signal amplitude is typically 0.5?hPa and explains about 10% of the yearly variability of the NAO in all models. The atmospheric response seems to be due primarily due to an increase of the heat loss along the North Atlantic Current and the subpolar gyre, associated with an AMOC-driven warming. Sea-ice changes appear to be less important. The stronger heating is associated to a southward shift of the lower-tropospheric baroclinicity and a decrease of the eddy activity in the North Atlantic storm track, which is consistent with the equivalent barotropic perturbation resembling the negative phase of the NAO. This study thus provides some evidence of an atmospheric signature of the AMOC in the cold-season, which may have some implications for the decadal predictability of climate in the North Atlantic region.     

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     

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.     

Tropical versus high latitude freshwater influence on the Atlantic circulation

We investigate the model sensitivity of the Atlantic meridional overturning circulation (AMOC) to anomalous freshwater flux in the tropical and northern Atlantic. Forcing in both locations leads to the same qualitative response: a positive freshwater anomaly induces a weakening of the AMOC and a negative freshwater anomaly strengthens the AMOC. Strong differences arise in the temporal characteristics and amplitude of the response. The advection of the tropical anomaly up to the deep water formation area leads to a time delayed response compared to a northern forcing. Thus, in its transient response, the AMOC is less sensitive to a constant anomalous freshwater flux in the tropics than in the north. This difference decreases with time and practically vanishes in equilibrium with constant freshwater forcing. The equilibrium response of the AMOC shows a non-linear dependence on freshwater forcing in both locations, with a stronger sensitivity to positive freshwater forcing. As a consequence, competitive forcing in both regions is balanced when the negative forcing is about 1.5 times larger than the positive forcing. The relaxation time of the AMOC after termination of a freshwater perturbation depends significantly on the AMOC strength itself. A strong overturning exhibits a faster relaxation to its unperturbed state. By means of a set of complementary experiments (pulse-perturbations, constant and stochastic forcing) we quantify these effects and discuss the corresponding time scales and physical processes.     

南极海冰涛动对北半球夏季大气环流的影响

南极海冰首要模态呈现偶极子型异常,正负异常中心分别位于别林斯高晋海/阿蒙森海和威德尔海。过去研究表明冬春季节南极海冰涛动异常对后期南极涛动（Antarctic Oscillation,AAO）型大气环流有显著影响,而AAO可以通过经向遥相关等机制影响北半球大气环流和东亚气候。本文中我们利用观测分析发现南极海冰涛动从5～7月（May–July,MJJ）到8～10月（August–October, ASO）有很好的持续性,并进一步分析其对北半球夏季大气环流的可能影响及其物理过程。结果表明,MJJ南极海冰涛动首先通过冰气相互作用在南半球激发持续性的AAO型大气环流异常,使得南半球中纬度和极地及热带之间的气压梯度加大,在MJJ至JAS,纬向平均纬向风呈现显著的正负相间的从南极到北极的经向遥相关型分布。对流层中层位势高度场上,在澳大利亚北部到海洋性大陆区域,出现显著的负异常,在东亚沿岸从低纬到高纬呈现南北走向的“? + ?”太平洋—日本（Pacific–Japan,PJ）遥相关波列,其对应赤道中部太平洋及赤道印度洋存在显著的降水和海温负异常,西北太平洋至我国东部沿海地区存在显著降水正异常和温度负异常;低纬度北美洲到大西洋一带存在的负位势高度异常和北大西洋附近存在的正位势高度异常中心,构成一个类似于西大西洋型遥相关（Western Atlantic,WA）的结构,对应赤道南大西洋降水增加和南撒哈拉地区降水减少。从物理过程来看,南极海冰涛动首先通过局地效应影响Ferrel环流,进而通过经圈环流调整使得海洋性大陆区域和热带大西洋上方的Hadley环流上升支得到增强,海洋性大陆区域特别是菲律宾附近的热带对流活动偏强,激发类似于负位相的PJ波列,影响东亚北太平洋地区的大气环流,而热带大西洋对流增强和北传特征,则通过激发WA遥相关影响大西洋和欧洲地区的大气环流。以上两种通道将持续性MJJ至ASO南极海冰涛动强迫的大气环流信号从南半球中高纬度经热带地区传递到北半球中高纬地区,从而对热带和北半球夏季大气环流产生显著影响。     

The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation

An OGCM hindcast is used to investigate the linkages between North Atlantic Ocean salinity and circulation changes during 1963–2003. The focus is on the eastern subpolar region consisting of the Irminger Sea and the eastern North Atlantic where a careful assessment shows that the simulated interannual to decadal salinity changes in the upper 1,500 m reproduce well those derived from the available record of hydrographic measurements. In the model, the variability of the Atlantic meridional overturning circulation (MOC) is primarily driven by changes in deep water formation taking place in the Irminger Sea and, to a lesser extent, the Labrador Sea. Both are strongly influenced by the North Atlantic Oscillation (NAO). The modeled interannual to decadal salinity changes in the subpolar basins are mostly controlled by circulation-driven anomalies of freshwater flux convergence, although surface salinity restoring to climatology and other boundary fluxes each account for approximately 25% of the variance. The NAO plays an important role: a positive NAO phase is associated with increased precipitation, reduced northward salt transport by the wind-driven intergyre gyre, and increased southward flows of freshwater across the Greenland–Scotland ridge. Since the NAO largely controlled deep convection in the subpolar gyre, fresher waters are found near the sinking region during convective events. This markedly differs from the active influence on the MOC that salinity exerts at decadal and longer timescales in most coupled models. The intensification of the MOC that follows a positive NAO phase by about 2 years does not lead to an increase in the northward salt transport into the subpolar domain at low frequencies because it is cancelled by the concomitant intensification of the subpolar gyre which shifts the subpolar front eastward and reduces the northward salt transport by the North Atlantic Current waters. This differs again from most coupled models, where the gyre intensification precedes that of the MOC by several years.     

Decadal-timescale changes of the Atlantic overturning circulation and climate in a coupled climate model with a hybrid-coordinate ocean component

A wide range of statistical tools is used to investigate the decadal variability of the Atlantic Meridional Overturning Circulation (AMOC) and associated key variables in a climate model (CHIME, Coupled Hadley-Isopycnic Model Experiment), which features a novel ocean component. CHIME is as similar as possible to the 3rd Hadley Centre Coupled Model (HadCM3) with the important exception that its ocean component is based on a hybrid vertical coordinate. Power spectral analysis reveals enhanced AMOC variability for periods in the range 15–30 years. Strong AMOC conditions are associated with: (1) a Sea Surface Temperature (SST) anomaly pattern reminiscent of the Atlantic Multi-decadal Oscillation (AMO) response, but associated with variations in a northern tropical-subtropical gradient; (2) a Surface Air Temperature anomaly pattern closely linked to SST; (3) a positive North Atlantic Oscillation (NAO)-like pattern; (4) a northward shift of the Intertropical Convergence Zone. The primary mode of AMOC variability is associated with decadal changes in the Labrador Sea and the Greenland Iceland Norwegian (GIN) Seas, in both cases linked to the tropical activity about 15 years earlier. These decadal changes are controlled by the low-frequency NAO that may be associated with a rapid atmospheric teleconnection from the tropics to the extratropics. Poleward advection of salinity anomalies in the mixed layer also leads to AMOC changes that are linked to processes in the Labrador Sea. A secondary mode of AMOC variability is associated with interannual changes in the Labrador and GIN Seas, through the impact of the NAO on local surface density.     

Mechanisms of the atmospheric response to North Atlantic multidecadal variability: a model study

The atmospheric circulation response to decadal fluctuations of the Atlantic meridional overturning circulation (MOC) in the IPSL climate model is investigated using the associated sea surface temperature signature. A SST anomaly is prescribed in sensitivity experiments with the atmospheric component of the IPSL model coupled to a slab ocean. The prescribed SST anomaly in the North Atlantic is the surface signature of the MOC influence on the atmosphere detected in the coupled simulation. It follows a maximum of the MOC by a few years and resembles the model Atlantic multidecadal oscillation. It is mainly characterized by a warming of the North Atlantic south of Iceland, and a cooling of the Nordic Seas. There are substantial seasonal variations in the geopotential height response to the prescribed SST anomaly, with an East Atlantic Pattern-like response in summer and a North Atlantic oscillation-like signal in winter. In summer, the response of the atmosphere is global in scale, resembling the climatic impact detected in the coupled simulation, albeit with a weaker amplitude. The zonally asymmetric or eddy part of the response is characterized by a trough over warm SST associated with changes in the stationary waves. A diagnostic analysis with daily data emphasizes the role of transient-eddy forcing in shaping and maintaining the equilibrium response. We show that in response to an intensified MOC, the North Atlantic storm tracks are enhanced and shifted northward during summer, consistent with a strengthening of the westerlies. However the anomalous response is weak, which suggests a statistically significant but rather modest influence of the extratropical SST on the atmosphere. The winter response to the MOC-induced North Atlantic warming is an intensification of the subtropical jet and a southward shift of the Atlantic storm track activity, resulting in an equatorward shift of the polar jet. Although the SST anomaly is only prescribed in the Atlantic ocean, significant impacts are found globally, indicating that the Atlantic ocean can drive a large scale atmospheric variability at decadal timescales. The atmospheric response is highly non-linear in both seasons and is consistent with the strong interaction between transient eddies and the mean flow. This study emphasizes that decadal fluctuations of the MOC can affect the storm tracks in both seasons and lead to weak but significant dynamical changes in the atmosphere.     

热带积云对流加热在全球大气遥响应中的重要作用──数值试验结果

利用ＩＡＰ两层大气环流模式模拟研究了热带地区积云对流加热在大气对赤道东太平洋海温正异常响应中的作用。通过对积分结果进行分析发现：热带地区积云对流在大气对赤道东太平洋海温正异常的响应过程中起着非常重要的作用。若热带地区的积云对流加热减弱则大气中的遥响应（相关）型也减弱。同时我们还发现，热带地区的积云对流加热加强则响应场的３０—６０ｄ低频振荡也得到加强。     

高纬度淡水强迫增强背景下大西洋经向翻转环流的响应及其机制

利用卑尔根海洋－大气－海冰耦合气候模式(Bergen Climate Model, 简称BCM), 研究在北冰洋及北欧海淡水强迫增强的背景下, 大西洋经向翻转环流(Atlantic Meridional Overturning Circulation, 简称AMOC)的响应及其机制, 着重讨论了海表热力性质、北大西洋深层水 (North Atlantic Deep Water, 简称NADW) 的生成率、 海洋内部等密度层间的垂直混合 (Diapycnal Mixing, 简称DM) 以及大气风场等物理过程随AMOC的响应所发生的时间演变特征。结果显示, 在持续150年增强 (强度为0.4 Sv) 的淡水强迫下 (淡水试验, FW1), AMOC的强度表现为前50年的快速减弱和在接下来100年中的逐渐恢复。同时, 在淡水试验的前50年北大西洋高纬度海表盐度 (Sea Surface Salinity, 简称SSS) 减小, 海水密度降低, 冬季对流混合减弱, 导致NADW生成率快速减弱; 在接下来的100年中, 尽管增强的淡水强迫依然维持, 由于海洋内部自身的调节和海气相互作用, 导致了AMOC的逐渐恢复。恢复机制可以概括为: (1) 随着向南的NADW的减少, 大西洋中低纬度海水垂直层结逐渐减弱, DM随之逐渐增强, 有利于中低纬度海盆内深层水的上升; (2) 南半球西风应力增强与东风应力的减弱及北半球东风的增强使得大西洋向北的埃克曼体积通量净传输恢复; (3) 大西洋向北的盐度传输逐渐恢复及次极地回旋区降水的减弱, 导致SSS和NADW生成率的恢复, 与之对应, AMOC逐渐恢复。研究还发现, 淡水试验中, NADW的恢复主要以厄尔明格海 (Irminger Sea) 为主, 冬季北大西洋海平面气压场 (SLP) 呈现类似正北大西洋涛动 (NAO+) 的模态, 热带降水中心移到赤道以南, 大西洋热带SSS增强。     

Quantifying the AMOC feedbacks during a 2×CO2 stabilization experiment with land-ice melting

The response of the Atlantic Meridional Overturning Circulation (AMOC) to an increase in atmospheric CO₂ concentration is analyzed using the IPSL-CM4 coupled ocean–atmosphere model. Two simulations are integrated for 70 years with 1%/year increase in CO₂ concentration until 2×CO₂, and are then stabilized for further 430 years. The first simulation takes land-ice melting into account, via a simple parameterization, which results in a strong freshwater input of about 0.13 Sv at high latitudes in a warmer climate. During this scenario, the AMOC shuts down. A second simulation does not include this land-ice melting and herein, the AMOC recovers after 200 years. This behavior shows that this model is close to an AMOC shutdown threshold under global warming conditions, due to continuous input of land-ice melting. The analysis of the origin of density changes in the Northern Hemisphere convection sites allows an identification as to the origin of the changes in the AMOC. The processes that decrease the AMOC are the reduction of surface cooling due to the reduction in the air–sea temperature gradient as the atmosphere warms and the local freshening of convection sites that results from the increase in local freshwater forcing. Two processes also control the recovery of the AMOC: the northward advection of positive salinity anomalies from the tropics and the decrease in sea-ice transport through the Fram Strait toward the convection sites. The quantification of the AMOC related feedbacks shows that the salinity related processes contribute to a strong positive feedback, while feedback related to temperature processes is negative but remains small as there is a compensation between heat transport and surface heat flux in ocean–atmosphere coupled model. We conclude that in our model, AMOC feedbacks amplify land-ice melting perturbation by 2.5.     

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.     

Global energy budget changes to high latitude North Atlantic cooling and the tropical ITCZ response

The intertropical convergence zone (ITCZ) in atmospheric general circulation models (coupled to slab ocean) shift southwards in response to northern extratropical cooling. Previous studies have demonstrated the utility of diagnosing the atmospheric energy fluxes in interpreting this teleconnection. This study investigates the nature of global energy flux changes in response to North Atlantic high latitude cooling applied to the Community Atmosphere Model version 3 coupled to a slab ocean, focusing on key local and remote feedbacks that collectively act to alter the energy budget and atmospheric energy transport. We also investigate the relative roles of tropical sea surface temperature (SST) and energy flux changes in the ITCZ response to North Atlantic cooling. Using a radiative kernel technique, we quantify the effects of key feedbacks—temperature, cloud and water vapor, to the top-of-the-atmosphere radiative flux changes. The results show only partial local energy flux compensation to the initial perturbation in the high latitudes, originating from the negative temperature feedback and opposed by positive shortwave albedo and longwave water vapor feedbacks. Thus, an increase in the atmospheric energy transport to the Northern extratropics is required to close the energy budget. The additional energy flux providing this increase comes from top-of-the-atmosphere radiative flux increase over the southern tropics, primarily from cloud, temperature and longwave water vapor feedbacks, and largely as a consequence of increased deep convection. It has been previously argued that the role of tropical SST changes was secondary to the role played by the atmospheric energy flux requirements in controlling the ITCZ shifts, proposing that the SST response is a result of the surface energy budget and not a driver of the precipitation response. Using a set of idealized simulations with the fixed tropical SSTs, we demonstrate that the ITCZ shifts are not possible without the tropical SST changes and suggest that the tropical SSTs are a more suitable driver of tropical precipitation shifts compared to the atmospheric energy fluxes. In our simulations, the ITCZ shifts are influenced mainly by the local (tropical) SST forcing, apparently independent of the actual high latitude energy demand.     

The impact of tropical western Pacific convection on the North Pacific atmospheric circulation during the boreal winter

In this study, we investigate the impact of atmospheric convection over the western tropical Pacific (100–145°E, 0–20°N) on the boreal winter North Pacific atmosphere flow by analyzing National Center for Environmental Prediction Reanalysis 1, Extended Reconstructed Sea Surface Temperature and Global Precipitation Climatology Project data. The western tropical Pacific convection is not only the main energy source driving the local Hadley and Walker circulations, but it also significantly influences North Pacific circulation, by modifying a mid-latitude Jet stream through the connection with the local Hadley circulation. On the one hand, this strong convection leads to a northward expansion of local Hadley cells simultaneous with a northward movement of the western North Pacific jet because of the close correlation between the Jet and Hadley circulation boundaries. On the other hand, this strong convection also intensifies tropical Pacific Walker circulation, which reduces the eastern Pacific sea surface temperature, resembling a La Nina state through the enhanced equatorial upwelling. The cooling of the eastern tropical Pacific has an inter-tropical convergence zone located further north; thus, the local Hadley circulation moves northward. As a result, the jet axis over the eastern North Pacific, which also corresponds to the boundary of the local Hadley circulation, moves to higher latitude. Consequently, this northward movement of the Jet axis over the North Pacific is reflected as a northwest–southeast dipole sea level pressure (SLP) pattern. The composite analysis of SLP over the North Pacific against the omega (Ω) (Pa/s) at 500 hPa over the western tropical Pacific actually reveals that this northwest-southeast dipole structure is attributed to the intensified tropical western Pacific convection, which pushes the Pacific Jet to the north. Finally we also analyzed south Pacific for the austral winter as did previously to North Pacific, and found that the results were consistent.