利用大气环流模式模拟北大西洋海温异常强迫响应

北大西洋地区的海温异常能够在多大程度上对大气产生影响，一直是一个有争议的问题。作者利用伴随北大西洋涛动出现的海温异常对大气环流模式CAM2.0.1进行强迫，考察了模式在冬季（12月、1月和2月）对三核型海温异常的响应。通过与欧洲中期天气预报中心提供的再分析资料的对比，发现该模式可以通过海温强迫在一定程度上再现具有北大西洋涛动特征的温度场和环流场。在北大西洋及其沿岸地区，模式模拟出了三核型的准正压响应，与经典的北大西洋涛动型大气异常是一致的。模式结果与北大西洋地区大气内部主导模态的差别主要体现在两个方面:一是异常中心位置多偏向于大洋上空，在陆地上的异常响应强度很弱；二是高纬地区对海温异常的响应不显著，没有强迫出与实际的大气模态相对应的异常中心，表明该地区海洋的反馈作用较弱。     

The North Atlantic Oscillation—What Role for the Ocean?

The extent to which the North Atlantic Oscillation (NAO) is influenced by changes in the ocean state is an issue that has attracted much recent attention. Although there have been counter claims, the weight of evidence clearly suggests that forcing by the ocean of year-to-year changes in the NAO is a weak influence by comparison with atmospheric internal variability. The NAO is thus very different in character to the Southern Oscillation (SO), and its predictability—at least on seasonal-to-interannual timescales—is almost certainly much lower.Although weak, the influence of the ocean on the NAO is not negligible. In a previous study we found that wintertime North Atlantic climate, including the NAO, was significantly influenced by a tripole pattern of North Atlantic SST anomalies. Here we report the results of experiments to further elucidate the nature of this influence. We show that the tripole pattern induces a significant response both in the tropical Atlantic and at mid-to-high latitudes. The low latitude response is forced by the low latitude SST anomalies, but the high latitude response is influenced by the extratropical SST anomalies as well as those in the tropics. Furthermore, we find evidence of nonlinear interaction between the influence of the tropical and extratropical SST anomalies. Lastly, we investigate the feedback from the atmosphere onto the SST tripole. We find that the expected negative feedback is significantly modified at low latitudes by the dynamical response of the atmosphere.     

Interannual-to-decadal variability of the North Atlantic from an ocean data assimilation system

An ocean analysis, assimilating both surface and subsurface hydrographic temperature data into a global ocean model, has been produced for the period 1958–2000, and used to study the time and space variations of North Atlantic upper ocean heat content (HC). Observational evidence is presented for interannual-to-decadal variability of upper ocean thermal fluctuations in the North Atlantic related to the North Atlantic Oscillation (NAO) variability over the last 40 years. The assimilation scheme used in the ocean analysis is a univariate, variational optimum interpolation of temperature. The first guess is produced by an eddy permitting global ocean general circulation forced by atmospheric reanalysis from the National Center for Environmental Prediction (NCEP). The validation of the ocean analysis has been done through the comparison with objectively analyzed observations and independent data sets. The method is able to compensate for the model systematic error to reproduce a realistic vertical thermal structure of the region and to improve consistently the model estimation of the time variability of the upper ocean temperature. Empirical orthogonal function (EOF) analysis shows that an important mode of variability of the wintertime upper ocean climate over the North Atlantic during the period of study is characterized by a tripole pattern both for SST and upper ocean HC. A similar mode is found for summer HC anomalies but not for summer SST. Over the whole period, HC variations in the subtropics show a general warming trend while the tropical and north eastern part of the basin have an opposite cooling tendency. Superimposed on this linear trend, the HC variability explained by the first EOF both in winter and summer conditions reveals quasi-decadal oscillations correlated with changes in the NAO index. On the other hand, there is no evidence of correlation in time between the NAO index and the upper ocean HC averaged over the whole North Atlantic which exhibits a substantial and monotonic warming trend during the last two decades of the analysis period. The maximum correlation is found between the leading principal component of winter HC anomalies and NAO index at 1 year lag with NAO leading. For SST anomalies significant correlation is found only for winter conditions. In contrast, for HC anomalies high correlations are found also in the summer suggesting that the summer HC keeps a memory of winter conditions.     

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.     

Can local linear stochastic theory explain sea surface temperature and salinity variability?

 Sea surface temperature (SST) and salinity (SSS) time series from four ocean weather stations and data from an integration of the GFDL coupled ocean-atmosphere model are analyzed to test the applicability of local linear stochastic theory to the mixed-layer ocean. According to this theory, mixed-layer variability away from coasts and fronts can be explained as a ‘red noise’ response to the ‘white noise’ forcing by atmospheric disturbances. At one weather station, Papa (northeast Pacific), this stochastic theory can be applied to both salinity and temperature, explaining the relative redness of the SSS spectrum. Similar results hold for a model grid point adjacent to Papa, where the relationships between atmospheric energy and water fluxes and actual changes in SST and SSS are what is expected from local linear stochastic theory. At the other weather stations, this theory cannot adequately explain mixed-layer variability. Two oceanic processes must be taken into account: at Panulirus (near Bermuda), mososcale eddies enhance the observed variability at high frequencies. At Mike and India (North Atlantic), variations in SST and SSS advection, indicated by the coherence and equal persistence of SST and SSS anomalies, contribute to much of the low frequency variability in the model and observations. To achieve a global perspective, TOPEX altimeter data and model results are used to identify regions of the ocean where these mechanisms of variability are important. Where mesoscale eddies are as energetic as at Panulirus, indicated by the TOPEX global distribution of sea level variability, one would expect enhanced variability on short time scales. In regions exhibiting signatures of variability similar to Mike and India, variations in SST and SSS advection should dominate at low frequencies. According to the model, this mode of variability is found in the circumpolar ocean and the northern North Atlantic, where it is associated with the irregular oscillations of the model’s thermohaline circulation. Received: 11 March 1996 / Accepted: 6 September 1996     

Air–Sea Coupling Enhances the East Asian Winter Climate Response to the Atlantic Multidecadal Oscillation

A simple air–sea coupled model,the atmospheric general circulation model(AGCM) of the National Centers for Environmental Prediction coupled to a mixed-layer slab ocean model,is employed to investigate the impact of air–sea coupling on the signals of the Atlantic Multidecadal Oscillation(AMO). A regional coupling strategy is applied,in which coupling is switched off in the extratropical North Atlantic Ocean but switched on in the open oceans elsewhere. The coupled model is forced with warm-phase AMO SST anomalies,and the modeled responses are compared with those from parallel uncoupled AGCM experiments with the same SST forcing. The results suggest that the regionally coupled responses not only resemble the AGCM simulation,but also have a stronger intensity. In comparison,the coupled responses bear greater similarity to the observational composite anomaly. Thus,air–sea coupling enhances the responses of the East Asian winter climate to the AMO. To determine the mechanism responsible for the coupling amplification,an additional set of AGCM experiments,forced with the AMO-induced tropical SST anomalies,is conducted. The SST anomalies are extracted from the simulated AMO-induced SST response in the regionally coupled model. The results suggest that the SST anomalies contribute to the coupling amplification. Thus,tropical air–sea coupling feedback tends to enhance the responses of the East Asian winter climate to the AMO.     

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.     

基于信息流理论的因果分析在辨析大西洋多年代际振荡物理机制中的应用

大西洋多年代际振荡（AMO）是指发生在北大西洋的海表温度（SST）冷暖异常多年代际（50～80年）振荡的现象。通常AMO被认为是受大西洋经向翻转环流（AMOC）及其对应的海洋动力过程（经向热量输运）的影响。近年来有观点认为,AMO是大气随机热力强迫的产物,大气主导了海气间的热量交换进而影响AMO。弄清AMO和北大西洋海表热通量的因果关系是辨析AMO动力和热力驱动机制的关键。本文利用基于信息流理论的因果分析方法,研究了1880年以来观测的AMO与北大西洋海表热通量间的因果关系。结果表明,在多年代际尺度上,从AMO到海表热通量的信息流要远大于二者相反方向的信息流,这说明AMO是北大西洋海表热通量异常的因,海洋主导了海气间的热量交换。大气随机热力强迫机制无法解释AMO与热通量两者因果分析的结果。对泛大西洋地区的陆地气温和AMO指数进行分析,进一步表明由于海洋主导了海气热量交换,AMO的海温异常加热/冷却控制了绝大多数地区气温的多年代际变化。利用海温驱动的大气环流模式的模拟结果验证了AMO的海温异常对周边陆地气温强迫作用。本文的结果为辨析AMO的动力和热力驱动机制提供了新线索,进一步表明AMO并非是大气随机热力强迫的产物,海洋环流可能是AMO的主要驱动因子。     

On decadal-scale ocean-atmosphere interactions in the extended ECHAM1/LSG climate simulation

 The last 810 years of a control integration with the ECHAM1/LSG coupled model are used to clarify the nature of the ocean-atmosphere interactions at low frequencies in the North Atlantic and the North Pacific. To a first approximation, the atmosphere acts as a white noise forcing and the ocean responds as a passive integrator. The sea surface temperature (SST) variability primarily results from short time scale fluctuations in surface heat exchanges and Ekman currents, and the former also damp the SST anomalies after they are generated. The thermocline variability is primarily driven by Ekman pumping. Because the heat, momentum, and vorticity fluxes at the sea surface are correlated in space and time, the SST variability is directly linked to that in the ocean interior. The SST is also modulated by the wind-driven geostrophic fluctuations, resulting in persistent correlation with the thermocline changes and a slight low-frequency redness of the SST spectra. The main dynamics are similar in the two oceans, although in the North Pacific the SST variability is more strongly influenced by advection changes and the oceanic time scales are larger. A maximum covariance analysis based on singular value decomposition in lead and lag conditions indicates that some of the main modes of atmospheric variability in the two oceans are sustained by a very weak positive feedback between the atmosphere, SST, and the strength of the subtropical and subpolar gyres. In addition, in the North Atlantic the main surface pressure mode has a small quasi-oscillatory component at 6-year period, and advective resonance occurs for SST around 10-year period, both periods being also singled out by multichannel singular spectrum analysis. The ocean-atmosphere coupling is however much too weak to redden the tropospheric spectra or create anything more than tiny spectral peaks, so that the atmospheric and oceanic variability is dominated in both ocean sectors by the one-way interactions. Received: 2 April 1999 / Accepted: 14 October 1999     

North Atlantic decadal regimes in a coupled GCM simulation

 The non-stationarity of the North Atlantic atmosphere-ocean coupling is investigated utilizing a long time integration of a coupled atmosphere-ocean general circulation model (GCM) and a consistent atmospheric experiment forced by the climatological sea surface temperature (SST) of the coupled GCM. The temporal behavior of the North Atlantic Oscillation (NAO) is non-stationary with two different decadal regimes being identified: (a) phases with enhanced (active) low-frequency variability of the NAO index are characterized by regional modes with a baroclinic Pacific-North America (PNA) and a dominant barotropic North Atlantic pattern; (b) in phases with reduced (passive) low-frequency variability a global mode connects tropics and midlatitudes. The characteristic space scales are similar in the coupled and the consistent atmospheric experiment; the time scales of the atmospheric eigenmodes are modified by ocean dynamics. In the active (passive) phase the corresponding atmospheric mode is reinforced by the North Atlantic (tropical Pacific) SST. Received: 15 September 2000 / Accepted: 30 March 2001     

Sensitivity of the Northern Hemisphere circulation to North Atlantic SSTs in the ARPÈGE Climate AGCM

In order to determine the response of the atmosphere to winter sea surface temperature (SST) anomalies in the North Atlantic area, we carried out ensemble runs of 20 years, forced with constant, perturbed, SST patterns using the climate version of the ARPèGE AGCM, at T42 resolution. A Monte Carlo technique was applied, in such a way that the control experiment, forced with observed climatological temperatures, and the four scenario experiments, forced with perturbed SSTs are equivalent to a length of 20 independent winters. Four anomalous winter North Atlantic sea surface temperature (SST) fields have been constructed by considering the observed SST variability in the main basins, namely the Labrador Sea and the Greenland Sea. Two patterns are of the `seesaw' type, while the two others have same the polarity in both basins. The patterns have been reinforced by a factor of 5–6 compared to presently observed multi-annual anomalies, in order to get SST anomalies which may have occurred during periods of the Little Ice Age. The differences between each of the four winter simulations with perturbed SSTs and the control run are analyzed in terms of tropospheric thickness, mean-sea-level pressure and storm activity. The `seesaw' type patterns give a weaker response in the tropospheric thickness fields than the two others. This is expected from simple considerations. In the mean circulation and synoptic activity, it appears that the Labrador Sea SST is important in determining the atmospheric response. This is probably due to enhanced temperature gradients east of New Foundland which enhances the storm activity. Received: 23 September 1998 / Accepted: 17 November 1999     

Extratropical control of recent tropical Pacific decadal climate variability: a relay teleconnection

Observations indicate that recent tropical Pacific decadal climate variability tends to be associated with the extratropical North Pacific through a relay teleconnection of a fast coupled ocean-atmosphere bridge and a slow oceanic tunnel. A coupled ocean-atmosphere model, forced by the observed decadal wind in the extratropical North Pacific, explicitly demonstrates that extratropical decadal sea surface temperature (SST) anomalies may propagate to the tropics through a coupled wind-evaporative-SST (WES) feedback. The WES feedback cannot only lead to a nearly synchronous change of tropical SST, but also force a delayed adjustment of the meridional overturning circulation in the upper ocean to further sustain the tropical SST change. The study further suggests that the extratropical–tropical teleconnection provides a positive feedback to sustain the decadal changes in both the tropical and extratropical North Pacific.     

Simulation of the response of the North Pacific Ocean to the anomalous atmospheric circulation associated with El Niño

During El Niño events when positive sea surface temperature (SST) anomalies form in the equatorial Pacific, SST anomalies also tend to develop in the North Pacific. This study attempts to model and explain the large-scale features of the observed SST anomaly field in the North Pacific during the fall and winter of the El Niño year. The experiment design consists of a mixed layer ocean model of the North Pacific which is forced by atmospheric surface fields from two sets of Community Climate Model (CCM) integrations: the El Niño set with prescribed positive SST anomalies in the tropical Pacific; and the control set which is obtained from an extended CCM integration with prescribed climatological SSTs. The response of the midlatitude ocean to atmospheric surface fields associated with El Niño is obtained by compositing each set of model integrations (El Niño and Control) and then taking the difference between the composites. The ocean model is able to reproduce the general features of the observed midlatitude SST anomaly pattern: warm water in the northeast Pacific and an elliptically shaped cold pool in the central Pacific. In these regions, a large fraction of the temperature anomalies are significant at the 95% level as indicated by a two tailed t-test. The ocean temperature anomalies simulated by the model are primarily caused by changes in the sensible and latent heat flux and to a lesser extent the longwave radiation flux. Entrainment of cold water from below the mixed layer also influences ocean temperatures. However, the entrainment anomaly pattern has a complex spatial structure which does not always coincide with the simulated mixed layer temperature anomalies.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

The North Atlantic thermohaline circulation simulated by the GISS climate model during 1970–99

Abstract

Evidence based on numerical simulations is presented for a strong correlation between the North Atlantic Oscillation (NAO) and the North Atlantic overturning circulation. Using an ensemble of numerical experiments with a coupled ocean‐atmosphere model including both natural and anthropogenic forcings, it is shown that the weakening of the thermohaline circulation (THC) could be delayed in response to a sustained upward trend in the NAO, which was observed over the last three decades of the twentieth century, 1970–99. Overall warming and enhanced horizontal transports of heat from the tropics to the subpolar North Atlantic overwhelm the NAO‐induced cooling of the upper ocean layers due to enhanced fluxes of latent and sensible heat, so that the net effect of warmed surface ocean temperatures acts to increase the vertical stability of the ocean column. However, the strong westerly winds cause increased evaporation from the ocean surface, which leads to a reduced fresh water flux over the western part of the North Atlantic. Horizontal poleward transport of salinity anomalies from the tropical Atlantic is the major contributor to the increasing salinities in the sinking regions of the North Atlantic. The effect of positive salinity anomalies on surface ocean density overrides the opposing effect of enhanced warming of the ocean surface, which causes an increase in surface density in the Labrador Sea and in the ocean area south of Greenland. The increased density of the upper ocean layer leads to deeper convection in the Labrador Sea and in the western North Atlantic. With a lag of four years, the meridional overturning circulation of the North Atlantic shows strengthening as it adjusts to positive density anomalies and enhanced vertical mixing. During the positive NAO trend, the salinity‐driven density instability in the upper ocean, due to both increased northward ocean transports of salinity and decreased atmospheric freshwater fluxes, results in a strengthening overturning circulation in the North Atlantic when the surface atmospheric temperature increases by 0.3°C and the ocean surface temperature warms by 0.5° to 1°C.     

Influence of Atlantic sea surface temperatures on persistent drought in North America

This study investigates the relationship between North Atlantic sea surface temperatures (SST) and persistent drought in North America using modern observations, proxy paleo-data, and simulations from multiple climate models. The observational results show that persistent droughts in the Great Plains and the southwest North America are closely related to multidecadal variations of North Atlantic SST (Atlantic Multidecadal Oscillations, AMO). During the AMO warm (cold) phases, most of North America is dry (wet). This relationship is persistent since at least 1567 AD, as based on proxy SST for the North Atlantic and the reconstructed drought index for North America. On centennial timescales, proxy SST records from the North Atlantic and proxy drought records for North America suggest that major periods of AMO-like warm (cold) SST anomalies during the last 7.0?ka correspond to dry (wet) conditions in the Great Plains. The influence of North Atlantic SST on North American droughts is examined using simulations made by five global climate models. When forced by warm North Atlantic SST anomalies, all models captured significant drying over North America, despite some regional differences. Specifically, dry summers in the Great Plains and the southwest North America are simulated by all models. The precipitation response to a cold North Atlantic is much weaker and contains greater disagreement among the models. Overall, the ensemble of the five models could well reproduce the statistical relationship between the dry/wet fluctuations in the North America and North Atlantic SST anomalies. Our results suggest that North Atlantic SSTs are likely a major driver of decadal and centennial timescale circulation, including droughts, in North America. Possible mechanisms that connect North Atlantic SST with North American drought, as well as interactions between North Atlantic and tropical Pacific SST and their relative roles on drought are also discussed.     

A 1,000-year ice core record of interannual to multidecadal variations in atmospheric circulation over the North Atlantic

Interannual to multidecadal modes in ocean/atmosphere dynamics in the North Atlantic region have been identified using sea salt aerosol proxy records from northern Greenland ice cores over the last 1,000 years. Sea salt concentrations show a consistent relationship with anomalies in the meridional pressure gradient over the North Atlantic region over all considered time scales. These pressure anomalies are connected to shifts in storm tracks, leading to lower pressure and higher storm activity, hence, higher sea salt export over the Greenland ice sheet. Two modes of long-term variability with a period of 10.4 years and 62 years could be identified. The latter is connected to long-term changes in sea surface temperature (SST) as documented by a high correlation of North Atlantic SST with our sea salt record over the last 150 years. Long-term reconstruction of these modes shows that the 10.4-year cycle has been a phenomenon persistent over the last millennium while the 62-year cycle has been mainly active after 1700. Accordingly, the longer-term persistence of this multidecadal variability in sea salt points also to significant variations in SST over the last 300 years.     

North Atlantic climate responses to perturbations in Antarctic Intermediate Water

Recent observations suggest Antarctic Intermediate Water (AAIW) properties are changing. The impact of such variations is explored using idealised perturbation experiments with a coupled climate model, HadCM3. AAIW properties are altered between 10 and 20°S in the South Atlantic, maintaining constant potential density. The perturbed AAIW remains subsurface in the South Atlantic, but as it moves northwards, it surfaces and interacts with the atmosphere leading to density anomalies due to heat exchanges. For a cooler, fresher AAIW, there is a significant decrease in the mean North Atlantic sea surface temperature (SST), of up to 1°C, during years 51?C100. In the North Atlantic Current region there are persistent cold anomalies from 2,000?m depth to the surface, and in the overlying atmosphere. Atmospheric surface pressure increases over the mid-latitude Atlantic, and precipitation decreases over northwest Africa and southwest Europe. Surface heat flux anomalies show that these impacts are caused by changes in the ocean rather than atmospheric forcing. The SST response is associated with significant changes in the Atlantic meridional overturning circulation (MOC). After 50?years there is a decrease in the MOC that persists for the remainder of the simulation, resulting from changes in the column-averaged density difference between 30°S and 60°N. Rather than showing a linear response, a warmer, saltier AAIW also leads to a decreased MOC strength for years 51?C100 and resulting cooling in the North Atlantic. The non-linearity can be attributed to opposing density responses as the perturbed water masses interact with the atmosphere.     

Pacific interdecadal variability driven by tropical–extratropical interactions

Interactions between the tropical and subtropical northern Pacific at decadal time scales are examined using uncoupled oceanic and atmospheric simulations. An atmospheric model is forced with observed Pacific sea surface temperatures (SST) decadal anomalies, computed as the difference between the 2000–2009 and the 1990–1999 period. The resulting pattern has negative SST anomalies at the equator, with a global pattern reminiscent of the Pacific decadal oscillation. The tropical SST anomalies are responsible for driving a weakening of the Hadley cell and atmospheric meridional heat transport. The atmosphere is then shown to produce a significant response in the subtropics, with wind-stress-curl anomalies having the opposite sign from the climatological mean, consistent with a weakening of the oceanic subtropical gyre (STG). A global ocean model is then forced with the decadal anomalies from the atmospheric model. In the North Pacific, the shallow subtropical cell (STC) spins down and the meridional heat transport is reduced, resulting in positive tropical SST anomalies. The final tropical response is reached after the first 10 years of the experiment, consistent with the Rossby-wave adjustment time for both the STG and the STC. The STC provides the connection between subtropical wind stress anomalies and tropical SSTs. In fact, targeted simulations show the importance of off-equatorial wind stress anomalies in driving the oceanic response, whereas anomalous tropical winds have no role in the SST signal reversal. We further explore the connection between STG, STC and tropical SST with the help of an idealized model. We argue that, in our models, tropical SST decadal variability stems from the forcing of the Pacific subtropical gyre through the atmospheric response to ENSO. The resulting Ekman pumping anomaly alters the STC and oceanic heat transport, providing a negative feedback on the SST. We thus suggest that extratropical atmospheric responses to tropical forcing have feedbacks onto the ocean dynamics that lead to a time-delayed response of the tropical oceans, giving rise to a possible mechanism for multidecadal ocean-atmosphere coupled variability.     

全球海洋蒸发量年代际变化归因：动力因子分析

本文基于动力调整方法,利用客观分析海气通量（OAFlux）资料研究了1958～2016年全球海洋蒸发量变化及其动力作用和辐射强迫分量的变化,发现海洋蒸发量及其动力作用分量具有一致性年代际变化特征,特别是在20世纪70年代及90年代末期存在明显的年代际转折。进一步分析发现:主要动力因子有太平洋—北美遥相关型（PNA）、北极涛动（AO）、北大西洋涛动（NAO）、厄尔尼诺—南方涛动（ENSO）和阿留申低压（AL）,并受到太平洋年代际振荡（PDO）的影响,其中,1970年代末期的转折与PNA、PDO、ENSO和AL密切相关,而1990年代末期的转折还与NAO变化有关。动力作用分量的前六个模态解释方差达到67.5%,其中,低纬北太平洋和印度洋蒸发异常主要与海表温度（SST）及其引起的环流异常有关,南太平洋、中纬北太平洋和北大西洋蒸发异常与环流异常直接相关。ENSO与PDO在全球海洋蒸发量上的影响要大于NAO。单因子相关分析发现南方涛动指数（SOI）、NAO和PDO与海洋蒸发年代际变化密切相关。总体来说,动力作用分量在海洋蒸发的年代际变化中起主导作用,其中,以ENSO、NAO和PDO的影响最大。     

Changes of Air–sea Coupling in the North Atlantic over the 20th Century

Changes of air–sea coupling in the North Atlantic Ocean over the 20 th century are investigated using reanalysis data,climate model simulations, and observational data. It is found that the ocean-to-atmosphere feedback over the North Atlantic is significantly intensified in the second half of the 20 th century. This coupled feedback is characterized by the association between the summer North Atlantic Horseshoe(NAH) SST anomalies and the following winter North Atlantic Oscillation(NAO). The intensification is likely associated with the enhancement of the North Atlantic storm tracks as well as the NAH SST anomalies. Our study also reveals that most IPCC AR4 climate models fail to capture the observed NAO/NAH coupled feedback.