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1.
Analyses of a 500-year control integration of the global coupled atmosphere–sea ice–ocean model ECHAM5.0/MPI-OM show a high variability in the ice export through Fram Strait on interannual to decadal timescales. This variability is mainly determined by variations in the sea level pressure gradient across Fram Strait and thus geostrophic wind stress. Ice thickness anomalies, formed at the Siberian coast and in the Chukchi Sea, propagate across the Arctic to Fram Strait and contribute to the variability of the ice export on a timescale of about 9 years. Large anomalies of the ice export through Fram Strait cause fresh water signals, which reach the Labrador Sea after 1–2 years and lead to significant changes in the deep convection. The associated anomalies in ice cover and ocean heat release have a significant impact on air temperature in the Labrador Sea and on the large-scale atmospheric circulation. This affects the sea ice transport and distribution in the Arctic again. Sensitivity studies, simulating the effect of large ice exports through Fram Strait, show that the isolated effect of a prescribed ice/fresh water anomaly is very important for the climate variability in the Labrador Sea. Thus, the ice export through Fram Strait can be used for predictability of Labrador Sea climate up to 2 years in advance.  相似文献   

2.
The ocean heat transport into the Arctic and the heat budget of the Barents Sea are analyzed in an ensemble of historical and future climate simulations performed with the global coupled climate model EC-Earth. The zonally integrated northward heat flux in the ocean at 70°N is strongly enhanced and compensates for a reduction of its atmospheric counterpart in the twenty first century. Although an increase in the northward heat transport occurs through all of Fram Strait, Canadian Archipelago, Bering Strait and Barents Sea Opening, it is the latter which dominates the increase in ocean heat transport into the Arctic. Increased temperature of the northward transported Atlantic water masses are the main reason for the enhancement of the ocean heat transport. The natural variability in the heat transport into the Barents Sea is caused to the same extent by variations in temperature and volume transport. Large ocean heat transports lead to reduced ice and higher atmospheric temperature in the Barents Sea area and are related to the positive phase of the North Atlantic Oscillation. The net ocean heat transport into the Barents Sea grows until about year 2050. Thereafter, both heat and volume fluxes out of the Barents Sea through the section between Franz Josef Land and Novaya Zemlya are strongly enhanced and compensate for all further increase in the inflow through the Barents Sea Opening. Most of the heat transported by the ocean into the Barents Sea is passed to the atmosphere and contributes to warming of the atmosphere and Arctic temperature amplification. Latent and sensible heat fluxes are enhanced. Net surface long-wave and solar radiation are enhanced upward and downward, respectively and are almost compensating each other. We find that the changes in the surface heat fluxes are mainly caused by the vanishing sea ice in the twenty first century. The increasing ocean heat transport leads to enhanced bottom ice melt and to an extension of the area with bottom ice melt further northward. However, no indication for a substantial impact of the increased heat transport on ice melt in the Central Arctic is found. Most of the heat that is not passed to the atmosphere in the Barents Sea is stored in the Arctic intermediate layer of Atlantic water, which is increasingly pronounced in the twenty first century.  相似文献   

3.
Atmospheric forcing of Fram Strait sea ice export: a closer look   总被引：2，自引：0，他引：2  
Fram Strait is the primary region of sea ice export from the Arctic and therefore plays an important role in regulating the amount of sea ice and freshwater within the Arctic. We investigate the variability of Fram Strait sea ice motion and the role of atmospheric circulation forcing using daily data during the period 1979–2006. The most prominent atmospheric driver of anomalous sea ice motion across Fram Strait is an east–west dipole pattern of Sea Level Pressure (SLP) anomalies with centers of action located over the Barents Sea and Greenland. This pattern, also observed in synoptic studies, is associated with anomalous meridional winds across Fram Strait and is thus physically consistent with forcing changes in sea ice motion. The association between the SLP dipole pattern and Fram Strait ice motion is maximized at 0-lag, persists year-round, and is strongest on time scales of 10–60 days. The SLP dipole pattern is the second empirical orthogonal function (EOF) of daily SLP anomalies in both winter and summer. When the analysis is repeated with monthly data, only the Barents center of the SLP dipole remains significantly correlated with Fram Strait sea ice motion. However, after removing the leading EOF of monthly SLP variability (e.g., the North Atlantic Oscillation), the full east–west dipole pattern is recovered. No significant SLP forcing of Fram Strait ice motion is found in summer using monthly data, even when the leading EOF is removed. Our results highlight the importance of high frequency atmospheric variability in forcing Fram Strait sea ice motion.  相似文献   

4.
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.  相似文献   

5.
The fourth version of the atmosphere-ocean general circulation (AOGCM) model developed at the Institut Pierre-Simon Laplace (IPSL-CM4) is used to investigate the mechanisms influencing the Arctic freshwater balance in response to anthropogenic greenhouse gas forcing. The freshwater influence on the interannual variability of deep winter oceanic convection in the Nordic Seas is also studied on the basis of correlation and regression analyses of detrended variables. The model shows that the Fram Strait outflow, which is an important source of freshwater for the northern North Atlantic, experiences a rapid and strong transition from a weak state toward a relatively strong state during 1990–2010. The authors propose that this climate shift is triggered by the retreat of sea ice in the Barents Sea during the late twentieth century. This sea ice reduction initiates a positive feedback in the atmosphere-sea ice-ocean system that alters both the atmospheric and oceanic circulations in the Greenland-Iceland-Norwegian (GIN)-Barents Seas sector. Around year 2080, the model predicts a second transition threshold beyond which the Fram Strait outflow is restored toward its original weak value. The long-term freshening of the GIN Seas is invoked to explain this rapid transition. It is further found that the mechanism of interannual changes in deep mixing differ fundamentally between the twentieth and twenty-first centuries. This difference is caused by the dominant influence of freshwater over the twenty-first century. In the GIN Seas, the interannual changes in the liquid freshwater export out of the Arctic Ocean through Fram Strait combined with the interannual changes in the liquid freshwater import from the North Atlantic are shown to have a major influence in driving the interannual variability of the deep convection during the twenty-first century. South of Iceland, the other region of deep water renewal in the model, changes in freshwater import from the North Atlantic constitute the dominant forcing of deep convection on interannual time scales over the twenty-first century.  相似文献   

6.
A coupled atmosphere-ocean-sea ice model is applied to investigate to what degree the area-thickness distribution of new ice formed in open water affects the ice and ocean properties. Two sensitivity experiments are performed which modify the horizontal-to-vertical aspect ratio of open-water ice growth. The resulting changes in the Arctic sea-ice concentration strongly affect the surface albedo, the ocean heat release to the atmosphere, and the sea-ice production. The changes are further amplified through a positive feedback mechanism among the Arctic sea ice, the Atlantic Meridional Overturning Circulation (AMOC), and the surface air temperature in the Arctic, as the Fram Strait sea ice import influences the freshwater budget in the North Atlantic Ocean. Anomalies in sea-ice transport lead to changes in sea surface properties of the North Atlantic and the strength of AMOC. For the Southern Ocean, the most pronounced change is a warming along the Antarctic Circumpolar Current (ACC), owing to the interhemispheric bipolar seasaw linked to AMOC weakening. Another insight of this study lies on the improvement of our climate model. The ocean component FESOM is a newly developed ocean-sea ice model with an unstructured mesh and multi-resolution. We find that the subpolar sea-ice boundary in the Northern Hemisphere can be improved by tuning the process of open-water ice growth, which strongly influences the sea ice concentration in the marginal ice zone, the North Atlantic circulation, salinity and Arctic sea ice volume. Since the distribution of new ice on open water relies on many uncertain parameters and the knowledge of the detailed processes is currently too crude, it is a challenge to implement the processes realistically into models. Based on our sensitivity experiments, we conclude a pronounced uncertainty related to open-water sea ice growth which could significantly affect the climate system sensitivity.  相似文献   

7.
Sea ice variability in the Barents Sea and its impact on climate are analyzed using a 465-year control integration of a global coupled atmosphere–ocean–sea ice model. Sensitivity simulations are performed to investigate the response to an isolated sea ice anomaly in the Barents Sea. The interannual variability of sea ice volume in the Barents Sea is mainly determined by variations in sea ice import into Barents Sea from the Central Arctic. This import is primarily driven by the local wind field. Horizontal oceanic heat transport into the Barents Sea is of minor importance for interannual sea ice variations but is important on longer time scales. Events with strong positive sea ice anomalies in the Barents Sea are due to accumulation of sea ice by enhanced sea ice imports and related NAO-like pressure conditions in the years before the event. Sea ice volume and concentration stay above normal in the Barents Sea for about 2 years after an event. This strongly increases the albedo and reduces the ocean heat release to the atmosphere. Consequently, air temperature is much colder than usual in the Barents Sea and surrounding areas. Precipitation is decreased and sea level pressure in the Barents Sea is anomalously high. The large-scale atmospheric response is limited with the main impact being a reduced pressure over Scandinavia in the year after a large ice volume occurs in the Barents Sea. Furthermore, high sea ice volume in the Barents Sea leads to increased sea ice melting and hence reduced surface salinity. Generally, the climate response is smallest in summer and largest in winter and spring.  相似文献   

8.
Freshwater (FW) leaves the Arctic Ocean through sea-ice export and the outflow of low-salinity upper ocean water. Whereas the variability of the sea-ice export is known to be mainly caused by changes in the local wind and the thickness of the exported sea ice, the mechanisms that regulate the variability of the liquid FW export are still under investigation. To better understand these mechanisms, we present an analysis of the variability of the liquid FW export from the Arctic Ocean for the period 1950–2007, using a simulation from an energy and mass conserving global ocean–sea ice model, coupled to an Energy Moisture Balance Model of the atmosphere, and forced with daily winds from the NCEP reanalysis. Our results show that the simulated liquid FW exports through the Canadian Arctic Archipelago (CAA) and the Fram Strait lag changes in the large-scale atmospheric circulation over the Arctic by 1 and 6 years, respectively. The variability of the liquid FW exports is caused by changes in the cyclonicity of the atmospheric forcing, which cause a FW redistribution in the Arctic through changes in Ekman transport in the Beaufort Gyre. This in turn causes changes in the sea surface height (SSH) and salinity upstream of the CAA and Fram Strait, which affect the velocity and salinity of the outflow. The SSH changes induced by the large-scale atmospheric circulation are found to explain a large part of the variance of the liquid FW export, while the local wind plays a much smaller role. We also show that during periods of increased liquid FW export from the Arctic, the strength of the simulated Atlantic meridional overturning circulation is reduced and the ocean heat transport into the Arctic is increased. These results are particularly relevant in the context of global warming, as climate simulations predict an increase in the liquid FW export from the Arctic during the twenty-first century.  相似文献   

9.
Tom Agnew 《大气与海洋》2013,51(2):259-280
Abstract

This study looks at simultaneous changes in atmospheric circulation and extremes in sea‐ice cover during winter. Thirty‐six years of ice‐cover data and 100‐kPa height and 50–100‐kPa thickness data are used. For the entire Arctic, the study found a general weakening of the Aleutian and Icelandic lows for heavy (i.e. severe) compared with light sea‐ice conditions suggesting reduced surface heating as a possible cause. The weakening of the two lows would also reduce meridional atmospheric circulation and poleward heat transport into the Arctic. The study also looks at three regions of high sea ice and atmospheric variability: the Bering Sea, the Davis Strait/Labrador Sea and the Greenland Sea. For the Bering Sea, heavy sea‐ice conditions were accompanied by weakening and westward displacement of the Aleutian Low again suggesting reduced surface heating and the formation of a secondary low in the Gulf of Alaska. This change in circulation is consistent with increased cold air advection over the Bering Sea and changes in storm tracks and meridional heat transport found in other studies. For the Davis Strait/Labrador Sea, heavy ice‐cover winters were accompanied by intensification of the Icelandic Low suggesting atmospheric temperature and wind advection and associated changes in ocean currents as the main cause of heavy ice. For the Greenland Sea no statistically significant difference was found. It is felt that this may be due to the important role that ice export through Fram Strait and ocean currents play in determining ice extent in this region.  相似文献   

10.
The stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean—atmosphere—sea ice coupled model. It turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates. The large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further South. The nature of the instability is of oceanic origin, identical to that found in ocean models under mixed boundary conditions. With similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates, the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters, the deep water temperatures being regulated by the temperature of freezing. Weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation. Within the unstable regime periodic millennial oscillations occur spontaneously. The climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients. Both states are unstable. The atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content. During the abrupt warming events that punctuate the course of a millennial oscillation, sea ice variations are shown respectively to damp (amplify) the amplitude of the oceanic (atmospheric) response. This sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period, the so called Dansgaard—Oeschger events, may be internal instabilities of the climate system.  相似文献   

11.
A scenario of the Mediterranean Sea is performed for the twenty-first century based on an ocean modelling approach. A climate change IPCC-A2 scenario run with an atmosphere regional climate model is used to force a Mediterranean Sea high-resolution ocean model over the 1960–2099 period. For comparison, a control simulation as long as the scenario has also been carried out under present climate fluxes. This control run shows air–sea fluxes in agreement with observations, stable temperature and salinity characteristics and a realistic thermohaline circulation simulating the different intermediate and deep water masses described in the literature. During the scenario, warming and saltening are simulated for the surface (+3.1°C and + 0.48 psu for the Mediterranean Sea at the end of the twenty-first century) and for the deeper layers (+1.5°C and + 0.23 psu on average). These simulated trends are in agreement with observed trends for the Mediterranean Sea over the last decades. In addition, the Mediterranean thermohaline circulation (MTHC) is strongly weakened at the end of the twenty-first century. This behaviour is mainly due to the decrease in surface density and so the decrease in winter deep-water formation. At the end of the twenty-first century, the MTHC weakening can be evaluated as −40% for the intermediate waters and −80% for the deep circulation with respect to present-climate conditions. The characteristics of the Mediterranean Outflow Waters flowing into the Atlantic Ocean are also strongly influenced during the scenario.  相似文献   

12.
Results from an ice-ocean coupled model are used to investigate the impact of long-term variability in sea ice transport at the Fram Strait on the intensity of the Atlantic deep circulation. An increase (or decrease) in sea ice transport through the Fram Strait leads to a stronger (or weaker) deep circulation in the Atlantic. Change in the sea ice transport is accompanied by a salinity anomaly in the surface layer of the Arctic Ocean. Such an anomaly could inversely affect the Atlantic circulation once it reaches deep water formation regions. If the Canadian Archipelago is closed, the anomaly is subsequently transported through the Fram Strait, and counters the initial changes in the Atlantic deep circulation. On the other hand, if the Canadian Archipelago is open, some of the anomaly is transported to the Canadian Archipelago, and the initial change in the Atlantic deep circulation persists. In the Arctic Ocean basin, the time scale and path of the salinity anomalys propagation depends on the large-scale flow at the surface of the Arctic Ocean. Our results suggest that the salinity anomaly transport and its propagation pathway out of the Arctic Ocean are important determinants of the role of sea ice transport variability through the Fram Strait in controlling the intensity of the Atlantic deep circulation.  相似文献   

13.
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.  相似文献   

14.
Climate change due to enhanced greenhouse warming has been calculated using the coupled GFDL general circulation model of the atmosphere and ocean. The results of the model for a sustained increase of atmospheric carbon dioxide of 1% per year over a century indicate a marked warming of the upper ocean. Results of the model are used to study the rise in sea level caused by increase in ocean temperatures and associated changes in ocean circulation. Neglecting possible contributions due to changes in the volume of polar ice sheets and mountain glaciers, the model predicts an average rise in sea level of approximately 15 ± 5 cm by the time atmospheric carbon dioxide doubles. Heating anomalies are greatest in subpolar latitudes. This effect leads to a weakening of the ocean thermohaline circulation. Changes in thermohaline circulation redistribute heat within the ocean from high latitudes toward the equator, and cause a more uniform sea level rise than would occur otherwise.  相似文献   

15.
Model studies point to enhanced warming and to increased freshwater fluxes to high northern latitudes in response to global warming. In order to address possible feedbacks in the ice-ocean system in response to such changes, the combined effect of increased freshwater input to the Arctic Ocean and Arctic warming--the latter manifested as a gradual melting of the Arctic sea ice--is examined using a 3-D isopycnic coordinate ocean general circulation model. A suite of three idealized experiments is carried out: one control integration, one integration with a doubling of the modern Arctic river runoff, and a third more extreme case, where the river runoff is five times the modern value. In the two freshwater cases, the sea ice thickness is reduced by 1.5-2 m in the central Arctic Ocean over a 50-year period. The modelled ocean response is qualitatively the same for both perturbation experiments: freshwater propagates into the Atlantic Ocean and the Nordic Seas, leading to an initial weakening of the North Atlantic Drift.Furthermore, changes in the geostrophic currents in the central Arctic and melting of the Arctic sea ice lead to an intensified Beaufort Gyre, which in turn increases the southward volume transport through the Canadian Archipelago. To compensate for this southward transport of mass, more warm and saline Atlantic water is carried northward with the North Atlantic Drift. It is found that the increased transport of salt into the northern North Atlantic and the Nordic Seas tends to counteract the impact of the increased freshwater originating from the Arctic, leading to a stabilization of the North Atlantic Drift.  相似文献   

16.
Interadapted fields of main hydrophysical characteristics in the vicinity of hydrological sections carried out in 1997–1998 in the Barents Sea are obtained on the basis of model computations. The complex analysis of these materials and atmospheric situation in the region during 1997–1998 enabled to estimate quantitatively the variations of significant hydrodynamic conditions: the decrease in the inflow of rather warm and salty North Atlantic waters and the compensation inflow of polar waters, the decrease in total heat content and the weakening of water dynamics in the system of the general cyclonic circulation, and the increase in the ice coverage of the Barents Sea in anomalously cold winter 1997/98. It is revealed with a high degree of the confidence probability that the significant deviations from the normal conditions occur in response to the global El Niño disturbance which took place in the same years with the maximum values of the Southern Oscillation Index in January–March 1998.  相似文献   

17.
The effects of Atlantic water inflow on the climate variability in the Barents Sea are studied. Initial data are the series of water temperature at the Kola meridian cross-section, monthly values of ice extent, air temperature at the stations, sea level pressure from the reanalysis data, and sea surface temperature. The methods of multivariate correlation, spectral, and factor analysis and EOF decomposition are used. It was found that variations in the Atlantic water inflow define the main part of interannual variability of sea ice extent, water temperature, and air temperature in the Barents Sea in the cold season. The influence of regional atmospheric circulation on the interannual variability of these parameters is small. The effects that water temperature anomalies in the area of Newfoundland and in the equatorial part of the North Atlantic have on climate parameters in the Barents Sea are discovered. The response of these parameters lags behind the respective anomalies by 9-58 months. The high correlation between them makes it possible to develop the method of statistical forecasting of sea ice extent and water temperature in the Barents Sea with the lead time up to 4 years.  相似文献   

18.
Interdecadal climate variability in the subpolar North Atlantic   总被引：1，自引：0，他引：1  
The statistical relationships between various components of the subpolar North Atlantic air-sea-ice climate system are reexamined in order to investigate potential processes involved in interdecadal climate variability. It is found that sea surface temperature anomalies concentrated in the Labrador Sea region have a strong impact upon atmospheric sea level pressure anomalies over Greenland, which in turn influence the transport of freshwater and ice anomalies out of the Arctic Ocean, via Fram Strait. These freshwater and ice anomalies are advected around the subpolar gyre into the Labrador Sea affecting convection and the formation of Labrador Sea Water. This has an impact upon the transport of North Atlantic Current water into the subpolar gyre and thus, also upon sea surface temperatures in the region. An interdecadal negative feedback loop is therefore proposed as an internal source of climate variability within the subpolar North Atlantic. Through the lags associated with the correlations between different climatic components, observed horizontal advection time scales, and the use of Boolean delay equation models, the time scale for one cycle of this feedback loop is determined to have a period of about 21 years.  相似文献   

19.
 In this study we investigate the role of heat, freshwater and momentum fluxes in changing the oceanic climate and thermohaline circulation as a consequence of increasing atmospheric CO2 concentration. Two baseline integrations with a fully coupled ocean atmosphere general circulation model with either fixed or increasing atmospheric CO2 concentrations have been performed. In a set of sensitivity experiments either freshwater (precipitation, evaporation and runoff from the continents) and/or momentum fluxes were no longer simulated, but prescribed according to one of the fully coupled baseline experiments. This approach gives a direct estimate of the contribution from the individual flux components. The direct effect of surface warming and the associated feedbacks in ocean circulation are the dominant processes in weakening the Atlantic thermohaline circulation in our model. The relative contribution of momentum and freshwater fluxes to the total response turned out to be less than 25%, each. Changes in atmospheric water vapour transport lead to enhanced freshwater input into middle and high latitudes, which weakens the overturning. A stronger export of freshwater from the Atlantic drainage basin to the Indian and Pacific ocean, on the other hand, intensifies the Atlantic overturning circulation. In total the modified freshwater fluxes slightly weaken the Atlantic thermohaline circulation. The contribution of the modified momentum fluxes has a similar magnitude, but enhances the formation of North Atlantic deep water. Salinity anomalies in the Atlantic as a consequence of greenhouse warming stem in almost equal parts from changes in net freshwater fluxes and from changes in ocean circulation caused by the surface warming due to atmospheric heat fluxes. Important effects of the momentum fluxes are a poleward shift of the front between Northern Hemisphere subtropical and subpolar gyres and a southward shift in the position of the Antarctic circumpolar current, with a clear signal in sea level. Received: 3 May 1999 / Accepted: 11 December 1999  相似文献   

20.
基于一个全球气-海-冰耦合模式数值模拟结果,对北半球高纬度地区年际尺度的气-海-冰相互作用进行了分析。在所使用的全球气-海-冰耦合模式中,大气环流模式和陆面过程模式来自国家气候中心,海洋环流模式和海冰模式来自中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室。采用一种逐日通量距平耦合方案实现次网格尺度海冰非均匀条件下大气环流模式和海洋环流模式在高纬地区的耦合。只对50 a模拟结果中的后30 a结果进行了分析。在分析中,首先对滤波后的北半球高纬度地区海平面气压、表面大气温度、海表面温度、海冰密集度及海表面感热通量的标准化距平做联合复经验正交函数分解,取第一模进行重建,然后讨论了在一个循环周期(约4 a)中北半球高纬度地区气-海-冰的作用关系。结果表明:(1)当北大西洋涛动处于正位相时,格陵兰海出现南风异常,使表面大气温度升高,海洋失去感热通量减少,海洋表面温度升高,海冰密集度减小;当北大西洋涛动处于负位相时,格陵兰海出现北风异常,使表面大气温度降低,海洋失去感热通量增多,海洋表面温度降低,海冰密集度增加。巴伦支海变化特点与格陵兰海相似,但在时间上并不完全一致。(2)多年平均而言,北冰洋内部靠近极点区域为冷中心。当北冰洋内部为低压异常时,因异常中心偏向太平洋一侧,使北冰洋内部靠近太平洋部分为暖平流异常,靠近大西洋一侧为冷平流异常。伴随着暖、冷平流异常,这两侧分别出现暖异常和冷异常,海表面给大气的感热通量分别偏少和偏多,上述海区海表面温度分别偏高和偏低,海冰密集度分别偏小和偏大。当北冰洋内部为高压异常时特点正好与上述相反。由上述分析结果可知,在海洋、大气年际循环中,大尺度大气环流变率起主导作用,海洋表面温度和海冰密集度变化主要是对大气环流变化的响应。  相似文献   

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