北大西洋海表面温度锋与大西洋风暴路径及大气大尺度异常的关系研究

运用NCEP、Had ISST再分析资料,北大西洋涛动(NAO)月指数序列,探讨了海表面温度(SST)锋的时空变化特征,揭示了北大西洋SST锋的主要气候变率及其与北大西洋风暴轴和大气大尺度环流异常的关系。研究表明,剔除季节循环后的SST锋显示其最主要变率为锋区的向南/北摆动,其对应的风暴轴发生相应的西南/东北移动,并同时在北大西洋上空对应一个跨海盆的位势高度负/正异常。这种环流异常可引起高纬度海平面气压(SLP)的反气旋/气旋式环流,这有利于增强海表面风对大洋副极地环流的负/正涡度异常输入,进一步减弱/加强了高纬度上层冷水向SST锋区的输送。北大西洋SST锋的另一主要模态为锋区在南北方向的分支和合并。当SST锋异常在40°N~45°N以单支形式加强时,对流层位势高度场和SLP南北梯度增大,对应NAO正位相,此时风暴轴也为单支型;同时SLP异常场促使冰岛附近具有气旋式风应力异常,亚速尔地区具有反气旋式风应力异常,导致副极地环流和副热带环流均加强,增加高纬度冷水和低纬度暖水在锋区的输入,从而进一步增强40°N~45°N附近的SST锋区。当SST锋异常在40°N~45°N纬带南北发生分支时,风暴轴也同时出现北强南弱的南北分支,此时对应了负位相NAO,来自北南的冷暖水输送减弱,SST锋也发生减弱分支。此外,位于大洋内区的SST锋东端也存在一个偶极子型的模态,尽管其解释方差相对较小,但仍与偏东北的NAO型具有显著相关。谱分析表明,北大西洋SST锋与风暴轴具有1~3年和年代际共振,与中高纬大尺度环流也存在周期1~3年的共变信号,其中准一年共变信号体现了SST锋和NAO之间的对应关系。进一步诊断分析表明,SST锋上空的近表层大气斜压性和经向温度梯度随着SST锋的增强而增强,经向热通量的向北输送导致涡动有效位能的增加;海洋的非绝热加热产生更强的垂直热量通量,这有利于涡动有效位能释放成为涡动动能,从而表现为该区域的风暴轴加强,并进一步影响风暴轴中的天气尺度扰动与下游大尺度环流异常的相互作用过程。     

Multi-stage onset of the summer monsoon over the western North Pacific

 The climatological summer monsoon onset displays a distinct step wise northeastward movement over the South China Sea and the western North Pacific (WNP) (110°–160°E, 10°–20°N). Monsoon rain commences over the South China Sea-Philippines region in mid-May, extends abruptly to the southwestern Philippine Sea in early to mid-June, and finally penetrates to the northeastern part of the domain around mid-July. In association, three abrupt changes are identified in the atmospheric circulation. Specifically, the WNP subtropical high displays a sudden eastward retreat or quick northward displacement and the monsoon trough pushes abruptly eastward or northeastward at the onset of the three stages. The step wise movement of the onset results from the slow northeastward seasonal evolution of large-scale circulation and the phase-locked intraseasonal oscillation (ISO). The seasonal evolution establishes a large-scale background for the development of convection and the ISO triggers deep convection. The ISO over the WNP has a dominant period of about 20–30 days. This determines up the time interval between the consecutive stages of the monsoon onset. From the atmospheric perspective, the seasonal sea surface temperature (SST) change in the WNP plays a critical role in the northeastward advance of the onset. The seasonal northeastward march of the warmest SST tongue (SST exceeding 29.5 °C) favors the northeastward movement of the monsoon trough and the high convective instability region. The seasonal SST change, in turn, is affected by the monsoon through cloud-radiation and wind-evaporation feedbacks. Received: 19 October 1999 / Accepted: 5 June 2000     

Time-dependent response of a zonally averaged ocean–atmosphere–sea ice model to Milankovitch forcing

An ocean–atmosphere–sea ice model is developed to explore the time-dependent response of climate to Milankovitch forcing for the time interval 5–3 Myr BP. The ocean component is a zonally averaged model of the circulation in five basins (Arctic, Atlantic, Indian, Pacific, and Southern Oceans). The atmospheric component is a one-dimensional (latitudinal) energy balance model, and the sea-ice component is a thermodynamic model. Two numerical experiments are conducted. The first experiment does not include sea ice and the Arctic Ocean; the second experiment does. Results from the two experiments are used to investigate (1) the response of annual mean surface air and ocean temperatures to Milankovitch forcing, and (2) the role of sea ice in this response. In both experiments, the response of air temperature is dominated by obliquity cycles at most latitudes. On the other hand, the response of ocean temperature varies with latitude and depth. Deep water formed between 45°N and 65°N in the Atlantic Ocean mainly responds to precession. In contrast, deep water formed south of 60°S responds to obliquity when sea ice is not included. Sea ice acts as a time-integrator of summer insolation changes such that annual mean sea-ice conditions mainly respond to obliquity. Thus, in the presence of sea ice, air temperature changes over the sea ice are amplified, and temperature changes in deep water of southern origin are suppressed since water below sea ice is kept near the freezing point.     

欧亚北部2004年以来频繁冷冬的特征分析及机理初探

利用1961—2013年NCEP/NCAR发布的月平均全球再分析资料,分析了欧亚北部(40°65°N,50°-120°E)2004年以来频繁冷冬的异常特征及形成机理。结果表明:欧亚北部2004年以来冷冬频繁发生,但温度异常的空间分布,尤其中心冷区的位置有显著差异,主要表现为全区偏冷型(2005、2009、2010、2012年)和南部偏冷型(2004、2007、2011年)。全区偏冷年主要由北极涛动(AO)显著负位相所致,对应海表温度特征为北大西洋高、中、低纬度成东北-西南走向的"+、-、+"带状分布,该分布有利于北极涛动/北大西洋涛动(AO/NAO)负位相维持和增强;南部偏冷年大气内部活动异常为乌拉尔-贝加尔湖阻塞高压偏强,北极涛动/北大西洋涛动以弱正位相为主,对应主要海表温度特征为北大西洋中部偏高,其次则为太平洋年代际振荡(PDO)负位相下"类拉尼娜事件",上述海表温度异常均可促进类似欧亚遥相关的罗斯贝波列形成,有利于乌拉尔贝加尔湖阻塞高压偏强、亚洲中部多低槽活动。2004年以来欧亚北部两种类型冷冬的大气环流与海表温度均表现出与历史典型年相类似的特征。     

全球变暖背景下亚非典型干旱区降水变化及其与水汽输送的关系研究

分析比较了中蒙(35°N~50°N,75°E~105°E)、中亚(28°N~50°N,50°E~67°E)和北非(15°N~32°N,17°W~32°E)三个典型干旱区水汽输送特征的异同,及其1961~2010年间的降水时空变化,分析了水汽来源和输送变化及其可能原因。结果显示,由于受不同的气候系统影响,中蒙、北非和中亚干旱区的降水在年内变化上有着显著不同。中蒙和北非干旱区降水呈现夏季风降水的特征;而中亚干旱区降水则为更多受到冬季风的影响。1961~2010年,随着全球气温上升,中蒙干旱区冬季纬向水汽输送增加而经向输送减少,总水汽输送增加;中亚干旱区冬季纬向输送减少而经向增加,总水汽输送减少;北非干旱区冬季纬向输送增加而经向输送减少,总水汽输送增加。夏季中蒙和北非干旱区经向、纬向输送均减小,中亚干旱区夏季纬向输送减少而经向减少,总输送增加。相应的,中蒙干旱区年、冬季和夏季降水分别以4.2、1.3和1.0 mm/10 a的趋势增加;而中亚干旱区冬季(1.2 mm/10 a)和夏季(0.1 mm/10 a)降水增加,年降水则呈减少趋势(-0.8 mm/10 a);北非干旱区年降水和夏季降水分别以0.5 mm/10 a和0.1 mm/10 a的速率增加。冬季中蒙干旱区主要水汽来源是水汽经向输送,而中亚干旱区水汽主要为纬向输送,经纬向水汽均为净输出是北非干旱区降水极少的主要原因,平均总水汽输送量约为-9.48×104 kg/s。冬季低纬度和高纬度环流通过定常波影响干旱区冬季降水。中蒙和中亚干旱区冬季降水主要受西太平洋到印度洋由南向北的波列影响,北非干旱区冬季降水主要和北大西洋上空由北到南的波列相联系。各干旱区的降水对海温变化有着不同的响应:中蒙干旱区冬季降水与冬季太平洋西海岸和印度洋海温呈显著正相关,夏季与海温相关不显著;中亚干旱区与地中海和阿拉伯海温相关,且与阿拉伯海温为正相关。     

耦合模式中潮汐对北大西洋模拟的影响研究

将8个主要平衡分潮加入到耦合模式中,对比研究潮汐对北大西洋模拟影响。由于潮汐的引入,模式模拟SST在北大西洋中纬度区域偏差显著减小,高纬度区域SST降温明显。SST模拟的改变使潮汐试验的海表净热通量模拟误差下降了约30%,但高纬度海冰显著增加。模式中引入潮汐对北大西洋上层环流,尤其是西边界流的路径模拟改进显著,这是SST及海表净热通量模拟改变的主要原因。同时,北大西洋上层和深层西边界流在潮汐的作用下,都表现出环流减弱的特点,这也使得大西洋经向翻转环流在26.5°N处上层2 km的输送减弱,与观测数据更为接近。较弱的大西洋经向翻转环流导致海洋热量在中低纬度聚集而无法输送到高纬度区域,这是造成潮汐试验模拟的海温在中低纬度偏高、高纬度偏低的原因,较弱的热输送也同时导致了潮汐试验中北半球海冰面积增加。     

An ocean–atmosphere interaction mechanism for the active break cycle of the Asian summer monsoon

A typical active–break cycle of the Asian summer monsoon is taken as beginning with maximum SST (pentad 0) over the north Bay of Bengal when the oceans to its west and east from longitude 40°–160°E, and between latitudes 10° and 25°N (area A) also has maximum SST. During this pentad the recently found “Cold Pool” of the Bay of Bengal (between latitudes 3°N and 10°N) has its minimum SST. An area of convection takes genesis over the Bay of Bengal immediately after pentad 0 in the zone of large SST gradient north of the Cold Pool and it pulls the monsoon Low Level Jetstream (LLJ) through peninsular India. Convection and the LLJ westerlies then spread to the western Pacific Ocean during pentads 1–4 taken as the active phase of the monsoon during which convection and LLJ have grown in a positive feed back process. The cyclonic vorticity to the north of the LLJ axis is hypothesized to act as a flywheel maintaining the convection during the long active phase against the dissipating effect of atmospheric stabilization by each short spell of deep convection. By the end of pentad 4 the SST over area A has cooled and the convection weakens there, when the LLJ turns clockwise over the Arabian Sea and flows close to the equator in the Indian ocean. A band of convection develops at pentad 5 between the equator and latitude 10°S over the Indian ocean and it is nourished by the cyclonic vorticity of the LLJ now near the equator and the moisture supply through it. This is taken as the break monsoon phase lasting for about three to four pentads beginning from pentad 5 of a composite active–break cycle of 40 day duration. With reduced wind and convection over the area A during the break phase, solar radiation and light winds make the SST there warm rapidly and a new active–break cycle begins. SST, convection, LLJ and the net heat flux at the ocean surface have important roles in this new way of looking at the active–break cycle as a coupled ocean–atmosphere phenomenon.     

冬季北太平洋海温主模态在1990年前后调整及其成因初探?

太平洋年代际振荡(PDO)和北太平洋涡旋振荡(NPGO)是北太平洋(20°~60°N,120°E~120°W)海温(SST)的EOF前两个模态,本文通过比较1990年前后北太平洋冬季SST EOF前两个模态,揭示了PDO和NPGO在1990年前后特征,并从关键区海温变化、北太平洋涛动(NPO)、赤道太平洋中部变暖(CPW)和北极涛动(AO)的影响,揭示了北太平洋主模态在1990年之后调整的成因。我们发现,1990年之前,北太平洋SST场的EOF前两个模态与PDO和NPGO的空间结构类似,但是在1990年之后,SST的EOF第一模态的最大荷载中心向日界线移动,40°N以北的太平洋被正的SST异常控制,表现出与NPGO模态的负位相相似的空间分布特征,而EOF第二模态由偶极子演变成了三极子结构。北太平洋中部(28°~36°N,152°~178°W)和北太平洋北部(44°~49°N,151°~177°W)海温距平在1990年之后呈显著的负相关变化,是导致在1990年之后冬季NPGO成为主模态的内部原因,而NPO在1990年之后的显著增强则是重要的外部原因。分析显示,NPO在1980年开始表现出增强趋势,通过风生流机制,NPO可以增强北太平洋45°N附近的气压梯度和西风异常幅度,从而导致了1990年之后NPGO海温模态的加强。虽然CPW和AO对NPO的南支(夏威夷)和北支(阿拉斯加)的海平面气压异常中心加强有贡献,但是上述两个因子与NPGO之间的关系在1990年之前并不明显。因此,CPW和AO与NPGO之间并不存在稳定的物理联系。     

Interannual variability of the upper layer of the tropical Atlantic Ocean from in situ data between 1979 and 1999

Interannual variability of the upper layers of the tropical Atlantic is described based on in situ data. An objective analysis used all available temperature observations of the upper tropical Atlantic between 1979 and 1999 to construct a 4D database. Wind data are used to investigate potential mechanisms which might explain the observed variability. Four remarkable events are described: 1983–1984, 1988–1990, 1994–1995 and 1997–1998. Three of them are characterised as equatorial (1983–1984, 1994–1995, and 1997–1998). The 1988–1990 event is a basin-wide phenomenon which does not involve the same mechanisms as the other three. Results of statistical decomposition in empirical orthogonal functions (EOFs) are discussed. There is no evidence of an inter-hemispheric mode on the depth of the 20?°C-isotherm (D20) and heat content comparable to the observed mode for sea surface temperature (SST) fields. Most energetic patterns for D20 and heat content are dominated by the stronger variability in the northern part of the basin. Influences of other climate signals are investigated. Correlations between the winter NAO (North Atlantic Oscillation) index and our standard variables is marginally significant. A positive NAOW (North Atlantic Oscillation of Winter) is associated with SST cooling in a latitude band between 10°N and 20°N. When applied to the El-Niño index, correlations are much more significant. We found two scales of maximum correlation: at the four month lag after the El-Niño mature phase when the thermocline slope and zonal heat content gradient are maximum along the equator, and at the ten month lag after the mature phase of El-Niño when the thermocline slope weakens and the equatorial gradient of heat content vanished. The correlation with a zonal wind index (average between 30°W–35°W and 2°N–2°S) has been computed. Correlation is maximum at the six month lag when the thermocline slope and the zonal heat content gradient are maximum in the equatorial band. This “Atlantic Niño” mode is influenced by the Pacific Ocean's variability and reaches a maximum one year after a warm event in the eastern Pacific.     

A comparison of PMIP2 model simulations and the MARGO proxy reconstruction for tropical sea surface temperatures at last glacial maximum

Results from multiple model simulations are used to understand the tropical sea surface temperature (SST) response to the reduced greenhouse gas concentrations and large continental ice sheets of the last glacial maximum (LGM). We present LGM simulations from the Paleoclimate Modelling Intercomparison Project, Phase 2 (PMIP2) and compare these simulations to proxy data collated and harmonized within the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface Project (MARGO). Five atmosphere–ocean coupled climate models (AOGCMs) and one coupled model of intermediate complexity have PMIP2 ocean results available for LGM. The models give a range of tropical (defined for this paper as 15°S–15°N) SST cooling of 1.0–2.4°C, comparable to the MARGO estimate of annual cooling of 1.7 ± 1°C. The models simulate greater SST cooling in the tropical Atlantic than tropical Pacific, but interbasin and intrabasin variations of cooling are much smaller than those found in the MARGO reconstruction. The simulated tropical coolings are relatively insensitive to season, a feature also present in the MARGO transferred-based estimates calculated from planktonic foraminiferal assemblages for the Indian and Pacific Oceans. These assemblages indicate seasonality in cooling in the Atlantic basin, with greater cooling in northern summer than northern winter, not captured by the model simulations. Biases in the simulations of the tropical upwelling and thermocline found in the preindustrial control simulations remain for the LGM simulations and are partly responsible for the more homogeneous spatial and temporal LGM tropical cooling simulated by the models. The PMIP2 LGM simulations give estimates for the climate sensitivity parameter of 0.67°–0.83°C per Wm⁻², which translates to equilibrium climate sensitivity for doubling of atmospheric CO₂ of 2.6–3.1°C.     

Biases in AMIP model simulations of the east China monsoon system

 AMIP model simulations of the east China (5–50°N; 105–122°E) monsoon system are analyzed to study coherent relationships between rainfall and wind annual cycle biases. A comparison with observed interannual variability patterns is carried out to identify the physical processes that explain the biases. The analyses show that poleward displacement of the simulated east Asian jet stream causes the ascending branch of the jet-induced transverse circulation to move north and, as a consequence, produces negative (positive) rainfall biases occur in central (northeast) China. The model simulations show decreased southwesterly flow and ITCZ rainfall over the South China Sea when weaker (versus observations) summer Hadley and Walker circulations are present. This results from diminished model tropical disturbance activity, and highlights the importance of air-sea interactions. In addition, during October–January, intensified model low-level easterlies enhance moisture transport and produce positive local rainfall biases over central and northeast China. Biases in the east China monsoon system are concurrently reflected in the planetary circulation. Enhanced northeast China rainfall results from increased surface pressure over the North Pacific and an amplified zonal pressure gradient along the east China coast. This bias pattern is associated with differences in model representations of topography. On the other hand, the South China Sea experiences an extensive elongated meridional rainfall bias dipole structure that straddles the equator. This is accompanied by a baroclinic vertical pattern over the tropics as well as a barotropic wave train that extends from Australia to the Antarctic, where the teleconnection is likely a direct atmospheric response to tropical convective heating. Received: 20 June 2000 / Accepted: 17 September 2000     

Observed and simulated multidecadal variability in the Northern Hemisphere

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

Evolution of the equatorial and dipole modes of the sea-surface temperature in the Tropical Atlantic at decadal scale

Summary ?One hundred and thirty six years (1856–1991) of monthly sea-surface temperature (SST) data in the Tropical Atlantic Ocean are used to investigate the propagating signals of the SST at a decadal (DD) time scale. The first and the third evolving modes show a relationship between the equatorial and the inter-hemispheric patterns, one evolving into the other mode and vice-versa. These modes describe two different evolutions of the SST at DD time-scale. The first EEOF features a 12-year period oscillatory regime with a strong 2-year duration inter-hemispheric pattern evolving into a weak 1-year duration equatorial pattern and vice-versa. This mode exhibits also a northward displacement of the anomalies in the band 15° S–15° N. The third EEOF also shows an oscillatory regime, but with a period of 10 years and with a relatively strong 2-year duration equatorial pattern evolving into a weak 1-year duration inter-hemispheric pattern and vice-versa. For this mode, the SST anomalies show a southward displacement in the band 15° S–15° N. These results have not yet been documented in previous works and explain some of the previous findings on the DD variability in the Tropical Atlantic. Received December 31, 2001; revised April 9, 2002; accepted September 4, 2002 Published online: March 20, 2003     

Holocene hydrological cycle changes in the Southern Hemisphere documented in East Antarctic deuterium excess records

Four Holocene-long East Antarctic deuterium excess records are used to study past changes of the hydrological cycle in the Southern Hemisphere. We combine simple and complex isotopic models to quantify the relationships between Antarctic deuterium excess fluctuations and the sea surface temperature (SST) integrated over the moisture source areas for Antarctic snow. The common deuterium excess increasing trend during the first half of the Holocene is therefore interpreted in terms of a warming of the average ocean moisture source regions over this time. Available Southern Hemisphere SST records exhibit opposite trends at low latitudes (warming) and at high latitudes (cooling) during the Holocene. The agreement between the Antarctic deuterium excess and low-latitude SST trends supports the idea that the tropics dominate in providing moisture for Antarctic precipitation. The opposite trends in SSTs at low and high latitudes can potentially be explained by the decreasing obliquity during the Holocene inducing opposite trends in the local mean annual insolation between low and high latitudes. It also implies an increased latitudinal insolation gradient that in turn can maintain a stronger atmospheric circulation transporting more tropical moisture to Antarctica. This mechanism is supported by results from a mid-Holocene climate simulation performed using a coupled ocean-atmosphere model. Received: 7 July 1999 / Accepted: 21 July 2000     

Climate variation and prediction of rapid intensification in tropical cyclones in the western North Pacific

Summary One of the greatest challenges in tropical weather forecasting is the rapid intensification (RI) of the tropical cyclone (TC), during which its one-minute maximum sustained wind speed increases at least 30 knots per 24 hours. Here we identify and elucidate the climatic conditions that are critical to the frequency and location of the RI on annual, intraseasonal, and interannual time scales. Whereas RI and formation share common environmental preferences, we found that the percentage of TCs with RI varies annually and from year to year. In August, only 30% of TC actually experiences RI, in contrast to the annual maximum of 47% in November. The proportion of RI in July–September is higher during El Ni?o years (53%) than the corresponding one in the La Ni?a years (37%). Three climate factors may contribute to the increase in the proportion of RI: the southward shift in the monthly or seasonal mean location of the TC formation, the increase in the low-level westerly meridional shear vorticity, and the decrease in northerly vertical shear. When the mean latitude of TC formation increases, the mixed-layer heat content decreases while TC’s inertial stability increases; both are more detrimental to the RI than to TC formation because the RI requires large amount of latent heat energy being extracted efficiently from the ocean mixed layer and requires accelerated low-level radial inflow that carries latent heat reaching the inner core region. We further demonstrate that the RI frequency in the Philippine Sea and South China Sea can be predicted 10 to 30 days in advance based on the convective anomalies in the equatorial western Pacific (5° S–5° N, 130°–150° E) on intraseasonal time scale. The Ni?o 3.4 SSTA in June is a potential predictor for the peak TC season (July–September) RI activity in the southeast quadrant of the western North Pacific (0–20° N, 140–180° E). The RI is an essential characteristic of category 4 and 5 hurricanes and super typhoons because all category 4 and 5 hurricanes in the Atlantic basin and 90% of the super typhoons in the western North Pacific experience at least one RI process in their life cycles. Over the past 40 years, the annual total of RI in the western North Pacific shows pronounced interdecadal variation but no significant trend. This result suggests that the number of supper typhoons has no upward trend in the past 40 years. Our results also suggest that when the mean latitude, where the tropical storms form, shifts southward (either seasonally or from year to year) the proportion of super typhoon or major hurricane will likely increase. This shift is determined by large scale circulation change rather than local SST effects. This idea differs from the current notion that increasing SST can lead to more frequent occurrence of category 4 or 5 hurricanes through local thermodynamics. Corresponding author’s address: Bin Wang, Department of Meteorology, University of Hawaii, 2525 Correa Rd., Honolulu, Hawaii 96822, USA (also visiting professor at the Ocean University of China)     

GFDL模式不同类型试验模拟海洋中CFC-11分布结果的评估

为了评估美国地球物理流体动力学实验室（GFDL）模式模拟海洋通风的能力,利用GFDL的物理气候系统模式和地球系统模式（GFDL-ESM2G、GFDL-ESM2M、GFDL-CM3）模拟海洋中CFC-11（一氟三氯甲烷,CCl3F）的资料,对CFC-11的海面浓度分布、单位面积水柱总量、全球总物质的量、最大穿透深度以及在大西洋、太平洋、南大洋的垂直剖面的特征进行了分析。本文将GFDL模拟结果与盐度、海温、CFC-11的观测资料比较,得到了如下重要结论：GFDL模式模拟的CFC-11海面高值中心集中在高纬度,如北大西洋、西北太平洋,但是在南大洋罗斯海、威德尔海模拟结果比观测值低了1.5 pmol·kg-1,这是CFC-11的溶解度与海面温度成负相关造成的,即随海面温度升高,CFC-11的溶解度降低;GFDL模拟的全球海洋中CFC-11总物质的量都比观测值高,尤其是CM3的模拟结果比观测高22.9%,GFDL模式平均值高于观测15.6%。通过对北太平洋46°N、北大西洋24°N和南大洋65°S的纬向断面的分析表明,目前GFDL模式在模拟一些重要水团时还有一定的改进空间,比如GFDL在24°N断面1 000 m以下模拟CFC-11浓度极大值位置过深。     

Signature of the Antarctic oscillation in the northern hemisphere

Using the ECWMF daily reanalysis data, this paper investigates signatures of the Antarctic Oscillation (AAO) in the upper troposphere of the northern hemisphere. It is found that during boreal winter, a positive (negative) phase of the AAO is associated with anomalous easterlies (westerlies) in middle-low latitudes (~30–40°N) and anomalous westerlies (easterlies) in middle-high latitudes (~45–65°N) of the upper troposphere about 25–40 days later. While there is also a response in zonal wind in the tropics, namely over the central-eastern Pacific, to some extent, these tropical zonal wind anomalies can trigger a Pacific/North American teleconnection patterns (PNA)-like quasi-stationary Rossby waves that propagate into the Northern Hemisphere and gradually evolve into patterns which resemble North Atlantic teleconnection patterns. Furthermore, these quasi-stationary Rossby waves might give rise to anomalous eddy momentum flux convergence and divergence to accelerate anomalous zonal winds in the Northern Hemisphere.     

亚太地区冬季海平面气压异常的偶极型结构及其与海温的关系

利用全球月平均海平面气压资料以及海表温度资料,采用旋转经验正交函数分解(REOF)、Morlet小波分析、相关分析及合成分析等方法研究了亚洲—太平洋地区(20°N～70°N,40°E～120°W)冬季海平面气压异常的空间结构与时间演变特征,并进—步分析了该地区冬季海平面气压异常与全球海温异常的关系.结果表明:亚太地区冬...     

Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses

 NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude 17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from about 3000 km/8 m s^-1 north of the jet to 5000 km/12 m s^-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics. The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength is higher, about 5000 km long, and the average phase speed is about 7 m s^-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component. The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR reanalyses, especially for the 3–5-day regime over the Atlantic. Received: 28 May 1998 / Accepted: 2 May 1999     

A predictability study of simulated North Atlantic multidecadal variability

 The North Atlantic is one of the few places on the globe where the atmosphere is linked to the deep ocean through air–sea interaction. While the internal variability of the atmosphere by itself is only predictable over a period of one to two weeks, climate variations are potentially predictable for much longer periods of months or even years because of coupling with the ocean. This work presents details from the first study to quantify the predictability for simulated multidecadal climate variability over the North Atlantic. The model used for this purpose is the GFDL coupled ocean-atmosphere climate model used extensively for studies of global warming and natural climate variability. This model contains fluctuations of the North Atlantic and high-latitude oceanic circulation with variability concentrated in the 40–60 year range. Oceanic predictability is quantified through analysis of the time-dependent behavior of large-scale empirical orthogonal function (EOF) patterns for the meridional stream function, dynamic topography, 170 m temperature, surface temperature and surface salinity. The results indicate that predictability in the North Atlantic depends on three main physical mechanisms. The first involves the oceanic deep convection in the subpolar region which acts to integrate atmospheric fluctuations, thus providing for a red noise oceanic response as elaborated by Hasselmann. The second involves the large-scale dynamics of the thermohaline circulation, which can cause the oceanic variations to have an oscillatory character on the multidecadal time scale. The third involves nonlocal effects on the North Atlantic arising from periodic anomalous fresh water transport advecting southward from the polar regions in the East Greenland Current. When the multidecadal oscillatory variations of the thermohaline circulation are active, the first and second EOF patterns for the North Atlantic dynamic topography have predictability time scales on the order of 10–20 y, whereas EOF-1 of SST has predictability time scales of 5–7 y. When the thermohaline variability has weak multidecadal power, the Hasselmann mechanism is dominant and the predictability is reduced by at least a factor of two. When the third mechanism is in an extreme phase, the North Atlantic dynamic topography patterns realize a 10–20 year predictability time scale. Additional analysis of SST in the Greenland Sea, in a region associated with the southward propagating fresh water anomalies, indicates the potential for decadal scale predictability for this high latitude region as well. The model calculations also allow insight into regional variations of predictability, which might be useful information for the design of a monitoring system for the North Atlantic. Predictability appears to break down most rapidly in regions of active convection in the high-latitude regions of the North Atlantic. Received: 28 October 1996 / Accepted: 21 March 1997