GCM response of the mean zonal surface heat and water budgets to a global sea surface temperature anomaly

Two sets of sensitivity experiments are presented. The first set consists of three 100-day integrations with perpetual January conditions: a reference integration, an integration with a uniform +2 K sea surface temperature (SST) anomaly, and an integration with an anomaly of reverse sign. The second set is similar, but with perpetual July conditions. The zonal mean components of the water and heat budgets at the surface are studied over ocean and over land separately.The values of the reference integration are very close to those obtained when the model is run with annual cycle conditions, and reasonably close to observed values over 60° N−40° S ocean. The SST anomalies produce generally a stronger response in July than in January. This response is linear for the averages over ocean, but if we consider the zonal distribution, only the longwave radiation, latent and sensible heat exhibit a linear response. The model response to temperature increase consists of an enhancement of the water cycle over ocean, and a heat transfer from the ocean, through the latent heat, to the continent. In January, we observe also a water transfer from the ocean to the continent. As a consequence of the heat transfer, the land surface temperature increases by the same magnitude as the SST.     

海温异常对台风形成的影响

本文利用地球流体力学实验室(GFDL)的低分辨气候模式进行数值试验,以研究海温异常对台风形成的影响.试验采用恒定8月气候条件和海表温度(SST).海温异常(SSTA)被置于北太平洋不同区域.结果表明,台风生成频率在暖SSTA区明显增加.这是由于暖SSTA区低层辐合的增强一方面使低空气旋式环流和高空反气旋式环流加大,另一方面导致低层水汽向该区辐合,使潜热释放加强,对流加剧所致.此一机制被用于解释台风频率和ENSO事件的相关.在冷ENSO年份,西北和西南太平洋台风增多不仅是由于赤道东太平洋SST异常冷,还与西太平洋SST异常暖有关.     

Polar amplification as a preferred response in an idealized aquaplanet GCM

An aquaplanet atmospheric general circulation model (GCM) coupled to a mixed layer ocean is analyzed in terms of its polar amplified surface temperature response to a 2×CO₂-like steady forcing and in terms of the phase space trajectory of the relaxation of a free perturbation to equilibrium. In earlier studies concerned with linear stability analysis of the same system we have shown that the least stable mode of the linearized surface budget operator has a polar amplified shape. We demonstrate that this shape of the least stable mode is responsible for the polar amplified shape of the response to a uniform forcing and for the manner in which the system relaxes back to equilibrium. Based on GCM and simple energy balance model results it is argued that the decay time-scale of this mode is determined by the sensitivity of the net top-of-atmosphere radiation to surface temperature while its shape (and thus the degree of polar amplification in a climate change experiment) is determined by the sensitivity of poleward heat transports to low- and high-latitude temperatures by the faster time-scale atmospheric dynamics. This implies that the underlying mechanisms for the polar amplification may be obscured when studying feedbacks during the slow evolution of climate change or considering only the new equilibrium state after introduction of a steady forcing.     

海表面盛行风背景下大气对黑潮海洋锋的响应特征

采用一系列高分辨率的卫星资料,应用高通滤波等方法,研究了春季不同海表面盛行风背景下,东海黑潮海洋锋区附近的海气关系。观测分析表明:在东海,春季3种不同海表面盛行风条件下海表面温度与海表面风速之间都存在明显的正相关关系,表现为海洋对大气的强迫作用。大气对海洋锋的响应在3种不同盛行风条件下也存在明显的差异。在西北盛行风和东南盛行风背景下,即当风向垂直于海洋锋由冷侧(暖侧)吹向暖侧(冷侧)时,海表面风的辐散(辐合)出现在海洋锋上空。同时,海洋锋对海平面气压(SLP)、降水和对流活动的影响较弱,表明大气对海洋锋的响应主要局限在大气边界层内。在东北盛行风背景下,即当风平行于海洋锋时,在海洋锋的暖(冷)水侧上空为海表面风的辐合(辐散),并与SLP的异常低(高)值相对应,主要雨带出现在黑潮暖舌上空。无论从总降水还是层云、对流降水频次的空间分布来看,盛行东北风时,海表面温度对其上雨带的影响最为明显。分析结果还表明,在不同盛行风背景下,海洋锋附近的海气关系由不同的物理机制在起主导作用。当盛行平行于海洋锋的东北风时,主要由SLP调整机制起作用;而盛行垂直于海洋锋的西北风时则主要由垂直混合机制起作用。     

The seasonal cycle of tropical Pacific sea surface temperature in a coupled GCM

 The mechanisms responsible for the seasonal cycle in the tropical central and eastern Pacific sea surface temperature (SST) are investigated using a coupled general circulation model. We find that the annual westward propagation of SST anomalies along the equator is explained by a two-stage process. The first stage sets the phase of the variation at the eastern boundary. The strengthening of the local Hadley Circulation in boreal summer leads to a strengthening of the northward winds that blow across the equator. These stronger winds drive enhanced evaporation and entrainment cooling of the oceanic mixed layer. The resulting change in SST is greatest in the east because the mixed layer is at its shallowest there. As the east Pacific SST cools the zonal SST gradient in the central Pacific becomes more negative. This development signals the onset of the second stage in the seasonal variation of equatorial SST. In response to the anomalous SST gradient the local westward wind stress increases. This increase drives cooling of the oceanic mixed layer in which no single mechanism dominates: enhanced evaporation, wind-driven entrainment, and westward advection all contribute. We discuss the role that equatorial upwelling plays in modulating mixed layer depth and hence the entrainment cooling, and we highlight the importance of seasonal variations in mixed layer depth. In sum these processes act to propagate the SST anomaly westward. Received: 22 February 1999 / Accepted: 20 March 2000     

The relationship between sea surface temperature anomalies and atmospheric circulation in GCM experiments

Several 19-year integrations of the Hamburg version of the ECMWF/T21 general circulation model driven by the monthly mean sea surface temperature (SST) observed in 1970–1988 were examined to study extratropical response of the atmospheric circulation to SST anomalies in the Northern Hemisphere in winter. In the first 19-years run SST anomalies were prescribed globally (GAGO run), and in two others SST monthly variability was limited to extratropical regions (MOGA run) and to tropics (TOGA run), respectively. A canonical correlation analysis (CCA), which select from two time-dependent fields optimally correlated pairs of patterns, was applied to monthly anomalies of SST in the North Alantic and Pacific Oceans and monthly anomalies of sea level pressure and 500 hPa geopotential height in the Northern Hemisphere. In the GAGO run the best correlated atmospheric pattern is global and is characterized by north-south dipole structures of the same polarity in the North Atlantic and the North Pacific sectors. In the MOGA and TOGA experiments the atmospehric response is more local than in the GAGO run with main centers in the North Atlantic and North Pacific, respectively. The extratropical response in the GAGO run is not equal to the sum of the responses in the MOGA and TOGA runs. The artificial meridional SST gradients at 25°–30°N probably influence the results of the MOGA and TOGA runs. The atmopsheric modes found by the CCA were compared with the normal modes of the barotropic vorticity equation linearized about the 500 hPa. winter climate. The normal modes with smallest eigenvalues are similar to the model leading variability modes and canonical patterns of 500 hPa geopotential height. The corresponding eigenvectors of the adjoint operator, which represent an external forcing optimal for exciting normal modes, have a longitudinal structure with maxima in regions characterized by enhanced high frequency baroclinic activity over both oceans.     

南印度洋热带气旋生成频数气候变率对海温异常模响应的研究

基于1980—2014年的哈德莱中心海冰及海温的月平均SST资料,美国联合台风警报中心(JTWC)的best-track资料以及NCEP/NCAR再分析月平均资料,利用广义平衡反馈方法(GEFA)研究南印度洋热带气旋(TC)生成频数对海表温度异常的响应特征。研究表明:(1)南印度TC生成频数对北太平洋第一模态(NP1)和热带大西洋第二模态(TA2)有显著响应,分别通过了置信度为99%和96%的Monte-Carlo检验,对应的响应振幅分别为0.67和0.49。(2)局地环境要素对关键SSTA模的GEFA响应结果显示:当NP1出现类似于太平洋年代际振荡(PDO)的正位相时,850 h Pa相对涡度在15°S附近的印度洋海域上都有一个自西向东的显著正响应带,垂直风切变在马达加斯加以东的大部分海域都表现为显著的负响应,600 h Pa相对湿度在马达加斯加以东的部分海域表现为显著的正响应;当TA2对应的时间系数为正异常时,850 h Pa相对涡度和600 h Pa相对湿度在澳大利亚的西北部印度洋海域表现为显著的正响应,垂直风切变在澳大利亚的西北部印度洋海域表现为显著的负响应。     

中国东部气温异常型与海表温度异常模关系的诊断

基于国家气候中心整编的160站常规观测气温资料和HADLEY中心的海表温度资料,应用最大协方差分析方法,诊断了中国东部各季节气温异常型和前期海表温度异常(SSTA)模的关系,并重点分析冬季气温与SSTA模的最佳耦合模态及海温异常对大气环流的影响。结果表明:中国东部四季气温异常型与前期海盆SSTA模的显著耦合关系表现出不同的特征。超前6个月的热带太平洋第二模和南印度洋第二模与东部地区冬季气温一致变化型耦合关系最佳。西南冷东北暖的气温异常型与超前4个月热带大西洋一致增暖模有最佳耦合关系。大气环流对与全区气温一致偏冷型对应的SSTA模的回归表现为:西伯利亚高压和阿留申低压增强,东亚大槽加深,中纬度西风加强。对与气温西南冷东北暖型对应的SSTA模的回归表现为:西伯利亚高压和阿留申低压略有增强,东亚大槽槽区附近位势升高,大槽变浅,槽线偏向西南,东部40°N以北风速加强,以南风速减弱。     

Distinct patterns of sea surface temperature anomaly in the South Indian Ocean during austral autumn

Climate Dynamics - A neural network-based cluster technique called self-organizing map (SOM) determines 12 distinguishable patterns derived from the sea surface temperature (SST) anomaly fields in...     

海表温度异常对南亚高压年代际变化影响的数值模拟

应用NCAR CAM3全球大气环流模式以及NCEP/NCAR再分析资料,研究了不同海域(全球、热带外、热带、热带印度洋—太平洋、热带印度洋及热带太平洋)的海表温度异常对夏季南压高压年代际变化的影响。结果表明,全球、热带、热带印度洋—太平洋和热带太平洋这些海域的海表温度异常都对南亚高压强度、面积、南界、西伸脊点和东伸脊点的1970s中后期年代际变化有重要影响:热带太平洋是关键海区,其海表温度第三模态(“三明治”式异常分布型)的变化与南亚高压的这些特征指数的年代际变化关系密切;热带印度洋的海表温度异常,主要是其第一模态(热带印度洋全区一致变化型)的变化与南亚高压强度、面积、南界和西伸脊点的年代际变化关系较密切,热带印度洋也是影响南亚高压年代际变化的关键海区;这两个关键海区的海表温度异常对南亚高压年代际变化影响的主要差异在于:热带太平洋海表温度异常能对南亚高压的东伸脊点的年代际变化有重要影响,而热带印度洋的海表温度异常对其影响小;热带太平洋和热带印度洋这两个海区的海表温度异常均可通过影响热带对流层大气温度的变化进而使南亚高压发生变化;热带外的海表温度异常对南亚高压的年代际变化影响小。     

Surface flux response to interannual tropical Pacific sea surface temperature variability in AMIP models

 A systematic comparison of observed and modeled atmospheric surface heat and momentum fluxes related to sea surface temperature (SST) variability on interannual time scales in the tropical Pacific is conducted. This is done to examine the ability of atmospheric general circulation models (AGCMs) in the Atmospheric Model Intercomparison Project (AMIP) to simulate the surface fluxes important for driving the ocean on interannual time scales. In order to estimate the model and observed response to such SST variability, various regression calculations are made between a time series representing observed ENSO SST variability in the tropical Pacific and the resulting surface flux anomalies. The models exhibit a range of differences from the observations. Overall the zonal wind stress anomalies are most accurately simulated while the solar radiation anomalies are the least accurately depicted. The deficiencies in the solar radiation are closely related to errors in cloudiness. The total heat flux shows some cancellation of the errors in its components particularly in the central Pacific. The performance of the GCMs in simulating the surface flux anomalies seems to be resolution dependent and low-resolution models tend to exhibit weaker flux responses. The simulated responses in the western Pacific are more variable than those of the central and eastern Pacific but in the west the observed estimates are less robust as well. Further improvements in atmospheric GCM flux simulation through better physical parametrization is clearly required if such models are to be used to their full potential in coupled modeling and climate forecasting. Received: 24 August 1999 / Accepted: 11 September 2000     

基于海温异常的南海夏季风爆发的可预报性分析

定义了综合评估南海地区大气对海温异常等外强迫响应的指数,并以此作为中间变量提出了利用前期海温异常预报南海夏季风爆发早晚的方法,进一步分析了基于海温异常的南海夏季风爆发的可预报性。分析表明:基于前期海温的异常,对于以日为单位预报对象的定量的南海夏季风爆发日期来说,基本没有可预报性;而对于定性的南海夏季风爆发早晚的预测,则可预报性大为提高。其预报时效可以提前至前期秋季的11月份。     

Impact of local sea surface temperature anomaly over the western North Pacific on extreme East Asian summer monsoon

In this study, the anomalous characteristics of observed large-scale synoptic fields in the extreme East Asian summer monsoon (EASM) years are analyzed, and the impact of the local sea surface temperature (SST) anomaly over the western North Pacific (WNP) on the extreme EASM is investigated through sensitivity experiments of 28?years EASM simulations to the local SST over the WNP. The observation analysis reveals that the extreme EASM is influenced more by anomalous large-scale atmospheric features such as monsoon circulations and the western North Pacific subtropical high than the local SST anomaly over the WNP. However, the results of the sensitivity experiments show that the local SST anomaly has an implicit impact on the extreme EASM. The patterns of differences in precipitation between the experiment forced by observed SST in each year and the experiment forced by climatological SST over the WNP are opposite to anomaly patterns of observed precipitation in the extreme EASM years. This is because the SST anomaly over the WNP plays a role in reducing precipitation anomaly by changing surface latent heat flux and monsoon circulations. In particular, the local SST anomaly over the WNP decreases anomalies of large-scale circulations, i.e., the local Hadley and the Walker circulations. Thus, the local SST anomaly over the WNP plays a role in decreasing the interannual variability of the EASM.     

The relation between the north pacific sea surface temperature anomaly during the sixties and hail damage in Alberta

Coincident with anomalously warm sea surface temperature in the North Pacific during the period 1962–67 was a period of low hail damage in the Province of Alberta. Investigation of thirty‐three years of hail insurance records reveals a reduction in hail damage during this period. Analysis of monthly mean upper air data indicates a significant change in the summer upper air circulation over Alberta. The mean jet stream is found to be farther south and much stronger than for the period 1950–61. Some evidence is found to support the claim that the hail damage shifted southward with the jet stream.     

Using idealized snow forcing to test teleconnections with the Indian summer monsoon in the Hadley Centre GCM

Anomalous heavy snow during winter or spring has long been regarded as a possible precursor of deficient Indian monsoon rainfall during the subsequent summer. However previous work in this field is inconclusive, in terms of the mechanism that communicates snow anomalies to the monsoon summer, and even the region from which snow has the most impact. In this study we explore these issues in coupled and atmosphere-only versions of the Hadley Centre model. A 1050-year control integration of the HadCM3 coupled model, which well represents the seasonal cycle of snow cover over the Eurasian continent, is analysed and shows evidence for weakened monsoons being preceded by strong snow forcing (in the absence of ENSO) over either the Himalaya/Tibetan Plateau or north/west Eurasia regions. However, empirical orthogonal function (EOF) analysis of springtime interannual variability in snow depth shows the leading mode to have opposite signs between these two regions, suggesting that competing mechanisms may be possible. To determine the dominant region, ensemble integrations are carried out using HadAM3, the atmospheric component of HadCM3, and a variety of anomalous snow forcing initial conditions obtained from the control integration of the coupled model. Forcings are applied during spring in separate experiments over the Himalaya/Tibetan Plateau and north/west Eurasia regions, in conjunction with climatological SSTs in order to avoid the direct effects of ENSO. With the aid of idealized forcing conditions in sensitivity tests, we demonstrate that forcing from the Himalaya region is dominant in this model via a Blanford-type mechanism involving reduced surface sensible heat and longwave fluxes, reduced heating of the troposphere over the Tibetan Plateau and consequently a reduced meridional tropospheric temperature gradient which weakens the monsoon during early summer. Snow albedo is shown to be key to the mechanism, explaining around 50% of the perturbation in sensible heating over the Tibetan Plateau, and accounting for the majority of cooling through the troposphere.     

北太平洋海温距平经向差对付热带高压影响的若干事实

本文在系统分析1951—1975年北太平洋海温距平特征及夏季西太平洋付高强度的基础上,得到如下主要结果:整个北太平洋海温距平基本上可以分为正型和负型两种类型。正型是指海温距平经向差为正,即南正北负或南区的距平值大于北区,负型反之。正型和负型各有很长的持续性、周期性和经常在春、秋发生转换的阶段性。此外,北太平洋海温距平经向差与付高之间存在非常密切的正相关,它们有共同的周期,同时付高比海温的变化落后3—7个月。在付高与海温相关普查的基础上,给出了用海温距平经向差予报付高趋势的拟合曲线。     

Fluctuations of surface air temperature in the Eastern Mediterranean

Summary  Changes in surface air temperature during the last century are widely discussed among researchers in the field of climatic change. Using various techniques, we investigate trends and periodicity of surface air temperature series from eight meteorological stations in the Eastern Mediterranean. For the analysis, we use the Mann-Kendall rank test, low-pass filtering, autocorrelation spectral analysis and maximum entropy spectral analysis. The latter two tests are compared. The study is based on series over one hundred years in length for four stations, and over fifty years in length for the other four. Increasing and decreasing surface temperature trends were found. These trends, however, are only significant for Malta, Jerusalem, and Tripoli at the 99% confidence level (positive trend) and for Amman at the 95% confidence level (negative trend). We also found inter-decadal variations in surface air temperature, including a fairly regular quasi 20-year oscillation, although its amplitude varied between different cycles. A period of warming began around 1910 at all stations. During the 1970s, the annual mean temperature series exhibit warming, but this warming was not uniform, continuous or of the same order at all the stations. The results of the Autocorrelation Spectral Analysis and the Maximum Entropy Spectral Analysis are similar, pointing to the reliability of the results. The quasi-biennial oscillation (QBO) exists at all stations during both increasing and decreasing trends. Similarly, a broad maximum from 3–8 years (related to El Ni?o) is found at Malta, Athens, Jerusalem, Beirut, and Latakia. An inverse relationship between El Ni?o and the North Atlantic Oscillation with surface air temperature over the Eastern Mediterranean is found at a highly significant confidence level. Received November 15, 1999 Revised August 29, 2000