土壤湿度年际异常对中国春、夏季气候模拟的影响

基于NCAR大气模式CAM3.1模式,设计了有、无土壤湿度年际异常两组试验对中国区域近40a(1961-2000年)气候进行了模拟。从气候态和年际变率的角度,通过分析两组试验的差值场来探讨土壤湿度年际异常对气候模拟的影响,并初步探讨了影响的可能机制。结果表明:模式模拟的温度和降水对土壤湿度的年际异常非常敏感,土壤湿度的年际变化对中国春夏季气候及其年际变率均有显著影响。当不考虑土壤湿度年际异常时,模式模拟的春夏季平均温度、最高温度、最低温度在我国大范围内降低,春夏季降水在东部大部分地区明显减少,西部增加。而模式模拟的春夏季温度、降水年际变率在中国大部分地区减弱。但当考虑土壤湿度的年际变化,则能在一定程度上提高模式对气候年际变率的模拟能力。在进一步分析表明土壤湿度年际异常时,主要通过改变地表能量通量和环流场,对温度、降水产生影响。当不考虑土壤湿度年际异常时,地表净辐射通量减少,地表温度降低,感热通量减少。感热通量差值场的空间变化和温度差值场的空间变化一致,感热通量对温度有一定影响。而潜热通量差值场的空间变化和降水的差值场的空间变化一致,可见降水受地表潜热通量的影响。土壤湿度年际异常引起的环流场的变化也是导致气候变化的原因之一,地表能量和环流场年际变率的改变对春夏季气候年际变率存在一定影响。     

青藏高原东坡理塘地区近地层湍流通量与微气象特征研究

简要介绍青藏高原东坡理塘大气综合观测站长期观测试验,并利用2006年1、7月资料分别代表该站冬季和夏季,初步分析和比较该地区冬、夏季近地层微气象特征和湍流通最输送情况,得到了以下结论:(1)风、温、湿均表现出明显的日变化特征.冬季风速值平均大于夏季,风速极大值均出现在下午;冬季温度梯度早晚大,白天小,而夏季均较小;湿度梯度早晚大于白天.(2)中件条件下风速廓线对数关系表现为一条直线而非中件条件下略偏离对数关系,晚上均有逆温现象出现.在一定高度能观测到较弱的逆湿现象.(3)冬季以感热为主,潜热值较小,夏季以潜热为主,但感热也较大,且冬季通量值要远小于夏季;冬季动量通量平均大于夏季,二氧化碳通量远小于夏季;浅层(地面以下2和5 cm)土壤热通量也具有明显的日变化特征,白天从土壤吸收热量,夜间则放出热量.(4)地面热源强度具有显著的日变化特征:白天为强热源,夜间冷热源特征不明显.冬季和夏季全天平均表现为热源,但夏季强度远大于冬季,平均达到134 W/m2左右,冬季仪约35.3 W/m2.     

不同辐射传输方案对P-σ RCM9区域气候模式性能的影响

将NCAR CCM3辐射传输方案CRM(Column Radiation Model)引入到P-σRCM9区域气候模式中,并对两组辐射传输方案下P-σRCM9区域气候模式模拟的东亚冬、夏季气候特征差异进行对比分析,发现采用CRM辐射传输方案能够改善P-σRCM9区域气候模式对中国区域冬、夏季地面温度的模拟,且在夏季的改善比冬季更明显,还改善了P-σ RCM9区域气候模式对对流层高层(200～100 hPa)短波辐射加热率和长波辐射加热率的计算,然后改进了模式对200～300 hPa垂直平均经向温度梯度的模拟,进而使得模拟的冬、夏季东亚高空西风急流比采用KQ辐射传输方案的模拟结果更加接近NCEP/NCAR再分析资料.同时还发现,采用CRM辐射传输方案后,P-σRCM9气候模式模拟的中国地区冬、夏季的降水率在强度和分布形势上均比采用KQ辐射传输方案的模拟结果更接近观测,CRM辐射传输方案能够较明显地提高P-σRCM9气候模式对中国地区冬、夏季降水的模拟能力.     

东亚区域气候变化的长期数值模拟试验

文中利用NCAR的中尺度模式MM 5V3对东亚地区进行了 10a的长期积分模拟试验 ,并着重对冬、夏两季东亚区域气候变化特征进行了分析。分析结果表明 :(1)模式能够合理地模拟出 10a冬、夏平均的区域气候特征。模拟的 10a冬季平均降水的分布和强度与实际比较一致 ,对夏季降水分布特征的模拟也比较合理 ,但模拟的夏季华北降水偏多。模式对冬季平均场的模拟要优于对夏季的模拟 ;(2 )模式对降水、地面气温年际变率的模拟较为合理 ,模拟的中高层环流、温度场等要素的距平相关系数都比较高 ;(3)模式对不同ElNi no年对东亚区域气候变化影响的模拟能力有所不同 ,模拟的 1992 ,1995年的结果比较合理 ,但对 1998年模拟得不理想 ;(4)MM5V3模式具备一定的区域气候模拟能力。     

青藏高原表层土壤热通量的时空分布特征

利用青藏高原(简称高原)9个站点的实测数据分析了表层土壤热通量G0的季节变化、日变化特征,然后利用MODIS数据(MOD13Q1和MOD09CMG)、中国西部逐日1 km空间分辨率全天候地表温度数据集和同化数据(ITPCAS-SRad和ITPCAS-LRad),借助G0遥感估算模型Ma模拟了高原四期(2014年7月12日和10月16日,2015年1月1日和4月7日)的G0空间分布特征。结果表明:G0振幅随季节变化,夏季较大,冬季最小,站点之间振幅不同可能与下垫面有关,下垫面植被覆盖度越高,振幅越小;G0在春、夏季以及全年整体为正,而秋、冬季G0则为负。高原G0呈现倒立的"U"型的日变化曲线,夜晚的变化相对白天而言比较平缓;G0日变化曲线为正值的时长存在明显季节差异,四个季节的顺序是夏季春季秋季冬季;高原G0的空间分布特征与高原地表温度的空间分布有较好的正相关,站点数据显示地表温度每增加1℃,G0随之增大2～5 W·m-2。     

阵风效应对冬季北太平洋涛动年际变化模拟效果的影响

利用观测海温驱动NCAR CAM5模式进行了两组数值试验,对比分析了在CAM5模式海气湍流热通量参数化方案中引入阵风效应前后,模式对冬季NPO(North Pacific Oscillation,北太平洋涛动)年际变化模拟效果的差异。结果表明,考虑了阵风效应后,CAM5模式能较好地模拟冬季NPO的年际变化特征,模拟的1979—1999年冬季NPO指数序列与观测结果的相关系数由0.09提高到0.57;热带东太平洋海表温度异常通过影响海表湍流热通量异常,对冬季NPO年际变化产生影响;引入阵风效应后,模式对热带东太平洋海表湍流热通量异常的模拟结果更趋合理,从而使得模式可以较好地模拟出冬季NPO的年际变化。     

IPCC A2情景下中国区域气候变化的数值模拟

在政府间气候变化委员会(IPCC)排放情景特别报告 (SRES)的A2情景下,利用CSIRO Mark3海气耦合模式模拟现代和未来2个10年的模拟结果,驱动MM5区域气候模式进行中国未来区域气候变化的数值模拟试验,研究了IPCC A2情景下未来中国温度、降水和环流等的变化趋势.结果表明,(1)区域气候模式MM5V3能够再现气候平均环流、降水和温度分布的主要特征,具有较好的区域气候变化模拟能力;(2)IPCC A2情景下,未来中国平均地面气温将有明显的升高,特别是中国的东北、西北和西南地区增幅超过了1 ℃.冬季,地面平均气温的增幅由南至北逐渐增加;夏季,在内蒙和中国西南地区有明显的增温.伴随温度的升高,降水也有明显的变化,年平均降水在中国的东北地区、江淮流域及以南大部分地区都有明显的增强,而中国华北部分地区及西南、西北大部分地区降水将呈减少趋势.不同季节不同地区的降水变化也不同,秋季华北、华南和江淮地区降水都增加,而冬季减少.降水的年内变化也有所增强.     

区域海气耦合模式对华北夏季大气水汽输送模拟结果的检验及其与单独气候模式的比较分析

将区域海气耦合模式RegCM3-POM和区域气候模式RegCM3 40年(1963-2002年)的模拟结果与NCEP/NCAR再分析资料进行对比,检验区域海气耦合模式对中国华北地区夏季大气水汽含量和水汽输送特征的模拟能力,比较耦合模式与单独区域气候模式的差异.结果表明,区域海气耦合模式RegCM3-POM的模拟性能相对于单独区域气候模式RegCM3,大气水汽输送特征的模拟能力有了较大的改进.分析显示两种模式都能够较好地再现东哑地区气候平均夏季大气水汽储量浅红和水汽输送的空间分布特征,而耦合模式对大气水汽输送的模拟更为合理.在对流层中低层更接近观测;耦合模式对中国华北地区夏季平均大气水汽输送通量在垂直方向卜的分布型及水平4个边界水汽输送收支的模拟,相对于单独大气模式有了一定的改进;耦合模式对伴随华北地区夏季早涝的大气水汽异常输送也具有较好的模拟能力,其模拟的水汽输送异常的来源与观测基本一致,尤其是在20°N以北地区,耦合模式结果相对于单独区域气候模式有了很大的改进.但同时耦合模式在低纬度海洋上对气候平均夏季大气水汽含量模拟的偏差比区域气候模式显著;与观测相比,耦合模式对来自孟加拉湾地区的大气水汽输送模拟偏弱,而对西太平洋副热带高压西侧水汽输送模拟偏强,与华北夏季旱涝相联系的水汽输送异常的模拟在低纬度海洋上也存在明显偏差.     

基于ENVI-met的广州街区温湿环境模拟

为研究华南湿热气候地区城市小区的温湿环境特征，基于微气候模式(ENVI-met)模拟广州市海珠区中山大学小区冬夏季温湿场的分布特征，并结合气象自动站连续监测资料进行检验订正，结果表明：(1)不同季节街区温湿场空间分布特征存在一定差异，白天水体和植被覆盖区为高湿和低温区；(2)ENVI-met模式能够较好预测小区微尺度温度和湿度环境，冬季温度和湿度实测值日变化幅度稍高于模拟值，夏季温度模拟值日变化幅度大于实测值；(3)冬夏季温度经过多项式函数拟合订正及冬季湿度经过指数函数和夏季湿度经过幂函数拟合订正后RMSE分别降至0.8℃以下和2.5%以下；MAPE分别降至0.26%和2.6%以下，模拟结果与实测值基本一致。研究结果表明ENVI-met可应用于城市小区规划建设。     

区域气候模式对东亚冬季风多年平均特征的模拟

利用改进的区域气候模式(RegCM-NCC)对东亚区域进行了连续5年(1998～2002年)的气候模拟,并对模拟的东亚冬季风情况进行了全面分析.结果表明该模式能够较好地模拟出东亚地区冬季平均环流特征,较真实反映出冬季低层大陆冷高压的平均位置与强度,也能够揭示出冬季风场变化的主要特征,如低层的冬季风气流及高层的西风急流;对冬季风强度及年际变化也有较好的反映,对冬季季风涌出现的频率、主要区域以及温度的演变等气候特征的模拟与实况一致.通过比较分析,对该模式在东亚冬季风等方面模拟性能有较全面的认识,便于模式的应用及进一步改进.     

我国近海和邻近海的海洋环境对最近全球气候变化的响应

鉴于全球气候变暖对海洋环境和海洋生态及对经济和社会可持续发展影响的严重性,作者首先利用ERA-40再分析的风场资料以及HadISST 和SODA等海洋高分辨率再分析资料,分析了近50年来全球气候变化对中国近海(包括渤海、黄海、东海和南海)和邻近海(主要是热带和副热带西太平洋)海面附近的风力、海表纬向和经向风应力和海表温度的影响.分析结果表明: 由于受全球气候变暖的影响,1976年之后中国近海和邻近海上空的冬、夏季风变弱,从而引起中国近海冬、夏季海表风应力减弱(尤其是经向风应力),而海表水温明显上升; 并且,冬、夏季海表风应力的减弱和海水温度的上升在中国东海反映尤其明显,这些为中国近海赤潮的频繁发生提供了有利的海洋环境.此外,从中国近海上空环流散度分布的变化可见,中国近海上空从1976年之后大气环流辐散增强,这不利于中国近海上升流的形成,从而会对沿岸水域营养盐的输送产生影响.     

Simulated impacts of afforestation in East China monsoon region as modulated by ocean variability

Using the National Center for Atmospheric Research Community Climate System Model Version 3.5, this paper examines the climatic effects of afforestation in the East China monsoon region with a focus on land–atmosphere interactions and the modulating influence of ocean variability. In response to afforestation, the local surface air temperature significantly decreases in summer and increases in winter. The summer cooling is attributed to enhanced evapotranspiration from increased tree cover. During winter, afforestation induces greater roughness and weaker winds over the adjacent coastal ocean, leading to diminished latent heat flux and increased sea-surface temperature (SST). The enhanced SST supports greater atmospheric water vapor, which is accompanied by anomalous wind, and transported into the East China monsoon region. The increase in atmospheric water vapor favors more cloud cover and precipitation, especially in the eastern afforestation region. Furthermore, the increase in atmospheric water vapor and cloud cover produce a greenhouse effect, raising the wintertime surface air temperature. By comparing simulations in which ocean temperature are either fixed or variable, we demonstrate that a significant hydrologic response in East China to afforestation only occurs if ocean temperatures are allowed to vary and the oceanic source of moisture to the continent is enhanced.     

A Regional Air--Sea Coupled Model and Its Application over East Asia in the Summer of 2000

A regional air-sea coupled model,comprising the Regional Integrated Environment Model System (RIEMS)and the Princeton Ocean Model(POM)was developed to simulate summer climate features over East Asia in 2000.The sensitivity of the model's behavior to the coupling time interval(CTI),the causes of the sea surface temperature(SST)biases,and the role of air-sea interaction in the simulation of precipitation over China are investigated.Results show that the coupled model can basically produce the spatial pattern of SST,precipitation,and surface air temperature(SAT)with five different CTIs respectively.Also,using a CTI of 3,6 or 12 hours tended to produce more successful simulations than if using 1 and 24 hours.Further analysis indicates that both a higher and lower coupling frequency result in larger model biases in air-sea heat flux exchanges,which might be responsible for the sensitivity of the coupled model's behavior to the CTI. Sensitivity experiments indicate that SST biases between the coupled and uncoupled POM occurring over the China coastal waters were due to the mismatch of the surface heat fluxes produced by the RIEMS with those required by the POM.In the coupled run,the air-sea feedbacks reduced the biases in surface heat fluxes,compared with the uncoupled RIEMS,consequently resulted in changes in thermal contrast over land and sea and led to a precipitation increase over South China and a decrease over North China.These results agree well observations in the summer of 2000.     

Individual and combined impacts of ENSO and East Asian winter monsoon on the South China Sea cold tongue intensity

A region of low sea surface temperature (SST) extends southward in the central part of southern South China Sea during boreal winter, which is called the South China Sea cold tongue (SCS CT). The present study investigates the factors of interannual variation of SST in the SCS CT region and explores the individual and combined impacts of El Niño-Southern Oscillation (ENSO) and East Asian winter monsoon (EAWM) on the SCS CT intensity. During years with ENSO alone or with co-existing ENSO and anomalous EAWM, shortwave radiation and ocean horizontal advection play major roles in the interannual variation of the SCS CT intensity. Ocean advection contributes largely to the SST change in the region southeast of Vietnam. In strong CT years with anomalous EAWM alone, surface wind-related latent heat flux has a major role and shortwave radiation is secondary to the EAWM-induced change of the SCS CT intensity, whereas the role of ocean horizontal advection is relatively small. The above differences in the roles of ocean advection and latent heat flux are associated with the distribution of low level wind anomalies. In anomalous CT years with ENSO, low level anomalous cyclone/anticyclone-related wind speed change leads to latent heat flux anomalies with effects opposite to shortwave radiation. In strong CT years with anomalous EAWM alone, surface wind-related latent heat flux anomalies are large as anomalous winds are aligned with climatological winds.

     

Response of the tropical Indian Ocean SST to decay phase of La Niña and associated processes

Delayed impact of El Niño on Tropical Indian Ocean (TIO) Sea Surface Temperature (SST) variations and associated physical mechanisms are well documented by several studies. However, TIO SST evolution during the decay phase of La Niña and related processes are not adequately addressed before. Strong cooling associated with La Niña decay over the TIO could influence climate over the Indian Oceanic rim including Indian summer monsoon circulation and remotely northwest Pacific circulation. Thus understanding the TIO basin-wide cooling and related physical mechanisms during decaying La Niña years is important. Composite analyses revealed that negative SST anomalies allied to La Niña gradually dissipate from its mature phase (winter) till subsequent summer in central and eastern Pacific. In contrast, magnitude of negative SST anomalies in TIO, induced by La Niña, starts increasing from winter and attains their peak values in early summer. It is found that variations in heat flux play an important role in SST cooling over the central and eastern equatorial Indian Ocean, Bay of Bengal and part of Arabian Sea from late winter to early summer during the decay phase of La Niña. Ocean dynamical processes are mainly responsible for the evolution of southern TIO SST cooling. Strong signals of westward propagating upwelling Rossby waves between 10°S to 20°S are noted throughout (the decaying phase of La Niña) spring and summer. Anomalous cyclonic wind stress curl to the south of the equator is responsible for triggering upwelling Rossby waves over the southeastern TIO. Further, upwelling Rossby waves are also apparent in the Arabian Sea from spring to summer and partly contributing to the SST cooling. Heat budget analysis reveals that negative SST/MLT (mixed layer temperature) anomalies over the Arabian Sea are mostly controlled by heat flux from winter to spring and vertical advection plays an important role during early summer. Vertical and horizontal advection terms primarily contribute to the SST cooling anomalies over southern TIO and the Bay of Bengal cooling is primarily dominated by heat flux. Further we have discussed influence of TIO cooling on local rainfall variations.     

Isolating mesoscale coupled ocean–atmosphere interactions in the Kuroshio Extension region

The Kuroshio Extension region is characterized by energetic oceanic mesoscale and frontal variability that alters the air–sea fluxes that can influence large-scale climate variability in the North Pacific. We investigate this mesoscale air-sea coupling using a regional eddy-resolving coupled ocean–atmosphere (OA) model that downscales the observed large-scale climate variability from 2001 to 2007. The model simulates many aspects of the observed seasonal cycle of OA coupling strength for both momentum and turbulent heat fluxes. We introduce a new modeling approach to study the scale-dependence of two well-known mechanisms for the surface wind response to mesoscale sea surface temperatures (SSTs), namely, the ‘vertical mixing mechanism’ (VMM) and the ‘pressure adjustment mechanism’ (PAM). We compare the fully coupled model to the same model with an online, 2-D spatial smoother applied to remove the mesoscale SST field felt by the atmosphere. Both VMM and PAM are found to be active during the strong wintertime peak seen in the coupling strength in both the model and observations. For VMM, large-scale SST gradients surprisingly generate coupling between downwind SST gradient and wind stress divergence that is often stronger than the coupling on the mesoscale, indicating their joint importance in OA interaction in this region. In contrast, VMM coupling between crosswind SST gradient and wind stress curl occurs only on the mesoscale, and not over large-scale SST gradients, indicating the essential role of the ocean mesocale. For PAM, the model results indicate that coupling between the Laplacian of sea level pressure and surface wind convergence occurs for both mesoscale and large-scale processes, but inclusion of the mesoscale roughly doubles the coupling strength. Coupling between latent heat flux and SST is found to be significant throughout the entire seasonal cycle in both fully coupled mode and large-scale coupled mode, with peak coupling during winter months. The atmospheric response to the oceanic mesoscale SST is also studied by comparing the fully coupled run to an uncoupled atmospheric model forced with smoothed SST prescribed from the coupled run. Precipitation anomalies are found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale.     

青藏高原及周边地区近地层小气候特征分析

利用中国气象局成都高原气象研究所建立的5个边界层站（湄潭、巴中、什邡、曲麻莱、狮泉河）2019年的观测资料，对比分析青藏高原及周边地区近地层大气要素变化和陆—气能量交换特征及异同点，探讨其原因。结果表明：（1）青藏高原及周边地区近地层大气温度、相对湿度、风速、感热通量、潜热通量、动量通量均符合一峰一谷的常规日变化特征，气压具有双峰双谷的日变化特征。（2）高海拔台站近地层温度日变幅（12℃）高于周边低海拔地区（4～6℃），季变幅与海拔高度的关系不显著。（3）相对湿度与温度关系密切，相对湿度的垂直结构和日变化都具有明显的区域差异，其垂直梯度夜间显著高于白天，峰值出现时间随夏、秋、春、冬季呈现季节性滞后，而谷值超前。（4）风速春季较大，夏、秋季次之，冬季小，季变幅略小于日变幅；低海拔区域的风速及其日变幅均显著低于高海拔区域。（5）低海拔区域气压季变幅（>13 hPa）远高于日变幅（2.5 hPa左右），而高海拔区域气压季变幅（>3 hPa）略低于日变幅（2 hPa左右）。（6）感热通量春季大、冬季小；感热通量和动量通量在高海拔区域均较高，二者具有较一致的日、季变化特征，表明大气动...     

东海黑潮区潜热变化对中国春季降水的影响及其影响过程

本文利用美国NCEP/NCAR再分析资料、哈德来(Hadley)中心海温数据、国家气候中心的观测站降水和客观分析海气通量(OAFlux)潜热感热通量资料,研究了1960~2010年春季黑潮区潜热输送对中国春季降水的影响及其影响过程。本文以黑潮流经的中国东部海域及邻近海域为研究对象,该区域是黑潮的主体区域,在文中简称为东海黑潮区。对中国东海以及邻近海域海温与降水的分析表明,在夏季该区域可能以大气强迫海洋为主,而在春冬两季可能主要为海洋强迫大气为主,秋季则可能为不明显的海气相互作用。在春季西北太平洋区域中感热和潜热都对黑潮流经的区域有比较好的敏感性,黑潮流经区域感热和潜热的气候平均值分别约为30 W m-2与120 W m-2;春季的感热通量标准差大值区主要集中在日本以西区域,潜热通量标准差主要集中在中国东海区域与日本东南区域(即东海黑潮区域)。春季潜热EOF第一模态的主要变化就集中在东海黑潮流域。相关分析与合成分析的结果表明,当黑潮潜热指数为正时,华南地区春季降水偏多,长江以北地区偏少,反之亦然。在物理过程分析中,黑潮潜热指数大于0.8时,长江以南的中国大陆有比较强盛的异常北风,使得水汽无法输送到更北的地区,导致在华南地区水汽的积累,并且在海面出现有利于降水的垂直运动异常延伸到大陆上,使华南地区降水增多,而长江以北的东部地区由于水汽输送偏弱,导致水汽积累偏少,从而降水减少。当黑潮指数小于-0.8时,有较强盛的异常南风,有利于水汽输送到北方地区,水汽在华北地区积累,导致长江以北出现降水正异常,而华南地区由于南风偏强,水汽输送加强,导致水汽无法在此区域积累,并且出现不利于降水的垂直运动异常,从而导致降水偏少。     

巢湖流域典型站点的风场特征分析

研究巢湖流域流场特征对于认识该地区热量、水汽交换和水流运动规律具有重要意义。利用2006年合肥、肥东、巢湖、庐江站以及姥山岛自动气象站的风场资料,分析了巢湖流域典型站点的风速和风向变化特征。结果表明,陆面站点年平均风速为2.17m/s,湖面站点风速为2.41m/s。所有站点春夏季风速大于秋冬季,陆上风速具有明显的日变化,白天风速大于夜间,而湖面风速日变化较不显著。陆面站点风向季节变化明显,春夏季以偏南风为主,秋冬季以偏北风为主,春夏季的风向日变化特征较秋冬季明显,湖面站风向没有明显的季节变化。陆面站点不同程度地受到湖陆风的影响,距离湖面较近的站点受到的影响较大。湖面和陆面站点风向差距平与气温差距平的日变化保持一致,表明湖陆温差是影响巢湖流域湖陆风的关键因子。     

Interdecadal variability of the Pacific Ocean: model response to observed heat flux and wind stress anomalies

Variability of the Pacific Ocean is examined in numerical simulations with an ocean general circulation model forced by observed anomalies of surface heat flux, wind stress and turbulent kinetic energy (TKE) over the period 1970-88. The model captures the 1976-77 winter time climate shift in sea surface temperature, as well as its monthly, seasonal and longer term variability as evidenced in regional time series and empirical orthogonal function analyses. Examination of the surface mixed-layer heat budget reveals that the 1976-77 shift was caused by a unique concurrance of sustained heat flux input anomalies and very strong horizontal advection anomalies during a multi-month period preceding the shift in both the central Pacific region (where cooling occurred) and the California coastal region (where warming occurred). In the central Pacific, the warm conditions preceding and the cold conditions following the shift tend to be maintained by anomalous vertical mixing due to increases in the atmospheric momentum flux (TKE input) into the mixed layer (which deepens in the model after the shift) from the early 1970s to the late 1970s and 1980s. Since the ocean model does not contain feedback to the atmosphere and it succeeds in capturing the major features of the 1976-77 shift, it appears that the midlatitude part of the shift was driven by the atmosphere, although effects of midlatitude ocean-atmosphere feedback are still possible. The surface mixed-layer heat budget also reveals that, in the central Pacific, the effects of heat flux input and vertical mixing anomalies are comparable in amplitude while horizontal advection anomalies are roughly half that size. In the California coastal region, in contrast, where wind variability is much weaker than in the central Pacific, horizontal advection and vertical mixing effects on the mixed layer heat budget are only one-quarter the size of typical heat flux input anomalies.This paper was presented at the Second International Conference on Modelling of Global Climate Variability, held in Hamburg 7–11 September 1992 under the auspices of the Max Planck Institute for Meteorology. Guest Editor for these papers is L. Dümenil