Effect of basic state on seasonal variation of convectively coupled Rossby wave

Convectively coupled equatorial Rossby (ER) waves display maximum varability over the northern hemisphere during boreal summer and over the southern hemisphere during boreal winter. It suggests that the seasonal variation of ER waves is significantly affected by the annual cycle of basic state. However, which specific environmental factor plays a determining role remains obscure. This study investigates the background influence on the seasonal variation of ER wave by employing an intermediate anomaly atmospheric model. By prescribing boreal summer/winter seasonal mean state as the model’s basic state, the authors found that the model is able to simulate the trapping of the ER wave purtrubation over the northern/southern hemisphere as in observation. Further sensitivity experiments suggest that the moisture distribution plays a major role in modulating the ER wave structure while the mosoonal flows play a minor role.     

Climate stability as deduced from an idealized coupled atmosphere-ocean model

The stability of an idealized climate system is investigated using a simple coupled atmosphere-ocean box model. Motivated by the results from general circulation models, the main physical constraint imposed on the system is that the net radiation at the top of the atmosphere is fixed. The specification of an invariant equatorial atmospheric temperature, consistent with paleoclimatic data, allows the hydrological cycle to be internally determined from the poleward heat transport budget, resulting in a model that has a plausible representation of the hydrological cycle-thermohaline circulation interaction. The model suggests that the stability and variability of the climate system depends fundamentally on the mean climatic state (total heat content of the system). When the total heat content of the climate system is low, a stable present-day equilibrum exists with high-latitude sinking. Conversely, when the total heat content is high, a stable equatorial sinking equilibrium exists. For a range of intermediate values of the total heat content, internal climatic oscillations can occur through a hydrological cycle-thermohaline circulation feedback process. Experiments conducted with the model reveal that under a 100-year 2 × CO₂ warming, the thermohaline circulation first collapses but then recovers. Under a 100-year 4 × CO₂ warming, the thermohaline circulation collapses and remains collapsed. Recent paleoclimatic data suggest that the climate system may behave very differently for a warmer climate. Our results suggest that this may be attributed to the enhanced poleward freshwater transport, which causes increased instability of the presentday thermohaline circulation.     

Parameter sensitivity of a coupled atmosphere-ocean model

 The sensitivity of a coupled model to the oceanic vertical diffusion coefficient κ _v is examined. This is compared to the sensitivity of an ocean-only model forced by mixed boundary conditions (BC). The atmospheric component of the coupled model is a moist energy balance model. The ocean component is a 12-level geostrophic model, defined on a midlatitude β-plane. Atmosphere and ocean are coupled through the fluxes of heat and moisture at their interface. The coupled model contains a number of feedback processes which are not represented in the ocean-only model. This results in a temperature and salinity response to κ _v which is stronger in the coupled model than in the ocean-only model. On the other hand, there is a weaker response in oceanic processes such as meridional heat transport, deep-water formation at high latitudes, etc. Ocean-only sensitivity experiments were also performed with modified BCs, which parametrise the feedback processes included in the coupled model. These are the modified thermal BC of Rahmstorf and Willebrand and a modified freshwater BC proposed in the present study. Large-scale features of the response in oceanic surface fields are well represented with modified BCs. However, the sensitivity of the deep ocean temperature is only partly captured due to local differences in the surface response. The scaling behavior of the zonal overturning stream function was found to depend on the surface BCs. In contrast to this, the meridional overturning stream function basically scales with κ^0.5 _v in all sensitivity experiments. Differences in the heat transport response among the experiments are thus primarily related to differences in the temperature response. Received: 28 February 1997/Accepted: 12 September 1997     

Low-frequency coupled atmosphere-ocean variability in the southern Indian Ocean

The low-frequency atmosphere-ocean coupled variability of the southern Indian Ocean(SIO) was investigated using observation data over 1958-2010.These data were obtained from ECMWF for sea level pressure(SLP) and wind,from NCEP/NCAR for heat fluxes,and from the Hadley Center for SST.To obtain the coupled air-sea variability,we performed SVD analyses on SST and SLP.The primary coupled mode represents 43% of the total square covariance and is featured by weak westerly winds along 45-30 S.This weakened subtropical anticyclone forces fluctuations in a well-known subtropical dipole structure in the SST via wind-induced processes.The SST changes in response to atmosphere forcing and is predictable with a lead-time of 1-2 months.Atmosphere-ocean coupling of this mode is strongest during the austral summer.Its principle component is characterized by mixed interannual and interdecadal fluctuations.There is a strong relationship between the first mode and Antarctic Oscillation(AAO).The AAO can influence the coupled processes in the SIO by modulating the subtropical high.The second mode,accounting for 30% of the total square covariance,represents a 25-year period interdecadal oscillation in the strength of the subtropical anticyclone that is accompanied by fluctuations of a monopole structure in the SST along the 35-25 S band.It is caused by subsidence of the atmosphere.The present study also shows that physical processes of both local thermodynamic and ocean circulation in the SIO have a crucial role in the formation of the atmosphere-ocean covariability.     

Effects of ocean particles on the upwelling radiance and polarized radiance in the atmosphere-ocean system

Based on a vector radiative transfer model of the atmosphere-ocean system, the influence of oceanic components on radiation processes, including polarization effects, was investigated in the wavelength region ranging from 0.380 to 0.865 μm. The components considered were phytoplankton, inorganic suspended material (sediment), and colored, dissolved organic matter. Due to their important roles in oceanic radiation processes, the sensitivity of the bidirectional reflectance to the rough ocean surface, represented by the wind velocity 10 m above the ocean surface, and aerosol, were taken into account. The results demonstrated that both radiance and polarized radiance just below the ocean surface were sensitive to the change of the concentrations of the considered components, while the dependence of polarized radiance on the observation geometry was more sensitive than radiance. Significant differences in the specular plane existed between the impacts of the phytoplankton and sediment on the degree of polarization just above the ocean surface at 670 nm. At the top of the atmosphere (TOA), polarization was relatively insensitive to changing concentrations of ocean particles at longer wavelengths. Furthermore, the radiance at the TOA in the solar plane was more sensitive to the aerosol optical thickness than wind velocity. In contrast, wind velocity strongly influenced the radiance at the TOA in the sun glint region, while the polarization degree showed less dependence in that region. Finally, a nonlinear optimal inversion method was proposed to simultaneously retrieve the aerosol and wind velocity using radiance measurement.     

On the cold start problem in transient simulations with coupled atmosphere-ocean models

Finite computer resources force compromises in the design of transient numerical experiments with coupled atmosphere-ocean general circulation models which, in the case of global warming simulations, normally preclude a full integration from the undisturbed pre-industrial state. The start of the integration at a later time from a climate state which, in contrast to the true climate, is initially in equilibrium then induces a cold start error. Using linear response theory a general expression for the cold start error is derived. The theory is applied to the Hamburg CO₂ scenario simulations. An attempt to estimate the global-mean-temperature response function of the coupled model from the response of the model to a CO₂ doubling was unsuccessful because of the non-linearity of the system. However, an alternative derivation, based on the transient simulation itself, yielded a cold start error which explained the initial retardation of the Hamburg global warming curve relative to the IPCC results obtained with a simple box-diffusion-upwelling model. In the case of the sea level the behaviour of the model is apparently more linear. The cold start error estimations based on a CO₂ doubling experiment and on an experiment with gradually increasing CO₂ (scenario A) are very similar and explain about two thirds of the coupled model retardation relative to the IPCC results.     

An investigation of the effects of wave state and sea spray on an idealized typhoon using an air-sea coupled modeling system

In this study, the impact of atmosphere-wave coupling on typhoon intensity was investigated using numerical simulations of an idealized typhoon in a coupled atmosphere-wave-ocean modeling system. The coupling between atmosphere and sea surface waves considered the effects of wave state and sea sprays on air-sea momentum flux, the atmospheric low-level dissipative heating, and the wave-state-affected sea- spray heat flux. Several experiments were conducted to examine the impacts of wave state, sea sprays, and dissipative heating on an idealized typhoon system. Results show that considering the wave state and sea-spray-affected sea-surface roughness reduces typhoon intensity, while including dissipative heating intensifies the typhoon system. Taking into account sea spray heat flux also strengthens the typhoon system with increasing maximum wind speed and significant wave height. The overall impact of atmosphere-wave coupling makes a positive contribution to the intensification of the idealized typhoon system. The minimum central pressure simulated by the coupled atmosphere-wave experiment was 16.4 hPa deeper than that of the control run, and the maximum wind speed and significant wave height increased by 31% and 4%, respectively. Meanwhile, within the area beneath the typhoon center, the average total upward air-sea heat flux increased by 22%, and the averaged latent heat flux increased more significantly by 31% compared to the uncoupled run.     

全球海气耦合模式对中国区域年代际气候变化预测能力的评估

利用中亚地区30个观测台站逐月降水资料及同期ERA-40再分析资料,结合8个CMIP5全球气候模式模拟与未来预估大尺度环流场,使用基于变形典型相关分析的统计降尺度方法（BP-CCA）建立降尺度模型,评估多个气候模式对当前气候下中亚地区春季降水的降尺度模拟能力,并对春季降水进行降尺度集合未来预估。结果表明,建立的降尺度模型能够很好地模拟出交叉检验期内春季降水的时间变化和空间结构：降尺度春季降水与相应观测序列的平均时间相关系数为0.35,最高为0.62,平均空间相关系数为0.87。气候模式对中亚春季降水的模拟能力通过降尺度方法得到了显著提高：8个模式降尺度后模拟的降水气候平均态相对误差绝对值降至0.2%—8%,相比降尺度前减小了10%—60%,模拟的降水量场与相应观测场的空间相关均超过0.77;对比降尺度前多模式集合结果,多模式降尺度集合模拟的相对误差绝对值由64%减小至4%,空间相关系数由0.47增大至0.81,标准化均方根误差降至0.59,且多模式降尺度集合结果优于大部分单个模式降尺度结果。多模式降尺度集合预估结果表明,在RCP4.5排放情景下,21世纪前期（2016—2035年）、中期（2046—2065年）和末期（2081—2100年）的全区平均降水变化率分别为-5.3%、3.0%和17.4%。21世纪前期中亚大部分地区降水呈减少趋势,降水呈增多趋势的站点主要分布在南部。21世纪中期整体降水变化率由减少变为增多趋势,21世纪末期中亚大部分台站降水增多较为明显。21世纪初期和末期可信度高的台站均主要位于中亚西部地区。     

Periodically synchronously coupled integrations with the atmosphere-ocean general circulation model ECHAM3/LSG

 A new periodically synchronous coupling scheme has been applied to an atmosphere-ocean general circulation model. Due to a temporary switching off of the atmospheric model this scheme can considerably reduce computer requirements of coupled model experiments. In order to evaluate the new coupling scheme the model results are compared to corresponding synchronously coupled integrations. Experiments with fixed present-day CO₂ concentration and a gradual increase of CO₂ show a good reproduction of the mean state and the climate-change pattern, respectively. The deviations from the synchronously coupled experiments are in the range of the variability of the corresponding synchronously coupled runs. Due to the forcing during the ocean-only periods the short-term fluctuations are underestimated and the long-term variability is overestimated. Received: 18 February 1997/Accepted: 27 October 1997     

A numerical study of a TOGA-COARE squall-line using a coupled mesoscale atmosphere-ocean model

1. Introduction Air-sea interaction plays an important role in theglobal seasonal to inter-annual climate variability,most notably, the El Ni?no and Southern Oscillation(ENSO) phenomenon (Webster and Lukas, 1992). Be-cause of its widespread impacts on …     

Effects of ice microphysics on a tropical coupled system

The effects of ice microphysics on tropical atmospheric and oceanic variability are investigated with a two-dimensional coupled ocean-cloud resolving atmosphere model forced by the large-scale vertical velocity and zonal wind derived from Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). The experiment without ice microphysics is compared to a control experiment with ice microphysics. Compared to the control experiment, the experiment without ice microphysics generates a more humid and colder atmosphere by suppressing stratiform clouds and rainfall and associated latent heating; the experiment without ice microphysics produces a saltier mixed layer by a larger saline forcing associated with a weaker stratiform rainfall. Ocean mixed-layer temperature is insensitive to the atmospheric variability associated with ice microphysics.     

The transport of CO2-induced warming into the ocean: an analysis of simulations by the OSU coupled atmosphere-ocean general circulation model

The OSU global coupled atmosphere-ocean general circulation model has been used to investigate a 2xCO₂-induced climate change. A previous analysis of the simulated 2xCO₂–1xCO₂ temperature differences showed that the CO₂-induced warming penetrated into the ocean and thereby caused a delay in the equilibration of the climate system with an estimatede-folding time of 50–75 years. The objective of the present study is to determine by what pathways and through which physical processes the simulated ocean general circulation produces the penetration of the CO₂-induced warming into the ocean.A global-mean oceanic heat budget analysis shows that the ocean gains heat at a rate of 3 W/m² due to the CO₂ doubling, and that this heat penetrates downward into the ocean predominantly through the reduction in the convective overturning. A zonal-mean oceanic heat budget analysis shows that the surface warming increases from the tropics toward the midlatitudes of both hemispheres and gradually penetrated into the deeper ocean, with a greater penetration in the subtropics and midlatitudes than in the equatorial region. The zonal-mean heat budget analysis also shows that the CO₂-induced warming of the ocean occurs predominantly through the down-ward transport of heat, with the meridional heat flux being only of secondary importance. In the tropics the penetration of the CO₂-induced heating is minimized by the upwelling of cold water. In the subtropics the heating is transported down-ward more readily by the downwelling existing there. In the high latitudes the suppressed convection plays the dominant role in the downward penetration of the CO₂-induced heating. The latter result should be considered as tentative, however, as the ocean component of the coupled model employed a prescribed surface salinity field and did not include the mechanism of brine rejection when sea water freezes into sea ice.     

全球对流层顶气压场的季节变化特征分析

利用经验正交函数分解(EOF)方法对1948-2004年全球对流层顶月平均气压场进行分析,讨论57 a来气压场的空间载荷向量场及其对应的标准化时间系数的演变,并且利用小波分析方法分析各个季节对流层顶气压周期变化特征.结果表明:春季和夏季整个时段内全球大部分地区对流层顶气压降低趋势比秋冬季明显,对应对流层顶高度有升高趋势,特别是东亚及北美等地在春季具有对流层顶气压升高的显著趋势,对应对流层顶高度可能降低;对流层顶气压场各个季节的标准化时间系数变化具有多时间尺度特征,在各个季节都有明显的3～5 a的周期振荡,春季还具有明显的准2 a周期特征.     

黑潮季节变化对次级海洋环流的影响

黑潮是全球大洋中和大气之间进行热量交换最多的海域之一,它所携带的热量对局地的海-气相互作用具有重要影响。20世纪50—60年代,中国学者就发现冬季黑潮流域的海表温度与中国长江流域汛期降水存在着密切联系。文中利用一个两层半海洋模式主要从动力学角度探讨黑潮流域热源的季节变化对次级海洋环流的影响。结果表明,在黑潮流域冬暖夏冷的外源强迫下,温跃层的海温异常峰值比热源的峰值滞后一个季节,春季的海温异常达到最大。混合层年平均海温异常为正,从冬季到夏季均偏高,春季最大,仅秋季为负异常,这与观测资料一致。混合层与温跃层的春季海温异常的量值大体相当,在其他季节混合层的海温异常均高于温跃层。从冬季到夏季,温跃层和混合层的次级环流呈现从气旋型环流到反气旋型环流的转变,这样的异常环流在夏季有利于向中国东部附近海域输送热量。     

How much the warming in Russia agrees with the output of coupled atmosphere-ocean general circulation models?

Climate variability parameters and air temperature trends in Russia, derived from observational data, are compared with those derived from climate modeling in the second half of the 20th-early 21st century, using the atmosphere-ocean general circulation model ensemble. The computation results from these models were used in the preparation of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. It is demonstrated that the ensemble averaging allowed us to efficiently filter the internal climate variability and get relatively stable estimates of trends. As a whole, for Russia, these estimates are in good agreement with the observational data, both for a year on average, and in individual seasons. The comparison of model and observed air temperature trends on a regional scale turns out to be irrelevant in a number of cases because of a high inadequacy of trend estimates derived from the observational data.     

Effects of atmosphere-ocean interaction on the interannual variability of winter temperature in Taiwan and East Asia

 This study investigated the ocean-atmosphere interaction effect on the winter surface air temperature in Taiwan. Temperature fluctuations in Taiwan and marine East Asia correlated better with a SST dipole in the western North Pacific than the SST in the central/eastern equatorial Pacific. During the warm (cold) winters, a positive (negative) SST anomaly appears in marine East Asia and a negative (positive) SST anomaly appears in the Philippine Sea. The corresponding low-level atmospheric circulation is a cyclonic (anticyclonic) anomaly over the East Asian continent and an anticyclonic (cyclonic) circulation in the Philippine Sea during the warm (cold) winters. Based on the results of both numerical and empirical studies, it is proposed that a vigorous ocean-atmosphere interaction occurring in the western North Pacific modulates the strength of the East Asian winter monsoon and the winter temperature in marine East Asia. The mechanism is described as follows. The near-surface circulation anomalies, which are forced by the local SST anomaly, strengthen (weaken) the northeasterly trade winds in the Philippine Sea and weaken (strengthen) the northeasterly winter monsoon in East Asia during warm (cold) winters. The anomalous circulation causes the SST to fluctuate by modulating the heat flux at the ocean surface. The SST anomaly in turn enhances the anomalous circulation. Such an ocean-atmosphere interaction results in the rapid development of the anomalous circulation in the western North Pacific and the anomalous winter temperature in marine East Asia. This interaction is phase-locked with the seasonal cycle and occurs most efficiently in the boreal winters. Received: 22 October 1999 / Accepted: 5 June 2000     

Intercomparsion of regional biases and doubled CO2-sensitivity of coupled atmosphere-ocean general circulation model experiments

 We compared regional biases and transient doubled CO₂ sensitivities of nine coupled atmosphere-ocean general circulation models (GCMs) from six international climate modeling groups. We evaluated biases and responses in winter and summer surface air temperatures and precipitation for seven subcontinental regions, including those in the 1990 Intergovernmental Panel on Climate Change (IPCC) Scientific Assessment. Regional biases were large and exceeded the variance among four climatological datasets, indicating that model biases were not primarily due to uncertainty in observations. Model responses to altered greenhouse forcing were substantial (average temperature change=2.7±0.9 ^°C, range of precipitation change =−35 to +120% of control). While coupled models include more climate system feedbacks than earlier GCMs implemented with mixed-layer ocean models, inclusion of a dynamic ocean alone did not improve simulation of long-term mean climatology nor increase convergence among model responses to altered greenhouse gas forcing. On the other hand, features of some of the coupled models including flux adjustment (which may have simply masked simulation errors), high horizontal resolution, and estimation of screen height temperature contributed to improved simulation of long-term surface climate. The large range of model responses was partly accounted for by inconsistencies in forcing scenarios and transient-simulation averaging periods. Nonetheless, the models generally had greater agreement in their sensitivities than their controls did with observations. This suggests that consistent, large-scale response features from an ensemble of model sensitivity experiments may not depend on details of their representation of present-day climate. Received: 9 September 1996 / Revised: 31 July 1997