Numerical Study for Potential Predictability of Short-Term Anomalous Climate Change Caused by E1 Nino

In this paper, the two-layer IAP model with sea surface temperature anomalies in the equatorial central-eastern Pacific is used to investigate potential predictability of global short-term anomalous climate change caused by El Nino via the "switching" experiments. The experimental results show that short-term anomalous climate change in the tropics is mainly caused by instantaneous response of tropical atmosphere to SSTA in the tropics. The effective period of this kind of anomalous climate is shorter and about monthly scale. In the high latitudes, the anomalous climate is mainly caused by the lag response of atmosphere to SSTA in the tropics. The strongest influence appears in the month after a half year when the SSTA in the tropics disappears. Therefore, potential predictability of short-term anomalous climate change may be reached to one year; anomalous climate change in the middle-latitudes is not only affected by instantaneous response to SSTA in the tropics, but also by lag response to that. There     

Recent Advances in Predictability Studies in China （1999-2002）

Since the last International Union of Geodesy and Geophysics (IUGG) General Assembly (1999), the predictability studies in China have made further progress during the period of 1999-2002. Firstly, three predictability sub-problems in numerical weather and climate prediction are classified, which are concerned with the maximum predictability time, the maximum prediction error, and the maximum allowable initial error, and then they are reduced into three nonlinear optimization problems. Secondly, the concepts of the nonlinear singular vector (NSV) and conditional nonlinear optimal perturbation (CNOP) are proposed,which have been utilized to study the predictability of numerical weather and climate prediction. The results suggest that the nonlinear characteristics of the motions of atmosphere and oceans can be revealedby NSV and CNOP. Thirdly, attention has also been paid to the relations between the predictability and spatial-temporal scale, and between the model predictability and the machine precision, of which the investigations disclose the importance of the spatial-temporal scale and machine precision in the study of predictability. Also the cell-to-cell mapping is adopted to analyze globally the predictability of climate,which could provide a new subject to the research workers. Furthermore, the predictability of the summer rainfall in China is investigated by using the method of correlation coefficients. The results demonstrate that the predictability of summer rainfall is different in different areas of China. Analysis of variance, which is one of the statistical methods applicable to the study of predictability, is also used to study the potential predictability of monthly mean temperature in China, of which the conclusion is that the monthly mean temperature over China is potentially predictable at a statistical significance level of 0.10. In addition,in the analysis of the predictability of the T106 objective analysis/forecasting field, the variance and the correlation coefficient are calculated to explore the distribution characteristics of the mean-square errors.Finally, the predictability of short-term climate prediction is investigated by using statistical methods or numerical simulation methods. It is demonstrated that the predictability of short-term climate in China depends not only on the region of China being investigated, but also on the time scale and the atmospheric internal dynamical process.     

Response of a Grassland Ecosystem to Climate Change in a Theoretical Model

The response of a grassland ecosystem to climate change is discussed within the context of a theoretical model.An optimization approach,a conditional nonlinear optimal perturbation related to parameter(CNOP-P) approach,was employed in this study.The CNOP-P,a perturbation of moisture index in the theoretical model,represents a nonlinear climate perturbation.Two kinds of linear climate perturbations were also used to study the response of the grassland ecosystem to different types of climate changes.The results show that the extent of grassland ecosystem variation caused by the CNOP-P-type climate change is greater than that caused by the two linear types of climate change.In addition,the grassland ecosystem affected by the CNOP-P-type climate change evolved into a desert ecosystem,and the two linear types of climate changes failed within a specific amplitude range when the moisture index recovered to its reference state.Therefore,the grassland ecosystem response to climate change was nonlinear.This study yielded similar results for a desert ecosystem seeded with both living and wilted biomass litter.The quantitative analysis performed in this study also accounted for the role of soil moisture in the root zone and the shading effect of wilted biomass on the grassland ecosystem through nonlinear interactions between soil and vegetation.The results of this study imply that the CNOP-P approach is a potentially effective tool for assessing the impact of nonlinear climate change on grassland ecosystems.     

MECHANISM FOR THE SUSTAINING ATMOSPHERIC RESPONSE TO WARM WINTERTIME SEA SURFACE TEMPERATURE ANOMALY IN THE KUROSHIO EXTENSION

Sensitivity experiments with atmospheric general circulation model CAM3 have been performed to investigate the atmospheric response to warm wintertime sea surface temperature anomalies in the Kuroshio Extension (KE). Mechanism for the sustaining abnormal atmospheric response to sea surface temperature anomaly (SSTA) is revealed. It is found that the warm wintertime SSTA in KE leads to soil moisture changes across the Asia continent. The abnormal soil moisture may possibly be one of the reasons for the sustaining of abnormal atmospheric response intrigued by SSTA. Oscillations of perturbations intrigued by warm wintertime SSTA in KE, which have similar frequencies with that of intrinsic atmospheric oscillations, are superposed on the atmospheric oscillations and propagate with primary periodic oscillation of the atmosphere. These SSTA-intrigued oscillations are coupled with natural atmospheric oscillation and finally become parts of it. This is probably another reason for the sustaining of abnormal atmospheric response to SSTA in KE.     

The role of atmospheric teleconnection in the subtropical thermal forcing on the equatorial pacific

The equatorial response to subtropical Pacific forcing was studied in a coupled climate model.The forcings in the western,central and eastern subtropical Pacific all caused a significant response in the equatorial thermocline,with comparable magnitudes.This work highlights the key role of air-sea coupling in the subtropical impact on the equatorial thermocline,instead of only the role of the ＂oceanic tunnel＂.The suggested mechanism is that the cyclonic （anticyclonic） circulation in the atmosphere caused by the subtropical surface warming （cooling） can generate an anomalous upwelling （downwelling） in the interior region.At the same time,an anomalous downwelling （upwelling） occurs at the equatorward flank of the forcing,which produces anomalous thermocline warming （cooling）,propagating equatorward and resulting in warming （cooling） in the equatorial thermocline.This is an indirect process that is much faster than the ＂oceanic tunnel＂ mechanism in the subtropical impact on the equator.     

ABRUPT CHANGE OF FLOW PATTERN IN BAROCLINIC ATMOSPHERE FORCED BY JOINT EFFECTS OF DIABATIC HEATING AND OROGRAPHY

Based on the catastrophic theory, the possible causes of abrupt change in the atmospheric circulation over the Northern Hemisphere during June and October have been explored by Li and Luo (1983) and Miao and Ding (1985). However these studies are confined to the barotropic atmosphere without consideration of orography. The purpose of this paper is to further study the physical mechanism of the abrupt change of flow pattern within the baroclinic atmosphere in the presence of orography. Results show that the abrupt change of flow pattern can be stimulated by the gradual variation of a diabatically heating parameter, which is similar to the observed fact about the rapid shift of position of the subtropical high center in the upper troposphere along the zonal direction during seasonal transition from the summer half year to the winter one.     

ANALYSIS ON EFFECT OF SOUTH ASIA HIGH ON MID-SUMMER EXTREME DROUGHT AND FLOOD IN SICHUAN-CHONGQING REGION

NCEP/NCAR data are utilized to analyze an extreme flood year （1998） and an extreme dry year （2006） in the Sichuan-Chongqing region （SCR） and the results are as follows. The positive divergence of South Asia High （SAH） is stronger in the flood year; the position of the ridge line of SAH is southward compared with the annual average; Western Pacific Subtropical High （WPSH） extends westward and its ridge line is southward. In the drought year, the positive divergence of SAH is weaker, its ridge line is northward, and the position of WPSH is also northward. As shown in the dynamics, in drought （flood） years, negative （positive） vorticity advection in the upper atmosphere can cause the atmosphere to ascend （descend）, and anomalous circulation of SAH displays divergence （convergence）, and anomalous circulation of the lower atmosphere shows convergence （divergence）. Thermal structure of the atmosphere shows that there is warm （cold） temperature advection in the lower atmosphere, and the vertical distribution of diabetic heating causes SAH＇s local circulation to display convergence （divergence） and affects vertical motion of the lower atmosphere circulation eventually. To some extent, the two extreme years in the SCR is closely related to the vertical motion of atmosphere circulation and the variation of such vertical motion is caused by differences of interactions between SAH and lower atmosphere circulations.     

Numerical Simulation of Regional Short-Range Climate Anomalies

With the high-speed development of numerical weather prediction, since the later 1980’s, the prediction of short-range climate anomalies has attracted worldwide meteorologists’ attention. What the so called short-range re-fers to the time scale from one month to one season or more. In dealing with the problem of short-range climate pre-diction, two points are needed noticing: one is the basic research to explore or investigate the mechanism of variability of the slow varying components which mainly include internal dynamics of extratropics, external forcings and tropical dynamics, and the other is the modeling efforts to simulate the process of the long-term evolution of the signal which include the improvement of model quality, stochastic prediction and the air-sea-coupled model (Miyakoda et al.,1986). Previous researches on the numerical prediction of short-term climate anomalies are mostly concentrated in the analysis of variables with global spatial scale, especially the global general atmospheric circulation analysis.As to the simulation or prediction of regional short-term climate anomalies, there exist many difficulties and problems. Though some meteorologists are devoting themself to this field, up to now, they have not reached satisfac-tory results. As a primary effort, by using the 2-level general atmospheric circulation model developed in the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP-AGCM) (Zeng et al., 1989), and taking the year of 1985 as a case, a numerical simulation of regional short-term climate change is completed. We pay high attention to the predictand of anomalous summer rainfall in the Yangtze River and Yellow River valleys, especially its month-to-month variation.     

Seasonal differences of model predictability and the impact of SST in the Pacific

Both seasonal potential predictability and the impact of SST in the Pacific on the forecast skill over China are investigated by using a 9-level global atmospheric general circulation model developed at the Institute of Atmospheric Physics under the Chinese Academy of Sciences (IAP9L-AGCM). For each year during 1970 to 1999, the ensemble consists of seven integrations started from consecutive observational daily atmospheric fields and forced by observational monthly SST. For boreal winter, spring and summer,the variance ratios of the SST-forced variability to the total variability and the differences in the spatial correlation coefficients of seasonal mean fields in special years versus normal years are computed respectively. It follows that there are slightly inter-seasonal differences in the model potential predictability in the Tropics. At northern middle and high latitudes, prediction skill is generally low in spring and relatively high either in summer for surface air temperature and middle and upper tropospheric geopotential height or in winter for wind and precipitation. In general, prediction skill rises notably in western China, especially in northwestern China, when SST anomalies (SSTA) in the Nino-3 region are significant. Moreover,particular attention should be paid to the SSTA in the North Pacific (NP) if one aims to predict summer climate over the eastern part of China, i.e., northeastern China, North China and southeastern China.     

CHARACTERISTICS OF ENVIRONMENTAL AND CLIMATE CHANGE IN CHANGJIANG DELTA AND ITS POSSIBLE MECHANISM*

Characteristics of climate change in the Changjiang Delta were analyzed based on the annual mean meteorological data since 1961,including air temperature,maximum and minimum air temperature,precipitation,sunshine duration and visibility at 48 stations in that area(southern Jiangsu and northern Zhejiang),and its adjacent areas(northern Jiangsu,eastern Anhui and southern Zhejiang),together with the environmental data.The results indicate that it is getting warmer in the Changjiang Delta and cooler in adjacent areas,thus the Changjiang Delta becomes a big heat island,containing many little heat islands consisting of central cities,in which Shanghai City is the strongest heat island.The intensity of heat islands enhances as economic development goes up.From the year 1978.the beginning year of reform and opening policy,to the year 1997,the intensity of big heat island of Changjiang Delta has increased 0.5℃ and Shanghai heat island increased 0.8℃.However.since 1978 the constituents of SO₂,NO_x and TSP(total suspended particles)in the atmosphere,no matter whether in the Changjiang Delta or in the adjacent areas,have all increased,but pH values of precipitation decreased.In the meantime,both sunshine duration and visibility are also decreased,indicating that there exists a mechanism for climate cooling in these areas.Our analyses show that the mechanism for climate warming in the Changjiang Delta may be associated with heating increase caused by,economic development and increasing energy consumption.It is estimated that up to 1997 the intensity of warming caused by this mechanism in the Changjiang Delta has reached 0.8-0.9℃,about 4-4.5 times as large as the mean values before 1978.Since then,the increase rate has become 0. 035℃/a for the Changjiang Delta.It has reached 1.3℃ for Shanghai in 1997,about 12-13 times as large as the mean values before 1978.This is a rough estimation of increasing energy consumption rate caused by economic development.     

Glacial-Interglacial Atmospheric CO2 Change—The Glacial Burial Hypothesis

Organic carbon buried under the great ice sheets of the Northern Hemisphere is suggested to be the missing link in the atmospheric CO2 change over the glacial-interglacial cycles. At glaciation, the advancement of continental ice sheets buries vegetation and soil carbon accumulated during warmer pe-riods. At deglaciation, this burial carbon is released back into the atmosphere. In a simulation over two glacial-interglacial cycles using a synchronously coupled atmosphere-land-ocean carbon model forced by reconstructed climate change, it is found that there is a 547-Gt terrestrial carbon release from glacial maximum to interglacial, resulting in a 60-Gt (about 30-ppmv) increase in the atmospheric CO2, with the remainder absorbed by the ocean in a scenario in which ocean acts as a passive buffer. This is in contrast to previous estimates of a land uptake at deglaciation. This carbon source originates from glacial burial,continental shelf, and other land areas in response to changes in ice cover, sea level, and climate. The input of light isotope enriched terrestrial carbon causes atmospheric δ^13C to drop by about 0.3‰ at deglaciation,followed by a rapid rise towards a high interglacial value in response to oceanic warming and regrowth on land. Together with other ocean based mechanisms such as change in ocean temperature, the glacial burial hypothesis may offer a full explanation of the observed 80-100-ppmv atmospheric CO2 change.     

Impact of SST Anomaly Events over the Kuroshio-Oyashio Extension on the “Summer Prediction Barrier”

The "summer prediction barrier"(SPB) of SST anomalies(SSTA) over the Kuroshio–Oyashio Extension(KOE) refers to the phenomenon that prediction errors of KOE-SSTA tend to increase rapidly during boreal summer, resulting in large prediction uncertainties. The fast error growth associated with the SPB occurs in the mature-to-decaying transition phase,which is usually during the August–September–October(ASO) season, of the KOE-SSTA events to be predicted. Thus, the role of KOE-SSTA evolutionary characteristics in the transition phase in inducing the SPB is explored by performing perfect model predictability experiments in a coupled model, indicating that the SSTA events with larger mature-to-decaying transition rates(Category-1) favor a greater possibility of yielding a more significant SPB than those events with smaller transition rates(Category-2). The KOE-SSTA events in Category-1 tend to have more significant anomalous Ekman pumping in their transition phase, resulting in larger prediction errors of vertical oceanic temperature advection associated with the SSTA events. Consequently, Category-1 events possess faster error growth and larger prediction errors. In addition, the anomalous Ekman upwelling(downwelling) in the ASO season also causes SSTA cooling(warming), accelerating the transition rates of warm(cold) KOE-SSTA events. Therefore, the SSTA transition rate and error growth rate are both related with the anomalous Ekman pumping of the SSTA events to be predicted in their transition phase. This may explain why the SSTA events transferring more rapidly from the mature to decaying phase tend to have a greater possibility of yielding a more significant SPB.     

A GLOBAL COUPLED OCEAN-ATMOSPHERE MODEL OF SHALLOW WATER WAVE AND ITS NUMERICAL EXPERIMENTS

A global coupled air-sea model of shallow water wave is developed based on coupled ocean-atmospheredynamics.The coupling is realized through the air-sea interaction process that the atmosphere acts on theocean by wind stress and the ocean acts on the atmosphere with heating proportional to sea surface temperature(SST)anomaly.The equation is harotropic primitive one.Response experiments of coupling system arealso carried out SSTA in two categories of intensities.Compared with the results of AGCM simulation ex-periment in which only the dynamic change of air system is considered,it demonstrates that the air-seainteraction between the tropical ocean and the global atmosphere plays a very important role in the evolutionof climate system.The results of numerical simulation show that it is encouraging.     

A MECHANISM FOR EFFECTS OF THE LARGE-SCALE MOTION AND HORIZONTAL INHOMOGENEITY OF THE MOIST STATIC INSTABILITY ON MCS AT MIDLATITUDES

Under the influence of the synoptic-scale environmental flows, the implicit nonthermal windvorticity imbalance with ξ_T>0 may arise when the change in the vertical shear vorticity of the flows,which is caused by the change of vorticity advection with altitude, is larger than that in the thermalwind vorticity of temperature fields, which is caused by the horizontal inhomogeneity of temperatureadvection. This kind of implicit imbalance will cause ascending motion to develop in the adjustmentprocess. If the static stratification is horizontally inhomogeneous with the presence of its mesoscalehigh-value regions or the conditionally unstable region where moisture can be fully provided, thedevelopment of mesoscale disturbance with scale L_m may result from the coupling of the ascendingmotion on synoptic scale L_1 with the parameter field of static stratification on mesoscale L_2. The disturbance possesses such a vertical structure as ascending motion in the center, horizontaldivergence in the upper and convergence in the lower atmosphere. This developmental process of insta-bility may be one of the most important processes responsible for the growing of heavy rains in summerhalf year in China.     

27.3-day and Average 13.6-day Periodic Oscillations in the Earth’s Rotation Rate and Atmospheric Pressure Fields Due to Celestial Gravitation Forcing

Variation in length of day of the Earth (LOD, equivalent to the Earth’s rotation rate) versus change in atmospheric geopotential height fields and astronomical parameters were analyzed for the years 1962-2006. This revealed that there is a 27.3-day and an average 13.6-day periodic oscillation in LOD and atmospheric pressure fields following lunar revolution around the Earth. Accompanying the alternating change in celestial gravitation forcing on the Earth and its atmosphere, the Earth’s LOD changes from minimum to maximum, then to minimum, and the atmospheric geopotential height fields in the tropics oscillate from low to high, then to low. The 27.3-day and average 13.6-day periodic atmospheric oscillation in the tropics is proposed to be a type of strong atmospheric tide, excited by celestial gravitation forcing. A formula for a Tidal Index was derived to estimate the strength of the celestial gravitation forcing, and a high degree of correlation was found between the Tidal Index determined by astronomical parameters, LOD, and atmospheric geopotential height. The reason for the atmospheric tide is periodic departure of the lunar orbit from the celestial equator during lunar revolution around the Earth. The alternating asymmetric change in celestial gravitation forcing on the Earth and its atmosphere produces a "modulation" to the change in the Earth’s LOD and atmospheric pressure fields.     

A new economic assessment index for the impact of climate change on grain yield

The impact of climate change on agriculture has received wide attention by the scientific community. This paper studies how to assess the grain yield impact of climate change, according to the climate change over a long time period in the future as predicted by a climate system model. The application of the concept of a traditional ＂yield impact of meteorological factor （YIMF）＂ or ＂yield impact of weather factor＂ to the grain yield assessment of a decadal or even a longer timescale would be suffocated at the outset because the YIMF is for studying the phenomenon on an interannual timescale, and it is difficult to distinguish between the trend caused by climate change and the one resulting from changes in non-climatic factors. Therefore, the concept of the yield impact of climatic change （YICC）, which is defined as the difference in the per unit area yields （PUAY） of a grain crop under a changing and an envisaged invariant climate conditions, is presented in this paper to assess the impact of global climate change on grain yields. The climatic factor has been introduced into the renowned economic Cobb-Douglas model, yielding a quantitative assessment method of YICC using real data. The method has been tested using the historical data of Northeast China, and the results show that it has an encouraging application outlook.     

ON THE RELATIONSHIP BETWEEN PRECIPITATION ANOMALIES IN THE FIRST RAINING SEASON (APRIL-JUNE) IN SOUTHERN CHINA AND SST OVER OFFSHORE WATERS IN CHINA

Precipitation anomalies in the first raining season of southern China were analyzed, with the suggestion that there are obvious interannual variation of peak values. In the raining season, the general tendency of precipitation is not obvious and the anomalous oscillation is multi-scale. Corresponding to years of more or less precipitation in the raining season, there are sharply opposite distribution across the nation in the simultaneous periods. In addition, by studying the distribution of correlation between anomalous precipitation in southern China in the first raining season and SSTA over offshore waters of China in the preceding period (June ~ August of the previous year), a sensitive zone of waters has been found that has steady effect on the precipitation of southern China in the season. Discussions are also made of the sensitive period, its simultaneous SSTA and subsequent anomalous circulation field in relation to precipitation anomalies and simultaneous circulation field in the first raining season of southern China. In the last part of the work, relationship between the SSTA in the sensitive zone and global SSTA is analyzed. A possible mechanism by which SSTA in offshore Chinese waters affects the precipitation anomalies in the first raining season of southern China is put forward.     

ENSO amplitude change in observation and coupled models

Observations show that the tropical E1 Nifio-Southern Oscillation （ENSO） variability, after removing both the long term trend and decadal change of the background climate, has been enhanced by as much as 60% during the past 50 years. This shift in ENSO amplitude can be related to mean state changes in global climate. Past global warming has caused a weakening of the Walker circulation over the equatorial Indo-Pacific oceans, as well as a weakening of the trade winds and a reduction in the equatorial upwelling. These changes in tropical climatology play as stabilizing factors of the tropical coupling system. However, the shallower and strengthening thermocline in the equatorial Pacific increases the SST sensitivity to thermocline and wind stress variabilities and tend to destabilize the tropical coupling system. Observations suggest that the destabilizing factors, such as the strengthening thermocline, may have overwhelmed the stabilizing effects of the atmosphere, and played a deterministic role in the enhanced ENSO variability, at least during the past half century. This is different from the recent assessment of IPCC-AR4 coupled models.     

“短期气候预测专刊”

A premonitory sign of an anomalous SST over the eastern equatorial Pacific shows up in the North Pacific Subtropical Mode Water (STMW) 18 months earlier,and the air-sea relationship between the STMW and the anomalous SST over the eastern equatorial Pacific is shown.This premonitory connection involves an air-sea coupling between the longtime persistent mid-latitude sea surface temperature anomaly (SSTA) induced by the remote re-emergence of the STMW and the following spring subtropical atmospheric circulation anomalies.An examination of the air-sea interaction reveals that the following spring subtropical atmospheric circulation,which responds to the longtime persistent SSTA,is dominated by the anomalous negative (positive) geopotential height downstream of the negative (positive) SSTA in the strong (weak) STMW case.Thus,the tropics adjust to these anomalies through coupled dynamics,producing positive (negative) SST anomalies over the eastern equatorial Pacific.A cold water event that occurred over the eastern equatorial Pacific during winter 2008-09 was successfully forecasted by the weak summer STMW index in 2007.The evolution of this process for the air-sea interactions from the autumn of 2007 to December 2008 is presented.     

ON THE PREDICTION OF TROPICAL CYCLONE LANDFALL AND INFLUENCE ON SOUTHERN CHINA USING MONTHLY OLR ANOMALIES FOR PRIME SUMMER

With the OLR data,the landfall and activity of tropical cyclones(TC) in southern China over a 20-year period(1975～1994) are studied.The result shows that the variation of the monthly anomalous OLR is somewhat teleconnected with the TC activity in southern China.The former is used to predict short-term climate for the latter over months with frequent or no TC influence.To some externt,the relationship between the TC activity in southern China and the monthly mean OLR anomalies is dependent on the climatological location of the subtropical high in northwestern Pacific region.