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1.
The simulation of precipitation in a general circulation model relying on relaxed mass flux cumulus parameterization scheme is sensitive to cloud adjustment time scale (CATS). In this study, the frequency of the dominant intra-seasonal mode and interannual variability of Indian summer monsoon rainfall (ISMR) simulated by an atmospheric general circulation model is shown to be sensitive to the CATS. It has been shown that a longer CATS of about 5 h simulates the spatial distribution of the ISMR better. El Niño Southern Oscillation–ISMR relationship is also sensitive to CATS. The equatorial Indian Ocean rainfall and ISMR coupling is sensitive to CATS. Our study suggests that a careful choice of CATS is necessary for adequate simulation of spatial pattern as well as interannual variation of Indian summer monsoon precipitation.  相似文献   

2.
The monthly mean sea surface temperature data of 6 areas are used to study the El Nino/Southern Oscillation signals in the global tropical ocean. These areas are in the 5oN-5oS latitude zone at 1) eastern Pacific (110o-l40oW), 2) western Atlantic (30o-50oW), 3) eastern Atlantic (10oW-10oE), 4) western Indian Ocean (30o-50oE), 5) central Indian Ocean (70o-90oE) and 6) far western Pacific (120o-140oE), and the data cover the 120-month period of December 1968 to November 1978.A power spectrum analysts shows that the characteristic time of the El Nino/Southern Oscillation (about 3-4 years) appears not only in the eastern Pacific but also in other areas of the tropics except for the western Pa-cific, where the spectrum is of white noise. The amplitude of oscillation in the eastern Pacific is about 4 times larger than the others, making the El Nino/Southern Oscillation signal the strongest in this area. According to a cross-spectrum analysis, there is no time lag between the variation in the central Indian Ocean and that in the eastern Pacific. These two areas oscillate simultaneously and comprise the main feature of the El Nino/ Southern Oscillation. Other tropical areas are related with time lags, as shown by correlation and coherence calculations.It should be noted that the sea surface temperature in the eastern Pacific oscillates in phase with that in the Indian Ocean, while the pressure oscillations in these two areas are out of phase with each other, according to the Southern Oscillation definition. It is suggested that the Southern Oscillation cannot be explained simply by the sea surface temperature anomalies.Variations in the far western equatorial Pacific do not have the time scale of the El Nino/Southern Oscilla-tion, perhaps because it is a buffer zone between the monsoon system and the trade wind system.  相似文献   

3.
Summary The global nature of the Madden-Julian Oscillations (MJOs) have been investigated by applying a frequency filter to daily data for the summer monsoon months (June to September) during two contrasting years—1987, a deficient monsoon year and 1988, an excess monsoon year. Several meteorological parameters at five levels in the troposphere have been examined. Regions with large amplitude of these oscillations are isolated for each year. The results indicate that the global spatial distribution of these oscillations is more in a deficient year than in an excess year, in particular over the Indian subcontinent and the EI Niño Southern Oscillation (ENSO) regions. The principal modes of variability during these two years have been investigated through Empirical Orthogonal Functions (EOFs). The first two eigenmodes of 850 hPa zonal wind explain nearly 50% of the variance. The dipole type of structure between the Indian and the Pacific region is more apparent in 1987 than in 1988. Time-longitude cross sections of the filtered zonal wind over the equatorial regions clearly show that eastward propagation is detected in 1987, but is virtually absent in 1988. It is also seen that the 30–60 day filtered winds are stronger during the monsoon of 1987 than in 1988.  相似文献   

4.
慢特征分析（SFA）方法可以从非平稳时间序列中提取出慢变的外强迫信息。近年来,SFA方法被应用于气候变化研究领域,用于探究气候变化的潜在驱动力及相关的动力学机制。本文基于SFA方法,提取全球陆地表面气温（LSAT）的慢变外强迫信息,研究全球LSAT慢变驱动力的空间结构特征及低频变率的主要驱动因子。SFA方法提取的LSAT慢变驱动力与历史时期全球辐射强迫（GRF）和全球海表温度（SST）的主模态（大西洋多年代际振荡AMO、热带太平洋ENSO变率和太平洋年代际振荡PDO）有显著的相关关系,表明全球大部分地区LSAT的变率受到GRF和三个SST模态的显著影响。GRF对LSAT变率的影响有全球一致性的特征,而三个SST模态对LSAT变率的影响则呈现出明显的区域特点。此外,由于SFA方法可以有效降低原始LSAT序列中随机噪声的干扰,GRF和SST模态对LSAT变率的解释方差显著提高,进一步表明GRF和SST模态是全球LSAT低频变率主要的驱动因子。最后,利用历史海温驱动AGCM试验（即AMIP试验）的结果,验证了三个SST模态对区域LSAT变率的显著影响。  相似文献   

5.
Summary Atmospheric variability in outgoing long-wave radiation (OLR) and tropospheric relative vorticity (VOR) over the South American region was studied from 1979 to 1996 using the complex Morlet wavelet function. The analyses focus on spatial variation in intraseasonal and submonthly scales. Scalograms were used to measure submonthly intraseasonal oscillations in convection, which were found to be predominant in the tropical regions. However, 7-day and 15-day oscillations were observed at tropical and extratropical latitudes in spring and winter, indicating that transient disturbances play a more prominent role. Regarding VOR, tropical energy intensities were highest in the spring and summer, whereas subtropical and extratropical energy intensities were highest in the autumn and winter. The dynamics of the 25-day and 45-day VOR oscillations indicates a possible correlation with Rossby waves over the eastern tropical Pacific Ocean, mainly during the summer. During winter, the 7-day and 15-day VOR oscillations are more frequent at higher latitudes and are enhanced along storm tracks. It was also observed that convection amplitudes in the regions of maximum intensity change appreciably from year to year and from season to season, showing that the behavior of the submonthly and intraseasonal oscillations is nonperiodic and correlates strongly with El Ni?o/Southern Oscillation years. These results confirm the efficiency of wavelet analysis for time-scale studies of atmospheric variability.  相似文献   

6.
The inverse relationship between the warm phase of the El Ni?o Southern Oscillation(ENSO) and the Indian Summer Monsoon Rainfall(ISMR) is well established. Yet, some El Ni?o events that occur in the early months of the year(boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Ni?o(springtime)...  相似文献   

7.
Among the key problems associated with the study of climate variability and its evolution are identification of the factors responsible for observed changes and quantification of their effects. Here, correlation and regression analysis are employed to detect the imprints of selected natural forcings (solar and volcanic activity) and anthropogenic influences (amounts of greenhouse gases—GHGs—and atmospheric aerosols), as well as prominent climatic oscillations (Southern Oscillation—SO, North Atlantic Oscillation—NAO, Atlantic Multidecadal Oscillation—AMO) in the Czech annual and monthly temperature and precipitation series for the 1866–2010 period. We show that the long-term evolution of Czech temperature change is dominated by the influence of an increasing concentration of anthropogenic GHGs (explaining most of the observed warming), combined with substantially lower, and generally statistically insignificant, contributions from the sulphate aerosols (mild cooling) and variations in solar activity (mild warming), but with no distinct imprint from major volcanic eruptions. A significant portion of the observed short-term temperature variability can be linked to the influence of NAO. The contributions from SO and AMO are substantially weaker in magnitude. Aside from NAO, no major influence from the explanatory variables was found in the precipitation series. Nonlinear forms of regression were used to test for nonlinear interactions between the predictors and temperature/precipitation; the nonlinearities disclosed were, however, very weak, or not detectable at all. In addition to the outcomes of the attribution analysis for the Czech series, results for European and global land temperatures are also shown and discussed.  相似文献   

8.
Climatic variability has profound effects on the distribution, abundance and catch of oceanic fish species around the world. The major modes of this climate variability include the El Niño-Southern Oscillation (ENSO) events, the Pacific Decadal Oscillation (PDO) also referred to as the Interdecadal Pacific Oscillation (IPO), the Indian Ocean Dipole (IOD), the Southern Annular Mode (SAM) and the North Atlantic Oscillation (NAO). Other modes of climate variability include the North Pacific Gyre Oscillation (NPGO), the Atlantic Multidecadal Oscillation (AMO) and the Arctic Oscillation (AO). ENSO events are the principle source of interannual global climate variability, centred in the ocean–atmosphere circulations of the tropical Pacific Ocean and operating on seasonal to interannual time scales. ENSO and the strength of its climate teleconnections are modulated on decadal timescales by the IPO. The time scale of the IOD is seasonal to interannual. The SAM in the mid to high latitudes of the Southern Hemisphere operates in the range of 50–60 days. A prominent teleconnection pattern throughout the year in the Northern Hemisphere is the North Atlantic Oscillation (NAO) which modulates the strength of the westerlies across the North Atlantic in winter, has an impact on the catches of marine fisheries. ENSO events affect the distribution of tuna species in the equatorial Pacific, especially skipjack tuna as well as the abundance and distribution of fish along the western coasts of the Americas. The IOD modulates the distribution of tuna populations and catches in the Indian Ocean, whilst the NAO affects cod stocks heavily exploited in the Atlantic Ocean. The SAM, and its effects on sea surface temperatures influence krill biomass and fisheries catches in the Southern Ocean. The response of oceanic fish stocks to these sources of climatic variability can be used as a guide to the likely effects of climate change on these valuable resources.  相似文献   

9.
Summary In this study the relationship between mid-tropospheric geopotential heights over the Northern Hemisphere (20° N to 90° N, around the globe) and Indian summer monsoon rainfall (ISMR: June to September total rainfall) have been examined. For this purpose, the monthly 500 hPa geopotential heights in a 2.5° lat./lon. grid over the Northern Hemisphere and the ISMR data for the period 1958 to 2003 have been used.The analysis demonstrates a dipole structure in the correlation pattern over the East Pacific Ocean in the month of January which intensifies in February and weakens in March.The average 500 hPa geopotential height over the eastern tropical Pacific Ocean during February (index one), has a significant positive relationship (r = 0.72) with the ISMR. In addition, the surface air temperature (SAT) anomaly over North-west Eurasia during January (index two) is found to be strongly related with the subsequent summer monsoon rainfall. These relationships are found to be consistent and robust during the period of analysis and these indices are found to be independent of each other.Hence, using index one and index two, a multiple linear regression model is developed for the prediction of the ISMR and the empirical relationships are verified on independent data. The forecast of the ISMR, using the above model, is found to be satisfactory.The dipole structure in the correlation pattern over the East Pacific region during February weakens once the ENSO (El-Nino and Southern Oscillation) events are excluded from the analysis. This suggests that the dipole type relationship between mid-tropospheric geopotential heights over the East Pacific Ocean and the ISMR may be a manifestation of the ENSO cycle.  相似文献   

10.
The Tahiti-Darwin Southern Oscillation index provided by Climate Analysis Center of USA has been used in numerous studies. But, it has some deficiency. It contains noise mainly due to high month-to-month variability. In order to reduce the level of noise in the SO index, this paper introduces a fully data-adaptive filter based on singular spectrum analysis. Another interesting aspect of the filter is that it can be used to fill data gaps of the SO index by an iterative process. Eventually, a noiseless long-period data series without any gaps is obtained.  相似文献   

11.
Summary The prediction of Indian Summer Monsoon Rainfall (ISMR) is vital for Indian economic policy and a challenge for meteorologists. It needs various predictors among which El Niño-Southern Oscillation (ENSO) is the most important. It has been established by various researchers that ENSO and ISMR relationship is weakening in recent years. It has been also argued that changes in ENSO-ISMR relationship may be due to decadal fluctuations, or it may be the indicative of longer-term trends related to anthropogenic-induced climate changes.In the present communication, an attempt is made to discuss the variability and predictability of ISMR in recent years. It is found that three different indices associated with different regions in the tropics and extra-tropics at different levels of the atmosphere-Asian land mass index represented by geopotential height at upper troposphere (A1), Caribbean-North Atlantic index represented by geopotential height at middle troposphere (A2) and tropical Pacific index at surface level (A3) – have different mechanisms to interact mutually and separately with ISMR in different periods. In recent years ISMR shows weak association with A1 and A3 while strong association with A2. Thus, if these three indices could be combined objectively, they can give rise to the predictability of ISMR. This objective combination is achieved here using Artificial Neural Network (ANN) and a model is developed to predict ISMR. This model has predicted reasonably well during the whole period of consideration (1958–2000) with a correlation coefficient of 0.92 in last 11 years (1990–2000) whereas most of the models fail to predict the variability in recent time.Current affiliation: Department of Physics, Federal University of Parana, Curitiba, Brazil.Received June 2002; revised October 1, 2002; accepted November 12, 2002 Published online: April 10, 2003  相似文献   

12.
The Interannual Variability of Climate in a Coupled Ocean-Atmosphere Model   总被引：2，自引：0，他引：2  
In this paper, the interannual variability simulated by the coupled ocean-atmosphere general circulation model of the Institute of Atmospheric Physics (IAP CGCM) in 40 year integrations is analyzed, and compared with that by the corresponding IAP AGCM which uses the climatic sea surface temperature as the boundary condition in 25 year integrations.The mean climatic states of January and July simulated by IAP CGCM are in good agreement with that by IAP AGCM, i.e., no serious ‘climate drift’ occurs in the CGCM simulation. A comparison of the results from AGCM and CGCM indicates that the standard deviation of the monthly averaged sea level pressure simulated by IAP CGCM is much greater than that by IAP AGCM in tropical region. In addition, both Southern Oscillation (SO) and North Atlantic Oscillation (NAO) can be found in the CGCM simulation for January, but these two oscillations do not exist in the AGCM simulation.The interannual variability of climate may be classified into two types: one is the variation of the annual mean, another is the variation of the annual amplitude. The ocean-atmosphere interaction mainly increases the first type of variability. By means of the rotated EOF, the most important patterns corresponding to the two types of interannual variability are found to have different spatial and temporal characteristics.  相似文献   

13.
Indian Summer Monsoon Rainfall(ISMR)exhibits a prominent inter-annual variability known as troposphere biennial oscillation.A season of deficient June to September monsoon rainfall in India is followed by warm sea surface temperature(SST)anomalies over the tropical Indian Ocean and cold SST anomalies over the western Pacific Ocean.These anomalies persist until the following monsoon,which yields normal or excessive rainfall.Monsoon rainfall in India has shown decadal variability in the form of 30 year epochs of alternately occurring frequent and infrequent drought monsoons since1841,when rainfall measurements began in India.Decadal oscillations of monsoon rainfall and the well known decadal oscillations in SSTs of the Atlantic and Pacific oceans have the same period of approximately 60 years and nearly the same temporal phase.In both of these variabilities,anomalies in monsoon heat source,such as deep convection,and middle latitude westerlies of the upper troposphere over south Asia have prominent roles.  相似文献   

14.
Our analyses partition the relative influence of progressive climate change and large-scale climate drivers that can be associated with the Quasi-Biennial Oscillation (QBO), El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), solar sunspot cycle, and multi-decadal oscillations on lake ice breakup dates for thirteen Northern Hemisphere lakes. Oscillatory dynamics explain 26 % of the total variance in the time series compared with 15 % for linear trends, leaving 60 % unexplained and likely attributable, in part, to local weather. Significant oscillatory dynamics include frequencies in 2–3 year periods (9.4 % of the total variance), 3–6 year periods (8.2 %), 10–12 year periods (1.6 %) and various multidecadal periods (0.4–1.3 %). All 13 study lakes, although widely scattered in the Northern Hemisphere, had similar oscillatory dynamics and linear trends, emphasizing that global processes influence lake ice breakup locally. We illustrate that while quasi-periodic dynamics associated with large-scale climate drivers are important, they do not mask the clear evidence for progressive climate change.  相似文献   

15.
The role of El Niño/Southern Oscillation (ENSO) and the mechanism through which ENSO influences the precipitation variability over northwest India and the adjoining (NWIA) region is well documented. In this study, the relative role of North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) and ENSO in modulating the Asian jet stream in the Northern Hemisphere winter and their relative impact on the precipitation variability over the region have been estimated through analysis of observed data. It is seen that interannual variations of NWIA precipitation are largely influenced by ENSO. An empirical orthogonal function (EOF) analysis has been carried out to understand dominant modes of interannual variability of zonal wind at 200 hPa of the Northern Hemisphere. The EOF-1 pattern in the tropical region is similar to that of an ENSO pattern, and the principal component (PC) time series corresponds to the ENSO time series. The EOF-2 spatial pattern resembles that of NAO/AO with correlation of PC time series with AO and NAO being 0.74 and 0.62, respectively. The precipitation anomaly time series over the region of interest has marginally higher correlation with the PC-2 time series as compared to that of PC-1. Regression analysis of precipitation and circulation parameters indicates a larger contribution of the second mode to variability of winds and precipitation over the NWIA. Moisture transport from the Arabian Sea during the active phase of NAO/AO and the presence of a cyclonic anomaly lead to higher precipitation over the NWIA region.  相似文献   

16.
应用奇异谱分析(SSA)方法,对全球及南北半球近100多年(1856～1997年)逐月地面气温距平序列的年际变化准周期性进行诊断分析,结果表明,全球平均气温序列中以准5～6年和准4年周期振荡最显著,其次是准两年周期振荡.各种准周期振荡年代际演变特征及其变率的阶段性,不但表现在振幅上,而且其波数亦很明显.上述特征在全球、南北半球都各有明显的差异.奇异交叉谱分析(SCSA)表明,全球平均地面气温的年际振荡与气候系统中其他各子系统所隐含的准周期信号具有各种耦合关系,尤其表现在与Nino区海温或南方涛动指数中的准周期信号的耦合关系上.  相似文献   

17.
This study has identified probable factors that govern ISMR predictability. Furthermore, extensive analysis has been performed to evaluate factors leading to the predictability aspect of Indian Summer Monsoon Rainfall (ISMR) using uncoupled and coupled version of National Centers for Environmental Prediction Coupled Forecast System (CFS). It has been found that the coupled version (CFS) has outperformed the uncoupled version [Global Forecast System (GFS)] of the model in terms of prediction of rainfall over Indian land points. Even the spatial distribution of rainfall is much better represented in the CFS as compared to that of GFS. Even though these model skills are inadequate for the reliable forecasting of monsoon, it imparts the capacious knowledge about the model fidelity. The mean monsoon features and its evolution in terms of rainfall and large-scale circulation along with the zonal and meridional shear of winds, which govern the strength of the monsoon, are relatively closer to the observation in the CFS as compared to the GFS. Furthermore, sea surface temperature–rainfall relation is fairly realistic and intense in the coupled version of the model (CFS). It is found that the CFS is able to capture El Niño Southern Oscillation ISMR (ENSO-ISMR) teleconnections much strongly as compared to GFS; however, in the case of Indian Ocean Dipole ISMR teleconnections, GFS has the larger say. Coupled models have to be fine-tuned for the prediction of the transition of El Niño as well as the strength of the mature phase has to be improved. Thus, to sum up, CFS tends to have better predictive skill on account of following three factors: (a) better ability to replicate mean features, (b) comparatively better representation of air–sea interactions, and (c) much better portrayal of ENSO-ISMR teleconnections. This study clearly brings out that coupled model is the only way forward for improving the ISMR prediction skill. However, coupled model’s spurious representation of SST variability and mean model bias are detrimental in seasonal prediction.  相似文献   

18.
High-frequency atmospheric variability depends on the phase of El Nino/Southern Oscillation (ENSO). Recently, there is increasing evidence that state-dependent high-frequency atmospheric variability significantly modulates ENSO characteristics. Hence, in this study, we examine the model simulations of high-frequency atmospheric variability and, further, its dependency on the El Nino phase, using atmospheric and coupled GCMs (AGCM and CGCM). We use two versions of physical packages here—with and without convective momentum transport (CMT)—in both models. We found that the CMT simulation gives rise to a large climatological zonal wind difference over the Pacific. Also, both the climate models show a significantly improved performance in simulating the state-dependent noise when the CMT parameterization is implemented. We demonstrate that the better simulation of the state-dependent noise results from a better representation of anomalous, as well as climatological, zonal wind. Our further comparisons between the simulations, demonstrates that low-frequency wind is a crucial factor in determining the state-dependency of high-frequency wind variability. Therefore, it is suggested that the so-called state-dependent noise is directly induced by the low-frequency wind anomaly, which is caused by SST associated with ENSO.  相似文献   

19.
We reconstructed decadal to centennial variability of maximum sea ice extent in the Western Nordic Seas for A.D. 1200–1997 using a combination of a regional tree-ring chronology from the timberline area in Fennoscandia and δ18O from the Lomonosovfonna ice core in Svalbard. The reconstruction successfully explained 59% of the variance in sea ice extent based on the calibration period 1864–1997. The significance of the reconstruction statistics (reduction of error, coefficient of efficiency) is computed for the first time against a realistic noise background. The twentieth century sustained the lowest sea ice extent values since A.D. 1200: low sea ice extent also occurred before (mid-seventeenth and mid-eighteenth centuries, early fifteenth and late thirteenth centuries), but these periods were in no case as persistent as in the twentieth century. Largest sea ice extent values occurred from the seventeenth to the nineteenth centuries, during the Little Ice Age (LIA), with relatively smaller sea ice-covered area during the sixteenth century. Moderate sea ice extent occurred during thirteenth–fifteenth centuries. Reconstructed sea ice extent variability is dominated by decadal oscillations, frequently associated with decadal components of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO), and multi-decadal lower frequency oscillations operating at ~50–120 year. Sea ice extent and NAO showed a non-stationary relationship during the observational period. The present low sea ice extent is unique over the last 800 years, and results from a decline started in late-nineteenth century after the LIA.  相似文献   

20.
Several observational and modeling studies indicate that the Indian summer monsoon rainfall (ISMR) is inversely related to the Eurasian snow extent and depth. The other two important surface boundary conditions which influence the ISMR are the Pacific sea surface temperature (SST) to a large extent and the Indian Ocean SST to some extent. In the present study, observed Soviet snow depth data and Indian rainfall data for the period 1951–1994 have been statistically analyzed and results show that 57% of heavy snow events and 24% of light snow events over west Eurasia are followed by deficient and excess ISMR respectively. Out of all the extreme monsoon years, care has been taken to identify those when Eurasian snow was the most dominant surface forcing to influence ISMR. During the years of high(low) Eurasian snow amounts in spring/winter followed by deficient(excess) ISMR, atmospheric fields such as temperature, wind, geopotential height, velocity potential and stream function based on NCEP/NCAR reanalyses have been examined in detail to study the influence of Eurasian snow on the midlatitude circulation regime and hence on the monsoon circulation. Results show that because of the west Eurasian snow anomalies, the midlatitude circulations in winter through spring show significant changes in the upper and lower level wind, geopotential height, velocity potential and stream function fields. Such changes in the large-scale circulation pattern may be interpreted as precursors to weak/strong monsoon circulation and deficient/excess ISMR. The upper level velocity potential difference fields between the high and low snow years indicate that with the advent of spring, the winter anomalous convergence over the Indian region gradually becomes weaker and gives way to anomalous divergence that persists through the summer monsoon season. Also the upper level anomalous divergence centre shifts from over the Northern Hemisphere and equator to the Southern Hemisphere over the Indian Ocean and Australia.  相似文献   

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