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1.
An operational weather forecast model, coupled to an oceanic model, was used to predict the initiation and propagation of two major Madden–Julian Oscillation(MJO) events during the dynamics of the MJO(DYNAMO)campaign period. Two convective parameterization schemes were used to understand the sensitivity of the forecast to the model cumulus scheme. The first is the Tiedtke(TDK) scheme, and the second is the Simplified Arakawa–Schubert(SAS) scheme. The TDK scheme was able to forecast the MJO-1 and MJO-2 initiation at 15-and45-day lead, respectively, while the SAS scheme failed to predict the convection onset in the western equatorial Indian Ocean(WEIO). The diagnosis of the forecast results indicates that the successful prediction with the TDK scheme is attributed to the model capability to reproduce the observed intraseasonal outgoing longwave radiation–sea surface temperature(OLR–SST) relationship. On one hand, the SST anomaly(SSTA) over the WEIO was induced by surface heat flux anomalies associated with the preceding suppressed-phase MJO. The change of SSTA, in turn,caused boundary layer convergence and ascending motion, which further induced a positive column-integrated moist static energy(MSE) tendency, setting up a convectively unstable stratification for MJO initiation. The forecast with the SAS scheme failed to reproduce the observed OLR–SST–MSE relation. The propagation characteristics differed markedly between the two forecasts. Pronounced eastward phase propagation in the TDK scheme is attributed to a positive zonal gradient of the MSE tendency relative to the MJO center, similar to the observed, whereas a reversed gradient appeared in the forecast with the SAS scheme with dominant westward propagation. The difference is primarily attributed to anomalous vertical and horizontal MSE advection.  相似文献   

2.
The Madden-Julian oscillation （MJO） is a dominant atmospheric low-frequency mode in the tropics. In this review article, recent progress in understanding the MJO dynamics is described. Firstly, the fundamental physical processes responsible for MJO eastward phase propagation are discussed. Next, a recent modeling result to address why MJO prefers a planetary zonal scale is presented. The effect of the seasonal mean state on distinctive propagation characteristics between northern winter and summer is discussed in a theoretical framework. Then, the observed precursor signals and the physical mechanism of MJO initiation in the western equatorial Indian Ocean are further discussed. Finally, scale interactions between MJO and higher- frequency eddies are delineated.  相似文献   

3.
The Madden–Julian Oscillation(MJO) has a significant impact on global weather and climate and can be used as a predictability resource in extended-term forecasting. We evaluate the ability of the Chinese Academy of Meteorological Sciences Climate System Model(CAMS-CSM) to represent the MJO by using the diagnostic method proposed by the US Climate Variability and Predictability Program(CLIVAR) MJO Working Group(MJOWG). In general,the model simulates some major characteristics of MJO well, such as the seasonality characteristics and geographical dependence, the intensity of intraseasonal variability(ISV), dominant periodicity, propagation characteristics, coherence between outgoing longwave radiation(OLR) and wind, and life cycle of MJO signals. However, there are a few biases in the model when compared with observational/reanalyzed data. These include an overestimate of precipitation in the convergence zone of the North and South Pacific, a slightly weaker eastward propagation, and a shift in the dominant periodicity toward lower frequencies with slower speeds of eastward propagation. The model gives a poor simulation of the northward propagation of MJO in summer and shows less coherence between the MJO convection and wind. The role of moistening in the planetary boundary layer(PBL) in the eastward/northward propagation of MJO was also explored. An accurate representation of the vertical titling structure of moisture anomalies in CAMS-CSM leads to moistening of the PBL ahead of convection, which accounts for the eastward/northward propagation of MJO. Poor simulation of the vertical structure of the wind and moisture anomalies in the western Pacific leads to a poor simulation of the northward propagation of MJO in this area. Budget analysis of the PBL integral moisture anomalies shows that the model gives a good simulation of the moisture charging process ahead of MJO convection and that the zonal advection of moisture convergence term has a primary role in the detour of MJO over the Maritime Continent.  相似文献   

4.
The first two series(RMM1 and RMM2) of RMM Index(all-Season Real-time Multivariate MJO Index) are computed to obtain the interannual variation of the preceding winter(preceding December to current February) MJO strength,according to which active(or inactive) years of preceding winter MJO are divided.By utilizing the data provided by NCEP/NCAR,CMAP and China’s 160 stations from 1979 to 2008,we studied the preceding winter MJO strength and discovered that the summer precipitation in the basin are of significantly negative correlation,i.e.when the preceding winter MJO is relatively active,the summer precipitation in the basin decreases,and vise verse.We also analyzed the causes.When the preceding winter MJO is relatively active,its release of potential heat facilities Inter-Tropical Convergence Zone(ITCZ) to strengthen and locate northward in winter and propagate northeastward.This abnormal situation lasts from winter to summer.In mid-May,ITCZ jumps northward to the South China Sea,the western Pacific subtropical high withdraws eastward,and the South China Sea summer monsoon sets off and strengthens.In summer,ITCZ propagates to South China Sea-subtropical western Pacific,the zonal circulation of subtropical Pacific strengthens,and a local meridional circulation of the South China Sea to the basin area forms,giving rise to the East Asia Pacific teleconnection wave-train.An East Asian monsoon trough and the Meiyu front show opposite features from south to north,the East Asian summer monsoon strengthens and advances northward.As a result,the summer monsoon is weakened as the basin is controlled by the subtropical high continually,with less rain in summer.On the contrary,when the preceding winter MJO is inactive,ITCZ weakens and is located southward,the subtropical high is located southward in summer,and the basin is in a region of ascending airflow with prevailing southwest wind.The East Asian monsoon trough and EASM weaken so that summer monsoon is reduced in the basin where precipitation increases.  相似文献   

5.
Observed outgoing longwave radiation (OLR) and ERA-Interim reanalysis data were analyzed to reveal the initiation processes associated with a successive and a primary MJO event during 2000-2001. It was found that the initiation of the successive event was caused by anomalous ascending motion induced by low-level horizontal temperature advection. The anomalous ascending motion, together with horizontal moisture advection, moistened lower troposphere and led to an unstable stratification and triggered convection. The initiation of the primary MJO event, on the other hand, was caused by the accumulation of anomalous moisture associated with three low-frequency modes, a convectively coupled Kelvin wave (CCKW), an westward-propagating equatorial Rossby wave (ER) and a weak planetary-scale MJO mode. It is the merging of the low-level specific humidity anomalies of the three modes that led to the rapid setup of large-scale convectively unstable stratification and favored the development of the eastward-propagating planetary-scale MJO mode.  相似文献   

6.
This paper investigates the contrasts between strong and weak Madden-Julian Oscillation (MJO) activity over the equatorial western Pacific during winter using the NCEP reanalysis data. It is shown that the MJO over the equatorial western Pacific in winter shows significant interannual and interdecadal variabilities. During the winters with strong MJO activity, an anomalous cyclonic circulation lies east of the Philippines, strong anomalous easterlies control the equatorial eastern Pacific, and anomalous westerlies extend from the Indian Ocean to the western Pacific in the lower troposphere, which strengthens the convergence and convection over the equatorial western Pacific. The moisture convergence in the lower troposphere is also enhanced over the western Pacific, which is favorable to the activity of MJO. Eastward propagation is a significant feature of the MJO, though there is some westward propagation. The space-time spectral power and center period of the MJO are higher during strong MJO activity winters. The anomalous activity of MJO is closely related to the sea surface temperature (SST) and East Asian winter monsoon (EAWM). During strong MJO activity winters, there are positive/negative anomalies at high/low latitudes in both sea level pressure and 500 hPa geopotential height, and the temperature is lower over the central part of the Chinese mainland, which indicates a strong EAWM. China experiences more rainfall between the Yellow and Yangtze Rivers, but less rainfall south of the Yangtze River. The SSTA is negative near the Taiwan Island due to the impact of strong EAWM and shows a La Ni?a pattern anomaly over the eastern Pacific. During the weak MJO activity winters, the situation is reversed.  相似文献   

7.
Taking into account moisture in virtue of general potential temperature,the author derive a three-dimensional（3D） pseudomomentum wave-activity relation for the moist atmosphere from the primitive equations in Cartesian coordinates using the Momentum-Casimir method.Since the wave-activity relation is constructed in an ageostrophic and non-hydrostatic dynamical framework,it may be applicable to diagnosing the evolution and propagation of mesoscale systems leading to heavy rainfall.The theoretical analysis shows that,besides the local change of wave-activity flux divergence and source or sink,the wave-activity relation includes two additional forcing terms.The first is the zonal gradient of difference between general potential temperature and potential temperature perturbations,and the second is the covariance of the solenoid and gradient of water vapor,denoting the direct influence of moisture on wave-activity density.The wave-activity density was applied to a heavy precipitation event occurring in the Jianghuai region of China.The calculation showed that the wave-activity density was consistent with 6-h accumulated precipitation observations,in terms of both spatial distribution and temporal tendency.This suggested that the disturbance represented by wave-activity density was closely related to the heavy precipitation.Although the wave-activity flux divergence and the covariance of the solenoid and gradient of water vapor made the primary contribution to the local change of wave-activity density,the covariance was more remarkable.The zonal gradient of difference between general potential temperature and potential temperature perturbations made a weaker contribution to the waveactivity density.  相似文献   

8.
Impacts of initial conditions on cloud-resolving model simulations are investigated using a series of sensitivity experiments. Five experiments with perturbed initial temperature, moisture, and cloud conditions are conducted and compared to the control experiment. The model is forced by the large-scale vertical velocity and zonal wind observed and derived from NCEP/Global Data Assimilation System （GDAS）. The results indicate that model predictions of rainfall are much more sensitive to the initial conditions than those of temperature and moisture. Further analyses of the surface rainfall equation and the moisture and cloud hydrometeor budgets reveal that the calculations of vapor condensation and deposition rates in the model account for the large sensitivities in rainfall simulations.  相似文献   

9.
A most striking summer–winter difference of evolution of the intraseasonal oscillation(ISO) over the equatorial Indian Ocean is a quasi-stationary oscillation in boreal summer but eastward propagation in boreal winter. This feature is consistent with the observational fact that maximum ISO variance appears only in the eastern Indian Ocean in boreal summer while it appears across the entire basin in boreal winter. The cause of the distinctive propagation and initiation characteristics is investigated through the diagnosis of observational and reanalysis data for the period of 1982–2012. It is found that when the ISO convection appears over eastern Indian Ocean, a positive(negative) moisture tendency appears to the east of the convection in boreal winter(summer). It is the moisture tendency difference that is responsible for different propagation behavior in the summer and winter. A further diagnosis of the moisture budget indicates that the major difference lies in anomalous moisture advection by the mean flow. In addition, air–sea interaction also plays a role. While boreal winter ISO starts over western Indian Ocean, boreal summer ISO is initiated over central–eastern equatorial Indian Ocean, due to boundary layer moistening. The moisture increase is caused primarily by the horizontal advection of mean specific humidity by anomalous easterlies induced by preceding suppressed-phase ISO over eastern Indian Ocean. Besides, a delayed SST feedback also plays a role. The overall difference of ISO evolution between the summer and winter is regulated by the seasonal mean state including the mean SST and water vapor content.  相似文献   

10.
A latent heating peak in the PBL was detected in a simulation by a global GCM that failed to reproduce Madden-Julian Oscillation(MJO).The latent heating peak in the PBL was generated by very shallow convection,which prevented moisture from being transported to the free troposphere.Large amount of moisture was therefore confined to the PBL,leading to a dry bias in the free atmosphere.Suffering from this dry bias,deep convection became lethargic,and MJO signals failed to occur.When the latent heating peak in the PBL was removed in another simulation,reasonable MJO signals,including the eastward propagation and the structure of its large-scale circulation,appeared.We therefore propose that the excessive latent heating peak in the PBL due to hyperactive shallow convection may be a reason for a lack of MJO signals in some simulations by other GCMs as well.  相似文献   

11.
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12.
Possible relationships between MJO and the severe rain-snow weather in Eastern China during November of 2009 are analyzed and results show that a strong MJO process is one of the strong impact factors.MJO is very active over the Indian Ocean in November 2009.Especially,it maintains 9 days in MJO phase 3,just corresponding to the two strongest rain-snow processes.Composites of MJO events show that when the MJO convective center is located over the Indian Ocean,the probability of rainfall is significantly increased and the temperature is lower than normal in eastern China,which is consistent with the situation in November of 2009.Atmospheric circulation anomalies of mid-and higher-latitudes can be influenced by the tropical MJO convection forcing and this influence could be realized by teleconnection.When the MJO is over the Indian Ocean,it is favorable for the maintenance of a circulation pattern of two ridges versus one trough at mid-and higher-latitudes.Meanwhile,the western Pacific subtropical high is stronger and more westward than normal,and a significant convective belt appears over eastern East Asia.All these circulation anomalies shown in the composite result also appeared in the observations in November 2009,which indicates the general features of relationships between the MJO and the circulation anomalies over the extratropics.Besides the zonal circulation anomalies,the MJO convection can also lead to meridional circulation anomalies.When the MJO convection is located over the Indian Ocean,the western Pacific is dominated by anomalous descending motion,and the eastern East Asia is controlled by strong convergence and ascending motion.Therefore,an anomalous meridional circulation is formed between the tropics and middle latitudes,enhancing the northward transportation of low-level moisture.It is potentially helpful to understanding and even forecasting such kind of rain-snow weather anomalies as that in November 2009 using MJO.  相似文献   

13.
The phenomenon of ENSO asymmetry has been recognized for many years, but most studies have focused on the asymmetry of surface temperature anomalies in the equatorial eastern Pacific. Here, the authors investigate the temperature asymmetry associated with ENSO in the subsurface of the western Pacific through analysis of observations and numerical experiments with an ocean GCM. Both the observation and simulation exhibit significant ENSO asymmetry, characterized by negative temperature skewness in the equatorial western Pacific and positive skewness in the eastern Pacific. Heat budget analysis reveals that nonlinear dynamical heating results in the positive temperature asymmetry in the equatorial eastern Pacific, but tends to weaken the negative temperature asymmetry in the equatorial western Pacific. The climatological meridional current transports the temperature anomalies and corresponding negative asymmetry from the off-equator region to the equator in the subsurface of the western Pacific. Through a sensitivity experiment with reversed wind stress forcing, the authors suggest that the skewness of the wind stress anomalies does not contribute to the negative temperature asymmetry in the western Pacific in the first-order approximation, while the internal nonlinear dynamics does play a key role. The study suggests that, as a result of nonlinear processes, the oceanic responses to anomalous wind stress are nonlinear and asymmetric in the tropical Pacific.  相似文献   

14.
The modulation of twin tropical cyclogenesis in the Indian-western Pacific Oceans by the Madden-Julian Oscillation (MJO) during the onset period of 1997/98 ENSO is explored for the period of September 1996 to June 1997 based on daily OLR, NCEP/NCAR wind vector, and JTWC best track datasets. The MJO westerly wind burst associated with its eastward propagation can result in a series of tropical cyclogeneses in a multi-day interval. Only in the transition seasons are pairs of tropical cyclones observed in both the tropical sectors of the Indian-western Pacific Oceans. Two remarkable twin tropical cyclogeneses probably modulated by the MJO westerly wind burst are found: one is observed in the Indian Ocean in the middle of October 1996, and the other is observed in the Western Pacific Ocean in late May 1997. The twin tropical cyclogenesis in mid-October 1996 is observed when the super cloud cluster separates into two isolated clusters by the enhanced westerly wind, which is accompanied by two independent vortices in the equatorial tropical sectors. The other one, in late-May 1997, however, is characterized by one cyclonic flow that later results in another cyclonic cell in its opposite equatorial sector. Thus, there are two very important conditions for twin cyclogenesis: one is the MJO westerly wind straddling the equator, and the other is the integral super cloud cluster, which later splits into two cloud convective clusters with independent vortices.  相似文献   

15.
The sensitivity of the simulated tropical intraseasonal oscillation or MJO （Madden and Julian oscillation） to different cumulus parameterizations is studied by using an atmospheric general circulation model （GCM）--SAMIL （Spectral Atmospheric Model of IAP LASG）. Results show that performance of the model in simulating the MJO alters widely when using two different cumulus parameterization schemes-the moist convective adjustment scheme （MCA） and the Zhang-McFarlane （ZM） scheme. MJO simulated by the MCA scheme was found to be more realistic than that simulated by the ZM scheme. MJO produced by the ZM scheme is too weak and shows little propagation characteristics. Weak moisture convergence at low levels simulated by the ZM scheme is not enough to maintain the structure and the eastward propagation of the oscillation. These two cumulus schemes produced different vertical structures of the heating profile. The heating profile produced by the ZM scheme is nearly uniform with height and the heating is too weak compared to that produced by the MCA, which maybe contributes greatly to the failure of simulating a reasonable MJO. Comparing the simulated MJO by these two schemes indicate that the MJO simulated by the GCM is highly sensitive to cumulus parameterizations implanted in. The diabatic heating profile plays an important role in the performance of the GCM. Three sensitivity experiments with different heating profiles are designed in which modified heating profiles peak respectively in the upper troposphere （UH）, middle troposphere （MH）, and lower troposphere （LH）. Both the LH run and the MH run produce eastward propagating signals on the intraseasonal timescale, while it is interesting that the intraseasonal timescale signals produced by the UH run propagate westward. It indicates that a realistic intraseasonal oscillation is more prone to be excited when the maximum heating concentrates in the middle-low levels, especially in the middle levels, while westward propagating disturbances  相似文献   

16.
Effect of the spatial distributions of chlorophyll-a concentration on upper ocean temperature and currents in the equatorial Pacific is investigated through a set of numerical experiments by using an ocean general circulation model. This study indicates that enhanced meridional gradient of chlorophyll-a between the equator and off-equatorial regions can strengthen zonal circulation and lead to a decrease in equatorial sea surface temperature （SST）. However, the circulation changes by themselves are not effective enough to affect SST in the equatorial cold tongue （CT） region. The comparison between the experiments indicates that the CT SST are more sensitive to chlorophyll-a distribution away from the equator. The off-equatorial chlorophyll-a traps more solar radiation in the mixed layer, therefore, the temperature in the thermoeline decreases. The cold water can then be transported to the equator by the meridional circulation within the mixed layer. Furthermore, the relation among CT SST, the surface heat flux, and the equatorial upwelling are discussed. The study implies the simulation biases of temperature on the equator are not only related to the local ocean dynamics but also related to some deficiency in simulating off-equatorial processes.  相似文献   

17.
A coupled earth system model(ESM) has been developed at the Nanjing University of Information Science and Technology(NUIST) by using version 5.3 of the European Centre Hamburg Model(ECHAM), version 3.4 of the Nucleus for European Modelling of the Ocean(NEMO), and version 4.1 of the Los Alamos sea ice model(CICE). The model is referred to as NUIST ESM1(NESM1). Comprehensive and quantitative metrics are used to assess the model's major modes of climate variability most relevant to subseasonal-to-interannual climate prediction. The model's assessment is placed in a multi-model framework. The model yields a realistic annual mean and annual cycle of equatorial SST, and a reasonably realistic precipitation climatology, but has difficulty in capturing the spring–fall asymmetry and monsoon precipitation domains. The ENSO mode is reproduced well with respect to its spatial structure, power spectrum, phase locking to the annual cycle, and spatial structures of the central Pacific(CP)-ENSO and eastern Pacific(EP)-ENSO; however, the equatorial SST variability,biennial component of ENSO, and the amplitude of CP-ENSO are overestimated. The model captures realistic intraseasonal variability patterns, the vertical-zonal structures of the first two leading predictable modes of Madden–Julian Oscillation(MJO), and its eastward propagation; but the simulated MJO speed is significantly slower than observed. Compared with the T42 version, the high resolution version(T159) demonstrates improved simulation with respect to the climatology, interannual variance, monsoon–ENSO lead–lag correlation, spatial structures of the leading mode of the Asian–Australian monsoon rainfall variability, and the eastward propagation of the MJO.  相似文献   

18.
The ECHAM5 model is coupled with the widely used Common Land Model(CoLM). ECHAM5 is a state-of-theart atmospheric general circulation model incorporated into the integrated weather and climate model of the Chinese Academy of Meteorological Sciences(CAMS-CSM). Land surface schemes in ECHAM5 are simple and do not provide an adequate representation of the vegetation canopy and snow/frozen soil processes. Two AMIP(Atmospheric Model Intercomparison Project)-type experiments using ECHAM5 and ECHAM5-CoLM are run over 30 yr and the results are compared with reanalysis and observational data. It is found that the pattern of land surface temperature simulated by ECHAM5-CoLM is significantly improved relative to ECHAM5. Specifically, the cold bias over Eurasia is removed and the root-mean-square error is reduced in most regions. The seasonal variation in the zonal mean land surface temperature and the in situ soil temperature at 20-and 80-cm depths are both better simulated by ECHAM5-CoLM. ECHAM5-CoLM produces a more reasonable spatial pattern in the soil moisture content, whereas ECHAM5 predicts much drier soils. The seasonal cycle of soil moisture content from ECHAM5-CoLM is a better match to the observational data in six specific regions. ECHAM5-CoLM reproduces the observed spatial patterns of both sensible and latent heat fluxes. The strong positive bias in precipitation over land is reduced in ECHAM5-CoLM, especially over the southern Tibetan Plateau and middle–lower reaches of the Yangtze River during the summer monsoon rainy season.  相似文献   

19.
Using 2 high-speed cameras, we have recorded 14 negative cloud-to-ground (CG) lightning flashes, half of which are natural and the others are artificially triggered. The two-dimensional (2D) propagation speed of different type leaders and the luminosity of lightning channel are analyzed in detail. Bidirectional leader processes are observed during the initial processes of two altitude triggered negative lightning (ATNL)flashes. The analysis shows: the propagation speed of the upward positive leader (UPL) before the initiation of the downward negative leader (DNL) is at the order of 104-105 m s-1; the UPL can be intensified by the initiation and development of the DNL in the way that the luminosity is enhanced and the speed is sped up; after initiation, the DNL in one ATNL flash propagates downward three times intermittently with interval of about 1 ms, while that in the other ATNL flash propagates downward continuously with a speed at the order of 105 m s-1. In the five classical triggered negative lightning (CTNL) flashes, the propagation speeds of the UPLs vary between 0.35×105 and 7.71×105 m s-1, and the variations of their luminosities and speeds are quite complex during the development processes. Among the four observed natural negative lightning flashes occurred on the land, three have only one return stoke (RS) each and all of their DNLs have many branches with an average speed at the order of 105 m s-1; while the another one has 13 RSs.In the CG flash with 13 RSs, the DNL before the first RS has no obvious branch below 1.4 km above the ground, and its speed ranges from 2.2×105 to 2.3×105 m s-1 between the heights of 0.7 and 1.4 km and exceeds 3.9×106 m s-1 below 0.7 km; preceding the 4th RS, an attempted leader is observed with a speed ranging from 1.1×105 to 1.1×106 m s-1 between 0.8 and 1.5 km. As for the three observed natural negative lightning flashes occurred on the sea, each has only one RS, and each DNL preceding the RS has a few branches, two of which have an average propagation speed at the order of 105 m s-1, and the other of 106m s-1, respectively. All the DNLs contained in the observed natural negative lightning flashes, except the attempted leader, propagate with gradually increasing luminosity and increasing speed in whole.  相似文献   

20.
Revisiting Asymmetry for the Decaying Phases of El Nino and La Nina   总被引：1，自引：0，他引：1       下载免费PDF全文
This study investigated the relationship be- tween the asymmetry in the duration of El Nifio and La Nina and the length of their decaying phases. The results suggested that the duration asymmetry comes from the long decaying ENSO cases rather than the short decaying ones. The evolutions of short decaying El Nino and La Nina are approximately a mirror image with a rapid decline in the following summer for the warm and cold events. However, a robust asymmetry was found in long decaying cases, with a prolonged and re-intensified La Nina in the following winter. The asymmetry for long decaying cases starts from the westward extension of the zonal wind anomalies in a mature winter, and is further contributed to by the air-sea interaction over the tropical Pacific in the following seasons.  相似文献   

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