首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 453 毫秒
1.
A self-contained derivation of the IPESD models [Majda, A.J., Klein, R., 2003. Systematic multi-scale models for the tropics. J. Atmos. Sci. 60, 393–408] governing synoptic and planetary scale tropical flows is provided. This derivation demonstrates the analytic tractability of the model and the effect of zonally and meridionally tilted synoptic scale heating on the forcing of planetary scale flows through upscale momentum and temperature fluxes. Exploiting the analytic tractability of the models, different aspects of the planetary scale forcing are traced to meridional and vertical tilts in the synoptic scale heating profile. Variants of the archetypal IPESD models for the Madden–Julian oscillation (MJO) presented in Majda and Biello [Majda, A.J., Biello, J.A., 2004. A multi-scale model for tropical intraseasonal oscillations. Proc. Natl. Acad. Sci. 101, 4736–4741; Biello, J.A., Majda, A.J., 2005. A new multi-scale model for the Madden–Julian oscillation. J. Atmos. Sci. 62, 1694–1721] are studied. In addition to vertically tilted synoptic scale heating, the models discussed herein incorporate upscale zonal momentum flux due to meridional flux convergence arising from meridionally tilted heating. The effect of a boundary layer momentum drag at the base of the free troposphere is also systematically incorporated into the IPESD models. Both meridional tilts and lower boundary layer drag are shown to meridionally confine the MJO westerly wind burst and drive a planetary scale barotropic flow. Meridionally tilted heating can also greatly strengthen the wind burst at the base of the troposphere and modify its vertical profile. The competing effects of meridionally tilted, and off-equatorial heating can also significantly weaken the MJO winds. Appendices are provided which discuss generalizations and a solution algorithm for the IPESD models.  相似文献   

2.
In recent years,significant progress has been made regarding theories of intraseasonal oscillations (ISOs) (also known as the Madden-Julian oscillation (MJO) in the tropics).This short review introduces the latest advances in ISO theories with an emphasis particularly on theoretical paradigms involving nonlinear dynamics in the following aspects:(1) the basic ideas and limitations of the previous and current theories and hypotheses regarding the MJO,(2) the new multi-scale theory of the MJO based on the intraseasonal planetary equatorial synoptic dynamics (IPESD) framework,and (3) nonlinear dynamics of ISOs in the extratropics based on the resonant triads of Rossby-Haurwitz waves.  相似文献   

3.
Summary The present paper addressed the issue of growth of planetary boundary-layer fluxes on the time scale of MJO based on ECMWF reanalysis daily data of 180 days covering April–September, 2001. Diagnostic analysis of this data set utilises computations of moisture and sensible heat fluxes in the frequency domain which involve nonlinear interactions phenomena of MJO time scale with the synoptic scales. Basically the whole computations performed are based on surface similarity theory and Richardson number dependent K-theory in the surface and planetary boundary layer (PBL), respectively, both invoke triple product nonlinearilies. Present observational study shows that among the totality of the triads participating in phase-locking phenomena, a prominent band of those reside in the MJO time scales (30 to 60 days) and the synoptic time scales (3 to 7 days). The study suggests that the low frequency variability on MJO time scale in moisture and sensible heat flux arises from its nonlinear interactions with synoptic time scales. The results show that the phases of the three interacting oscillations associated with specific humidity/SST, Richardson number dependent instability factor and wind shear are positive and reasonably close to each other. The amplitudes of the synoptic scale oscillations are not insignificant compared to that of MJO. These dynamical aspects regarding the phases and amplitudes of the three participating oscillations favour the nonlinear interactions of MJO to the synoptic scales and thus lead to rapid exchange of energy transfer to the former.Visiting scientist from Indian Institute of Tropical Meteorology, Pune-411008, India.  相似文献   

4.
The diurnal cycle is a fundamental time scale in the climate system, at which the upper ocean and atmosphere are routinely observed to vary. Current climate models, however, are not configured to resolve the diurnal cycle in the upper ocean or the interaction of the ocean and atmosphere on these time scales. This study examines the diurnal cycle of the tropical upper ocean and its climate impacts. In the present paper, the first of two, a high vertical resolution ocean general circulation model (OGCM), with modified physics, is developed which is able to resolve the diurnal cycle of sea surface temperature (SST) and current variability in the upper ocean. It is then validated against a satellite derived parameterization of diurnal SST variability and in-situ current observations. The model is then used to assess rectification of the intraseasonal SST response to the Madden–Julian oscillation (MJO) by the diurnal cycle of SST. Across the equatorial Indo-Pacific it is found that the diurnal cycle increases the intraseasonal SST response to the MJO by around 20%. In the Pacific, the diurnal cycle also modifies the exchange of momentum between equatorially divergent Ekman currents and the meridionally convergent geostrophic currents beneath, resulting in a 10% increase in the strength of the Ekman cells and equatorial upwelling. How the thermodynamic and dynamical impacts of the diurnal cycle effect the mean state, and variability, of the climate system cannot be fully investigated in the constrained design of ocean-only experiments presented here. The second part of this study, published separately, addresses the climate impacts of the diurnal cycle in the coupled system by coupling the OGCM developed here to an atmosphere general circulation model.  相似文献   

5.
Rainfall characteristics of the Madden–Julian oscillation (MJO) are analyzed primarily using tropical rainfall measuring mission (TRMM) precipitation radar (PR), TRMM microwave imager (TMI) and lighting imaging sensor (LIS) data. Latent heating structure is also examined using latent heating data estimated with the spectral latent heating (SLH) algorithm.The zonal structure, time evolution, and characteristic stages of the MJO precipitation system are described. Stratiform rain fraction increases with the cloud activity, and the amplitude of stratiform rain variation associated with the MJO is larger than that of convective rain by a factor of 1.7. Maximum peaks of both convective rain and stratiform rain precede the minimum peak of the outgoing longwave radiation (OLR) anomaly which is often used as a proxy for the MJO convection. Stratiform rain remains longer than convective rain until 4000 km behind the peak of the mature phase. The stratiform rain contribution results in the top-heavy heating profile of the MJO.Associated with the MJO, there are tri-pole convective rain top heights (RTH) at 10–11, 7 and 3 km, corresponding to the dominance of afternoon showers, organized systems, and shallow convections, respectively. The stratiform rain is basically organized with convective rain, having similar but slightly lower RTH and slightly lags the convective rain maximum. It is notable that relatively moderate (7 km) RTH is dominant in the mature phase of the MJO, while very tall rainfall with RTH over 10 km and lightning frequency increase in the suppressed phase. The rain-yield-per flash (RPF) varies about 20–100% of the mean value of 2–10 × 109 kg fl−1 over the tropical warm ocean and that of 2–5 × 109 kg fl−1 over the equatorial Islands, between the convectively suppressed phase and the active phase of MJO, in the manner that RPF is smaller in the suppressed phase and larger in the active phase.  相似文献   

6.
Summary Lower tropospheric (1000–500) hPa kinetic energy (KE), temporal variations of KE and nonlinear KE transfer of rotational and divergent flows and energy conversion between them, partitioning further into stationary and transient components in the Fourier spectral domain and the mechanism for the evolution of significant transient waves for the month July 1979 in the latitudinal belt 10° S–30° N are studied.Divergent zonal and eddy KE show their maxima at the lowest level 1000 hPa. Lower tropospheric monsoon motion provides a non-divergent level close to 850 hPa. The daily flow patterns bear little resemblence to the climatology over tropics at 500 hPa. Although the transient mode of synoptic scale waves is stronger than that of planetary scale waves they are comparable. Analysis of energetics over global tropics can get signature of transient activities embedded in the large scale system. Summer momentum flux in the lower troposphere is essentially associated with stationary planetary and transient synoptic scale waves. Waves 1, 3 and 6 are the most preferred transient waves. Divergent to rotational KE conversion is the most dominating mechanism for the maintenance of planetary and synoptic scale waves. All categories of waves contribute towards the maintenance of zonal flows. The primary source of energy for transient synoptic scale waves is the transient divergent rotational KE transfer whereas the interaction between zonal stationary and transient wave is likely to be secondary source. Transient KE and all transient interactions, stationary KE and all stationary interactions are found to be strongest at 500 hPa and 850 hPa respectively. Growth and decay of transient waves 1 and 3 are mainly controlled by divergent-rotational KE conversions whereas those of transient wave 6 are controlled by KE transfer due to zonal-wave interaction.With 13 Figures  相似文献   

7.
Abstract

Intervention experiments using the Coupled Forecast System model, version 2 (CFSv2), have been performed in which various Madden-Julian Oscillation (MJO) evolutions were added to the model’s internally generated heating: Slow Repeated Cycles, Slow Single Cycle, Fast Repeated Cycles, and Fast Single Cycle. In each experiment, one of these specified MJO evolutions of tropical diabatic heating was added in multiple ensemble reforecasts of boreal winter (1 November to 31 March for 31 winters: 1980–2010). Since in each experiment, multiple re-forecasts were made with the identical heating evolution added, predictable component analysis is used to identify modes with the highest signal-to-noise ratio. Traditional MJO-phase analysis of total model heating (dominated by internally generated heating) shows that the MJO-related heating structure compares well with heating estimated from observed fast and slow episodes; however, the model heating is larger by a factor of two. The evolution of Euro-Atlantic circulation regimes indicates a clear response due to the added heating, with a robust increase in the frequency of occurrence of the negative phase of the North Atlantic Oscillation (NAO?) after the heating crosses into the Pacific and a somewhat less robust increase in the positive phase of the NAO (NAO+) following Indian Ocean heating. In the Fast Cycle experiments, the model response is somewhat muted compared with the Slow Cycle experiments. The Scandinavian Blocking regime becomes more frequent prior to the NAO? regime. The two leading modes in the predictable component analysis of 300?hPa height (Z300), synoptic scale feedback (DZ300), and planetary wave diabatic heating in all experiments form an oscillatory pair with high statistical significance. The oscillatory pair represents the cyclic response to the particular MJO signal (Fast or Slow, Single, or Repeated Cycles) in each case. The period is about 64 days for the Slow Cycle and 36 days for the Fast Cycle, consistent with the imposed periods. The time series of one of the leading modes of Z300 is highly anti-correlated with the frequency of occurrence of the NAO– in the Repeated Cycle experiments. A clear cycle involving the Z300 and DZ300 leading modes is identified.  相似文献   

8.
The problem of the orbit of a parcel from its initial location at which there might be a momentum anomaly is here considered for a β-plane. The solutions represent a generalisation of the recent work of Wan and Yang (1990, Adv. Atmos. Sci., 7:409–422). A physical interpretation of the orbits, in which they are related to flow stability, is given for both extratropics and tropics. The possible effect of dissipation on these results is also discussed.  相似文献   

9.
This note relates to the paper Circulation and boundary layers in differentially heated rotating stratified fluid by Whitehead and Pedlosky [Whitehead, J.A., Pedlosky, J., 2000. Circulation and boundary layers in differentially heated rotating stratified fluid. Dyn. Atmos. Ocean. 31, 1–21]. Here, we describe an alternative method of solution for the theoretical model developed therein, and provide a comparison with the original method used in the paper.  相似文献   

10.
Summary This work deals with idealized modelling experiments designed to understand the dynamical evolution of low frequency intraseasonal monsoonal oscillations that result from interactions between the large scale monsoon Reverse Hadley Cell (RHC) and moist convective processes. The monsoon differential heating, which primarily determines the low-level convergence of the large-scale monsoon flow, is found to play a decisive role in affecting the northward progression of the monsoonal modes. A strong north-south differential heating leads to a robust generation and steady maintenance of northward propagating monsoonal oscillations. A weaker land-ocean thermal contrast leads to feeble low frequency monsoonal modes that have relatively longer periods in the 30–50 day band. This increase in the period of the monsoonal oscillations due to weak north-south thermal contrast is in good agreement with the observational findings of Yasunari (1980) and Kasture and Keshavamurty (1987). It is speculated that such an increase in the oscillatory period may be an outcome from an elongation in the meridional scale of the transient Hadley type cells which act as resonating cavities for the monsoonal modes.A Mobile Wave CISK (MWC) form of interaction between the large scale monsoon and the transient circulations associated with the Madden Julian Oscillation (MJO) is projected as a viable physical mechanism for the northward movement of low frequency modes. It is demonstrated that the effective low level convergence, following such an interaction, tends to shift northward relative to the site of interaction. This enables the heating perturbations to be displaced northward which in turn causes the secondary circulations and wind perturbations to follow. The essential criterion for the occurrence of a prolonged northward propagation of the low frequency modes is that the heating perturbations should phase lead the wind perturbations at all times.An examination of the - interactions on the 30–50 day time scale reveals that the conversion from the transient divergent motions to rotational motions is quite intense (feeble) in the strong (weak) monsoon differential heating experiments. Because of the closer proximity to the monsoon heat source and also due to the latitudinal variation of earth's rotational effects, the - interactions tend to be more pronounced to the north of 15°N while they are less robust in the near equatorial latitudes.The regularity of the monsoonal modes is found to depend on the strength of the monsoon differential heating and also on the periodic behaviour of the equatorial intraseasonal oscillations. The monsoonal modes are quite steady and exhibit extreme regularity in the presence of a weak north-south differential heating provided the equatorial forcing due to the MJO varies in a periodic manner. This result supports the findings of Mehta and Krishnamurti (1988) who found greater regularity of the 30–50 day modes during bad monsoon years.The low frequency monsoonal modes are found to be quite sensitive to the moisture availability factor (m) and the vertical profile of heating used in the MWC parameterization. A small increase in the value of (m) is found to significantly intensify the amplitude of the monsoonal oscillations while there is no considerable shift in the spectral frequency within the 30–50 day band as such. The 30–50 day motions show significant enhancement, with a relatively sharp spectral peak around 45 days, when the vertical profile of MWC heating has a maximum in the lower troposphere. However an upward displacement of the heating maximum tends to weaken the low frequency oscillations.With 19 Figures  相似文献   

11.
Summary The relationship between global circulation, temperature distribution and weather variability as a mechanism of meridional heat exchange over the northern hemisphere has been examined for the period 1967–1991 using analysis data from the German Weather Service. From geopotential heights five parameters, relating to different climatic features, were computed. The data were filtered using a Principal Component Analysis (PCA) to omit the random noise. The time series of amplitudes for the Principal Patterns were investigated with respect to their linear trends and extreme events.The period 1967–1991 is characterised by a transition from a relatively cool period-beginning in the early 1960s-to a warmer one. The strongest warming took place at different latitudes in the Atlantic and the Pacific section, respectively. Due to the warming the meridional temperature gradient and the geostrophic zonal wind intensified. Strengthened baroclinic conditions reinforced the activities on the synoptic scale and the meridional eddy heat flux. The results corroborate the hypothesis that the enlarged synoptic activity is responsible for the accumulated occurrences of extreme midlatitude storms over the Atlantic and Europe within the last few years and, therefore, that the observation of more frequent deep cyclones is neither random nor due to improved observation techniques.With 6 Figures  相似文献   

12.
Considering linearized motion about a resting basic state, we derive analytical solutions of the equatorial β-plane primitive equations under the assumption that the flow is steady in a reference frame moving eastward with a diabatic forcing resembling a Madden–Julian Oscillation (MJO) convective envelope. The solutions are analyzed in terms of potential vorticity (PV) dynamics. Because the diabatic source term for PV contains a factor βy, the diabatic heat source is ineffective at generating a PV anomaly at the equator but maximizes the PV response near the poleward edges of the heat source. In this way a moving heat source can produce two ribbons of lower tropospheric PV anomaly, a positive one off the equator in the northern hemisphere and a negative one off the equator in the southern hemisphere, with oppositely signed PV anomalies in the upper troposphere. Associated with these PV anomalies are geopotential anomalies that are shifted several hundred kilometers poleward. In the lower troposphere these zonally elongated geopotential anomalies resemble ITCZ trough zones, which demonstrates the close connection between the MJO wake dynamics and the formation of double ITCZs.To demonstrate that the MJO wake response can be described by simple PV dynamics, we propose an invertibility principle relating the PV to the streamfunction, which in turn is locally related to the geopotential. This equatorial invertibility principle accurately recovers the balanced wind and mass fields found in the MJO wake in the primitive equation model. However, while the invertibility principle highlights the ability of simple PV dynamics to accurately describe the flow in the wake of an MJO convective envelope, it also clearly illustrates the inability of such dynamics to describe the Kelvin-like flow pattern ahead of the convection.  相似文献   

13.
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   

14.
Previous studies have suggested that the South China Sea (SCS) summer monsoon onset is concurrent with the arrival of a 30–60-day northward-propagating trough. On the other hand, from a synoptic viewpoint, some studies pointed out that the arrival of a mid-latitude front may be the triggering mechanism of the SCSSM onset. This study attempts to link these two viewpoints and to investigate their relative role in inducing the SCSSM onset. Composites of low-level zonal winds, geopotential heights and temperatures during the 1991–1999 SCSSM onsets based on the European Centre for Medium Range Weather Forecast ERA-40 data indicate that both the Madden and Julian Oscillation (MJO)/Kelvin waves and mid-latitude trough are apparently involved in the onset. The MJO/Kelvin waves play a major role in inducing the large-scale easterly-westerly shift over the central SCS, while the effect of the acceleration of westerlies ahead of the mid-latitude trough is limited to the northern SCS only. Numerical experiments using a regional climate model further demonstrate that the MJO/Kelvin waves control the timing of the onset by changing the background meridional geopotential height gradient over the SCS. When the MJO is at its peak phase over the Maritime continent, it imposes a positive meridional geopotential height gradient over the SCS such that easterly winds are induced, which significantly reduces the strength of a mid-latitude trough. After the equatorial convection has dissipated, a Rossby-wave response is induced, leading to the formation of a northward-moving trough. When this trough moves northward, the meridional geopotential height gradient is reversed and westerly winds are induced. At the same time, if a mid-latitude trough arrives in south China, the westerlies associated with the mid-latitude trough will strengthen because of the background meridional geopotential height gradient, which gives the impression that both the northward-moving trough and mid-latitude trough are in phase and work together to induce the onset.  相似文献   

15.
Madden-Julian variability in NCAR CAM2.0 and CCSM2.0   总被引：1，自引：0，他引：1  
The Madden-Julian Oscillation (MJO) dominates tropical variability on time scales of 30–70 days. During the boreal winter/spring it is manifested as an eastward propagating disturbance, with a strong convective signature over the eastern hemisphere. Here, 20–100 day bandpass filtered outgoing longwave radiation (OLR) for the months of November–March from the National Center for Atmospheric Research Community Atmospheric Model Version 2.0 (NCAR CAM2.0) and the Community Coupled System Model Version 2.0 (CCSM2.0) models is projected onto the observed patterns of MJO convection. This provides for the analysis of the models within a standard framework. Additionally, only analyzing years when the lead/lag relationship of the simulated principal components lie in the observed phase-space better isolates the simulated MJO signal. CCSM2.0 yields a better representation of the MJO than CAM2.0 due to the presence of air-sea interaction. Even so, the amplitude and spatial extent of the intraseasonal convection are underestimated relative to observed OLR, with a pronounced underestimate of the near-equatorial convection. Due to the development of a split inter-tropical convergence zone in the western Pacific, which is independent of the MJO, the models are precluded from representing the low-level moisture convergence that is central to the eastward propagation of the MJO. Once the systematic model error is remedied the underlying capability of the models to simulate the MJO will be possible.  相似文献   

16.
Even though multi-model prediction systems may have better skill in predicting the interannual variability (IAV) of Indian summer monsoon (ISM), the overall performance of the system is limited by the skill of individual models (single model ensembles). The DEMETER project aimed at seasonal-to-interannual prediction is not an exception to this case. The reasons for the poor skill of the DEMETER individual models in predicting the IAV of monsoon is examined in the context of the influence of external and internal components and the interaction between intraseasonal variability (ISV) and IAV. Recently it has been shown that the ISV influences the IAV through very long breaks (VLBs; breaks with duration of more than 10 days) by generating droughts. Further, all VLBs are associated with an eastward propagating Madden–Julian Oscillation (MJO) in the equatorial region, facilitated by air–sea interaction on intraseasonal timescales. This VLB-drought–MJO relationship is analyzed here in detail in the DEMETER models. Analyses indicate that the VLB-drought relationship is poorly captured by almost all the models. VLBs in observations are generated through air–sea interaction on intraseasonal time scale and the models’ inability to simulate VLB-drought relationship is shown to be linked to the models’ inability to represent the air–sea interaction on intraseasonal time scale. Identification of this particular deficiency of the models provides a direction for improvement of the model for monsoon prediction.  相似文献   

17.
Summary Through the use of a zonal balance model we investigate the properties of the tropical meridional circulation to a range of specified diabatic forcing fields for climatologically observed zonal winds. As in earlier studies, the solutions show that latent heat release away from the equator forces an asymmetric meridional circulation in response the anisotropy in the inertial stability parameter with respect to the meridional location of the forcing. The presence of strong zonal flows appears to play a relatively minor role in determining the magnitude and asymmetry of the meridional circulation, whereas the structure of the diabatic heating, particularly the meridional breadth, proves to be of much greater importance.A dynamic efficiency factor, which provides an analytic measure of the efficacy of diabatic heating at generating zonal kinetic energy, generally exhibits a meridionally symmetric structure except during Northern Hemisphere summer. This asymmetry gives rise to a pronounced sensitivity of zonal kinetic energy generation to the meridional location of ITCZ convection. Further examination of the flow pattern suggests that for zonal flows representative of those over the Indian Ocean during the Northern Hemisphere summer months, meridional displacements of the heating of less than 20° latitude can result in as much as an order of magnitude difference in the rate of kinetic energy generation. Solution of the balance system also implies the existence of a feedback mechanism, between zonally-organized convection and the energetics properties of the large-scale flow, that is highly sensitive to the meridional location of the convection.With 11 FiguresThe National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

18.
Summary In order to improve our understanding of the interannual variability of the 30–50 day oscillations of the northern summer monsoon, we have performed numerical experiments using a 5-level global spectral model (GSM). By intercomparing the GSM simulations of a control summer experiment (E1) and a warm ENSO experiment (E2) we have examined the sensitivity of the low frequency intraseasonal monsoonal modes to changes in the planetary scale component of the monsoon induced by anomalous heating in the equatorial eastern Pacific during a warm ENSO phase.It is found that the anomalous heating in the equatorial eastern Pacific induces circulation changes which correspond to weakening of the time-mean divergent planetary scale circulation in the equatorial western Pacific, weakening of the east-west Walker cell over the western Pacific ocean, weakening of the time-mean Reverse Hadley circulation (RHC) over the summer monsoon region and strengthening of the time-mean divergent circulation and the subtropical jet stream over the eastern Pacific and Atlantic oceans. These changes in the large scale basic flow induced by the anomalous heat source are found to significantly affect the propagation characteristics of the 30–50 day oscillations. It is noticed that the reduction (increase) in the intensity of the time-mean divergent circulation in the equatorial western (eastern) Pacific sectors produces weaker (stronger) low-level convergence as a result of which the amplitude of the eastward propagating 30–50 day divergent wave decreases (increases) in the western (eastern) Pacific sectors in E2. One of the striking aspects is that the eastward propagating equatorial wave arrives over the Indian longitudes more regularly in the warm ENSO experiment (E2). The GSM simulations reveal several small scale east-west cells in the longitudinal belt between 0–130°E in the E1 experiment. On the other hand the intraseasonal oscillations in E2 show fewer east-west cells having longer zonal scales. The stronger suppression of small scale east-west cells in E2 probably accounts for the greater regularity of the 30–50 day oscillations over the Indian longitudes in this case.The interaction between the monsoon RHC and the equatorial 30–50 day waves leads to excitation of northward propagating modes over the Indian subcontinent in both cases. It is found that the zonal wind perturbations migrate northward at a rate of about 0.8° latitude per day in E1 while they have a slightly faster propagation speed of about 1° latitude per day in E2. The low frequency monsoonal modes have smaller amplitude but possess greater regularity in E2 relative to E1. As the wavelet trains of low latitude anomalies progress northward it is found that the giant meridional monsoonal circulation (RHC) undergoes well-defined intraseasonal oscillations. The amplitude of the monsoon RHC oscillations are significantly weaker in E2 as compared to E1. But what is more important is that the RHC is found to oscillate rapidly with a period of 40 days in E1 while it executes slower oscillations of 55 days period in E2. These results support the observational findings of Yasunari (1980) who showed that the cloudiness fluctuations on the 30–60 day time scale over the Indian summer monsoon region are associated with longer periods during El Nino years. The oscillations of the monsoon RHC show an enhancement of the larger scale meridional cells and also a stronger suppression of the smaller scale cells in E2 relative to E1 which seems to account for the slower fluctuations of the monsoon RHC in the warm ENSO experiment. It is also proposed that the periodic arrival of the eastward propagating equatorial wave over the Indian longitudes followed by a stronger inhibition of the smaller meridional scales happen to be the two primary mechanisms that favour steady and regular northward propagation of intraseasonal transients over the Indian subcontinent in the warm ENSO experiment (E2). This study clearly demonstrates that the presence of E1 Nino related summertime SST anomalies and associated convection anomalies in the tropical central and eastern Pacific are favourable criteria for the detection and prediction of low frequency monsoonal modes over India.With 11 Figures  相似文献   

19.
孟加拉湾季风爆发对南海季风爆发的影响Ⅰ:个例分析   总被引：11，自引：4，他引：11       下载免费PDF全文
利用南海季风试验分析场和NCAR向外长波辐射通量(OLR)资料研究了1998年孟加拉湾季风和南海季风爆发期间副热带环流的大尺度和天气尺度特征,探讨了孟加拉湾季风爆发与南海季风爆发之间的物理联系及孟加拉湾季风气旋的对流凝结潜热释放对副热带高压“撤出”南海的影响。结果表明,1998年5月爆发的东亚季风展现出典型的从孟加拉湾地区东传发展到南海地区的过程。随着孟加拉湾季风爆发和对流活动增强、北移,南海北部出现了低层西风和对流活动,领先于副热带高压在南海地区减弱和撤退。结果还显示南海北部地区的对流凝结加热有助于该地区经向温度梯度的反转,在热成风关系的制约下南海上空副热带高压脊面的垂直倾斜由冬季型转向夏季型,季风爆发。  相似文献   

20.
When a broad ocean current encounters a large-scale topographic feature, standing Rossby wave patterns can be generated. Short Rossby waves with a scale Li = √ Q/β (Q is the speed of the approaching flow; β is the meridional gradient of f) are generated east of the topography. If the zonal scale of the topography, L, is planetary, long standing Rossby waves can be generated west of the topography, when the current has a meridional component. The long waves focus the disturbance zonally and produce alternating regions of intensified or reduced zonal flow. The meridional scale that characterizes these zonal bands is the intermediate scales, L = Li2/3L1/3. When the meridional topographic scale is comparable to L, the amplitude of the long-wave disturbance is dominant. Using multiple-scale methods to exploit the scale gap between the planetary, intermediate and Rossby wave scales, the topographically induced pressure and velocity fields due to a zonal ridge are obtained. When the planetary-scale flow field is directed poleward, a westward counterflow can occur along the poleward flank of the ridge. The meridional scales of these topographically induced flows are comparable to those observed along the Indian-Antarctic Ridge by Callahan (1971).  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号