Upper and lower tropospheric energetics of standing and transient eddies in wave number domain during summer monsoon of 1991

Kinetic energy exchange equations (Saltzman 1957) in wave number domain are partitioned into standing, transient and standing-transient components following Murakami (1978, 1981). These components are computed for the 1991 summer monsoon using dailyu andv grid point data at 2.5° latitude-longitude interval between the equator and 40°N at 200 hPa and 850 hPa levels for the period June through August. The data are obtained from NCMRWF, New Delhi. The study shows that at 200 hPa wave number 1 over Region 3 (30°N to 40°N), wave number 2 over Region 2 (15°N to 30°N) and wave number 3 over Region 1 (equator to 15°N) dominate the spectrum of transport of momentum and wave to zonal mean flow interaction. Wave number 1 over Region 1 and Region 3 and wave number 2 over Region 2 are the major sources of kinetic energy to other waves via wave-to-wave interaction. At 850 hPa wave number 1 over Region 3 has maximum contribution in the spectrum of transport of momentum and kinetic energy and more than 90% of its contribution is from the standing component. This indicates that standing wave number 1 over Region 3 plays a very important role in the dynamics of monsoon circulation of the lower troposphere. The study further shows that although the circulation patterns at 200 hPa and 850 hPa levels are opposite in character, a number of energy processes exhibit a similar character at these levels. For example, (i) transport of momentum by most of the waves is northward, (ii) small scale eddies intensify northward, (iii) eddies are sources of kinetic energy to zonal mean flow over Region 1 and (iv) standing eddies are sources of kinetic energy to transient eddies. Besides the above similarities some contrasting energy processes are also observed. Over Region 2 and Region 3 standing and transient eddies are sources of kinetic energy to zonal mean flow at 200 hPa, while at 850 hPa the direction of exchange of kinetic energy is opposite i.e. zonal mean flow is a source of kinetic energy to standing as well as transient eddies. L(n) interaction indicates that at 200 hPa waves over R2 maintain waves over R1, while at 850 hPa waves over R1 maintain waves over R2. It has been found that the north-south gradient of zonal mean of zonal wind is the deciding factor of wave to zonal mean flow interaction.     

Spectral analysis of monsoonal global energy and enstrophy and their nonlinear exchanges in the zonal wavenumber domain

Vertical distributions of various components of the tropospheric global rotational kinetic energy, enstrophy and available potential energy during July 1979, and the contributions to these from different zonal wave categories were studied. Representative levels in the lower and upper troposphere for the stationary and transient energetics were identified on the basis of different components of energy and enstrophy. The eddy energy and enstrophy contained in different zonal scale components in the lower and upper troposphere were studied to find out the preferred scales for stationary and transient monsoonal motion in the two atmospheric layers. The role of different zonal wave categories in the nonlinear exchanges of energy and enstrophy arising due to stationary-stationary, transient-transient, stationary-transient and observed flow interactions was examined. Stationary and transient global spectra of the aforesaid dynamical variables in terms of the zonal wavenumber(m) with triangular truncation atm = 42 were utilized for this purpose. It was found from the global average kinetic energy in lower and upper troposphere that the global stationary and transient motions were comparable in the lower troposphere while in the upper troposphere stationary motion dominated over the transient motion. The computed zonal and eddy energy confirmed that the stationary motion was predominantly zonal while the transient motion was dominated by eddies. From the time mean nonlinear interaction of kinetic energy (enstrophy) of observed flow it was seen that the long and short waves as well as the zonal flow gained kinetic energy (enstrophy) from medium waves due to nonlinear interactions. The transfer of available potential energy due to nonlinear interaction was down the scale except for short waves in the upper troposphere. The stationary-transient interaction was found to be an important element of the spatial-temporal varying atmospheric flow.     

Energetics of lower tropospheric planetary waves over mid latitudes: Precursor for Indian summer monsoon

Based on NCEP/NCAR reanalysis data, kinetic energy and momentum transport of waves 0 to 10 at 850 hPa level are computed from monthly mean zonal (u) and meridional (v) components of wind from equator to 90?N. Fourier technique is used to resolve the wind field into a spectrum of waves. Correlation analysis between All India Seasonal Monsoon Rainfall (AISMR) and energetics of the waves indicates that effective kinetic energy of waves 1, 3 and 4 around 37.5?N in February has significant correlation (99.9%) with the subsequent AISMR. A simple linear regression equation between the effective kinetic energy of these three waves and AISMR is developed. Out of 47 years’ (1958–2004) data, 32 years (1958–1989) are utilized for developing the regression model and the remaining 15 years (1990–2004) are considered for its verification. Predicted AISMR is in close agreement with observed AISMR. The regression equation based on the dynamics of the planetary waves is thus useful for Long Range Forecasting (LRF) of AISMR. Apart from the regression equation, the study provides qualitative predictors. The scatter diagram between AISMR and effective kinetic energy of waves 1, 3 and 4 around 37.5?N indicates that if the kinetic energy is more (less) than 5m²s^-2, the subsequent monsoon will be good (weak). Stream function fields indicate that high latitude trough axis along 40?E (70?E) leads to a good (weak) monsoon over India.     

Tropical systematic and random error energetics based on NCEP (MRF) analysis-forecast system — A barotropic approach

Systematic and random error and their growth rate and different components of growth rate budget in energy/variance form are investigated at wavenumber domain for medium range tropical (30‡S-30‡N) weather forecast using daily horizontal wind field of 850 hPa up to 5-day forecast for the month of June, 2000 of NCEP (MRF) model. Part I of this paper deals with the study at physical domain. The following are the major findings in this paper:

Equatorial modes observed in atmospheric variables

Wavenumber-frequency spectral analysis of different atmospheric variables has been carried out using 25 years of data. The area considered is the tropical belt 25°S–25°N. A combined FFT-wavelet analysis method has been used for this purpose. Variables considered are outgoing long-wave radiation (OLR), 850 hPa divergence, zonal and meridional winds at 850, 500 and 200 hPa levels, sea level pressure and 850 hPa geopotential height. It is shown that the spectra of different variables have some common properties, but each variable also has few features different from the rest. While Kelvin mode is prominent in OLR and zonal winds, it is not clearly observed in pressure and geopotential height fields; the latter two have a dominant wavenumber zero mode not seen in other variables except in meridional wind at 200 hPa and 850 hPa divergences. Different dominant modes in the tropics show significant variations on sub-seasonal time scales.     

Global spectra of energy and enstrophy and their fluxes during July 1979

Transient and stationary spectra of kinetic energy (KE), available potential energy (APE) and enstrophy (EN), and their spectral fluxes as a function of the two-dimensional wavenumbern were computed for July 1979. Triangular truncation at zonal wavenumber 42 was used for computation. The slopes of various spectra in the wavenumber range 14≤n≤25 were obtained by fitting a straight line in log-log scale by the least square method. The transientKE, APE andEN spectra in the lower (upper) troposphere had slopes −2·21 (−2·30), −2·65 (−2·64) and −0·36 (−0·46), respectively. The effect of stationary and divergent motion on the slope values was investigated. The possible correlation between the slope and percentage of transient component in the combined energy and enstrophy was examined to identify the transient motion of the atmosphere with the two-dimensional homogeneous isotropic turbulence. The vertically averaged slope of kinetic energy and enstrophy in the lower (upper) troposphere was close to the value at 700 (200) hPa level. The spectral fluxes of kinetic energy and enstrophy in the wavenumber range 14≤n≤25 satisfied, to a very rough approximation, the criteria of inertial subrange. The stationary fluxes were small. The estimated stationary-transient component of flux was larger, comparable and less than the corresponding transient flux of APE, KE and EN. Representative levels for computation of energy and enstrophy spectra and their fluxes in the lower and upper troposphere were identified.     

Comparative study of MLT mean winds using MF radars located at 16.8°N and 8.7°N

The main aim of the paper is to describe mesosphere and lower thermosphere (MLT) wind field observed between June 2000 and May 2002 by medium frequency (MF) radars at two locations near the equatorial region and at tropical latitude. We have presented and compared observations of mean horizontal winds in the mesosphere and lower thermosphere (MLT) region over Kolhapur (16.8°N, 74.2°E) and Tirunelveli (8.7°N, 77.8°E). Our analysis reveals annual oscillation in the zonal wind and semi-annual oscillation in the meridional wind over Kolhapur. On the other hand, results over Tirunelveli reveal semi-annual oscillation (SAO) in the zonal wind and annual oscillation in the meridional wind. Also we have observed enhanced magnitude of wind speed in spring equinox period of 2002 exhibiting the signatures of the quasi-biennial oscillation (QBO) over Tirunelveli.     

Stability of a stationary Rossby wave embedded in barotropic monsoon zonal flow

Barotropic stability of a stationary Rossby wave of wavelength 30° longitude superposed on the uniform monsoon zonal flow has been examined. The wave is unstable to perturbations and the growth rate depends on the meridional scale. These perturbations grow by drawing on the kinetic energy of the stationary Rossby wave.     

Harmonic analysis of summer mean wind at 200 mbar level during contrasting monsoon years over India

Summer (June–August) mean zonal and meridional wind components at 200 mbar level are subjected to harmonic analysis for the years 1970, 1971, 1972 and 1979. It is found that the small scale disturbances are intense during normal monsoon years. The westerlies in the belt 10°S to 30°S are stronger during drought years. During normal monsoon years (1970, 1971) the northward transport of westerly momentum by wave number 1 at 19.6°N is large as compared to drought years (1972, 1979). The transport of westerly momentum by standing eddies is northward for all the years between 5°S and 28.7°N but large during the normal monsoon years.     

Energetics of lower tropospheric ultra-long waves: A key to intra-seasonal variability of Indian monsoon

Analysis of fifty four (1951–2004) years of daily energetics of zonal waves derived from NCEP/NCAR wind (u and υ) data and daily rainfall received over the Indian landmass (real time data) during southwest monsoon season (1 June–30 September) indicate that energetics (momentum transport and kinetic energy) of lower tropospheric ultra-long waves (waves 1 and 2) of low latitudes hold a key to intra-seasonal variability of monsoon rainfall over India. Correlation coefficient between climatology of daily (122 days) energetics of ultra-long waves and climatology of daily rainfall over Indian landmass is 0.9. The relation is not only significant but also has a predictive potential. The normalised plot of both the series clearly indicates that the response period of rainfall to the energetics is of 5–10 days during the onset phase and 4–7 days during the withdrawal phase of monsoon over India. During the established phase of monsoon, both the series move hand-in-hand. Normalised plot of energetics of ultra-long waves and rainfall for individual year do not show marked deviation with respect to climatology. These results are first of its kind and are useful for the short range forecast of rainfall over India.     

Momentum transport of wave zero during March: A possible predictor for the Indian summer monsoon

Analysis of monthly momentum transport of zonal waves at 850 hPa for the period 1979 to 1993, between ‡S and ‡N for January to April, using zonal (u) and meridional (v) components of wind taken from the ECMWF reanalysis field, shows a positive correlation (.1% level of significance) between the Indian summer monsoon rainfall (June through September) and the momentum transport of wave zero TM(0) over latitudinal belt between 25‡S and 5‡N (LB) during March. Northward (Southward) TM(0) observed in March over LB subsequently leads to a good (drought) monsoon season over India which is found to be true even when the year is marked with the El-Nino event. Similarly a strong westerly zone in the Indian Ocean during March, indicates a good monsoon season for the country, even if the year is marked with El-Nino. The study thus suggests two predictors, TM(0) over LB and the strength of westerly zone in the Indian Ocean during March.     

Moist baroclinic instability and the growth of monsoon depressions— linear and nonlinear studies

The objective of this study was two-fold: the first to investigate the role of moist convection and nongeostrophic effects on the growth of the monsoon depressions using a linearized multi-level moist primitive equation (PE) model and quasi-geostrophic (QG) model with only vertical shear. The second was to study the nonlinear evolution, growth, movement and detailed energetics of the monsoon depressions using a nonlinear moist global spectral model. Our linear studies using both models revealed lower as well as upper tropospheric growing modes. For the lower tropospheric modes the shorter scales were found to grow faster. While the PE model showed faster growth rate for shorter scales, as compared to longer scales, the QG model showed less tendency for scale selection. The shorter scales in PE model had phase speeds ranging from 4 to — 1 ms−1 and in QG model from 8 to — 4 ms−1. The nongeostrophic effects were found to be, in general, important. One of the lower tropospheric modes with wavelength 2500 km was found to have many features similar to the observed monsoon depression of the Bay of Bengal. In the upper troposphere the PE model showed much faster growth rates compared to the QG model. Also the fastest growing mode with a doubling time of 2.5 days had a scale of 6000 km. This was shorter than the scale predicted in the QG model. This mode had many characteristics similar to the observed features of the monsoon upper tropospheric easterly waves. Using a nonlinear global spectral model, we simulated the monsoon depression around 21°N starting from an antisymmetric heating distribution (with respect to the equator) and with a specific vertical structure with and without basic flows. The model was integrated for a period of five days incorporating a simple form of cumulus heating. The simulated model disturbance showed a pronounced growth and a westward movement in the presence of cumulus heating. The detailed energetics calculations revealed that the baroclinic energy exchange is the primary energy exchange process and cumulus heating is the driving force for the generation of available potential energy.     

The interannual variability of mid-latitude meridional circulation and its teleconnection with Indian monsoon activity

The statistical relationship between the summer monsoon rainfall over all India, northwest India and peninsular India, onset dates of monsoon and the index of mid latitude, (35° to 70°N) meridional circulation at 500 hPa level over different sectors and hemisphere based on 19 years (1971–1989) data, have been examined. The results indicate that (i) the summer monsoon rainfalls over all India, northwest India and peninsular India show a significant inverse relationship with the strength of meridional index during previous January over sector 45°W to 90°E. (ii) The summer monsoon rainfalls over all India and peninsular India show a significant inverse relationship with the strength of meridional index during previous December over sector 90°E to 160°E, (iii) The summer monsoon rainfall over northwest India shows a significant direct relationship with the meridional index during previous May over sector 160°E to 45°W. Significant negative relationships are also observed between the meridional circulation indices of previous October (sector 3 and 4), previous December (sectors 1, 3 and 4), previous winter season (sector 3 and 4) and the onset dates of summer monsoon over India. The meridional circulation index thus can have some possible use for long range forecasting of monsoon rainfall over all India, northwest India and peninsular India, as well as the onset dates of monsoon.     

Climate-model evaluation of the contribution of sea-surface temperature and carbon dioxide to the Middle Miocene Climate Optimum as a possible analogue of future climate change

The Middle Miocene Climate Optimum (MMCO), which occurred at about 15 Ma, is the most recent global warming episode. Given the fact that no dramatic tectonic movement had taken place, this historical warming episode mirrors the present warming event induced mostly by human activities. Proxy data indicate that the MMCO had a global mean surface temperature of ～3–4°C higher than the present, equivalent to the warming predicted for the next century by the mid-range scenarios of the IPCC Fourth Report (AR4). With this comparable magnitude of warming, it is therefore of scientific interest to examine whether or not the present warming is similar to the MMCO warming. Since the MMCO boundary conditions such as paleogeography and paleobathymetry were not greatly different from today, contentious scientific issues on possible forcing mechanisms can be assessed. The MMCO climate was simulated using the latest National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM3.1 and Land Model CLM3.0 coupled to a slab ocean model. Two simulations were conducted, the MG700 with a near-present equator–pole meridional sea-surface temperature (SST) gradient and a CO₂ level of 700 ppmv and the HG350 with a higher equator–pole meridional SST gradient and a CO₂ level of 350 ppmv. These two simulations give the closest proxy global mean surface temperature of 18.4°C with a difference of ±0.6°C. This study analyses these two simulations and provides an insight into possible mechanisms of the MMCO. Results show that the north and south poles respond differently to a 50% reduction in CO₂ from the upper bound value of 700 ppmv (an approximate doubling of the present CO₂) and a change in equator–pole meridional SST gradient. The Arctic response to a 50% reduction in CO₂ is balanced by a 50% decrease in surface temperature but shows no response to the increase in equator–pole meridional SST gradient. The Antarctic situation is opposite to the Arctic: the western Antarctic's response to the increase in the equator–pole meridional SST gradient is nearly balanced by a doubling of the maximum surface temperature, but shows no response to a 50% reduction in CO₂; the eastern Antarctic shows a weak response to either a 50% reduction in CO₂ or to an increase in the equator–pole meridional SST gradient. This may explain why the Arctic warms much more than the Antarctic in the MMCO and the decoupling of CO₂ with temperature as determined by the proxy SST.     

The role of boundary layer momentum advection in the mean location of the ITCZ

The inter-tropical convergence zones (ITCZ) form closer to the equator during equinoxes while they form well away from the equator during the boreal summer. A simple three-way balance between the pressure gradients, Coriolis force and effective Rayleigh friction has been classically used to diagnose the location of maximum boundary layer convergence in the near equatorial ITCZ. If such a balance can capture the dynamics of off-equatorial convergence was not known. We used idealized aqua planet simulations with fixed, zonally symmetric sea surface temperature boundary conditions to simulate the near equatorial and off-equatorial ITCZ. As opposed to the convergence of inter-hemispheric flows in the near equatorial convergence, the off-equatorial convergence forms due to the deceleration of cross-equatorial meridional flow. The detailed momentum budget of the off-equatorial convergence zone reveals that the simple balance is not sufficient to capture the relevant dynamics. The deceleration of the meridional flow is strongly modulated by the inertial effects due to the meridional advection of zonal momentum in addition to the terms in the simple balance. The simple balance predicts a spurious near equatorial convergence and a consistent off-equatorial convergence of the meridional flow. The spurious convergence disappears when inertial effects are included in the balance. As cross equatorial meridional flow decelerates to form convergence, the inertial effects cancel the pressure gradient effects near the equator while they add away from the equator. The contribution to the off-equatorial convergence induced by the pressure gradients is significantly larger than the contribution due to the inertial effects and hence pressure gradients appear to be the primary factor in anchoring the strength and location of the off-equatorial convergence.     

Wind gradients at meteor heights over mid-latitude stations

The gradients of amplitude and phase of diurnal and semidiurnal components of zonal and meridional winds in the height intervals of 80–90 km and 90–100 km are studied using the meteor wind radar data collected at Atlanta (34°N, 84°W) during the period August 1974–March 1978. The results are compared with those at Adelaide (35°S, 139°E). It is found that the gradients vary in an opposite manner between the two height intervals.     

Has influence of extratropical waves in modulating Indian summer monsoon rainfall (ISMR) increased?

In the paper, influence of extratropical circulation features on Indian Summer Monsoon Rainfall (ISMR) is examined. Energetics of extratropics, north of Indian subcontinent for deficient and non-deficient ISMR years, during two periods 1951–1978 and 1979–2005, are calculated and critically analyzed. It is observed that for the period 1951–1978, only two out of the 10 energetics parameters, viz., the zonal available potential energy (high) and conversion of zonal kinetic energy to eddy kinetic energy (low) differed significantly in JJA months of the deficient years from that of the non-deficient years. However, during the 1979–2005 period, as many as six out of the 10 energetics parameters, viz., eddy available potential energy, zonal available potential energy, eddy kinetic energy, generation of zonal available potential energy, conversion of zonal available potential energy to zonal kinetic energy and conversion of zonal kinetic energy to eddy kinetic energy differed significantly in JJA months of the deficient years from that of the non-deficient years. These results confirm growing influence of the transient stationary waves in deficient years after the climate shift year, 1979. Analysis of energetics parameters of the pre-monsoon season of the two periods also reveals similar results. This suggests that forcings apparently responsible for energetics in JJA months of the deficient years of the later period were present even before the advent of Indian summer monsoon season.     

Simulation of marine boundary layer characteristics using a 1-D PBL model over the Bay of Bengal during BOBMEX-99

The characteristic features of the marine boundary layer (MBL) over the Bay of Bengal during the southwest monsoon and the factors influencing it are investigated. The Bay of Bengal and Monsoon Experiment (BOBMEX) carried out during July–August 1999 is the first observational experiment under the Indian Climate Research Programme (ICRP). A very high-resolution data in the vertical was obtained during this experiment, which was used to study the MBL characteristics off the east coast of India in the north and south Bay of Bengal. Spells of active and suppressed convection over the Bay were observed, of which, three representative convective episodes were considered for the study. For this purpose a one-dimensional multi-level PBL model with a TKE-ε closure scheme was used. The soundings, viz., the vertical profiles of temperature, humidity, zonal and meridional component of wind, obtained onboard ORV Sagar Kanya and from coastal stations along the east coast are used for the study. The temporal evolution of turbulent kinetic energy, marine boundary layer height (MBLH), sensible and latent heat fluxes and drag coefficient of momentum are simulated for different epochs of monsoon and monsoon depressions during BOBMEX-99.The model also generates the vertical profiles of potential temperature, specific humidity, zonal and meridional wind. These simulated values compared reasonably well with the observations available from BOBMEX.     

地下浅部异质体对表面波场谱扰动分析

均匀半无限体中表面源激发近表面波场由瑞利波主导,当浅部存在异质体,前行瑞利波发生散射,异质体周围表面波场响应谱发生变化。当异质体长度相对波长较大时,由异质体上方表面质点响应谱提取绕射波频散曲线可分析异质体与周围介质刚度差异对绕射波传播特性的影响。与含软层或硬层半无限体中瑞利波特性比较,可以发现绕射波具有与瑞利波类似的传播特性。异质体上方谱变化特征与异质体和周围介质间刚度差异有关,本研究基于异质体绕射波与入射瑞利波位移结构差异解释谱变化现象。结果表明：异质体上方谱与周围谱明显不同,在偏移距−波长域,相对于前方波场,软质体上方谱密度整体呈减弱趋势,硬质体上方谱密度整体呈增强趋势。根据谱密度变化对应偏移距、波长及谱密度强弱,可以预估异质体在传播方向的位置、埋深,并识别异质体类型。     

Palaeogeographic Position of the South China Plate During the Early-Middle Devonian1

Abstract In Guangxi, the Lower- Middle Devonian boundary beds yield abundant corals, brachiopods, stromatoporoids and conodonts. The former three were common in tropical and subtropical zones, while the last occurred only round the equator. This reveals that the South China Plate in the Early-Middle Devonian was at or near the equator, which was also supported by palaeomagnetic study giving the palaeolatitude of the Guangxi Region at the time to be 0 ° 36 ′N.