This thesis deals with tropical intraseasonal variation (TISV) having time scales in 20-80 day range. Variations on this time scale have been observed to have profound influence on the weather and climate of the entire globe, and hence its study forms an important area of current research. A large number of studies have been carried out on this topic since the pioneering work of Madden and Julian in 1971. However, the observational studies are biased towards using the outgoing longwave radiation (OLR) as the variable of interest, and other variables, pressure in particular, have received less attention. The present thesis explores features of intraseasonal variations in sea level pressure (SLP) with the following main objectives. 1. Compare and contrast wavenumber – frequency spectra of OLR, zonal winds and SLP. 2. Quantify temporal and spatial variations of different tropical modes observed in the above variables. 3. Investigate intraseasonal variations in sea level pressure in the tropics and its meridional connections. 4. Document the movement of cloud bands during the periods of high and low TISV activity during different seasons. 5. Explore the relations between intraseasonal variations in SLP and monsoon rainfall over India. The study considered global data for a time period of 25 years from 1979 to 2003. Spectral analysis and correlations are the main tools of analysis. A combined FFT-wavelet spectral method, which uses FFT in longitude and wavelet transform in time, was developed for this purpose. This method provided an effective way of obtaining wavenumber - frequency spectra as well as in quantifying temporal variations of different modes. The transform gives spectral intensity as a function of wavenumber, frequency and time. The analysis is applied to OLR, zonal wind and SLP to understand spectral characteristics of different modes and their temporal variations. The thesis shows that the nature of spectra for OLR, SLP and wind is different although these variables are physically connected. OLR spectrum shows many of the equatorial modes observed from the previous studies for an equivalent depth of 40 m. Spectra of zonal winds at three vertical levels (850 mb, 500 mb and 200 mb) shows peaks corresponding to MJO, Kelvin modes at an equivalent depth of 75 m and Rossby Haurwitz modes. SLP spectrum is different from others. It has peaks at wavenumber zero and at MJO and Rossby Haurwitz modes. Another important new result of the thesis is the spatial and temporal behavior of SLP on intraseasonal time scales. It is shown that the the global atmosphere exhibits quasi-periodic oscillations in SLP with variations in the tropics and high latitudes strongly correlated but in opposite phases. Importantly, the strength of TISV is correlated with sea surface temperature (SST) anomalies in the equatorial Pacific Ocean. This may have some predictive value for predicting the active and weak TISV activity.

The east-west oriented cloud bands in the tropics are called the Inter-tropical Con-vergence Zones (ITCZ). Till recently, the ITCZ has been assumed to have a simple vertical structure with convergence near the surface boundary layer and divergence near the tropopause. Recent work has shown that the ITCZ can have a complex ver-tical structure with multi-level ows. This complex structure has a profound impact on the mass, momentum and energy budget in the ITCZ. This thesis addresses the factors that govern the shallow meridional circulation that occurs in the ITCZ and the mechanisms that govern the abrupt poleward transition and the gradual poleward migration . The shallow meridional circulation forms when the boundary layer ow that con-verges in the ITCZ, rises above the boundary layer and diverges in the lower tropo-sphere. The ow above the boundary layer is in the direction opposite to the direction of the ow within the boundary layer. Some authors have argued that this is caused by the reversal of pressure gradients just above the boundary layer in response to strong sea surface temperature gradients. This hypothesis neglects the eect of plan-etary rotation on the ow and was found to be insucient to explain the formation of shallow meridional circulation. In the east Pacic ocean, the shallow circulation forms only to the south of the ITCZ when the ITCZ forms away from the equator, while it is absent when the ITCZ forms close to the equator. The aqua-planet simulations of the equatorial and the o-equatorial ITCZ were conducted using Community Atmosphere Model (CAM 3.0). The model used the Eulerian dynamical core with T42 horizontal resolution and 26 levels in vertical. Each simulation was run for 3 years and analysis of last six months was presented. The simulations reproduced the contrast in the vertical structure of the equatorial and o-equatorial ITCZ. The shallow circulation was simulated with-out the reversal of pressure gradients and the SST gradients were weakest when the shallow circulation was simulated. We have proposed a new mechanism for the exis-tence of shallow meridional circulation in the ITCZ. We have argued that, in Earth's atmosphere, the mean horizontal ow generally occurs in the direction perpendicular to the direction of applied pressure gradient due to the action of Coriolis force. If the local rotational eects of the ow (relative vorticity) cancels the action of the Coriolis force, then a ow along the pressure gradient is possible. We demonstrated that this condition was satised only to the south of the ITCZ when it forms away from the equator. The ITCZ is characterized by the maximum mass convergence in the boundary layer. The mass convergence is mainly caused by the deceleration of poleward ow in the boundary layer. When the ITCZ forms close to the equator, the ow in the boundary layer is a resultant of vector addition of three forces, a pressure gradient force in the north-south direction (i.e., the ow towards low pressure), a Coriolis force which acts in the east-west direction( perpendicular to the direction of the ow), and surface friction which opposes the resultant ow. When the ITCZ forms away from the equator a three way balance does not capture the dynamics of ow. As the poleward ow is accelerated towards low pressure, it has to advect a considerable amount of zonal momentum with it which acts to retard the poleward ow. This eect of advection of zonal momentum has to be included in the force balance to obtain an accurate estimate of the ow and associated convergence. The ITCZ acts like a heat engine. The energy is gained near the surface, some energy is transported towards pole while some is utilized in driving the meridional circulation. The rest is rejected near the tropopause. The transport within the troposphere occurs through the vertical or horizontal advection of the energy due to vertical and horizontal motions respectively. Our analysis of the ITCZ suggests that; a large amount of transport occurs through horizontal motions that was neglected in the previous studies. The detailed analysis suggests that the latent energy in the form of mass of water vapor is exported out of the ITCZ at dierent levels in association with the multilevel ows. The equatorial and the o-equatorial ITCZ are dierent because, evaporation is larger in the o-equatorial ITCZ when compared to the equatorial ITCZ. The ITCZ shows a strong sub-seasonal variability in its location in the Indian Ocean and the west Pacic Ocean during boreal summer. There are two favorable locations, one near the equator and another away from the equator, for formation of the ITCZ. The equatorial ITCZ either propagates abruptly or gradually to the o-equatorial location. A detailed analysis of moisture and momentum budget of the simulated abrupt and gradual propagations enabled us to separate the role of thermo-dynamic and dynamic processes. We found that, if the equatorial ITCZ would propa-gate abruptly or gradually to the o-equatorial location is decided by the availability of the water vapor in the boundary layer between the two locations of the ITCZ, i.e., by the thermodynamic processes. But, such a transition to the o-equatorial location is allowed only when the constraints imposed by the re-adjustment in the circulation are satised. In simple terms, these constraints emerge due to two processes. 1. The Earth (lower boundary of the atmosphere) spins at maximum eective radius near the equator. As a result, the atmosphere gains maximum angular momentum near the equator (`zonal momentum' in Cartesian co-ordinates) . The ITCZ is one of the primary avenues to transport the zonal momentum from the lower troposphere to the upper troposphere. When the favorable location of ITCZ is near the equator, the location of ITCZ and the location where atmosphere gains maximum zonal momentum are coincident. The ITCZ and associated meridional circulation transports the zonal momentum upwards which is then transported polewards. As the favorable location of ITCZ moves away from the equator, the two locations are die rent. As a result, the atmospheric ow has to re-adjust so that the zonal momentum is transported from the equator to the favorable location of the ITCZ which then transports it upwards and polewards. In summary, this thesis proposes a new mechanism for the generation of shallow meridional circulation, the abrupt transition and the gradual propagations of the ITCZ.

O objetivo deste trabalho foi estudar a aplicação de reator anaeróbio serial (RAS) no processamento de vinhaça de cana-de-açúcar para a remoção de matéria orgânica e produção de biogás, submetido ao aumento gradativo de carregamento orgânico. Para tanto, foi utilizado um reator composto por quatro câmaras em série denominadas C1, C2, C3 e C4, com capacidade de 5 L cada, totalizando um volume de 20 L. A primeira etapa do trabalho (Etapa 1) consistiu na operação e monitoramento do reator RAS partindo-se da aplicação da carga orgânica volumétrica (COV) de 0,5 kg DQO m-3.d-1 até a obtenção da COV de 2,5 kg DQO m-3.d-1. Na segunda etapa (Etapa 2) foram operados reatores em batelada sequencial, simulando a operação contínua do reator RAS, visando estudar o efeito do aumento de taxas de carregamento orgânico aplicado. O reator RAS operou sob baixos valores de COVglobal e COALglobal aplicados na Etapa 1, obtendo-se máximas de 2,50 kg DQO m-3.d-1 e 0,167 kg STV m-3.d-1, respectivamente. Os resultados obtidos da Etapa 1 indicam satisfatória remoção global média de matéria orgânica na forma de DQO, superiores a 90 %, propiciada pela adaptação da biomassa ao longo dos reatores sequenciais. O metano apresentou-se como o gás predominante na composição do biogás produzido, com valores acima de 75 %, que confirmam o estabelecimento do sistema metanogênico pleno. Em todos os compartimentos dos reatores sequenciais verificou-se maior abundância relativa para os domínios Archaea e Bacteria os gêneros Methanosaeta, produtores de metano e, T78, responsável pela degradação de açúcares, respectivamente. Principalmente para os reatores C3 e C4, observou-se adaptação de algumas populações para ambos os domínios Bacteria e Archaea indicado pelo menor índice de diversidade e aumento da dominância. O aumento das taxas de carregamento orgânico aplicado ao lodo, estipuladas nos ensaios em batelada na Etapa 2, propiciou o aumento dos valores de carga orgânica aplicada para todos os reatores. Com o aumento do carregamento orgânico aplicado, observa-se o aumento e acúmulo de ácidos orgânicos para todas as fases dos ensaios, principalmente o acético. A produção de metano foi significativa na operação dos reatores em batelada, principalmente em C1. De forma geral, os resultados obtidos demonstraram o bom desempenho do processamento da vinhaça de cana-de açúcar em reator compartimentado tipo RAS para a remoção de matéria orgânica e produção de biogás. / The aim of this study was to investigate the application of an anaerobic serial reactor (ASR) in the sugarcane stillage processing for the removal of organic matter and biogas production, submitted to the gradual increase in organic loading rate. ASR reactor consisted of four reactors operated in series (C1, C2, C3 and C4), each with a reaction volume of 5 L (total volume of 20 L). The first step (Step 1) consisted of reactor operation and monitoring; organic loading rate was gradually increased from 0.5 kg COD m-3d-1 up to 2.5 kg COD m-3 d-1. In the second step (Step 2), sequential batch reactors were operated, simulating the continuous ASR reactor operation, aiming to study the effect of the increase of organic load applied to sludge (OLS). The ASR reactor operated under low values of COVglobal and COALglobal applied in Step 1, obtaining a maximum 2.50 kg COD m-3d-1 and 0.167 kg TVS/m-3d-1, respectively. The results obtained from Step 1 indicated satisfactory removal of the global average organic matter in the form of COD more than 90 %, caused by adaptation of the biomass along the sequential reactors. Methane appeared to be the predominant gas in biogas composition, with values above 75%, which confirmed the establishment of the full methanogenic system. In all compartments of the sequential reactors, there was a higher relative abundance for the domains Archaea and Bacteria of the genera Methanosaeta, methane producers, and T78, which is responsible for degradation of sugars, respectively. Especially for the reactors C3 and C4, the adaptation of some populations for both domains Bacteria and Archaea indicated was observed by the lower diversity index and increase in dominance. The increase of the organic loading rates applied to the sludge, as established in the batch tests, increased the organic load values applied to all reactors. With the increase of organic loading applied, an increase in the accumulation of organic acids, mainly acetic, for all stages and the batch tests. Methane production was significant in the operation of batch reactors, mainly in the C1. In general, the results obtained showed good performance of the processing of the sugarcane stillage in serial reactor (ASR) for the removal of organic matter and biogas production.