The intraseasonal Oscillation (ISO) is a very strong and coherent mode of variability observed in the Earths climate. Rainfall variability in the intraseasonal timescale is particularly strong in the Tropics and it directly interacts with the South Asian monsoon during boreal summer and with the Australian monsoon during winter. A detailed analysis of the horizontal and vertical structure of the ISO during both summer and winter is presented in this work considering the coupled ocean-atmosphere system. In addition, the role of the intraseasonal variability of the Southeast Asian monsoon is studied in detail. From the applications point of view, the intraseasonal time scale is arguably the most important period of variability. However, extended forecasting of intraseasonal activity has proven to be a difficult task for the state of the art numerical models. In order to improve the forecasts of the ISO activity over the Southeast Asian monsoon region, a physically based empirical scheme was designed. The scheme uses wavelet banding to separate the predictand and predictors into physically significant bands where linear regression followed by recombination of the bands is used to generate the forecast. Results of the empirical scheme suggest that isolating the evolution of the intraseasonal signal from higher frequency variability and noise improve the skill of the prediction. The hypothesis is that a similar phenomenon occurs in numerical models: The strong intraseasonal signal is eroded by high frequency errors due to the model parameterizations, especially in convection. To evaluate the hypothesis, a coupled ocean-atmosphere model was run in ensemble mode for 30 day periods initialized daily for 20 days before to 20 days after major intraseasonal oscillations, allowing the examination of the skill of the model relative to the phase of the oscillation. The results, which confirm the previous hypothesis, represent well the observations for about 7 days after which the magnitude of the errors is greater than the signal itself. An integration scheme was developed for the coupled ocean-atmosphere general circulation model in order to mimic the philosophy of the empirical scheme and use for 30-day forecasts. The propagation features associated to ISO activity are improved.

Intraseasonal oscillations (ISOs) are important elements of the tropical climate with time-scales of 20-80 day. The ISO is poorly simulated and predicted by numerical models. This work presents a joint diagnostic and modeling study of the ISO that examines the hypothesis that local coupling between the ocean and the atmosphere is essential to the existence and evolution of the ISO in the Indo-Pacific warm pool region. Low-level moistening during the transition phase preconditions the atmosphere for deep convection. The vertical structure of ISO from the ECMWF coupled model during different phases of the oscillation as well as the skill of the model in simulating the processes that occur during the transition phase were studied. The forecast skill of the vertical structure associated with the ISO is greater for winter than for summer events. Predictability of the convective period is poor when initialized before the transitional phase. When initialized within the transition period including lower tropospheric moistening, predictability increases substantially, although the model parameterizations appears to trigger convection quickly without allowing an adequate buildup of CAPE during the transition. The model tends to simulate a more stable atmosphere compared to data, limiting the production of deep convective events. Two different one-dimensional coupled models are used to analyze the role of local ocean-atmosphere coupling in generating ISO. The ocean component is a one-dimensional mixed layer model. In the first model the atmospheric component corresponds to the SCCM. Results suggest that convection in the model tends to be "overactive," inhibiting development of lower frequency oscillations in the atmosphere. In the second case, the atmospheric component is a semi-empirical model that allows reproducing the coupled ISO over long integration periods including only local mechanisms. In the semi-empirical scheme the rate of change of atmospheric variables is statistically related to changes in SST. The stable state of this model is a quasi-periodic oscillation with a time scale between 25 and 80 days that matches well the observed ISO. Results suggest that the period of the oscillation depends on the characteristics of the ocean mixed layer, with a higher frequency oscillation for a shallow mixed layer.

Information from the TRMM-CSH and TRMM-2A12 datasets is used to examine the four-dimensional latent heating (LH) structures over the Asian monsoon region between 1998 and 2006. High sea surface temperatures, ocean-land contrasts and complex terrain produce large precipitation and atmospheric heating rates whose spatial and temporal characteristics are relatively undocumented. Analyses identify interannual and intraseasonal LH variations, with a large fraction of the interannual variability induced by internal intraseasonal variability. Also, the analyses identify a spatial dipole of LH anomalies between the equatorial Indian Ocean and the Bay of Bengal regions occurring during the summer active and suppressed phases of the monsoon intraseasonal oscillation. Comparisons made between the TRMM-CSH and TRMM-2A12 datasets indicate significant differences in the shape of the vertical profile of LH. Comparison of TRMM-LH retrievals with sounding budget observations made during the South China Sea Monsoon experiment shows a high correspondence in the timing of positive LH episodes during rainy periods. Negative values of LH, associated with radiative cooling and with higher troposphere cooling from non-precipitating clouds, are not captured by any of the TRMM datasets. In summary, LH algorithms based on satellite information are capable of representing the spatial and temporal characteristics of the vertically integrated heating in the Asian monsoon region. The TRMM-CSH presents better performance than TRMM-2A12. However, the vertical distribution of atmospheric heating is not captured accurately throughout all different convective phases. It is suggested that satellite derived radiative heating/cooling products are needed to supplement the LH products in order to give an overall better depiction of atmospheric heating.

Submitted by Maria Medeiros (maria.dilva1@ufcg.edu.br) on 2018-08-17T14:30:53Z No. of bitstreams: 1 EDICARLOS PEREIRS DE SOUSA - TESE (PPGMet) 2016.pdf: 7117312 bytes, checksum: 96b3ee401a9f1387b098676da0e967eb (MD5) / Made available in DSpace on 2018-08-17T14:30:53Z (GMT). No. of bitstreams: 1 EDICARLOS PEREIRS DE SOUSA - TESE (PPGMet) 2016.pdf: 7117312 bytes, checksum: 96b3ee401a9f1387b098676da0e967eb (MD5) Previous issue date: 2016-05-31 / Capes / O estado de Minas Gerais, localizado na região Sudeste do Brasil, possui um território planáltico, com áreas mais elevadas situadas na porção sul. O clima é predominantemente tropical e tropical de altitude, com temperaturas oscilando, normalmente, entre 17 e 20°C. Tais características o tornam, muitas vezes, susceptível a fortes chuvas, provocando transtornos diversos à população. Desse modo, buscou-se analisar a variabilidade da chuva mineira através de técnicas que fossem capazes de definir padrões espaciais e temporais de eventos secos e chuvosos, assim como modulá-los nas escalas tempo-frequência. Para isso, calculou-se o Índice de Precipitação Normalizada (IPN) mensal e trimestral no período 1977-2012. A fim de encontrar padrões espaço-temporais e regiões homogêneas (RH) do IPN mensal e trimestral, utilizaram-se as técnicas da Análise em Componentes Principais (ACP) e Agrupamentos (AA). A técnica da Transformada de Ondeletas (TO) foi aplicada para algumas localidades das diferentes RH em diversas escalas. A TO possibilitou encontrar oscilações importantes no sinal da precipitação, mostrando as componentes dominantes da variabilidade da chuva na área pesquisada. Nos espectros de fase e global de energia da TO, para a chuva mensal, predominou o ciclo anual em todas as localidades. Além da escala anual, observaram-se interações com escalas inferiores a doze meses, possivelmente decorrentes da sazonalidade da precipitação. A TO da precipitação e da Radiação de Onda Longa (ROL) diárias evidenciou frequências maiores que as da chuva mensal. A análise dos espectros de fase e de energia global, nas regiões centro-sul e oeste, apresentou maior energia no início das séries da precipitação e de ROL. Contribuições mais elevadas ocorreram nas escalas sinóticas e intrassazonais, principalmente nas localidades de Viçosa, Bom Despacho, Ituiutaba e Lavras. Esses padrões temporais estão associados com a Oscilação de Madden-Julian (OMJ), intensificando a atuação de sistemas moduladores do clima de Minas Gerais. Oscilações em escalas menores que dez dias foram detectadas nos anos de 2011 e 2012 e podem estar associadas aos sistemas de curto prazo que contribuíram para a chuva mineira. No caso de 2011, os eventos de chuva observados em meados de janeiro foram modulados pelos episódios da Zona de Convergência do Atlântico Sul (ZCAS) e reforçados pela atuação da OMJ. / The state of Minas Gerais, located in southeastern Brazil, has a plateaux area, with higher areas in the southern part. The climate is predominantly tropical and tropical altitude, with temperatures ranging normally between 17 and 20°C. These characteristics make it often susceptible to heavy rains, causing many disorders the population. Thus, it sought to analyze the variability of the rain of Minas Gerais through techniques that were able to define spatial and temporal patterns of dry and wet events, and modulate them in the time-frequency ranges. For this, we calculated the Standardized Precipitation Index (SPI) monthly and quarterly in the period 1977-2012. In order to find spatiotemporal patterns and homogeneous regions (HR) of the monthly and quarterly SPI, we used the techniques of Principal Components Analysis (PCA) and Cluster Analysis (CA). The technique of Wavelets Transform (WT) has been applied to some locations of the different HR at various scales. The WT possible find important variations in rainfall signal, showing the dominant components of rainfall variability in the studied area. In phase spectra and global energy of WT, for the monthly rainfall, dominated the annual cycle in all locations. In addition to the annual scale, there were interactions with scales less than twelve months, possibly due to the seasonality of precipitation. The WT of precipitation and Outgoing Longwave Radiation (OLR) daily reported higher frequencies than the monthly rainfall. Analysis of phase spectra and global power, in south-central and western regions, showed higher energy at the beginning of the series of precipitation and OLR. Higher contributions occurred in the synoptic and intraseasonal scales, especially in the localities of Viçosa, Bom Despacho, Ituiutaba and Lavras. These temporal patterns are associated with the Madden-Julian Oscillation (MJO), intensifying the activity of modulators systems climate of Minas Gerais. Oscillations at smaller scales than ten days were detected in 2011 and 2012 and may be associated with short-term systems that contributed to the Minas Gerais rainfall. In the case of 2011 rain events observed in mid-January they were modulated by episodes of South Atlantic Convergence Zone (SACZ) and reinforced by the performance of MJO.

Interannual variation of Indian summer (June-September: JJAS) monsoon rainfall (ISMR) depends on its relative intensity during early (June-July: JJ; contribution 52%) and late (August-September: AS; contribution 49%) phases. Apart from variations in sea surface temperature (SST), the primary reasons behind the variability during JJ and AS can be very different due to change in climatic conditions on account of post-onset processes. Here, using a high resolution general circulation model with seasonally varying climatological SST, mechanisms those govern the intensity of rainfall during JJ and AS are investigated. There is no significant relation-ship between intensity of precipitation over Indian region in JJ and AS. Moreover, the factors determining early monsoon (JJ) precipitation are different than that for late monsoon (AS). In absence of interannual SST variation, pre-monsoon soil moisture do not play a significant role for the interannual variation of monsoon precipitation over India. A large scale oscillation of the ITCZ is noticed on interannual time scale spanning from around 60◦E to 150◦E that brings spatially coherent flood and drought over this region. Early monsoon precipitation has a larger dependency on spring snow depth over Eurasia and phase of the upper tropospheric Rossby wave in May. However, late monsoon precipitation over India is mainly governed by the intensity and time scale of the intraseasonally varying convective cloud bands. This study suggests that early monsoon (JJ) precipitation over Indian region is more correlated with pre-monsoon signatures of land-atmosphere parameters. However, in later parts after the onset (AS), the monsoon intensity is primarily driven by its internal dynamics and characteristics of intraseasonal oscillation.

In this thesis, we investigated modes of intraseasonal variability (ISV) observed in the Indian monsoon rainfall and how these modes modulate rainfall over India. We identified a decreasing trend in the intensity of low-frequency intraseasonal mode with increasing strength in synoptic variability over India. We also made an attempt to understand the reason for these observed trends using numerical simulations. In the first part of the thesis, satellite rainfall estimates are used to understand the spatiotem-poral structures of convection in the intraseasonal timescale and their intensity during boreal sum-mer over south Asia. Two dominant modes of variability with periodicities of 10–20-days (high-frequency) and 20–60-days (low-frequency) are found, with the latter strongly modulated by sea surface temperature. The 20–60-day mode shows northward propagation from the equatorial In-dian Ocean linked with eastward propagating modes of convective systems over the tropics. The 10–20-day mode shows a complex space-time structure with a northwestward propagating anoma-lous pattern emanating from the Indonesian coast. This pattern is found to be interacting with a structure emerging from higher latitudes propagating southeastwards. This could be related to ver-tical shear of zonal wind over northern India. The two modes exhibit variability in their intensity on the interannual time scale and contribute a significant amount to the daily rainfall variability in a season. The intensities of the 20–60-day and 10–20-day modes show significantly strong inverse and direct relationship, respectively, with the all-India June–September rainfall. This study also establishes that the probability of occurrence of substantial rainfall over central India increases significantly if the two intraseasonal modes simultaneously exhibit positive anomalies over the region. There also exists a phase-locking between the two modes. In the second part of the thesis, we investigated the changing nature of these intraseasonal modes over Indian region, and their association with extreme rainfall events using ground based observed rainfall. We found that the relative strength of the northward propagating 20–60-day mode has a significant decreasing trend during the past six decades, possibly attributed to the weakening of large-scale circulation in the region during monsoon. This reduction is compensated by a gain in synoptic-scale (3–9 days) variability. The decrease in the low-frequency ISV is associated with a significant decreasing trend in the percentage of extreme events during the active phase of the monsoon. However, this decrease is balanced by a significant increasing trend in the percentage of extreme events in break phase. We also find a significant rise in occurrence of extremes during early- and late-monsoon months, mainly over the eastern coastal regions of India. We do not observe any significant trend in the high-frequency ISV. In the last part of the thesis, we used numerical simulations to understand the observed changes in the ISV features. Using the atmospheric component of a global climate model (GCM), we have performed two experiments: control experiment (CE) and heating experiment (HE). The CE is the default simulation for 10 years. In HE, we prescribed heating in the atmosphere in such a way that it mimics the conditions for extreme rainfall events as observed over central India during June– September. Heating is prescribed primarily during the break phase of the 20–60-day mode. This basically increases the number of extremes, majority of which are in break phase. The design of the experiment reflects the observed current scenario of increased extreme events during breaks. We found that the increased extreme events in the HE decreased the intensity of the 20–60-day mode over the Indian region. This reduction is associated with a reduction of rainfall in active phase and increase in the length of break phase. A reduction in the seasonal mean over India is also observed. The reduction of active phase rainfall is linked with an increased stability of the atmosphere over central India. Lastly, we propose a possible mechanism for the reduction of rainfall in active phase. We found that there is a significant reduction in the strength of the vertical easterly shear over the northern Indian region during break–active transition phase. This basically weakens the conditions for the growth of Rossby wave instability, thereby elongating break phase and reducing the rainfall intensity in the following active phase. This study highlights the redistribution of rainfall intensity among periodic (low-frequency) and non-periodic (extreme) modes in a changing climate scenario, which is further tested in a modeling study. The results presented in this thesis will provide a pathway to understand, using observations and numerical model simulations, the ISV and its relative contribution to the Indian summer monsoon. It can also be used for model evaluation.