Este estudo discute uma climatologia de 59 anos (1949-2007) de bloqueios no Hemisfério Sul (SH) usando dados de altura geopotencial em 500-hPa das reanálises do National Center for Environmental Prediction / National Center for Atmospheric Research (NCEP-NCAR). A variabilidade espaço-temporal dos eventos de bloqueio e associações com o El Niño/Oscilação do Sul (ENOS) também foram examinadas. Adicionalmente, os bloqueios foram investigados em dois Modelos de Circulação Geral Acoplados Atmosfera-Oceano de clima (MCGAO) do Intergovernamental Painel for Climate Change (IPCC), o ECHAM5/MPI-OM e o MIROC 3.2. Dois cenários simulados foram analisados: O clima do século XX e o cenário de emissão A1B. Os episódios do ENOS foram identificados usando dois métodos. O primeiro foi o Índice Oceânico Mensal do Niño (ONI) do Climate Prediction Center (CPC-NCEP). O segundo método foi baseado em Funções Empíricas Ortogonais (EOF) e foi aplicado nos MCGAOs. Similarmente, também foi examinado a influencia combinada do ENOS e a Oscilação Antártica (AAO) na ocorrência e características dos bloqueios. O índice diário da AAO foi obtido pelo CPC-NCEP. Os índices convencionais de bloqueios detectam principalmente variações longitudinais. Este trabalho propõe um índice de bloqueio que detecta, além de variações longitudinais também as variações latitudinais dos bloqueios. Cinco setores relevantes de bloqueios foram examinados em detalhes: Indico Sudoeste (SB1), Pacífico Sudoeste (SB2), Pacífico Central (SB3), Pacífico Sudeste (SB4) e Atlântico Sudoeste (SB5). Além disso, foram investigados duas grandes regiões do Pacífico Sul: Pacífico Oeste e Pacífico Leste. Foi encontrado que a escala média típica dos eventos de bloqueio varia entre 1,5 e 2,5 dias. Além disso, a duração dos eventos depende da latitude, com eventos de maior duração observados em latitudes mais altas. Diferenças longitudinais estatisticamente significativas na freqüência do escoamento bloqueado foram observadas entre as fases Quente e Neutra do ENOS desde o outono a primavera. Episódios intensos da fase Quente do ENOS (isto é, moderados a fortes) tendem a modificar o local preferencial de bloqueio, mas não a freqüência. Por outro lado, os episódios fracos da fase Quente do ENOS estiveram associados relativamente com alta freqüência. Os Eventos de bloqueio durante o ENOS+ duram, em média, mais um dia relativamente aos episódios Neutros. Em contraste, a fase Fria do ENOS (ENOS-) caracterizou-se pela redução dos eventos de bloqueio sobre o setor do Pacífico Central, exceto durante os meses do verão austral. Entretanto, nenhuma diferença estatisticamente significativa foi detectada na duração dos eventos. Composições de anomalias de vento em 200-hPa indicam que o enfraquecimento (fortalecimento) do jato polar em torno de 60ºS durante a AAO negativa (positiva) em ambas as fases do ENOS tem uma importância significativa no aumento (redução) dos eventos de bloqueio. Um significativo aumento estatístico dos eventos sobre o setor do Pacífico Sudeste foi observado durante a AAO negativa em ambas as fases do ENOS. Ainda, um aumento (redução) dos eventos foi observado sobre a região do Pacífico Oeste na fase negativa (positiva) da AAO durante o ENOS-. Em contraste, durante o ENOS+ não houve diferenças estatisticamente significativas na distribuição longitudinal dos eventos separado de acordo com as fases opostas da AAO, embora haja um aumento (redução) dos eventos da região do Pacífico Oeste para o Pacífico Leste durante a fase negativa (positiva) da AAO. Os MCGAOs simularam corretamente a amplitude do ciclo anual observado. Também, ambos os MCGAOs simularam melhor a duração e o local preferencial do que freqüência. Nenhum MCGAO simulou adequadamente a freqüência durante a fase Neutra do ENOS. O ECHAM5/MPI-OM (rodada 2) mostra um erro sistemático que levam a uma superestimativa na freqüência de eventos sobre o Pacífico Leste durante as fases Neutra e Fria do ENOS. As diferenças entre as duas versões do MIROC 3.2 indicam que a alta resolução nos modelos melhora o desempenho em simular a freqüência de bloqueios. / This study discusses 59-yr climatology (1949-2007) of Southern Hemisphere (SH) blockings using daily 500-hPa geopotential height data from National Center for Environmental Prediction / National Center for Atmospheric Research (NCEP-NCAR reanalysis. The spatiotemporal variability of blocking events and associations with El Nino/Southern Oscillation (ENSO) are examined. Additionally, blockings were examined in two Intergovernmental Panel for Climate Change (IPCC) Coupled General Circulation Models (CGCM), ECHAM5/MPI-OM and MIROC 3.2. Two sets of simulations were examined: the climate of the 20th century and the A1B emission scenario. ENSO episodes were identified using two methods. The first method was the Monthly Oceanic Niño Index (ONI) from the Climate Prediction Center (CPC-NCEP). The second method was based on Empirical Orthogonal Function (EOF) and was applied to identify ENSO episodes in the CGCMs. Similarly, the combined influence of ENSO and the Antarctic Oscillation (AAO) on the occurrence and characteristics of blockings was also examined. The daily AAO index was obtained from CPC/NCEP. Most conventional blocking indices detect longitudinal variations of blockings. In this study we propose a new blocking index that detects longitudinal and latitudinal variations of blockings. The following relevant sectors of blocking occurrence were identified and examined in detail: Southeast Indian (SB1), Southwest Pacific (SB2), Central Pacific (SB3), Southeast Pacific (SB4) and Southwest Atlantic (SB5) oceans. In addition, we investigated two large regions of South Pacific: West Pacific and East Pacific. We found that the typical timescale of a blocking event is about ~1.5 2.5 days. Nonetheless, the duration of events depends on the latitude, with larger durations observed at higher latitudes. Statistically significant differences in the longitude of blockings are observed between Warm (ENSO+) and Neutral ENSO phases from the Austral fall to spring. Moderate to strong Warm ENSO episodes modulate the preferred locations of blockings but do not play a significant role for variations in their frequency. On the other hand, weak ENOS+ episodes were associated with relatively high frequency of blockings. Blocking events during ENSO+ last on average one more day compared to events that occur during Neutral episodes. In contrast, Cold (ENOS-) ENSO episodes are characterized by a decrease of blockings over the Central Pacific sector, except during the Austral summer months. However, no statistically significant differences are detected in the duration. Composites of 200-hPa zonal wind anomalies indicate that the weakening (strengthening) of the polar jet around 60oS during negative (positive) AAO phases in both ENSO phases plays a major role for the relative increase (decrease) of blocking events. A statistically significant increase of events over Southeast Pacific is observed during negative AAO phases in both ENSO phases. Moreover, an increase (decrease) of events is observed over West Pacific region when negative (positive) AAO phases occur during ENSO-. In contrast, during ENSO+ there is no statistically significant difference in the longitudinal distribution of events separated according to opposite AAO phases, although there is an increase (decrease) in the events from West Pacific region to East Pacific during negative (positive) AAO phase. The CGCMs investigated in this study correctly simulated the amplitude of observed annual cycle of geopotential height in the extratropics. Also, both CGCMs show a better performance in simulating the duration and preferred locations of blockings than their frequency. None of these CGCMs simulated well the frequency during Neutral ENSO phase. The ECHAM5/MPI-OM (run2) shows systematic biases in some regions. For instance, this model overestimates the frequency of blockings over East Pacific region during Cold and Neutral ENSO phases. The differences between the two versions of MIROC 3.2 indicate that the increase in resolution improves the performance of the model in simulating the frequency of blockings.

The effects of a non-zero background state on atmospheric dynamics is explored through simple models and observations. Firstly, we examine the effects of moisture gradients on the stability and propagation of Rossby waves in a mid-latitude -plane. We begin by a consistent derivation of the forced quasi-geostrophic equations on a -plane to understand the constraints placed by geostrophy on the time scale of condensation. We see that the presence of meridional gradients of moisture results in a slowdown of the waves. On the introduction of zonal gradients of moisture, the waves become unstable, and for certain parameters which are representative of the real atmosphere, they propagate eastward and mature on an intra-seasonal timescale. The mechanism of the in hence of moisture on waves is understood by thinking of condensation as providing an \equivalent" potential vorticity (PV) gradient which opposes the dynamical PV gradient. Secondly, we look at the effects of a mean background ow on the Matsuno-Gill response in the spherical shallow water system. The mean ow is prescribed to resemble the climatological upper tropospheric zonal wind structure in the atmosphere. As the strength of the ow increases, the equatorially trapped Matsuno-Gill response rst transforms into a poleward propagating Rossby wavetrain. As the strength of the mean ow reaches values similar to that observed in the atmosphere, the stationary wave response becomes a zonally oriented quadrupole structure. This structure bears a striking resemblance to the observed upper level structure of the Madden-Julian oscillation (MJO). The time evolution of this quadrupole structure is quick enough to be relevant on MJO timescales, and the structure is quite robust across a range of values for the drag coefficient. Finally, we look at the role played by low frequency variability in the Pacific in the recent expansion of the Hadley cell. We find that the dominant effect of the low frequency variability is a stationary dispersive Rossby wavetrain extending from the tropical Paci. We further find that most of the observed expansion of the Hadley cell can be accounted for by this low frequency variability. We nd that large scale changes such as the changes in the equator-pole temperature gradient or midlatitude static stability need not be invoked to understand the observed expansion.

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The ENSO-monsoon system is known to have a biennial component. Here we show using high resolution satellite data, mainly daily rainfall and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM), and daily scatterometer surface winds from QuickSCAT, that there is a clear biennial oscillation (TBO) in summer monsoon rainfall over Central India – Bay of Bengal (Cl-BoB) and the far west Pacific in the period 1999-2005. Summer (JJAS) mean rainfall oscillates between high and low values in alternate years; the rainfall is high in the odd years 1999, 2001, 2003, and 2005, and low in even years 2000, 2002 and 2004. The amplitude of the oscillation is significant, as measured against the long term standard deviation of seasonal rain based on 1979-2005 Global Precipitation Climatology Project (GPCP) data. We find that the TBO in rainfall is associated with TBO of SST over the tropical Indian, west Pacific and Atlantic Oceans in different seasons. There is no TBO in east Pacific SST, and no strong El Nino in this period. The TBO of SST is related to change in evaporation due to TBO of surface wind speed. A TBO of the surface branch of the Walker circulation in the eastern Indian and western Pacific basins is clearest in the autumn season during 1999-2005. There is a clear relation between a large-amplitude TBO of winter surface air temperature over north Asia associated with TBO of the Arctic oscillation (AO), and the TBO of summer monsoon rainfall. High rainfall over CI-BoB lin summer is followed by a relatively high value of the AO Index, and warm air termperature over north Asia in the succeeding winter. The Inter Tropical Convergence Zone(ITCZ) over the central Pacific and Atlantic Oceans shift north by about two degrees when the northern hemisphere is warm, reminiscent of the behaviour of the climate system of ENSO, decadal and palaeoclimate time scales. In this thesis we document the biennial oscillation of monsoon rain and its spatial structure in the recent period, and its relation with biennial oscillation of surface climate over the global tropics and extratropical regions. The existence of TBO in the tropical Atlantic, and its relation with the monsoon, is a new finding. We demonstrate that the interannual variability of the summer monsoon during 1999-2005, including the drought of 2002, is part of a pervasive TBO of global surface climate.

Indian summer monsoon (ISM) is an important component of the tropical climate system, known for its regular seasonality and abundance of rainfall over the country. The droughts and floods associated with the year-to-year variation of the average seasonal rainfall have devastating effect on people, agriculture and economy of this region. The demand for prediction of seasonal monsoon rainfall, therefore, is overwhelming. A number of attempts to predict the seasonal mean monsoon have been made over a century, but neither dynamical nor empirical models provide skillful forecasts of the extremes of the monsoon such as the unprecedented drought of 2002. This study investigates the problems and prospects of extended range monsoon prediction. An evaluation of the potential predictability of the ISM with the aid of an ensemble of Atmospheric General Circulation Model (AGCM) simulations indicates that the interannual variability (IAV) of ISM is contributed equally by the slow boundary forcing (‘externally’ forced variability) and the inherent climate noise (‘internal’ variability) in the atmosphere. Success in predicting the ISM would depend on our ability to extract the predictable signal from a background of noise of comparable amplitude. This would be possible only if the ‘external’ variability is separable from the ‘internal’ variability. A serious effort has been made to understand and isolate the sea surface temperature (SST) forced component of ISM variability that is not strongly influenced by the ‘internal’ variability. In addition, we have investigated to unravel the mechanism of generation of ‘internal’ IAV so that the method of isolating it from forced variability may be found. Since the primary forcing mechanism of the monsoon is the large-scale meridional gradient of deep tropospheric heat sources, large-scale changes in tropospheric temperature (TT) due to the boundary forcing can induce interannual variations of the timing and duration of the monsoon season. The concept of interannually varying monsoon season is introduced here, with the onset and withdrawal of monsoon definitions based on the reversal of meridional gradient of TT between north and south. This large scale definition of the monsoon season is representative of the planetary scale influence of the El Ni˜no Southern Oscillation (ENSO) on monsoon through the modification of TT and the cross equatorial pressure gradient over the ISM region. A sig- nificant relationship between ENSO and monsoon, that has remained steady over the decades, is discovered by which an El Ni˜no (La Ni˜na) delays (advances) the onset, advances (delays) the withdrawal and suppresses (enhances) the strength of the monsoon. The integral effect of the meridional gradient of TT from the onset to withdrawal proves to be a useful index of seasonal monsoon which isolates the boundary forced signal from the influence of internal variations that has remained steady even in the recent decades. However, consistent with the estimates of potential predictability, the boundary forced variability isolated with the above definitions explains only about 50% of the total interannual variability of ISM. Detailed diagnostics of the onset and withdrawal processes are performed to understand how the ENSO forcing modifies the onset and withdrawal, and thus the seasonal mean monsoon. It is found that during an El Ni˜no, the onset is delayed due to the enhanced adiabatic subsidence that inhibits vertical mixing of sensible heating from the warm landmass during pre-monsoon months, and the withdrawal is advanced due to the horizontal advective cooling. This link between ENSO and monsoon is realized through the advective processes associated with the stationary waves in the upper troposphere set up by the tropical ENSO heating. The remaining 50% of the monsoon IAV is governed by internal processes. To unravel the mechanism of the generation of internal IAV, we perform another set of AGCM simulations, forced with climatological monthly mean SSTs, to extract the pure internal IAV. We find that the spatial structure of the intraseasonal oscillations (ISOs) in these simulations has significant projection on the spatial structure of the seasonal mean and a common spatial mode governs both intraseasonal and interannual variability. Statistical average of ISO anomalies over the season (seasonal ISO bias) strengthens or weakens the seasonal mean. It is shown that interannual anomalies of seasonal mean are closely related to the seasonal mean of intraseasonal anomalies and explain about 50% of the IAV of the seasonal mean. The seasonal mean ISO bias arises partly due to the broadband nature of the ISO spectrum, allowing the intraseasonal time series to be aperiodic over the season and partly due to a non-linear process where the amplitude of ISO activity is proportional to the seasonal bias of ISO anomalies. The later relationship is a manifestation of the binomial character of the rainfall time series. The remaining part of IAV may arise due to the complex land-surface processes, scale interactions, etc. We also find that the ISOs over the ISM region are not significantly modulated by the Pacific and Indian Ocean SST variations. Thus, even with a perfect prediction of SST, only about 50% of the observed IAV of ISM could be predicted with the best model in forced mode. Even so, prediction of all India rainfall (AIR) representing the average conditions of the whole country and the season may not always serve the purposes of monsoon forecasting. One reason is the large inhomogeneities in the rainfall distribution during a normal seasonal monsoon. Agriculture and hydrological sector could benefit more if provided with regional scale forecasts of active/break spells 2-3 weeks ahead. Therefore, we advocate an alternative strategy to the seasonal prediction. Here, we present a method to estimate the potential predictability of active and break conditions from daily rainfall and circulation from observations for the recent 24 years. We discover that transitions from break to active conditions are much more chaotic than those from active to break, a fundamental property of the monsoon ISOs. The potential predictability limit of monsoon breaks (∼20 days) is significantly higher than that of the active conditions (∼10 days). An empirical real- time forecasting strategy to predict the sub-seasonal variations of monsoon up to 4 pentads (20 days) in advance is developed. The method is physically based, with the consideration that the large-scale spatial patterns and slow evolution of monsoon intraseasonal variations possess some similarity in their evolutions from one event to the other. This analog method is applied on NOAA outgoing longwave radiation (OLR) pentad mean data which is available on a near real time basis. The elimination of high frequency variability and the use of spatial and temporal analogs produces high and useful skill of predictions over the central and northern Indian region for a lead-time of 4-5 pentads. An important feature of this method is that, unlike other empirical methods to forecast monsoon ISOs, this uses minimal time filtering to avoid any possible end-point effects, and hence it has immense potential for real-time applications.

We used long-term observations of grassland aboveground net plant production (ANPP, 19392016), growing seasonal advanced very-high-resolution radiometer remote sensing normalized difference vegetation index (NDVI) data (1982-2016), and simulations of actual evapotranspiration (1912-2016) to evaluate the impact of Pacific Decadal Oscillation (PDO) and El Nino-Southern Oscillation (ENSO) sea surface temperature (SST) anomalies on a semiarid grassland in northeastern Colorado. Because ANPP was well correlated (R-2 = 0.58) to cumulative April to July actual evapotranspiration (iAET) and cumulative growing season NDVI (iNDVI) was well correlated to iAET and ANPP (R-2 = 0.62 [quadratic model] and 0.59, respectively), we were able to quantify interactions between the long-duration (15-30 yr) PDO temperature cycles and annual-duration ENSO SST phases on ANPP. We found that during cold-phase PDOs, mean ANPP and iNDVI were lower, and the frequency of low ANPP years (drought years) was much higher, compared to warm-phase PDO years. In addition, ANPP, iNDVI, and iAET were highly variable during the cold-phase PDOs. When NINO-3 (ENSO index) values were negative, there was a higher frequency of droughts and lower frequency of wet years regardless of the PDO phase. PDO and NINO-3 anomalies reinforced each other resulting in a high frequency of above-normal iAET (52%) and low frequency of drought (20%) when both PDO and NINO-3 values were positive and the opposite pattern when both PDO and NINO-3 values were negative (24% frequency of above normal and 48% frequency of drought). Precipitation variability and subsequent ANPP dynamics in this grassland were dampened when PDO and NINO-3 SSTs had opposing signs. Thus, primary signatures of these SSTs in this semiarid grassland are (1) increased interannual variability in ANPP during cold-phase PDOs, (2) drought with low ANPP occurring in almost half of those years with negative values of PDO and NINO-3, and (3) high precipitation and ANPP common in years with positive PDO and NINO-3 values.

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