No abstract

Hurricane Catarina (2004) and subtropical storm Anita (2010) called attention to the development of subtropical cyclones (SCs) over the South Atlantic basin. Besides strong and organized storms, a large number of weaker, shallower cyclones with both extratropical and tropical characteristics form in the region, impacting the South American coast. The main focus of this study is to simulate a climatology of subtropical cyclones and their synoptic pattern over the South Atlantic, proposing a broader definition of these systems. In addition, a case study is presented to discuss the main characteristics of one weak SC. The Interim ECMWF Re-Analysis (ERAInterim) and NCEP-NCAR reanalysis are used to construct the 33-yr (1979-2011) climatology, and a comparison between them is established. Both reanalyses show good agreement in the SCs' intensity, geographical distribution, and seasonal variability, but the interannual variability is poorly correlated. Anomaly composites for austral summer show that subtropical cyclogenesis occurs under a dipole-blocking pattern in upper levels. Upward motion is enhanced by the vertical temperature gradient between a midtropospheric cold cutoff low/trough and the intense low-level warm air advection by the South Atlantic subtropical high. Turbulent fluxes in the cyclone region are not above average during cyclogenesis, but the subtropical high flow advects great amounts of moisture from distant regions to fuel the convective activity. Although most of the SCs develop during austral summer (December-February), it is in autumn (March-May) that the most ''tropical'' environment is found (stronger surface fluxes and weaker vertical wind shear), leading to the most intense episodes.

ABSTRACT:This work is an assessment of frequency of extreme values (EVs) of daily rainfall in the city of Sao Paulo, Brazil, over the period , based on the peaks-over-threshold (POT) and Generalized Pareto Distribution (GPD) approach. Usually, a GPD model is fitted to a sample of POT values selected with a constant threshold. However, in this work we use time-dependent thresholds, composed of relatively large p quantiles (for example p of 0.97) of daily rainfall amounts computed from all available data. Samples of POT values were extracted with several values of p. Four different GPD models (GPD-1, GPD-2, GPD-3, and GDP-4) were fitted to each one of these samples by the maximum likelihood (ML) method. The shape parameter was assumed constant for the four models, but time-varying covariates were incorporated into scale parameter of GPD-2, GPD-3, and GPD-4, describing annual cycle in GPD-2, linear trend in GPD-3, and both annual cycle and linear trend in GPD-4. The GPD-1 with constant scale and shape parameters is the simplest model. For identification of the best model among the four models we used rescaled Akaike Information Criterion (AIC) with second-order bias correction. This criterion isolates GPD-3 as the best model, i.e. the one with positive linear trend in the scale parameter. The slope of this trend is significant compared to the null hypothesis of no trend, for about 98% confidence level. The non-parametric Mann-Kendall test also showed presence of positive trend in the annual frequency of excess over high thresholds, with p-value being virtually zero. Therefore, there is strong evidence that high quantiles of daily rainfall in the city of Sao Paulo have been increasing in magnitude and frequency over time.

ABSTRACT.A climatology of cold fronts at the coast of State do Santa Catarina, Brazil, is established based on 10-year period (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999) of NCEP-NCAR reanalysis data. The cold front passages were objectively identified taking into account the wind shifts to a southerly direction, as well as the persistence of southerly wind for at least one day, and a decreasing of air temperature. The results show that, on the average, 3 to 4 cold fronts reach coast of Santa Catarina, each month, with an interval of 8 days. The composite data analysis, using the date of days of cold front passages at Santa Catarina as reference, shows clearly a climatological pattern of evolution, with cold front moving typically from southwest to northeast. In terms of seasonal mean, during winter, one day before reaching coast of Santa Catarina (day -1) the cold front is observed at Rio Grande do Sul. After passing at Santa Catarina (day 0), it continues to move equatorward, reaching the coast of Rio de Janeiro one day later (day +1), until it reaches latitudes close to 20˚S (day +2), in the its stage of dissipation. In the third day, after the passage at Santa Catarina, the cold front is no more observed. Similar pattern of evolution is observed during summer, though the cold fronts are less intense and of slightly more rapid dissipation.Keywords: climatology of cold fronts, movement of cold fronts, cold fronts at the coast of Santa Catarina, Brazil. RESUMO. Uma climatologia de frentes friasé estabelecida para o litoral do Estado de Santa Catarina, com base nos dados de reanálises do NCEP-NCAR, do períodode 10 anos, de 1990 a 1999. As passagens de frentes frias foram objetivamente identificadas levando em conta o giro do vento para direção sul, persistência do vento de sul por pelo menos um dia, e uma queda de temperatura do ar simultânea ao giro do vento ou até dois dias depois. Os resultados mostram que, na média, de 3 a 4 frentes frias atingem a costa de Santa Catarina, mensalmente, com um intervalo de 8 dias. As análises da composição de dados, usando como referência os dias de passagens de frentes frias em Santa Catarina, mostram claramente um padrão climatológico de evolução com frente fria movendo-se tipicamente de sudoeste para nordeste. Em termos das médias sazonais, durante o inverno, um dia antes da passagem em Santa Catarina (dia -1), a frente friaé observada no Rio Grande do Sul, e depois de passar por Santa Catarina (dia 0), continua avançando para latitudes mais baixas, alcançando o litoral do Rio de Janeiro um dia depois (dia +1), até atingir posteriormente latitude de aproximadamente 20˚S (dia +2), já em fase de dissipação. No terceiro dia após a passagem por Santa Catarina, ela nãoé mais identificada.Padrão climatológico de evolução semelhanteé observado no verão, embora nessaépoca as frentes frias tenham menor intensidade e dissipação ligeiramente mais rápida.Palavras-chave: climatologia das frentes frias, movimento das frentes frias, frentes frias no litoral de Santa Catar...

This work presents analyses of the atmospheric conditions and the hindcast of the surface wave field when six extratropical cyclones formed and displaced over the South Atlantic Ocean (10ЊN, 60ЊS; 75ЊW, 15ЊE) between April and September 1999. These events caused high sea waves associated with hazardous conditions along the south and southeast coast of Brazil. The meteorological composite fields for these cyclones show a strong nearsurface wind velocity (up to 14 m s Ϫ1) during its mature phase. The sea-state wave hindcast was obtained using a third-generation wave model forced by the 10-m above ground level wind field from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis dataset. Closer to the south and southeast Brazilian coast, the hindcast results showed significant wave heights of up to 5 m in some of the events. The wave hindcast results for the significant wave height were compared against satellite altimeter data at 6 h intervals. The statistical index showed a systematic underestimation of the significant wave height by 0.5 m. The correlation between wave hindcast and altimeter measurements was greater than 90%, showing a good phase reproduction by the wave model.