Subtropical cyclones are hybrid systems presenting a warm core at low levels, as tropical cyclones, and a cold core at upper levels, as extratropical cyclones. Their genesis can be as proper subtropical system or from the transitions (extratropical to subtropical or tropical to subtropical). Subtropical cyclones occur mostly over the sea and generate intense near surface winds with great impacts on human activities and ecosystems. We present a review of the main features, as obtained from observations and numerical simulations, for subtropical cyclone development over the major oceanic basins.

No abstract

No abstract

This work investigates how regional climate models REMO and RegCM4 simulate the frequency of cold fronts (CFs) over southern Brazil and the contribution of these systems to the seasonal precipitation over the southern part of La Plata Basin (SLPB) in South America. Simulations were driven by the ERA-Interim reanalysis and compared with local observations (from Rio Grande station; RG), ERA-Interim, and rainfall analysis from the Climate Prediction Center (CPC). CF identification was carried out objectively by considering (1) the turning of the meridional wind component from north to south and (2) a decrease in air temperature between 1 d before and 1 d after the CF passage. For the period 1991−2008, a mean of 55.3, 53.1 and 51.8 CFs yr −1 were identified, respectively, for RG (observed), REMO and RegCM4. These values show a small bias in the simulated annual frequency of CFs (around −5%), but the underestimation reaches −17% during summer in the RegCM4. In the simulations, it is possible to find some association between seasonal bias of rainfall and the bias of rainfall during CF periods (defined as extending from 2 d before to 2 d after the CF passage) over SLPB. Except in summer, RegCM4 presents stronger biases than REMO for rainfall for both CF periods and overall seasonal climatology. For the winter season, the CF composites indicate that deeper low pressure and larger availability of moisture at low levels (from 2 d before to the day of the CF) contribute to the small underestimation of rainfall in REMO (−5%) compared with RegCM4 (−25%).

Southeastern Brazil is the most populated and economically developed region of this country. Its climate consists of two distinct seasons: the dry season, extending from April to September, the precipitation is significantly reduced in comparison to that of the wet season, which extends from October to March. However, during nine days of the 2016 dry season, successive convective systems were associated with atypical precipitation events, tornadoes and at least one microburst over the southern part of this region. These events led to flooding, damages to buildings, shortages of electricity and water in several places, many injuries, and two documented deaths. The present study investigates the synoptic and dynamical features related to these anomalous events. The convective systems were embedded in an unstable environment with intense low-level jet flow and strong wind shear and were supported by a sequence of extratropical cyclones occurring over the Southwest Atlantic Ocean. These features were intensified by the Madden–Julian oscillation (MJO) in its phase 8 and by intense negative values of the Pacific South America (PSA) 2 mode.