Seasonal cycle of precipitation variability in South America on intraseasonal timescales

The seasonal cycle of the intraseasonal (IS) variability of precipitation in South America is described through the analysis of bandpass filtered outgoing longwave radiation (OLR) anomalies. The analysis is discriminated between short (10--30 days) and long (30--90 days) intraseasonal timescales. The seasonal cycle of the 30--90-day IS variability can be well described by the activity of first leading pattern (EOF1) computed separately for the wet season (October--April) and the dry season (May--September). In agreement with previous works, the EOF1 spatial distribution during the wet season is that of a dipole with centers of actions in the South Atlantic Convergence Zone (SACZ) and southeastern South America (SESA), while during the dry season, only the last center is discernible. In both seasons, the pattern is highly influenced by the activity of the Madden--Julian Oscillation (MJO). Moreover, EOF1 is related with a tropical zonal-wavenumber-1 structure superposed with coherent wave trains extended along the South Pacific during the wet season, while during the dry season the wavenumber-1 structure is not observed. The 10--30-day IS variability of OLR in South America can be well represented by the activity of the EOF1 computed through considering all seasons together, a dipole but with the stronger center located over SESA. While the convection activity at the tropical band does not seem to influence its activity, there are evidences that the atmospheric variability at subtropical-extratropical regions might have a role. Subpolar wavetrains are observed in the Pacific throughout the year and less intense during DJF, while a path of wave energy dispersion along a subtropical wavetrain also characterizes the other seasons. Further work is needed to identify the sources of the 10--30-day-IS variability in South America

Windthrow variability in central Amazonia

Windthrows are a recurrent disturbance in Amazonia and are an important driver of forest dynamics and carbon storage. In this study, we present for the first time the seasonal and interannual variability of windthrows, focusing on Central Amazonia, and discuss the potential meteorological factors associated with this variability. Landsat images over the 1998-2010 time period were used to detect the occurrence of windthrows, which were identified based on their spectral characteristics and shape. Here, we found that windthrows occurred every year but were more frequent between September and February. Organized convective activity associated with multicell storms embedded in mesoscale convective systems, such as northerly squall lines (that move from northeast to southwest) and southerly squall lines (that move from southwest to northeast) can cause windthrows. We also found that southerly squall lines occurred more frequently than their previously reported ~50 year interval. At the interannual scale, we did not find an association between El Niño-Southern Oscillation (ENSO) and windthrows

Balances in the Atmosphere and Ocean: Implications for Forecasting and Reliability

Scale interactions between a variety of motions in the atmosphere and ocean have many theoretical and practical implications from predictability at the weather scales to reliability at the slow seasonal and climate scales. Two classes of wavy motions are prominent at the hydrostatic limit, for instance: the fast inertia-gravity waves and the slow Rossby waves. Although only Rossby waves are believed to be of direct meteorological significance, neglecting the fast oscillations may corrupt numerical integrations leading to unrealistic results and eventually to a complete model crash. Reliability of long seasonal and climate scales depends upon a proper representation of, at least, the statistics of the weather scale phenomena under given boundary conditions. The predictability of the weather scale phenomena, on the other hand, depends on the proper evolution of the system from a given initial condition. It has long been shown that a balance between stringent and permissive control of the high-frequency oscillations can allow improvements to weather forecasting. Behind these concepts are the ways by which Rossby waves can interact, horizontally and vertically, with high-frequency oscillations, or with other slow frequency oscillations and even with topography. Thus, in the present work we make a review of Rossby wave theory, considering its generation mechanisms and their interactions, including a brief discussion of some applications for the atmosphere