This paper reviews recent progress made in our understanding of the functioning and variability of the South American Monsoon System (SAMS) on time scales varying from synoptic to long-term variability and climate change. The SAMS contains one of the most prominent summertime climate patterns in South America, featuring a strong seasonal variability in a region lying between the Amazon and the La Plata Basin. Much of the recent progress is derived from complementary international programs, such as the Monsoon Experiment South America (MESA), as well as from ongoing international programs such as the Large Scale Biosphere Atmosphere Experiment in the Amazon Basin (LBA) and the La Plata Basin (LPB) Regional Hydroclimate Project, which includes the CLARIS LPB Europe-South America Network for Climate Change Assessment and Impact Studies in La Plata Basin Project. The latter assesses atmosphere-land surface interactions, the role of land use changes and aerosols from biomass burning considered as sources of variability and change in the SAMS functioning, characteristics and behaviour.The SAMS region is particularly susceptible to variations of climate due to the importance of hydroelectricity generation and the agricultural base of local economies. Also addressed in this report are projections of climate change and extremes, which are important for impact and vulnerability assessments. This discussion includes the need to identify and understand important processes that control the monsoonal climate, how these processes may vary and change, and how they may interact with key societal sectors, including water resource management, hydroelectric generation, agriculture, and agribusiness. This paper reports on the major contributions of MESA to the knowledge of characteristics, functioning and variability of the SAMS, and is based on recent studies and publications, and can be considered as an update of a previous review by C. S. Vera et al. (2006a).

An objective index of the onset and demise of the Indian summer monsoon (ISM) is introduced. This index has the advantage of simplicity by using only one variable, which is the spatially averaged all‐India rainfall, a reliably observed quantity for more than a century. The proposed onset index is shown to be insensitive to all historic false onsets. By definition, now the seasonal mean rainfall anomalies become a function of variations in onset and demise dates, rendering their monitoring to be very meaningful. This new index provides a comprehensive representation of the seasonal evolution of the ISM by capturing the corresponding changes in large‐scale dynamic and thermodynamic variables. We also show that the interannual variability of the onset date of the ISM is associated with El Niño–Southern Oscillation (ENSO) with early (late) onsets preceded by cold (warm) ENSO.

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We present an analysis of the seasonal, subseasonal, and diurnal variability of rainfall from COAPS LandAtmosphere Regional Reanalysis for the Southeast at 10-km resolution (CLARReS10). Most of our assessment focuses on the representation of summertime subseasonal and diurnal variability. Summer precipitation in the Southeast United States is a particularly challenging modeling problem because of the variety of regional-scale phenomena, such as sea breeze, thunderstorms and squall lines, which are not adequately resolved in coarse atmospheric reanalyses but contribute significantly to the hydrological budget over the region. We find that the dynamically downscaled reanalyses are in good agreement with station and gridded observations in terms of both the relative seasonal distribution and the diurnal structure of precipitation, although total precipitation amounts tend to be systematically overestimated. The diurnal cycle of summer precipitation in the downscaled reanalyses is in very good agreement with station observations and a clear improvement both over their ''parent'' reanalyses and over newer-generation reanalyses. The seasonal cycle of precipitation is particularly well simulated in the Florida; this we attribute to the ability of the regional model to provide a more accurate representation of the spatial and temporal structure of finer-scale phenomena such as fronts and sea breezes. Over the northern portion of the domain summer precipitation in the downscaled reanalyses remains, as in the ''parent'' reanalyses, overestimated. Given the degree of success that dynamical downscaling of reanalyses demonstrates in the simulation of the characteristics of regional precipitation, its favorable comparison to conventional newer-generation reanalyses and its cost-effectiveness, we conclude that for the Southeast United states such downscaling is a viable proxy for high-resolution conventional reanalysis.

This study attempts to explain the considerable spatial heterogeneity in the observed linear trends of monthly mean maximum and minimum temperatures (T max and T min ) from station observations in the southeastern (SE) United States (specifically Florida, Alabama, Georgia, South Carolina, and North Carolina). In a majority of these station sites, the warming trends in T min are stronger in urban areas relative to rural areas. The linear trends of T min in urban areas of the SE United States are approximately 78F century 21 compared to about 5.58F century 21 in rural areas. The trends in T max show weaker warming (or stronger cooling) trends with irrigation, while trends in T min show stronger warming trends. This functionality of the temperature trends with land features also shows seasonality, with the boreal summer season showing the most consistent relationship in the trends of both T max and T min . This study reveals that linear trends in T max in the boreal summer season show a cooling trend of about 0.58F century 21 with irrigation, while the same observing stations on an average display warming trends in T min of about 3.58F century 21 . The seasonality and the physical consistency of these relationships with existing theories may suggest that urbanization and irrigation have a nonnegligible influence on the spatial heterogeneity of the surface temperature trends over the SE United States. The study also delineates the caveats and limitations of the conclusions reached herein due to the potential influence of perceived nonclimatic discontinuities (which incidentally could also have a seasonal cycle) that have not been taken into account.

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In this paper a methodology is proposed to downscale coarse-resolution atmospheric general circulation model (AGCM) seasonal simulations. Anomaly nesting involves replacing the climatology of the driving AGCM with observed (in this case the National Centers for Environmental Prediction reanalysis) climatology at the lateral boundaries of the nested regional climate model (the regional spectral model). In this study the methodology is tested over South America and the neighboring ocean basins. A comparison of the austral summer seasonal simulation with the conventional way of nesting, namely driving the regional model with full AGCM forcing, reveals that substantial gains in the deterministic skill are realized through anomaly nesting. It is also shown that the high-frequency variance (at 3-30-and 30-40-day time scales) is more realistic from the anomaly nesting procedure.

In this article, we focus on the analysis of the climate variability of the Southeastern Asian Summer Monsoon (SEAM) region encompassing Myanmar, Thailand, Cambodia, Vietnam, Laos and parts of southern China. This region is climatologically found to have one of the longest wet seasons in the Asian monsoon region (of nearly 160 d) and also exhibits one of the strongest interannual variations in the length of the monsoon (wet) season. The interannual variations of the length of the SEAM are characterized by corresponding variations in the onset and demise pentad dates of the wet season, with the former dominating slightly over the latter except over Myanmar. Our study reveals that the pentad of late onset of SEAM is characterized by anomalous increase in remote moisture source from Bay of Bengal and Arabian Sea while a substantial decrease of moisture source from the near Andaman Sea and Gulfs of Martaban and Thailand. Furthermore, anomalously strong June-August Somali Jet is found to be associated with earlier than normal onset of the SEAM. Similarly, the pentad of late demise of the SEAM features excess moisture source from the South China Sea associated with a slow eastward withdrawal of the north Pacific subtropical high. We suggest on the basis of the findings of this study that careful monitoring of the onset the SEAM season will provide important information on the evolution of an ongoing SEAM. Likewise observing low level winds over the northern equatorial Indian Ocean, Bay of Bengal, Gulfs of Martaban and Thailand and South China Sea could be very useful in understanding the seasonal variability of the SEAM. Finally, monitoring of the demise would be equally helpful in characterizing the variation of the concluded SEAM as the length of the wet season seems to be a very robust climate feature of the region. KEY WORDS Monsoon onset; Monsoon demise; length of wet season; interannual variation