05 The South American Monsoon System: Climatology and Variability

A typical Monsoon System is characterized by a reversal in the low-level wind direction between summer and winter seasons, and distinct wet (summer) and dry (winter) periods. The changes in low-level atmospheric circulation are related to changes in the thermal contrast between oceans and continents. During summer, the air over continents is warmer and more convectively unstable than air over adjacent oceanic regions. Consequently, lower pressure occurs over land and higher pressure occurs over nearby oceanic areas. This pressure pattern causes low-level moist air to converge onto the land, resulting in precipitation, especially during the late afternoon and evening hours. During winter the temperature contrasts and low-level atmospheric circulation are reversed, resulting in dry conditions over continents.https://digitalcommons.usu.edu/modern_climatology/1004/thumbnail.jp

The South American Monsoon System: Climatology and Variability

A typical Monsoon System is characterized by a reversal in the low-level wind direction between summer and winter seasons, and distinct wet (summer) and dry (winter) periods. The changes in low-level atmospheric circulation are related to changes in the thermal contrast between oceans and continents. During summer, the air over continents is warmer and more convectively unstable than air over adjacent oceanic regions. Consequently, lower pressure occurs over land and higher pressure occurs over nearby oceanic areas. This pressure pattern causes low-level moist air to converge onto the land, resulting in precipitation, especially during the late afternoon and evening hours. During winter the temperature contrasts and low-level atmospheric circulation are reversed, resulting in dry conditions over continents.https://digitalcommons.usu.edu/modern_climatology/1004/thumbnail.jp

Ciclone no Atlantico sul: análise sinótica e observação

Ponencia presentada en: XXX Jornadas Científicas de la AME y el IX Encuentro Hispano Luso de Meteorología celebrado en Zaragoza, del 5 al 7 de mayo de 2008

Tropical circulation variability with emphasis on interannual and intraseasonal time scales

Tropical atmosphere variability is analyzed by computing the variance (total, interannual-IA, and intraseasonal-IS) of outgoing longwave radiation (OLR), sea levei pressure (SLP), 250-hPa streamfunction and 250-hPa velocity po- tential, and the percent of total variance in the IA and IS bands, for the period 1985-1994 for ali seasons and for the austral summer and winter seasons, separately. High OLR variability in the tropics and subtropics is mainly associated with the IS band, whereas the maximum IA variance observed in the equatorial central and eastern Pacific is related to the EI Nirio-Southern Os- cillation (ENSO). The SLP variance is largest in the extratropics, however, the pattern of percent of total variance in the IA band shows maxima in the tropics consistent with the centers of action of the Southern Oscillation (SO). A substantial portion of the IS variance in SLP is related to the Madden- Julian oscillation. The greatest variance of the 250-hPa streamfunction is found in the subtropics and lower midlatitudes. The small regions in the tropics with more than 30% of the total variance in the IA band, during the austral summer correspond to the main anomalous ENSO-related circulation centers. The 250-hPa velocity potential shows maxima in total and IS variance centered in the tropics. The IA variance is generally quite small, except over Indonesia and the eastem tropical Pacific, consistent with ENSO-re- lated anomalies. Seasonality is evident in the variance patterns for ali variables. Maximum variability is observed in the winter hemisphere for SLP and 250-hPa streamfunction and during the austral summer for OLR and 250- hPa velocity potential.Pages: 06 -1

Principal modes of outgoing longwave radiation and 250-mb circulation for the South American sector

Principal modes of anomalous upper-tropospheric circulation and outgoing longwave radiation (OLR) are determined for the South American sector. A combined EOF analysis is performed using anomalous 250-mb zonal and meridional wind components plus anomalous OLR. The data are filtered so that modes on both intraseasonal and interannual timescales can be studied separately. The resulting pattems show a consistent relationship between anomalous OLR and anomalous upper-tropospheric circulation features. The first and second interannuaI modes contain many atmospheric features related to extremes in the Southem Oscillation. These include I) variations in the pattem of tropical convection, and 2) changes in the intensity of upper-tropospheric zonal flow in the equatorial band and in the subtropics. The first intraseasonal mode has its greatest loadings in the Tropics and is most active during the southem summer season. For positive (negative) amplitudes ofthis mode, anomalous upper-tropospheric westerly (easterly) flow dominates the Tropics throughout the region, and positive (negative) OLR anomalies are observed over northeastem South America and over the eastem equatorial Pacifico These features are associated with the 30-60 day (Madden-Julian) oscillation. An extended combined EOF analysis was performed to study the evolution of intraseasonal oscillations in the South American sector. The first rotated combined extended EOF mode describes a sequence of pattems in which anomalous equatorial westerlies and positive OLR anomalies over tropical Brazil gradually weaken and then reverse. The largest loadings throughout the evolution of the pattems contained in the first mode remain in the Tropics and subtropics. These results provide information on the combined relationship between atmospheric circulation and convection over the South American sector and on the evolution of pattems associated with 30-60 day (intraseasonal) oscillations. By projecting the combined fields of anomalous OLR and 250-mb zonal and meridional wind onto the individual pattems contained in the first rotated combined extended EOF mode, one obtains indices that should enhance real-time monitoring efforts and may lead to improved monthly forecasts for the South American sector.Pages: 1131-114

Sobre o monitoramento das oscilações intrasazonais

Variabilidade intra-sazonal nos trópicos, como refletida nas anomalias de radiação de onda longa (ROL), é investigada utilizando análise de funções ortogonais empíricas estendidas. A seqüência de padrões do primeiro modo claramente indica uma propagação dos "loadings" para leste, de um passo em tempo para o seguinte. A principal característica deste modo é um dipólo nos "loadings", orientado na direção leste- oeste, sobre o oceano índico/Pacífico oeste que se assemelha à associada à oscilação intra-sazonal (período de 30 a 60 dias) de Madden e Julian. A série temporal de amplitudes deste modo apresenta flutuações mais pronunciadas durante o verão e outono do hemisfério sul com períodos variando de 40 a 55 dias. Tais flutuações apresentam marcada variabilidade interanual, com amplitudes relativamente reduzidas durante anos de extremos na Oscilação Sul. As amplitudes projetadas do primeiro modo fornecem informações quanto à evolução e intensidade das oscilações intra-sazonais. Os resultados deste estudo indicam que as amplitudes projetadas podem ser utilizadas como índices para monitorar estas oscilações e para inferir épocas favoráveis à convecção em certas regiões tropicais. ABSTRACT: Intraseasonal variability in the tropics, as indicated by outgoing longwave radiation (OLR) anomalies, is investigated using extended empirical orthogonal function analysis. The sequence of loadings of the first mode clearly shows eastward propagation from one time-step to the next. The main feature of the tirst mode is an east-west dipole in the Indian Ocean/western acitic, which resembles that associated with the Madden and Julian intraseasonal (30-60 day) oscillation. The amplitude time series for this mode presents more pronounced fluctuations during the southern summer and fali seasons, with periods ranging from 40 to 55 days. These fluctuations feature marked interannual variability, with reduced amplitudes during years of extremes in the Southern Oscillation. The projected amplitudes for the tirst mode provide information on the evolution and intensity of intraseasonal oscillations. The results of this study indicate that the projected amplitudes many be used as índices to monitor these oscillations, and to infer favorable periods for convection in certain tropical regions.Pages: 593-60

The Diurnal Cycle of Precipitation over South America based on CMORPH

The diurnal cycle of precipitation over the region of South America is examined using high spatial and temporal resolution analyses that have been produced by NOAAs Climate Prediction Center morphing technique (CMORPH) (Joyce et al. 2004). The 8-km spatial resolution (at the equator) and 30-minute temporal resolution of these analyses permit an in-depth look at the diurnal cycle of precipitation. Major features of the summertime diurnal cycle, as depicted by CMORPH for the South American Monsoon, include an afternoon maximum in precipitation over the Andes and the high terrain in central and eastern Brazil, a nocturnal maximum in precipitation over areas just east of the Andes (western Argentina, central Bolivia and western Paraguay), and a nocturnal maximum over the Atlantic Ocean in the vicinity of the South Atlantic Convergence Zone (Fig. 1). A remarkable diurnal cycle in precipitation occurs in coastal areas of northern and northeastern South America. With daytime heating, precipitation rapidly develops along and just inland from the coast (Fig. 1, lower left panel), probably related to the sea breeze. This precipitation advances westward and southward, producing a nocturnal maximum in areas approximately 500 km inland from the coast. The inland propagation of sea-breeze-induced rainfall systems is a feature most frequently found during late SH summer (December-February) and fall (March-May). The seasonal average diurnal cycle for equatorial South America (Eq. 5\ub0N) for March-May 2003 indicates that sea-breeze-induced precipitation systems propagate westward, reaching the western Amazon Basin in about two days. As these systems propagate inland they contribute to a nocturnal precipitation maximum in some areas and a diurnal precipitation maximum in other areas. A nocturnal or early morning precipitation maximum also occurs along the immediate coast and offshore in the vicinity of the Atlantic ITCZ and over the Pacific near the west coast of South America. The seasonality of the diurnal cycle will also be included in this presentation.Pages: 1113-111