Ozone and stratospheric height waves for opposite phases of the QBO

The stratospheric quasi-biennial oscillation (QBO) provides an important source of interannual variations in the Northern Hemisphere. O'sullivan and Salby (1990) related extra-tropical eddy transport with the phase of the tropical QBO. When the tropical wind is easterly, the zero wind line is shifted into the winter hemisphere. Enhanced wave activity in middle latitudes acts to weaken the polar vortex. When the tropical wind is in the westerly phase the situation reverses. Heights at 30 mb and ozone configurations are contrasted in this paper for these two QBO phases. When the winter vortex deforms due to the amplification of planetary waves 1 and 2, extends westward and equatorward, the complementary band of low vorticity air spirals in toward the pole from lower latitudes. Sometimes, these planetary waves break (Juckes and McIntyre, 1987) and an irreversible mixing of air takes place between high and mid-latitudes. Global ozone patterns, as obtained form satellite observations, appear to be affected by planetary wave breaking (Leovy et al. 1985). This mixing results on regions with uniform ozone and potential vorticity. In the Southern Hemisphere (SH), Newman and Randel (1988) using Total Ozone Mapping Spectrometer (TOMS) data and the NMC analyses have found strong spatial correlation between the October mean temperature in the lower stratosphere and total ozone for the 1979 through 1986 years. Recently Nogues-Paegle et al.(1992) analyzed SH ozone and height data from 1986 to 1989. They found that leading empirical orthogonal functions (EOFs) for both ozone and 50 mb heights exhibit zonal wave 1 and 2 and that the correlations between ozone and 50 mb principal components (PCs) are high. The results were found to be consistent with a linear planetary wave advecting a passive tracer. In this paper, the dominant patterns of variability for 30 mb NMC heights and TOMS total ozone are obtained for the winter to summer transition (January to May) in the Northern Hemisphere (NH) for the years 1987-1990

Variability of the South American Low Level Jet (SALLJ) in Various Time and Spatial Scales

Variability of the structure and spatial extension of the SALLJ is studies using a combination of various data sets (global reanalyses, PACS-SONET observations, OLR) and the data generated during the SALLJEX field experiment during the austral summer of 2003. On the circulation characteristics, SALLJ composites during the warm season show the enhanced low-level meridional moisture transport coming from equatorialSouth America as well as an upper level wave train emanating from the West Pacific propagating towards South America. The intensification of the warm season SALLJ obeys to the establishment of an upper-level ridge over southern Brazil and a trough over most of Argentina. The circulation anomalies at upper and lower levels suggest that the intensification of the SALLJ would lead to an intensification of the South Atlantic Convergence Zone SACZ later on, and to a penetration of cold fronts with an area of enhanced convection ahead at the exit region of the SALLJ. Regarding the time variability, SALLJ seems to occur all year long, with the SALLJs bringing tropical moist air masses from the Amazon into southern Brazil-Northern Argentina more frequent in the warm season, and the SALLJs bringing tropical maritime air less humid than the tropical air masses coming from the Subtropical Atlantic High more frequent during the cold season. SALLJs are detected mostly during the warm season to the North of 20S, while to the south the SALLJs seem to occur all year long. The diurnal cycle shows that SALLJs are more frequent and intense between 06 and 12 Z for the warm season north of 20 S, while at the region downstream the maximum is detected between 00 and 06 Z. during the cold season. At interannual time scales, even though there is a weak tendency for stronger and more frequent warm season SALLJ episodes in years with anomalously warm surface waters in the tropical Pacific, we cannot affirm with large degree of certainty that there is a strong relationship between the occurrence of El Niño events and the number and/or intensity of SALLJ episodes. However, the El Nino 1998 featured more frequent and intense warm season jet episodes than during La Nina 1999, and this has been demonstrated by the reanalyses, the available PACS-SONET upper-air observations and by other studies using independent data sets and regional modeling. RESUMO: O ciclo diurno do SALLJ mostra que o jato é mais forte e frequente entre as 0600 e 1200 Z durante o verão em latitudes ao norte de 20 °S, enquanto que na região de saída do jato o máximo é observado entre as 0000 e 0600 Z durante a estação de inverno. A variabilidade intrasazonal mostra associações entre a presença da SACZ, o SALLJ e a modulação de eventos extremos de chuva na região sudeste do Brasil. Aparentemente, o SALLJ ocorre durante todo o ano, mais os jatos trazendo umidade da Amazônia para o sul do Brasil são mais intensos no verão, e os jatos trazendo ar marítimo menos úmido (não tropical) do anticiclone do Atlântico Sul sub-tropical são mais frequentes no inverno. Na escala de tempo inter-anual, ainda que exista uma tendência de existir mais episódios de jatos em anos com temperaturas do Pacífico equatorial tropical, não existe uma evidência forte que indique com um alto grau de certeza que existe uma forte associação entre o número e intensidade de eventos jatos e a ocorrência do El Nino. Em escalas de tempo mais longas, existe uma tendência de ter mais episódios de jatos desde meados da década de 1970 s, consistente com tendências negativas de chuva no sul da Amazônia e no sul do Brasil-Norte da Argentina

The Monsoon Systems Of The Americas

In recent years the WCRP/CLIVAR/VAMOS and U.S. CLIVAR programs have made major contributions to our understanding of the American Monsoon systems via focused research activities in South, Central, and North America. In this paper we will review these CLIVAR achievements, with emphasis on common features of the American monsoon systems and intersections amongst the activities. Over tropical and subtropical South America, there has been considerable effort to understand the diurnal cycle and mesoscale variability of the precipitation and atmospheric flow. Precipitation in both regions is strongly controlled by the continentalscale gyre that transports moisture from the tropical Atlantic Ocean, first westward across the Amazon Basin, and then southward across the extratropical continent. That gyre displays a regional intensification just to the east of the Andes Mountains, usually in the form of the South American Low-Level Jet (SALLJ). The VAMOS/SALLJEX field experiment (Nov 02-Feb 03) provided a unique dataset for improved understanding and more realistic simulations of the jet and related precipitation patterns. Variability on both intraseasonal and interannual time scales produces a strong modulation of the low-level circulation mainly through zonal-wind changes at the SA tropics and meridional-wind changes at the subtropics, both associated with a dipole-like precipitation anomaly structure, being one of its centers associated with the SACZ activity. Over the core region of the North American monsoon similar efforts are underway to understand the diurnal cycle of convection, intraseasonal variability, and the influence of oceanic and continental boundary forcing on the atmospheric circulation and precipitation patterns in the region. As in South America, a low-level jet is an important feature for transporting moisture onto the continent, so a common research goal on both continents is to improve understanding and predictability of such jet circulations. The diurnal cycle in this region is larger than the amplitude of the annual cycle. There are large-scale shifts in the regions of deep convection during the day from over high topography on the continent (western Mexico) to over the eastern Pacific Ocean. The intraseasonal and interannual fluctuations of monsoon precipitation are in turn linked to continent-scale precipitation patterns. The North American Monsoon Experiment (NAME) field campaign (summer 2004) will provide a unique dataset for improved understanding and more realistic simulations of warm season precipitation and atmospheric circulation patterns over the region. Associated NAME modeling, data assimilation and predictability studies will serve to accelerate progress towards measurably improved climate models that predict North American monsoon variability out to months to seasons in advance.Pages: 13-1

Toward a unified view of the American Monsoon Systems

An important goal of the Climate Variability and Predictability (CLIVAR) research on the American monsoon systems is to determine the sources and limits of predictability of warm season precipitation, with emphasis on weekly to interannual time scales. This paper reviews recent progress in the understanding of the American monsoon systems and identifies some of the future challenges that remain to improve warm season climate prediction. Much of the recent progress is derived from complementary international programs in North and South America, namely, the North American Monsoon Experiment (NAME) and the Monsoon Experiment South America (MESA), with the following common objectives: 1) to understand the key components of the American monsoon systems and their variability, 2) to determine the role of these systems in the global water cycle, 3) to improve observational datasets, and 4) to improve simulation and monthly-to-seasonal prediction of the monsoons and regional water resources. Among the recent observational advances highlighted in this paper are new insights into moisture transport processes, description of the structure and variability of the South American low-level jet, and resolution of the diurnal cycle of precipitation in the core monsoon regions. NAME and MESA are also driving major efforts in model development and hydrologic applications. Incorporated into the postfield phases of these projects are assessments of atmosphere–land surface interactions and model-based climate predictability experiments. As CLIVAR research on American monsoon systems evolves, a unified view of the climatic processes modulating continental warm season precipitation is beginning to emerge.UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigaciones Geofísicas (CIGEFI

Progress in Pan American CLIVAR research: Understanding the South American monsoon / Progresos en las investigaciones de Pan American CLIVAR: Entendiendo el monzón sudamericano

A review of recent findings on the South American Monsoon System (SAMS) is presented. SAMS develops over a large extension of land mass crossed by the equator with surface conditions that vary from the world's largest tropical forest in Amazonia to a high desert in the Altiplano. The high Andes mountains to the west effectively block air exchanges with the Pacific Ocean, but plentiful moisture transport from the Atlantic maintains intense precipitation that is strongest over central Brazil. There is algo abundant precipitation over the subtropical plains of South America in association with moisture transport from tropical latitudes. Furthermore, midlatitude systems are important modulators of the tropical precipitation. The combination of alI these factors results in a unique seasonal evolution of convection and rainfall. The findings presented emphasize the system's complexity, and highlight the importance ofthe South American continent as the core of atmospheric linkages with the adjacent oceans. A discussion on directions for research on SAMS is algo presented. There are still outstanding questions on the relative roles played on the system evolution by the orography, local and remote heat sources, and sea surface temperature anomalies. Other remaining questions address the impact of Amazon-deforestation on water and energy cycles over the two largest river basins of South America (Amazon and La Plata). RESUMEN: Se presenta un resumen actualizado acerca del Sistema Monzónico en Sudamerica (SMS). El SMS se desarrolla sobre un continente tropical, con variadas condiciones de superficies que incluyen grandes bosques tropicales (en Amazonia), y el elevado desierto en el Altiplano. Los Andes en el oeste del continente bloquean eficientemente el intercambio de aire con el Océano Pacífico, y canalizan el transporte de humedad desde el Océano Atlántico produciendo intensa precipitación con máximo valores en el centro de Brasil y un maximo secundario sobre las llanuras subtr opicales de Sudamerica. Además, sistemas de latitudes medias son importantes moduladores de la precipitación tropical. De la combinación de todos estos factores resulta una singular evolución estacional de la convección y precipitación. Los resultados presentados enfatizan la complejidad del sistema y destacan la im portancia del continente Sudamericano como centro de las conexiones atmosféricas con los océanos adjacentes. Una discusión sobre las futuras lineas de investigación a seguir en SMS es también presentada. Existen aun importantes preguntas acerca del papel relativo que ejercen sobre la evolución del sistema la orografía, fuentes de calor locales y remotas y las anomalias de la temperatura de la superficie del mar. El impacto de la desforestación del Amazonas en al balance hídrico y de energía de las dos más grandes cuencas en Sudamerica (de los rios Amazonas y de la Plata) tambien necesita ser estudiado con mayor profundidad.Pages: 1-3