Remote and Local Effects on
South American Summer Rains
Dr. Julia Nogués-Paegle
University of Utah
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INTRODUCTION The focus of this project is on understanding climate variability over South-America. This is not only of intrinsic interest to the South-American continent but also to North-America and surrounding oceans due to interacting atmospheric patterns over this extended region. Several past studies have documented the influence of interannual variations in the tropical Pacific (ENSO) with rainfall variability over South-America. More recent work links sea-surface temperature in the Atlantic with wet season rainfall over the continent. Furthermore, teleconnections from tropical South America northward have an impact on atmospheric patterns onto the Northern hemisphere. The morphology of the South-American continent, with steep orogaphy (the Andes) on its western boundary, a meridional extension that spans from equatorial to mid-latitudes with a variety of surface vegetation and a large tropical land mass, presents unique challenges to numerical simulations. This is of special importance over South America where lack of an adequately dense observational system has resulted in an increased dependance of empirical studies on products of data assimilation systems. The work plan emphasizes the intraseasonal and interannual fluctuations of phenomena such as Amazon Basin convection and the SACZ using traditional diagnostics and numerical experiments applied to special high resolution output obtained with mesoscale and global variable resolution models. Modeling studies are particularly important because of the paucity of surface-based observations over South America; where radiosonde data is available it is insufficient to resolve key topographically bound circulations and their diurnal evolution and related horizontal and vertical fluxes of moisture, heat, and momentum. Advanced numerical models can be used to fill this gap in both prognostic and diagnostic modes. With regard to South American climate it is essential that models be able to adequately resolve the extremely corrugated Andes mountain range, the low-level jets (LLJ) which connect tropical South-America with the fertile sub-tropical plains to the South and surface fluxes that are important both over mountains and sea. The Variability of American Monsoon Systems (VAMOS), a CLIVAR program, has stimulated interest in a number of scientific issues regarding the variability of summer rains. With PACS's support, the P.I has promoted activities aimed at improving understanding of the 1) South American Monsoon System (SAMS), (see http://www.met.utah.edu/jnpaegle/research/miami_report.html), and 2) the LLJ east of the Andes (see http://www.met.utah.edu/jnpaegle/research/ALLS.html) during the last two years. GOALS 1) Identify possible interannual signals in long-lasting precipitation events over South-America in summer, and their correlation with LLJ east of the Andes, 2) Describe the character of interhemispheric connections over the Americas and the North-Atlantic, 3) Assess the dependence of numerical simulations of regional climate on model design and surface conditions. METHODOLOGY A meso-scale model and a global primitive equation variable resolution model developed at the University of Utah are used in the numerical experiments to address the third goal of our proposal. RESULTS AND ACCOMPLISHMENTS 1. Alternating wet and dry summer periods over South-America Nogués-Paegle and Mo (1997) describe the horizontal structure of alternating periods of wet and dry conditions over South-America. A seesaw pattern was found in the SACZ, with strong convective activity over the SACZ associated with rainfall deficits over the subtropical plains of South America. When the SACZ weakens, precipitation increases over the plains. This phase is characterized by moisture influx from the tropics into central Argentina and southern Brazil, which is caused by low level northerlies east of the Andes. 2. Interhemispheric connections over the Pan-American region Nogués-Paegle et al (1998) study the predictability of the NCEP/NCAR reanalysis model during austral summer. This model is similar to NCEP's climate model and therefore it is expected to similarly respond to erroneous simulations of tropical convection. It was found that the underprediction of tropical convection over South-America and associated weakened overturning circulation produced a meridional wave pattern that extended into North-America causing a shift of the subtropical jet over North-America and the Atlantic. This casual relationship was established with a simplified version of the Utah global primitive equation model. Nogués-Paegle et al (1999) extend the analysis of Nogués-Paegle and Mo (1997) to examine variability patterns of convection in the South Atlantic Convergence Zone (SACZ) and the subtropical plains of South-America and define the role of the Pacific in this oscillation . Two dominant periods of variability are found: a 36-40 day (the MJO) and a 22-28 day mode. The latter leads the varibility over the subtropical plains. Figure 1 shows outgoing long wave radiation and 200 mb streamfunction anomalies keyed to cases with enhanced precipitation over subtropical South America. The figure shows the eastward extension of a wave that originates in the South Pacific, it propagates south-eastwardly towards South America and curves towards the north-east as it reaches the continent. Linkages between the Americas are established in the convective phase of the SACZ when the 22 day and 40 day mode overlap and the streamfunction pattern exhibits meridional propagation onto the Northern Hemisphere (Figure 2). 3. Meso-scale model simulations. A regional model was provided with climatological summer boundary conditions and large internal scales and then integrated to obtain more detailed forecast fields. Experiments were run with 2o, 1o, and .5o resolution with and without surface evapotranspiration and with and without precipitation. Results indicate that the cross-mountain moisture flux over North America is more sensitive to resolution than it is the meridional moisture flux. In contrast, the meridional moisture flux east of the Andes Cordillera is sensitive to resolution. Precipitation feedback is most important in these experiments to describe local circulations. Furthermore, the higher the resolution, the closer the LLJ is found to the mountains, indicating the sensitivity of analyzed LLJs to horizontal resolution. Variable grid models allow adequate resolution of local precipitation events and retain globally interacting influences. Figure 3 illustrates one possible grid-generation mechanism in a global, variable resolution approach and Figure 4 displays resulting vertical wind structure. FUTURE WORK 1) Determine the principal time scales of interhemispheric connections over the Americas and the North-Atlantic and diagnose its dynamical and thermodynamic structure. Determination of cause-effect relationships will be obtained with controlled numerical simulations. 2) Assess the extent to which the meso-structure, regional climate variability over South-America depends on its highly variable surface, is influenced by large scale flow and has an effect on conterminuos regions. 3) Identify potential winter precursors to summer rains, and the dependance of this evolution on surface conditions and remote influences. REFERENCES Nogués-Paegle. J. and K.-C. Mo, 1997: Alternating wet and dry conditions over South America during summer. Mon. Wea. Rev., 125, 279-291. Nogués-Paegle. J., K.-C. Mo and J. Paegle, 1998: Predictability of the NCEP-NCAR reanalysis model during austral summer. Mon. Wea. Rev., 126, 3135-3152. Nogués-Paegle. J., L. A. Byerle and K.-C. Mo, 1999: Intraseasonal modulation of South American summer precipitation. Mon. Wea. Rev., 127, In press CONTACT Principal Investigator: Julia Nogués-Paegle LINKS http://www.met.utah.edu/jnpaegle/homepages/jnpaegle.html Return to CLIVAR PACS Home Page |
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