Rossby Wave Propagation Tracks in Southern Hemisphere Mean Basic Flows Associated to Generalized Frosts over Southern South America

A partir de los estudios observacionales previos acerca de los eventos extremos fríos en el sudeste de América del Sur, surge la hipótesis la cual plantea que los patrones de gran escala condicionan la frecuencia de ocurrencia de heladas, a través de la propagación de ondas de Rossby excitadas remotamente. Aplicando los conceptos de la teoría lineal de propagación de ondas de Rossby, el objetivo en este artículo es analizar la propagación de tales ondas en dos estados básicos diferentes referidos a los inviernos con una máxima y mínima frecuencia de ocurrencia de heladas generalizadas en la Pampa Húmeda (centro–noreste de Argentina). Basado en los conceptos teóricos acerca de la dispersión de onda y el trazo de trayectorias, se identifican los caminos preferenciales de las ondas de Rossby que alcanzan América del Sur a través de la técnica conocida como trazado de rayos. El análisis del flujo básico desde una perspectiva teórica, basado en los cálculos de las trayectorias, permite comprobar que las ondas de Rossby excitadas en forma remota son el mecanismo que favorece la máxima ocurrencia de heladas generalizadas. Siendo los lugares de excitación de tales ondas condicionados por el estado básico en que se propagan. Ellas son excitadas en lugares determinados de la atmósfera, desplazándose hasta América del Sur a lo largo de los jets que actúan como guías de dichas ondas, propiciando la generación de heladas generalizadas. En suma, este artículo presenta una revisión de la técnica de trazado de rayos y como puede ser usada para investigar la ocurrencia de importantes eventos sinópticos tales como las heladas en una región específica y su relación con la propagación de ondas planetarias de gran escala.Based on previous observational studies on cold extreme events over southern South America, some recent studies suggest a possible relationship between Rossby wave propagation remotely triggered and the occurrence of frost. Using the concept of linear theory of Rossby wave propagation, this paper analyzes the propagation of such waves in two different basic states that correspond to austral winters with maximum and minimum generalized frost frequency of occurrence in the Wet Pampa (central–northwest Argentina). In order to determine the wave trajectories, the ray tracing technique is used in this study. Some theoretical discussion about this technique is also presented. The analysis of the basic state, from a theoretical point of view and based on the calculation of ray tracings, corroborates that remotely excited Rossby waves is the mechanism that favors the maximum occurrence of generalized frosts. The basic state in which the waves propagate is what conditions the places where they are excited. The Rossby waves are excited in determined places of the atmosphere, propagating towards South America along the jet streams that act as wave guides, favoring the generation of generalized frosts. In summary, this paper presents an overview of the ray tracing technique and how it can be used to investigate an important synoptic event, such as frost in a specific region, and its relationship with the propagation of large scale planetary waves.Fil: Muller, Gabriela Viviana. Provincia de Entre Ríos. Centro de Investigaciones Científicas y Transferencia de Tecnología a la Producción. Universidad Autónoma de Entre Ríos. Centro de Investigaciones Científicas y Transferencia de Tecnología a la Producción. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Centro de Investigaciones Científicas y Transferencia de Tecnología a la Producción; ArgentinaFil: Ambrizzi, Tércio. Universidade de Sao Paulo; Brasi

Changes in the Austral Winter Hadley Circulation and the Impact on Stationary Rossby Waves Propagation

The present study investigates how changes in the Hadley Cell (HC) intensity impact the stationary Rossby waves energy propagation in the Southern Hemisphere (SH) extratropics. Composites for weak and strong HC Intensity Index (HCI) were used in this analysis. The results for weak HC cases showed a wave train emanating from the subtropical central-west Indian Ocean in an arc-like route, with zonal wavenumber three in the polar jet waveguide, and reaching the north of South America. For strong HC cases, the wave train is also trapped inside the polar jet waveguide with zonal wavenumber four, emanating from subtropical central-east Indian Ocean and reaching the subtropical west coast of Africa. A weaker zonally oriented wave train with zonal wavenumber five has been found in the subtropical region with opposite polarity for weak and strong HC cases. Over the South America, the results show that an HC weakening can lead to a very cold and rainy winter in the southwest of the continent and a mild warm and dry winter on Brazilian states of Minas Gerais and Bahia. A pattern almost opposite was observed when the CH strengthens

Moisture Sources and Life Cycle of Convective Systems over Western Colombia

This paper describes life cycle and moisture sources of mesoscale convective systems (MCSs) observed over western Colombia. Results show that, in general, MCS are more frequent during boreal summer and autumn, and particularly, systems observed in summer season present longer life and larger extension. On the continent, MCS genesis is strongly affected by sea breeze and diurnal heating and presents a peak from 15 to 18 LST. For oceanic systems, the main genesis period is later, from 00 to 03 LST. Continental and oceanic systems present a tendency of westward displacement. Analysis using a Lagrangian approach implemented to estimate air parcel trajectories suggests that, during boreal winter, the main moisture sources are from the Caribbean Sea and tropical north Atlantic, possibly resulting from the moisture-laden trade winds and the land-ocean temperature contrast over northern South America. In summer, it is clear the influence of ITCZ positioning with moisture particles traveling from the tropical Atlantic over Amazonian river basin. In Autumn, Chilean-Peruvian Pacific is the main moisture source, confirming the importance of Chocó low level jet to MCS genesis

Sistemas Frontais Sobre a América do Sul parte II: Monitoramento Mensal em Dados da Reanálise i do NCEP/NCAR

A tracking of the frontal systems over South America is performed by the Climate Studies Group at the University of São Paulo  (GrEC/USP) since January 2014. This analysis used an objective methodology for tracking the frontal systems that consists in analyze daily data from I NCEP / NCAR Reanalysis I and verify the occurrence of decrease in temperature at 925hPa, simultaneous to a shift in the meridional wind in 925hPa and sea level pressure increase one day to another. This monitoring is done through maps with the number and anomalies of frontal systems and also analyzing the occurrence of these systems in grid points on the coast, inland and center of the continent.Um monitoramento dos sistemas frontais para a América do Sul é realizada pelo Grupo de Estudos Climáticos da Universidade de São Paulo (GrEC/USP) desde janeiro de 2014. Para esta análise é utilizada uma metodologia objetiva de rastreamento dos sistemas frontais que consiste em analisar dados diários da Reanálise I do NCEP/NCAR e verificar a ocorrência de queda na temperatura em 925hPa, simultânea a um giro no vento meridional em 925hPa e aumento na pressão ao nível médio do mar de um dia para o outro. Este monitoramento é feito através de mapas com o número e a anomalias de sistemas frontais e também para a análise da ocorrência destes sistemas em pontos de grade sobre o litoral, interior e centro do continente.

Rossby wave propagation during winters of maximum frequency of frost occurrence over the wet pampa

Neste trabalho estudou-se a influência dos padrões de onda extratropicais, que favorecem o desenvolvimento de eventos extremos frios no sudeste Sul-Americano, e em particular na região conhecida como Pampa Úmida. O aquecimento anômalo observado na região do oceano Pacífico tropical ocidental a nordeste da Austrália, durante os invernos de máxima freqüência de ocorrência de Geadas Generalizadas (GG) no centro-leste da Argentina, (região conhecida como Pampa Úmida - PU), atua como disparador de ondas de Rossby, as quais se propagam até o continente, favorecendo assim a ocorrência daqueles eventos. O padrão de propagação obtido nas simulações numéricas com um modelo baroclínico global, mostra o predomínio de um número de onda 3. Adicionalmente, foram analisadas as correlações do vento meridional em altos e baixos níveis observados para os eventos de GG, selecionados dentro dos invernos de máxima freqüência de ocorrência desses eventos. O vento meridional global em 250hPa apresenta regiões com correlação estatisticamente significativa com o vento meridional médio na PU. A configuração obtida no caso do vento meridional global em 250hPa, correlacionado com o vento meridional na PU, pode estar associada ao padrão de propagação das ondas simuladas numericamente a partir da forçante tropical. Igualmente importantes e significativos são os valores de correlação do vento sul nos baixos níveis, em particular para toda região da PU. O padrão de ondas simulado está bem representado pelas significativas correlações entre o vento meridional hemisférico em altos níveis e a temperatura no dia de evento de GG.The influence of extratropical waves on the development of extreme cold events over southeast South America, particularly on the Argentinean Wet Pampa region is studied in this work. An anomalous heating observed over the occidental tropical Pacific Ocean, northeast of Australia, during Austral winters of maximum frequency of Generalized Frost (GF) occurrence over the center-east of Argentina, known as Wet Pampa (WP), acts as a Rossby wave source which generate waves that propagate towards the South American continent favoring the frost events. The wave propagation pattern obtained from simulations using a Global Baroclinic Model shows wavenumber 3 dominance. Additional, upper and lower levels meridional wind correlations during the GF events selected were analyzed. The 250hPa global meridional wind shows a significant correlation (0.9) with the meridional wind at the WP region. The wave propagation pattern observed in this case agrees with that simulated by the model when a heating source is located at the Pacific tropical ocean. Also, significant correlation values were found for the low level southern winds at the WP region. The wave pattern simulated shows a good correlation between the hemispheric meridional wind at higher levels and the air temperature in the day of GF events.Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET

Relação entre o modo anular sul e os sistemas atmosféricos no hemisfério sul

Seasonal relationship between the Southern Annular Mode (SAM) and the spatial distribution of the cyclone systems over Southern Hemisphere is investigated for the period 1980 to 1999. In addition, seasonal frontogenesis and rainfall distribution over South America and South Atlantic Ocean during different SAM phases were also analyzed. It is observed that during negative SAM phases the cyclone trajectories move northward when compared to the positive one, and in the South America and South Atlantic sector there is intense frontogenetic activity and positive anomaly precipitation over the Southeast of the South America. In general, SAM positive phase shows opposite signals.A relação sazonal entre o Modo Anular Sul (Southern Annular Mode - SAM) e a distribuição espacial dos sistemas ciclônicos sobre o Hemisfério Sul foi investigada no período de 1980 a 1999. Também foram analisados os padrões sazonais de frontogêneses e de distribuição de precipitação, sobre a América do Sul e oceano Atlântico Sul, durante as diferentes fases da SAM. Na fase negativa da SAM foi observado que a trajetória dos ciclones em todo o Hemisfério Sul move-se para norte, comparada à fase positiva, e que no setor da América do Sul e Atlântico Sul há intensa atividade frontogenética e anomalias positivas de precipitação na costa sudeste da América do Sul. Em geral, na fase positiva da SAM são observadas condições inversas.FAPESPCoordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)CNP

Precipitation regimes in South America: a bibliography review

Como a América do Sul se estende por diferentes latitudes e possui formas de relevo variadas, proporciona a atuação e o desenvolvimento de diferentes sistemas atmosféricos, os quais contribuem para a não homogeneidade climática da região. Portanto o objetivo deste estudo é apresentar uma revisão dos sistemas atmosféricos que atuam nos diferentes setores do continente sul-americano e que contribuem para a precipitação.The distribution of South American lands on different latitudes and its diversified topography can influence the development and action of many atmospheric systems contributing to a non-homogeneous climate in this region. This work presents a review on the atmospheric systems present in the different sectors of South America contributing to the precipitation variability in these regions

Influence of decadal sea surface temperature variability on northern Brazil rainfall in CMIP5 simulations

The Amazonia and Northeast regions of northern Brazil are characterized by very different rainfall regimes but have certain similarities in terms of their variability. The precipitation variability in both regions is strongly linked to the tropical Atlantic sea surface temperature (SST) gradient and the tropical Pacific SST anomalies, which at decadal timescales are modulated by the Atlantic Multidecadal Variability (AMV) and the Interdecadal Pacific Oscillation (IPO) modes of SST, respectively. On the other hand, it has been found that state-of-the-art models from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) are able to reproduce some of the characteristics of the low-frequency SST variability modes. In this work we analyze how CMIP5 models simulate the observed response of precipitation in the Amazonia and Northeast regions to the AMV and the IPO and the atmospheric mechanisms involved. Results show that, in both CMIP5 simulations and observations, Amazonia and Northeast rainfall response to the AMV is the opposite, due to the modulation of the intertropical convergence zone (ITCZ) position. Conversely, the IPO affects equally both regions as a consequence of anomalous subsidence over the entire northern Brazil triggered by warm SST anomalies in the tropical Pacific. Such results suggest that an improvement of the predictability of decadal SST modes will directly revert into a better prediction of changes in the Amazonia and Northeast rainfall at longer timescales

Analysis of a semi-stationary cyclone on the coast of southern Brazil associated to an atmospheric blocking

Na primeira semana de maio de 2008, durante quatro dias, um ciclone em superfície permaneceu semi-estacionário na costa da região sul do Brasil. Este sistema foi responsável por chuvas e ventos fortes no Rio Grande do Sul e Santa Catarina, os quais causaram muitos danos (queda de árvores, enchentes e desabamentos). O objetivo deste trabalho é avaliar o processo de formação e entender os mecanismos responsáveis pelo lento deslocamento do ciclone, já que a maioria dos ciclones nesta região possui deslocamento mais rápido. A equação de desenvolvimento de Sutcliffe mostrou que a advecção de vorticidade absoluta ciclônica na média troposfera e a advecção de ar quente na camada entre 1000-500 hPa foram mecanismos importantes para a ciclogênese. Neste período, o intenso aquecimento diabático também contribuiu para a ciclogênese, à medida que se contrapôs ao intenso resfriamento adiabático devido aos movimentos verticais ascendentes. A advecção de vorticidade absoluta ciclônica que favoreceu a ciclogênese esteve associada a um Vórtice Ciclônico em Altos Níveis (VCAN), que se formou numa região de anomalia de vorticidade potencial. O VCAN se manteve semi-estacionário e compôs o setor norte de um bloqueio do tipo dipolo. Tal bloqueio intensificou um anticiclone em superfície, situado a sul/leste do ciclone, o que contribuiu para o ciclone se manter semi-estacionário. O movimento atípico e lento do ciclone para sul, e em alguns períodos para sudoeste, esteve associado com advecções de vorticidade absoluta ciclônica na média troposfera e de ar quente no seu setor sul. Somente quando o bloqueio em níveis médios e a anomalia de vorticidade potencial em níveis médios/altos se enfraqueceram, o ciclone em superfície se afastou da costa sul do Brasil.On the first week of May 2008, during four days, a semi-stationary surface cyclone acted over the coast of southern Brazil. This system was responsible for large amount of rain and strong near surface winds over Rio Grande do Sul and Santa Catarina causing many damages (fallen trees, floodings and landslides). This work evaluated cyclone development process to understand the reasons for its slow displacement, once most cyclones developed in this region move faster. The Sutcliffe development equation showed that cyclonic absolute vorticity advection in middle troposphere, and positive thermal advection in layer between 1000-500 hPa were important mechanisms for cyclogenesis processes. In this same period, diabatic heating also contributes to cyclogenesis as far as it was opposite to adiabatic cooling due to strong upward movement. Cyclonic absolute vorticity advection which propitiated cyclogenesis was associated with an Upper Level Cyclonic Vortex (ULCV) developed in a potential vorticity anomaly region. The ULCV was semi-stationary and part of a blocking dipole pattern. This blocking intensified a surface anticyclone at south/east of the cyclone, contributing to the cyclone staying semi-stationary near the coast of southern Brazil. The slow and unusual surface cyclone movement to south and sometimes to southwest was associated with middle level cyclonic absolute vorticity and warm air advections in their southern sector. Only when middle levels blocking and middle/high levels potential vorticity anomaly weakened, surface cyclone moved away from the coast of Southern Brazil.Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)CNP