Relationship between the southern annular mode and southern hemisphere atmospheric systems

Rosmeri Porfírio da Rocha,Michelle Simões Reboita,Tércio Ambrizzi

doi:10.1590/s0102-77862009000100005

Rosmeri Porfírio da Rocha, Michelle Simões Reboita + Show 1 more

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https://doi.org/10.1590/s0102-77862009000100005

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Abstract

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.

Highlights

The Southern Annular Mode (SAM) is the major mode of variability of the extratropical circulation in the Southern Hemisphere and is characterized by zonally symmetric or annular structures, with geopotential height perturbations of opposing signs in Antarctica and in the surrounding zonal ring centered near 45o latitude (Thompson and Wallace, 2000a)
The objective of this study is to identify the relationship between the seasonal spatial variability of the cyclones distribution in the Southern Hemisphere and the frontogenetic regions in South America and South Atlantic Ocean in different SAM phases
The main frontogenetic regions as well as the rainfall distribution over southeast South America and how they are modified during different SAM phases were analyzed

Summary

Introduction

The Southern Annular Mode (SAM) is the major mode of variability of the extratropical circulation in the Southern Hemisphere and is characterized by zonally symmetric or annular structures, with geopotential height perturbations of opposing signs in Antarctica and in the surrounding zonal ring centered near 45o latitude (Thompson and Wallace, 2000a). According to Thompson and Wallace (2000a) the SAM can be observed through the leading empirical orthogonal function (EOF) in many atmospheric fields (for example, sea level pressure, surface temperature, geopotential height and zonal wind). In order to understand the variability of SAM and Revista Brasileira de Meteorologia its possible connection with southern climatic anomalies in detail, an index was built by the Climate Prediction Center (CPC – NOAA) using EOF analysis in the monthly mean 700 hPa height anomalies poleward 20oS, for the period of 1979 to 2000. Similar definition to the SAM index was employed by Thompson and Wallace (2000 a, b). In their analysis from 1968 to 1997 they found a trend toward the positive phase of the SAM, but any clear seasonality

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