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-mean zonal wind will be referred to as the zonal index mode. The leading EOF of Northern Hemisphere (NH) extratropical sea level pressure (SLP) was defined by Thompson and Wallace (1998) as the Arctic Oscillation, or Northern Annular Mode [respectively the Antarctic Oscillation or Southern Annular Mode for the Southern Hemisphere (SH) SLP]. The EOF spatial structures are equivalent barotropic and dipole-like, and are approximately zonally symmetric with oppositely signed anomalies in the mid- and high
-mean zonal wind will be referred to as the zonal index mode. The leading EOF of Northern Hemisphere (NH) extratropical sea level pressure (SLP) was defined by Thompson and Wallace (1998) as the Arctic Oscillation, or Northern Annular Mode [respectively the Antarctic Oscillation or Southern Annular Mode for the Southern Hemisphere (SH) SLP]. The EOF spatial structures are equivalent barotropic and dipole-like, and are approximately zonally symmetric with oppositely signed anomalies in the mid- and high
1. Introduction The annular modes are the leading mode of variability in the extratropical atmosphere of each hemisphere ( Thompson and Wallace 2000 ). They take the form of dipole structures, centered on the time-mean jets and representing north–south fluctuations of the maximum jet speed. With a decorrelation time of 10–30 days, the annular modes are a key source of predictability for midlatitude weather, and their persistence has been linked to the extratropical atmosphere’s response to
1. Introduction The annular modes are the leading mode of variability in the extratropical atmosphere of each hemisphere ( Thompson and Wallace 2000 ). They take the form of dipole structures, centered on the time-mean jets and representing north–south fluctuations of the maximum jet speed. With a decorrelation time of 10–30 days, the annular modes are a key source of predictability for midlatitude weather, and their persistence has been linked to the extratropical atmosphere’s response to
1. Introduction In this study, we pursue the ongoing question of the zonal structure of the annular modes. The annular modes (AMs) are usually defined as the first EOF of the hemisphere-wide streamflow in the Northern Hemisphere (NH) or Southern Hemisphere (SH) extratropics ( Thompson and Wallace 1998 , 2000 ). The hemisphere-wide EOF analysis produces hemispheric-scale streamflow patterns with a dipolar meridional structure and relatively little zonal structure. On the other hand
1. Introduction In this study, we pursue the ongoing question of the zonal structure of the annular modes. The annular modes (AMs) are usually defined as the first EOF of the hemisphere-wide streamflow in the Northern Hemisphere (NH) or Southern Hemisphere (SH) extratropics ( Thompson and Wallace 1998 , 2000 ). The hemisphere-wide EOF analysis produces hemispheric-scale streamflow patterns with a dipolar meridional structure and relatively little zonal structure. On the other hand
1. Introduction The variability of Southern Hemisphere (SH) atmospheric circulation is dominated by an approximately zonally symmetric pattern, referred to as the southern annular mode (SAM), characterized by a dipole in the zonal wind strength with opposing centers of action near 40° and 65°S and an equivalent barotropic structure in the vertical ( Kidson 1988 ; Karoly 1990 ; Shiotani 1990 ; Hartmann and Lo 1998 ; Feldstein and Lee 1998 ; Limpasuvan and Hartmann 2000 ; Thompson and
1. Introduction The variability of Southern Hemisphere (SH) atmospheric circulation is dominated by an approximately zonally symmetric pattern, referred to as the southern annular mode (SAM), characterized by a dipole in the zonal wind strength with opposing centers of action near 40° and 65°S and an equivalent barotropic structure in the vertical ( Kidson 1988 ; Karoly 1990 ; Shiotani 1990 ; Hartmann and Lo 1998 ; Feldstein and Lee 1998 ; Limpasuvan and Hartmann 2000 ; Thompson and
1. Introduction “What is meant by the words annular and mode?” asked Ambaum et al. (2001 , p. 3506), with respect to atmospheric variability. This is a fair question, since the words are hard to find in meteorological glossaries. A typical dictionary will offer “ring-shaped” and “most frequent value in a dataset” (matching the meaning for mode given in the American Meteorological Society Glossary of Meteorology ). A geographical pattern of variability that features a zonal band does form a
1. Introduction “What is meant by the words annular and mode?” asked Ambaum et al. (2001 , p. 3506), with respect to atmospheric variability. This is a fair question, since the words are hard to find in meteorological glossaries. A typical dictionary will offer “ring-shaped” and “most frequent value in a dataset” (matching the meaning for mode given in the American Meteorological Society Glossary of Meteorology ). A geographical pattern of variability that features a zonal band does form a
1. Introduction a. Background The leading mode of atmospheric variability in the extratropics of both hemispheres is the meridional vacillation of the equivalent barotropic eddy-driven jet streams and embedded storm tracks ( Kidson 1988 ; Mo and White 1985 ; Thompson and Wallace 2000 ; Baldwin 2001 ; Wallace 2000 ). This variability is variously referred to as the annular modes ( Limpasuvan and Hartmann 1999 ), the Antarctic or Arctic Oscillations ( Thompson and Wallace 1998 ; Gong and
1. Introduction a. Background The leading mode of atmospheric variability in the extratropics of both hemispheres is the meridional vacillation of the equivalent barotropic eddy-driven jet streams and embedded storm tracks ( Kidson 1988 ; Mo and White 1985 ; Thompson and Wallace 2000 ; Baldwin 2001 ; Wallace 2000 ). This variability is variously referred to as the annular modes ( Limpasuvan and Hartmann 1999 ), the Antarctic or Arctic Oscillations ( Thompson and Wallace 1998 ; Gong and
1. Introduction In the extratropical circulation of both hemispheres, a long-recognized dominant pattern of variability at the intraseasonal to interannual time scales is the “annular mode,” which is often derived from empirical orthogonal function (EOF) analysis of meteorological fields such as zonal-mean zonal wind or geopotential heights and has been known for decades ( Kidson 1988 ; Thompson and Wallace 1998 ; Feldstein 2000 ). The leading EOF (EOF1) of features an equivalent
1. Introduction In the extratropical circulation of both hemispheres, a long-recognized dominant pattern of variability at the intraseasonal to interannual time scales is the “annular mode,” which is often derived from empirical orthogonal function (EOF) analysis of meteorological fields such as zonal-mean zonal wind or geopotential heights and has been known for decades ( Kidson 1988 ; Thompson and Wallace 1998 ; Feldstein 2000 ). The leading EOF (EOF1) of features an equivalent
sea level rise (e.g., De Angelis and Skvarca 2003 ; Rignot et al. 2004 ). The atmospheric circulation of the peninsula region has also changed significantly during the last few decades. The principal mode of variability in the Southern Hemisphere (SH) circulation, the SH Annular Mode (SAM), began a positive phase shift in the mid-1960s ( Marshall 2003 ), which has resulted in increased (reduced) pressures in the SH midlatitudes (the Antarctic region). The trends in the SAM are statistically
sea level rise (e.g., De Angelis and Skvarca 2003 ; Rignot et al. 2004 ). The atmospheric circulation of the peninsula region has also changed significantly during the last few decades. The principal mode of variability in the Southern Hemisphere (SH) circulation, the SH Annular Mode (SAM), began a positive phase shift in the mid-1960s ( Marshall 2003 ), which has resulted in increased (reduced) pressures in the SH midlatitudes (the Antarctic region). The trends in the SAM are statistically
1. Introduction The annular modes are the leading modes of extratropical variability of the Northern and Southern Hemisphere tropospheric winds on intraseasonal time scales. This annular pattern exhibits an equivalent barotropic dipolar structure and is often described as a zonally symmetric latitudinal shift of the midlatitude jets (e.g., Hartmann and Lo 1998 ; Limpasuvan and Hartmann 2000 ; Thompson and Wallace 2000 ). The observed anomalous zonal winds in the annular modes are often more
1. Introduction The annular modes are the leading modes of extratropical variability of the Northern and Southern Hemisphere tropospheric winds on intraseasonal time scales. This annular pattern exhibits an equivalent barotropic dipolar structure and is often described as a zonally symmetric latitudinal shift of the midlatitude jets (e.g., Hartmann and Lo 1998 ; Limpasuvan and Hartmann 2000 ; Thompson and Wallace 2000 ). The observed anomalous zonal winds in the annular modes are often more
1. Introduction At the intraseasonal to interannual time scales, the variability of the large-scale atmospheric circulation in the midlatitudes of both hemispheres is dominated by the “annular modes,” which are usually defined based on empirical orthogonal function (EOF) analysis of zonal-mean meteorological fields (e.g., Kidson 1988 ; Thompson and Wallace 1998 , 2000 ; Lorenz and Hartmann 2001 , hereafter LH01 , 2003 ; Thompson and Woodworth 2014 ; Thompson and Li 2015 ). The
1. Introduction At the intraseasonal to interannual time scales, the variability of the large-scale atmospheric circulation in the midlatitudes of both hemispheres is dominated by the “annular modes,” which are usually defined based on empirical orthogonal function (EOF) analysis of zonal-mean meteorological fields (e.g., Kidson 1988 ; Thompson and Wallace 1998 , 2000 ; Lorenz and Hartmann 2001 , hereafter LH01 , 2003 ; Thompson and Woodworth 2014 ; Thompson and Li 2015 ). The
