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1. Introduction Even a cursory analysis of the geopotential height in the mid- and upper troposphere shows the presence of waves with wavelengths varying from planetary scale to mesoscale. These waves have a fundamental role in maintaining the heat, momentum, and moisture balance and might generate cyclones at the surface level. In general, the initial formation of surface cyclones or cyclogenesis in the Southern Hemisphere (SH) occurs between 35° and 55°S. The matured systems are found between
1. Introduction Even a cursory analysis of the geopotential height in the mid- and upper troposphere shows the presence of waves with wavelengths varying from planetary scale to mesoscale. These waves have a fundamental role in maintaining the heat, momentum, and moisture balance and might generate cyclones at the surface level. In general, the initial formation of surface cyclones or cyclogenesis in the Southern Hemisphere (SH) occurs between 35° and 55°S. The matured systems are found between
; Seidov and Haupt 2002 ), and is thought to arise from net evaporation in the Atlantic ( Warren 1983 ) and the THC itself ( Manabe and Stouffer 1988 ). Exchange of thermocline water between basins is controlled by the Antarctic Circumpolar Current (ACC), a mighty current that is strongly linked with the Southern Hemisphere (SH) subpolar westerly winds (SWWs). The location of the fronts at its northern boundary controls the warm, saline influence of Indian Ocean thermocline water resulting from the
; Seidov and Haupt 2002 ), and is thought to arise from net evaporation in the Atlantic ( Warren 1983 ) and the THC itself ( Manabe and Stouffer 1988 ). Exchange of thermocline water between basins is controlled by the Antarctic Circumpolar Current (ACC), a mighty current that is strongly linked with the Southern Hemisphere (SH) subpolar westerly winds (SWWs). The location of the fronts at its northern boundary controls the warm, saline influence of Indian Ocean thermocline water resulting from the
1. Introduction One peculiar feature in the atmosphere, which does not seem to have received much attention, is the fact that large-scale westerly jets, at times, take on a spiral form. An example of the spiral jet structure is shown in Fig. 1 , which displays the 275-hPa Southern Hemisphere (SH) zonal wind field, corresponding approximately to a 40-yr calendar mean of 27 April. A more precise description of the data and averaging procedure will be given in section 2 . Starting from the
1. Introduction One peculiar feature in the atmosphere, which does not seem to have received much attention, is the fact that large-scale westerly jets, at times, take on a spiral form. An example of the spiral jet structure is shown in Fig. 1 , which displays the 275-hPa Southern Hemisphere (SH) zonal wind field, corresponding approximately to a 40-yr calendar mean of 27 April. A more precise description of the data and averaging procedure will be given in section 2 . Starting from the
estimating, from monthly mean data, spatial patterns of the slow and intraseasonal components. This methodology provides a way to better identify and understand the sources of predictive skill as well as the sources of uncertainty in climate variability. By applying this methodology to reanalysis datasets, they successfully identified the potentially predictable and unpredictable patterns of the 500-hPa geopotential height field for the Northern Hemisphere ( Frederiksen and Zheng 2004 ) and the Southern
estimating, from monthly mean data, spatial patterns of the slow and intraseasonal components. This methodology provides a way to better identify and understand the sources of predictive skill as well as the sources of uncertainty in climate variability. By applying this methodology to reanalysis datasets, they successfully identified the potentially predictable and unpredictable patterns of the 500-hPa geopotential height field for the Northern Hemisphere ( Frederiksen and Zheng 2004 ) and the Southern
region such that blocking is suppressed during the warm phase of ENSO. In contrast to the Northern Hemisphere (NH), Renwick (1998) found that the number of days of blocking tends to increase on average during the warm phase of ENSO cycle, particularly over the southeast Pacific during the southern spring and summer, using a 16-yr record of 500-hPa height field data. This result is not only confirmed by the updated work of Renwick and Revell (1999) , using a 39-yr record of 500-hPa height dataset
region such that blocking is suppressed during the warm phase of ENSO. In contrast to the Northern Hemisphere (NH), Renwick (1998) found that the number of days of blocking tends to increase on average during the warm phase of ENSO cycle, particularly over the southeast Pacific during the southern spring and summer, using a 16-yr record of 500-hPa height field data. This result is not only confirmed by the updated work of Renwick and Revell (1999) , using a 39-yr record of 500-hPa height dataset
boundary currents and cool equatorward currents off the west coasts of the continents. The strong equatorward alongshore winds to the east of the anticyclones also contribute to the maintenance of underlying cool sea surface temperatures (SSTs) by enhancing surface evaporation, coastal upwelling, and entrainment at the bottom of the oceanic mixed layer. In the Southern Hemisphere (SH), as well, the summertime surface subtropical anticyclones are in a cell-type configuration (as shown below), suggesting
boundary currents and cool equatorward currents off the west coasts of the continents. The strong equatorward alongshore winds to the east of the anticyclones also contribute to the maintenance of underlying cool sea surface temperatures (SSTs) by enhancing surface evaporation, coastal upwelling, and entrainment at the bottom of the oceanic mixed layer. In the Southern Hemisphere (SH), as well, the summertime surface subtropical anticyclones are in a cell-type configuration (as shown below), suggesting
1. Introduction Quasi-annular patterns, often called annular modes, dominate atmospheric extratropical low-frequency variability ( Thompson and Wallace 1998 ). For both hemispheres, these modes are characterized by pressure anomalies of one sign over the polar region, surrounded by a band of opposing polarity with peak amplitude in the midlatitudes. They also appear as the favored response to a wide range of climate forcings, such as the observed trend in the Southern Hemisphere ( Thompson and
1. Introduction Quasi-annular patterns, often called annular modes, dominate atmospheric extratropical low-frequency variability ( Thompson and Wallace 1998 ). For both hemispheres, these modes are characterized by pressure anomalies of one sign over the polar region, surrounded by a band of opposing polarity with peak amplitude in the midlatitudes. They also appear as the favored response to a wide range of climate forcings, such as the observed trend in the Southern Hemisphere ( Thompson and
1. Introduction The climatological stationary waves in the Southern Hemisphere (SH) winter are much weaker than that in the Northern Hemisphere (NH), due in large part to less zonal structure in the land–sea distribution in the SH ( James 1988 ; Rao and Ren 2020 ). It has been reported that wave forcing from the SH troposphere is too weak to exert significant influence on the strong SH stratospheric polar vortex, especially during the austral winter, before it begins to weaken through
1. Introduction The climatological stationary waves in the Southern Hemisphere (SH) winter are much weaker than that in the Northern Hemisphere (NH), due in large part to less zonal structure in the land–sea distribution in the SH ( James 1988 ; Rao and Ren 2020 ). It has been reported that wave forcing from the SH troposphere is too weak to exert significant influence on the strong SH stratospheric polar vortex, especially during the austral winter, before it begins to weaken through
1. Introduction The Northern and Southern Hemisphere annular modes play a prominent role in the climate of their respective hemispheres. Both modes are characterized by approximately zonally symmetric, equivalent barotropic seesaws in the strength of the zonal flow between ∼55°–60° and ∼35°–40° latitude. The structure of the Southern Hemisphere annular mode (SAM; also referred to as the Antarctic Oscillation or High Latitude Mode) is documented in, for example, Trenberth (1979) , Rogers and
1. Introduction The Northern and Southern Hemisphere annular modes play a prominent role in the climate of their respective hemispheres. Both modes are characterized by approximately zonally symmetric, equivalent barotropic seesaws in the strength of the zonal flow between ∼55°–60° and ∼35°–40° latitude. The structure of the Southern Hemisphere annular mode (SAM; also referred to as the Antarctic Oscillation or High Latitude Mode) is documented in, for example, Trenberth (1979) , Rogers and
personal biases and prejudices. To make matters even more complex, a number of studies have shown that the diagnoses of Southern Hemisphere (SH) synoptic systems (of central interest here) in different reanalyses are often considerably at variance (e.g., Bromwich et al. 2007 ). The concept of a front dates back to the work of Bjerknes (1919) and can be understood in its simplest form as a (sloping) quasi-discontinuity in the density field. Over the twentieth century the concept was progressively
personal biases and prejudices. To make matters even more complex, a number of studies have shown that the diagnoses of Southern Hemisphere (SH) synoptic systems (of central interest here) in different reanalyses are often considerably at variance (e.g., Bromwich et al. 2007 ). The concept of a front dates back to the work of Bjerknes (1919) and can be understood in its simplest form as a (sloping) quasi-discontinuity in the density field. Over the twentieth century the concept was progressively
