Characteristics of southern hemisphere blocking as determined from a time series of observational data

Lejenäs, Harald

doi:10.1256/smsqj.46608

Cited by 20 publications

(29 citation statements)

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“…Preferred Northern Hemisphere locations are to the north and east of the Pacific and Atlantic storm tracks. Similar studies for the Southern Hemisphere are much smaller in number [ Lejenäs , 1984; Trenberth and Mo , 1985; Rutland and Fuenzalida , 1991; Sinclair , 1996; Marques and Rao , 2000]. A preferred region for blocking action is found on the Australian‐New Zealand sector, as well as over the southeast of South America and the southern Indian Ocean.…”

Section: Introductionmentioning

confidence: 69%

Sensitivity of the Northern Hemisphere blocking frequency to the detection index

2002

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[1] The sensitivity of Northern Hemisphere blocking features to the use of two different blocking indices is investigated using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses (ERA) and data from a climate simulation carried out with the atmospheric general circulation model Action de Recherche, Petite Echelle, Grande Echelle ARPEGE. The seasonal cycle of blocking frequency shows a good agreement between both indices, for both analyses and model, when the seasonal cycle of the 500-hPa geopotential height is previously removed from the data. Furthermore, the blocking frequency seasonal cycle agrees well with the 5-to 45-day bandpass-filtered standard deviation (a proxy indicator of blocking activity) seasonal cycle. In contrast to previous results, this demonstrates that a consensus estimate of blocking features may be obtained with two different blocking indices. Nevertheless, some differences in the results from the two indices have been detected, particularly for the Euro-Atlantic blocking. Regional differences suggest that on some occasions, an index may detect blocking events that the other index misses, and vice versa. An important consequence is that uncertainty in blocking interannual variability estimates, which depend on the index used, is expected to be high. The study shows that the model underestimates the blocking frequency regardless of the index used. However, the model proves to be able to simulate realistic blocks when removing an estimate of the geopotential time-mean bias. Blocking drawbacks in the model are interpreted as a consequence of a specific zonal wind forcing associated with a decrease in the frequency of large-scale ridges over the main blocking regions. These results strengthen the interest of analyzing model blocking as a complex phenomenon with regional dynamical differences and tight links with other atmospheric phenomena.

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Section: Introductionmentioning

confidence: 69%

Sensitivity of the Northern Hemisphere blocking frequency to the detection index

2002

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“…These intraseasonal variations are similar to the so-called "breaks" in the monsoon activity known in other parts of global tropics. As yet, there are no systematic studies on these phenomena, and it is difficult to say if and how they are related to the blocking phenomena in the Pacific (Lejenas 1984;Kayano and Kousky 1990) and the MJO. During blocking episodes in the eastern Pacific, rain-producing transient disturbances steer to the south downstream of the region and avoid advancing to the north of 40 0 S over SA.…”

Section: C9 Intraseasonal Variabilitymentioning

confidence: 99%

South America

Satyamurty¹,

Nobre²,

Dias³

1998

Meteorology of the Southern Hemisphere

144

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“…For the SH, the first combined technique (subjective and objective) was originally proposed by Lejenäs (), in which the criterion derived from the kinetic properties of the flow was quantified using the well‐established concept of the zonal index (Namias, ). For instance, for a certain longitude to be blocked, Lejenäs () required easterly latitudinal zonal wind average. In other words, the height difference between 35 o and 55 o S should be less than zero, that is:

Z {((), λ)}_{φ_{N}} - Z {((), λ)}_{φ_{S}} < 0

…”

Section: Introductionmentioning

confidence: 99%

A new climatology for Southern Hemisphere blockings in the winter and the combined effect of ENSO and SAM phases

Oliveira¹,

Carvalho

Ambrizzi

2013

Intl Journal of Climatology

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This study presents 53-year climatology of Southern Hemisphere (SH) blockings in the winter using daily 500-hPa geopotential height data from NCEP-NCAR reanalysis. The variability of SH blocking events and their relationships with combined phases of El Niño/southern oscillation (ENSO) and the southern annular mode (SAM) are examined. Conventional indices were revised and a slightly modified index is proposed to detect latitudinal variations of SH blockings. The South Pacific region is examined in detail. There is no statistically significant long-term trend in the SH blockings. During moderate El Niño, the preferred location SH blocking is observed over East Pacific, and we show that the blocking frequency increases during negative SAM phases. During moderate La Niña the SH blockings are significantly suppressed over Central Pacific, with lower blocking frequency during positive SAM phases. These results indicate that the daily variability of SH blocking is strongly modulated by both ENSO and SAM phases.

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Characteristics of southern hemisphere blocking as determined from a time series of observational data

Cited by 20 publications

References 0 publications

Sensitivity of the Northern Hemisphere blocking frequency to the detection index

Sensitivity of the Northern Hemisphere blocking frequency to the detection index

South America

A new climatology for Southern Hemisphere blockings in the winter and the combined effect of ENSO and SAM phases

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