Northern and Southern Hemisphere Seasonal Variability of Blocking Frequency and Predictability

Tibaldi, S.; Tosi, E.; Navarra, Antonio; Pedulli, L.

doi:10.1175/1520-0493(1994)122<1971:nashsv>2.0.co;2

Cited by 127 publications

(157 citation statements)

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“…Such deficiencies have often been attributed to insufficient horizontal resolution, where small-scale features, such as atmospheric eddies important for maintaining larger-scale blocking patterns through positive feedbacks, are insufficiently resolved (Kug and Jin 2009;Scaife et al 2011, and references therein). While higher horizontal resolution does improve biases in blocking (e.g., Tibaldi et al 1997;Ringer et al 2006;Matsueda et al 2010), underestimates of blocking activity were linked to errors in the mean model climatology (Scaife et al 2010(Scaife et al , 2011. This echoes early work by Kaas and Branstator (1993), who found the zonal mean state in numerical simulations to be highly influential for blocking activity.…”

Section: Introductionmentioning

confidence: 52%

“…The BI in Eq. (1) is appropriate for use in the Australian region given its specific design for the region and inclusion of a northern component measuring the strength of the subtropical jet, which is missing in other definitions of the BI (e.g., Tibaldi and Molteni 1990;Tibaldi et al 1994). Hence, the BI used here is a more comprehensive measure of blocked flow for the AustraliaNew Zealand sector.…”

Section: Blocking Indexmentioning

confidence: 99%

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Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

et al. 2013

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Characteristics of atmospheric blocking in the Southern Hemisphere (SH) are explored in atmospheric general circulation model (AGCM) simulations with the Community Atmosphere Model, version 3, with a particular focus on the Australia-New Zealand sector. Preferred locations of blocking in SH observations and the associated seasonal cycle are well represented in the AGCM simulations, but the observed magnitude of blocking is underestimated throughout the year, particularly in late winter and spring. This is related to overly zonal flow due to an enhanced meridional pressure gradient in the model, which results in a decreased amplitude of the longwave trough/ridge pattern. A range of AGCM sensitivity experiments explores the effect on SH blocking of tropical heating, midlatitude sea surface temperatures, and land-sea temperature gradients created over the Australian continent during austral winter. The combined effects of tropical heating and extratropical temperature gradients are further explored in a configuration that is favorable for blocking in the Australia-New Zealand sector with warm SST anomalies to the north of Australia, cold to the southwest of Australia, warm to the southeast, and cool Australian land temperatures. The blockingfavorable configuration indicates a significant strengthening of the subtropical jet and a reduction in midlatitude flow, which results from changes in the thermal wind. While these overall changes in mean climate, predominantly forced by the tropical heating, enhance blocking activity, the magnitude of atmospheric blocking compared to observations is still underestimated. The blocking-unfavorable configuration with surface forcing anomalies of opposite sign results in a weakening subtropical jet, enhanced midlatitude flow, and significantly reduced blocking.

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Section: Introductionmentioning

confidence: 52%

Section: Blocking Indexmentioning

confidence: 99%

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

et al. 2013

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“…This indicates that the cyclones that reached the region were weaker than what would have been expected based on the climatology due to the subsidence associated with the persistent blocking structure. The SA region is not known as a preferred centre of blocking action in the SH (Tibaldi et al, 1994;Wiedenmann et al, 2002;Renwick, 2005), but the blocking in March 2004 was demonstrated to be within the 0.62% range, being most intense over the last 25 years (Pezza and Simmonds, 2005). The hemispheric view of the DP also shows that most of the high latitudes around Antarctica presented above-average values.…”

Section: The 'South Atlantic Index' and High-latitude Interactionsmentioning

confidence: 95%

Climate perspective on the large‐scale circulation associated with the transition of the first South Atlantic hurricane

Pezza

Simmonds

Filho

2008

Intl Journal of Climatology

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ABSTRACT:The landfall of Cyclone Catarina on the Brazilian coast in March 2004 became known as the first documented hurricane in the South Atlantic Ocean, promoting a new view on how large-scale features can contribute to tropical transition. The aim of this paper is to put the large-scale circulation associated with Catarina's transition in climate perspective. This is discussed in the light of a robust pattern of spatial correlations between thermodynamic and dynamic variables of importance for hurricane formation. A discussion on how transition mechanisms respond to the present-day circulation is presented. These associations help in understanding why Catarina was formed in a region previously thought to be hurricane-free.Catarina developed over a large-scale area of thermodynamically favourable air/sea temperature contrast. This aspect explains the paradox that such a rare system developed when the sea surface temperature was slightly below average. But, although thermodynamics played an important role, it is apparent that Catarina would not have formed without the key dynamic interplay triggered by a high latitude blocking. The blocking was associated with an extreme positive phase of the Southern Annular Mode (SAM) both hemispherically and locally, and the nearby area where Catarina developed is found to be more cyclonic during the positive phase of the SAM. A conceptual model is developed and a 'South Atlantic index' is introduced as a useful diagnostic of potential conditions leading to tropical transition in the area, where large-scale indices indicate trends towards more favourable atmospheric conditions for tropical cyclone formation.

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“…Highlatitude Pacific cyclones, such as the 960-hPa center shown, tend to be steered into the Bellingshausen Sea region, rather than continuing through the Drake Passage. This is a condition of interest, as Northern Hemisphere studies have shown that blocking events produce conditions that result in lower quality numerical weather prediction (e.g., Tibaldi and Molteni 1990). Additionally the quality of forecasts during blocking conditions has been found to be related to low model resolution, which results in a feedback loop between regional and planetary scales (Tracton 1990).…”

Section: Model Description and Approachmentioning

confidence: 99%

“…Additionally the quality of forecasts during blocking conditions has been found to be related to low model resolution, which results in a feedback loop between regional and planetary scales (Tracton 1990). In the Southern Hemisphere, however, the sparse observational data coverage over the Southern Ocean has been previously shown to be a significant obstacle to the prediction of blocking events (Tibaldi et al 1994). …”

Section: Model Description and Approachmentioning

confidence: 99%

An Assessment of the NCEP Operational Global Spectral Model Forecasts and Analyses for Antarctica during FROST*

Bromwich

Cullather²,

Grumbine³

1999

Wea. Forecasting

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Analyses and medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction are evaluated poleward of 50ЊS during the July 1994 special observing period of the Antarctic First Regional Observing Study of the Troposphere project. Over the Antarctic plateau, the poor representation of the continent's terrain creates ambiguity in assessing the quality of surface variables. An examination of the vertical temperature profile, however, finds the near-surface temperature inversion strength to be substantially smaller than the observed climatology at the zero forecast hour. This arises from surface temperatures that are warmer than expected. Significant adjustment occurs in a variety of fields over the first few days of the mediumrange forecast, which likely results from the initial hour's suspect temperature profile. A spatially oscillating series of forecast anomalies in the zonally averaged temperature cross section stretches to middle latitudes by day 3. Near-surface and upper-troposphere values are found actually to improve at the South Pole with forecast time, although some fields continue to adjust through day 7. Although the examination presented here does not give a complete diagnosis, differences between observations and analyses suggest deficiencies with the model initial fields have a major role in producing the substantial model drift found. Atmospheric moisture over the continental interior does not change significantly with forecast hour, although the distinct contrast between nearshore and interior conditions lessens with forecast time. A spurious high-latitude wave pattern is found for a variety of variables. The pattern of this distortion remains constant with forecast hour. Over the ocean, large forecast pressure and height differences with analyses are associated with blocking conditions. However, it is unclear whether this results from deficiencies in the forecast model or the meager observational network over the Southern Ocean.

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Northern and Southern Hemisphere Seasonal Variability of Blocking Frequency and Predictability

Cited by 127 publications

References 0 publications

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

Climate perspective on the large‐scale circulation associated with the transition of the first South Atlantic hurricane

An Assessment of the NCEP Operational Global Spectral Model Forecasts and Analyses for Antarctica during FROST*

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