Each term a review of the global climate during the past season takes place, organised at the University of Reading by Mike Blackburn (NCAS-Climate). The seasonal climate discussion for summer 2010 took place on the 26th November. The aim was to discuss physical factors that may have influenced the seasonal climate in different parts of the globe. The session was not short of material; several spells of extreme weather around the world were reported in the media throughout the summer. Some of these are summarised here.
One significant feature of the global circulation during summer 2010 was a persistent ‘omega’ blocking pattern over Asia, with a ridge over Russia and troughs to the west over the Mediterranean and to the east over Kazakhstan. This pattern resulted in a strong heatwave in western Russia, with temperatures at least 5°C higher than normal during July and August. The eastern trough, situated over Kazakhstan, appeared to enhance anomalous south-westerly flow from the Arabian Sea, which was linked to the floods in Pakistan at the end of July.
Top: ECMWF analysis of a measure of temperature on the tropopause between 23-30th July 2010. The omega (Ω) shaped block has been highlighted. A clear wave pattern is evident within the ellipse, with warm (red) subtropical air pushed north over western Russia and cooler (blue) air moved south over the Mediterranean and Kazakhstan. Bottom: Global map of temperature anomalies for July 2010.
ENSO
Thomas Toniazzo outlined the evolution of the El Nino-Southern Oscillation (ENSO) over the year. In spring, sea-surface temperatures (SSTs) shifted from the anomalously warm (El Nino) to anomalously cold phase (La Nina) in the tropical east Pacific. In the Nino4 region (which straddles the date line in the central Pacific), the transition to La Nina conditions was the most rapid in the observational record, with an average change in SSTs of 0.5°C per month. The La Nina was also unusual because, despite cold anomalies developing in the central and east Pacific, warm ocean heat content anomalies persisted in the west Pacific. It is well known that ENSO alters the atmospheric Walker circulation over the south Pacific, which influences circulation patterns around the globe (see Ricardo’s blog on Atlantic hurricanes for one example of these global ‘teleconnections’). Thus the shift to La Nina conditions is likely to have played a role in seasonal climate experienced elsewhere in the world.
Timeseries of the Nino 4 index in the central Pacific. The series shows a rapid transition from positive (El Nino) to negative (La Nina) conditions in 2010. Source: NOAA Climate Diagnostics Bulletin.
Indian monsoon
Andy Turner (NCAS-Climate) gave an overview of the Indian summer monsoon season. In India itself, the monsoon season was fairly typical with total rainfall around 2.7% above average, well within the year-to-year variability. There is some indication that Madden-Julian Oscillation events were associated with the onset of the monsoon in May and a slight delay in the monsoon offset, which led to September being slightly wetter than normal in the north-west.
However, despite the average Indian rainfall, Pakistan received anomalously high summer rainfall, which caused wide spread damage and disruption and featured heavily in the media. For more details see Andy’s blog on the Pakistan floods.
Tim Hewson (ECMWF) showed that a low pressure system tracked into the Arabian Sea before the main precipitation event which caused the flooding in Pakistan. The influence of the low pressure system, alongside the pre-existing trough situated to the north of Pakistan (associated with the large-scale ‘omega’ blocking pattern), was associated with inflow of moist air from the south-west. This led to strong precipitation over a few days at the end of July which was enhanced by the complex orography in the region. This was the main precipitation event that resulted in widespread flooding.
Australia
Nick Klingaman (NCAS-Climate) presented an overview of the Australian winter climate in 2010. The dominant pattern for the season was a strong anticyclone to the south-west of Australia. This resulted in a storms being anomalously diverted to the north and south around the anticyclone. The increase in cyclonic activity in north-west Australia meant that regional rainfall was 4 times higher than usual (although this is not large absolute amounts because the region is very dry in general). In contrast, south-west Australia had the lowest January to October rainfall on record; this is part of a drying trend observed in recent decades that is thought to be linked to long-term changes in the large-scale circulation in the Southern Ocean. Studies suggest this may be linked to stratospheric ozone depletion and atmospheric CO2 increases.
There has been shown to be a link between SSTs in the Indian Ocean (Indian Ocean Dipole) and Australian rainfall patterns; however, it does not appear that this mechanism played a significant role this season.
Temperatures in north-east Australia were around 2°C above average whilst central regions were up to 3°C cooler.
Maps of June to August rainfall and temperature anomalies in Australia. Source: Australian Bureau of Meteorology.
Africa
Many coastal parts of western Africa (e.g. Ivory Coast, Liberia, Ghana) and the Sahel experienced above average rainfall, whereas central Africa (e.g. Central African Republic, Chad) had below average rainfall in summer 2010. This pattern of anomalies is consistent with the above average SSTs observed in the tropical north Atlantic and a strengthened and northward shifted jet stream. The early onset of the African monsoon in May was coincident with a Madden-Julian Oscillation event.
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The review of the upcoming winter season will take place in the spring. Keep your eyes peeled for further information!